human reproduction//male reproductive system, male germ cell, scrotal cell, testes, penis.

 Scrotal sac

In human males, the testes are situated outside the abdominal or pelvic cavity in a sac of skin called the scrotal sac. It acts as thermoregulation to keep the testicular temperature 2-3c lower than the optimum temperature necessary for sperm production (spermatogenesis).  It is biologically homologous to the major in females. 

Testes

These are the primary male gonads in man. Each testis is oval in shape, with a length of about 4-5 cm and a width of 2-3 cm. They develop in the scrotum by a spermatic cord in the later stages of life.

Cremaster muscles and dortas muscles of the scrotal sac help in the positioning of the testes. Whenever the temperature is low, these contract to move the testes close to the abdomen or pelvic cavity and vice-versa.

Note * Cremaster muscle and connective tissues surround all structures passing through the inguinal canal including the spermatic cord. 

* If testes fail to descend, this condition is called cryptorchidism, which leads to sterility. inflammation of the testis is called orchitis.

* Gubernaculum is a fibrous cord that extends from the caudal end of the testis to the scrotal wall.

Tunica of testis

The testis is surrounded by three protective coverings known as tunicates. These are as follows:

(1) Tunica vaginalis It is the outer covering of the testes.

(2) Tunica albuginea It is the white fibrous capsule that sends partitions through its interior surface to divide the testis onto lobules.

(3) Tunica vasculosa contains a network of capillaries that are supported by delicate connective tissue.

Seminiferous tubules

Each testies has about 250 compartments called testicular lobules and each lobule contains one to three highly coiled seminiferous tubules, where the sperms are produced. Seminiferous tubules from loops at both ends, which continue as a short segment of straight tubules, the tubuli recti. These tubules connect the seminiferous tubules to a highly anastomosing labyrinth of cuboidal epithelium-lined channels, the rete testis, which gives rise to 10-12 fine tubules called ductules efferentes.

Tubuli recti, rete testis, and ductile efferentes from the intertentacular genital duct systems. the tubules of ductuli efferentes combine to form the ductus epididymis.

The stratified epithelium of the seminiferous tubule is made up of three types of cells.

(1) Sertoli cells (supporting cells) These are also called sustentacular cells or nurse cells. They are elongated and pyramidal, which partially envelope the spermatogenic cells. This cell secret a glycoprotein hormone called inhibin, which is involved in the negative feedback control of sperm production by checking FSH activity.

(2) Spermatogenic cells Line the space between the basal lamina and the lumen of the tubule. These cells divide several times and differentiate to produce spermatozoa.

(3) Leydig cells (interstitial cells) They are a group of polyhedral cells and are present between the seminiferous tubules.

These cells secrete the testosterone hormone that controls spermatogenesis. These are endocrine in nature and are a characteristics feature of the testes of mammals.

Vasa efferentia(Sing. vas efferens)

A set of about 10-20 vasa efferentia collects from inside the testis and transfers them to the epididymis through rete testis. It has large columnar ciliated cells and small non-ciliated cells with endocytic activity, which help in conducting sperms.

Epididymis 

The epididymis is a coiled tube, which consists of three parts. The anterior end called caput epididymis, middle part corpse epididymis, and the posterior end is cauda epididymis. It is lined with pseudostratified columnar epithelium. It stores by sperm temporarily and produce essential chemicals required for the nourishment and maturation of sperms.

Vasa deferentia

It is a straight tube, which carries sperms to the urethra. Most of the sperms are stored in vas deferens. vas deferens is also called ductus deferens or sperm duct. It arises from cauda epididymis, ascends to the abdomen, passes over the urinary bladder, and receives the duct from the seminal vesicle behind the urinary bladder forming an ejaculatory duct. it passes through the prostate to open the ejaculatory duct. it passes through the prostate to open into the urethra shortly after its origin from the urinary bladder.

Urethra

It carries urine from the bladder as well as, sperms from the vas deferentia, through the penis. Thus, it is known as urinogenital as it provides a common pathway for the flow of both urine and semen.

It comprises three regions

(1) A short proximal prostatic urethra surrounded by the prostate gland. 

(2) A very short middle membranous urethra without covering.

(3) A long distal penile urethra that passes through penis.

penis

It is male external genitalia. It is vascular, spongy contains erectile tissue and a pensile organ suspended from the public region in from public region in front of the scrotum. During sexual arousal, erectile tissue fills with blood, causing the penis to become hard and erect to facilitate copulation and transmit sperms into the vagina of the female. Therefore, it is also called a copulatory organ. it has a glance penis at the tip, which is covered with a loose fold of skin called the prepuce or foreskin. The margin of glance penis is known as Corona.

The interior of the penis consists of three cylindrical cords made of spongy and erectile tissue. Two of them are parallel and called corpora cavernosa, which become superior-posterior when the penis is erect. The third, spongiosum corpora urethrae remain inferior-anterior in an erect penis. It is called so because the urethra runs from this cord. The pineal mass is covered by a dense connective tissue,m called tunica albuginea. Tyson's gland or preputial gland present in the penis neck secret a white sebaceous substance called smegma.

Regeneration, regeneration in animals, regeneration in planaria, regeneration in hydra.

 REGENERATION

It is the capacity of the body to produce the lost part of the proliferation of the cells. Regeneration leads to the healing of wounds, replacing worn outcall, and even the formation of the complete individual. 

Types of regeneration 

There are mainly two types of regeneration. These are as follows 

1. Reparative regeneration 

It is localized cell proliferation and cell migration leading to repair or headline. It is quite common in both invertebrate and vertebrate groups, e.g earthworms, amphibians, salamanders, and axolotl larvae. Reparative regeneration also involves compensatory hypertrophy, i.e. growth of a part that is removed or damaged. 

2. Restorative regeneration

It is the restoration or replacement of a lost body part. In this process, even a complete organism can be formed from a fragment of a body, e.g. in Hydra, Planaria, and sponges.

Restorative regeneration decreases with the increase in complexity of the organization of animals. It involves the differentiation of cells, mitotic divisions, cell movements, etc.

Mechanism of regenration

TH Morgan recognized two primary mechanisms of regeneration. These are as follows

(1) Morphallaxis  (Morpholloactic regeneration)

It involves the reconstitution of the body from small fragments by the reorientation and reorganization of the existing body cells. The regenerated animals are smaller than the original ones after the completion of the process, e.g. hydra.

(2) Epimorphosis (Epimorphic regeneration)

It takes place by the proliferation of new cells from the surface, tail in lizard, etc. In the case of crustacean, if the antenna is cut, etc. In the case of crustacean, if the antenna is cut, it may be replaced by an eye or vice-versa. This is called heteromorphisms. in the regeneration of mammalian liver or kidney, the cells divide, but do not form an undifferentiated mass of cells or tissues. These produce cells similar to themselves and maintain their differentiated function. This intermediate type of regeneration is called compensatory regeneration.

Examples of regeneration 

Regeneration occurs in the organism of almost every class some example of regeneration occurring to the groups are given below

Protozoans:

Ciliated like Stentor, Blenpharisma, and Spirostomum required only cell cortex for regeneration. About 1/100th part of the body of Acetabularia can regeneration the entire body.

Poriferans

They possess the remarkable power of regeneration. Any part of the body cut off or injured is readily repaired. Small pieces of sponges (containing amoebocyte) can grow into a compiled individual. 

Coelenterates

Hydra has excellent power of regeneration that was discovered by Trembley. he cut the body into sections and the lost part is regenerated with exact polarity. in hydro, it is the interested cell, which is totipotent and responsible for Completed regeneration. in hydra, regeneration will be faster if it is cut off from tentacles. hydra has a unique capacity of regenerating its hypostome (Ireland) again and again. this is called repetitive regeneration that has made it virtually immortal.

Planarians

(Platyhelminthes) They have a stock of undifferentiated cells called neoblasts, which are omnipotent. They may regeneration the cut portions, lost parts, and whole body. Planaria show super regeneration, i.e. the development of cells or organs in a number that is more than required.

Annelids Sealing of the wounds by neoblast cells.

Mollusca Regeneration is poor in gastropods. Eye stalk with the eye may be regenerated. Cephalopods may regeneration their arms.

Nematodes Regeneration is poor, only closure of the superficial wound is observed.

Arthropods Limited renewal of lost appendages as in certain crustaceans and spider takes place by regeneration.

Echinoderms  Starfish, brittle stars, and sea lilies can regenerate arms and parts of the disc.

Fishes   Regeneration restricted to fish fins.

Amphibian Limb regeneration. 

Reptiles Lizards are known to regenerate their tails which they lose by autotomy, i.e. self-amputation of a part of the body when threatened by a predator.

Birds Parts of the beak can be regenerated, otherwise, the regenerative ability is poor.

Mammals Extremely poor regenerative capacity, few marsupials can regenerate part of the limbs and liver.

                                          

                                                             

Pre-Mendelian experiment || Mendel's Laws of Inheritance// Heredity and variation

 Pre-Mendelian experiment 

As we know, reproduction in living organisms is done to facilitate the continuity of life. The progeny produced, often resemble their parents in most of the characteristics. This passage or transmission of characters from parents to their offsprings is called Heredity and the process by which characters from one generation to the next is called inheritance. However, all characteristics of offspring are not similar to their parents. Some of them show variations or changes also. These variations signify their own characteristics.

The study genetical of heredity and variation is called genetics. This term was proposed by William Basten's in 1905. The basic principles of genetics were discovered by Mendel in (1822-84). The medal is also known as the 'father of genetics'.

                                                              

Branches of genetics

Geneticists (the scientists who the process of genetics) often divide genetics into the following three main branches,

(1) Trancemission genetics is also called classics genetics. It involved the transmission of genetic material from one generation to another. it is further subdivided into Mendelian genetics, Morganton genetics (i.e. recombination in all kinds of organisms), non-Mendelian genetics, and mutation.

(2) Molecular and biochemical genetics involve the study of structural and functional aspects of genes at a molecular level.

(3) Population and biometric genetics involve the study of behaviour and effect of genes in various biological populations.

Heredity

It is defined as the transmission of structural, functional and behavioural characteristics from one generation to another. Thus, for a generation, it is the sum of genetic endowment obtained from their parents. Thus, for a generation, it is the sum of generation endowment obtained from their parents. The initiation of genetics work is attributed to Mendel. Genetics can be categorised into Pre-Mendelian, Mendelian and Post-Mendelian eras of working.

Pre-mendelian concepts of inheritance(theories of blending inheritance)

Before the idea of Mendel was accepted, there were many philosophers, thinkers and workers who gave various theory to explain heredity or inheritance. These theories are called Pre-Mendelian concept of heredity. Some of these are as follows, 

1.Moist vapour theory Pythagoras (580-500 bc) propose that various body part emit certain vapour, which gets aggregated to form a new individual. 

2. Flud theory Empedocles (504-433) through that every part of the body produces a fluid. This fluid then mixes up to form an offspring. Any defect in this fluid result in the formation of defective organs.

3. Preformation theory this was given by Swammerdam (1679) and by Malgiphi (1673). They believe that the Minature form of an individual is already present in the sperm or egg called 'homunculus'. Fertilisation is required and nassory to stimulate its growth. 

4. particulate theory Maupertius (1689-1759) Propose that various part of the body produce minute particles for reproduction. An individual is formed when the particles from the male and female unit.

5. Theory of pangenesis Darwin (1868) proposed that each body part produces a gemmule or Pantene. These pangenes carry parental strategies as well as variation. Their aggregation result to form gametes. On fusion, these rise to a new individual.

Pre-Mendelian experiment

Among the workers who performed the experiment before Mendel was a German botanist who works on the tobacco plant.

Other persons who worked on hybridisation before Mendal, include Bartner (1722-1850), Naudin (1815-1899) caught (1799) and goss (1824). but none of these workers was able to rationalise and explain the result which a sure degree of repetitions.

what is the growth? factory of growth|| imp information about information?????

 growth

                                                 

growth in living beings can be defined as the irreversible increase in overall size and weight of an organism due to the synthesis of new protoplasm.

forms of growth

In living organisms, growth can occur in two following forms.

Intussusception: It is an internal type of growth. In this form, the surface area of the cell increasing via deposition of any external material, that is synthesized by the existing body cell.

Apposition It is an external type of growth in which the surface area of the cell increase via deposition of any external material, this is not the constituent of the cell. Non-living, like mountains, sand mouth, and snow also grow by apposition, if the increase in body mass is the criterion. however, this growth is by the accumulation of material on the surface only.

Features of growth

At the cellular level, growth involves

1. Increase in size of the cell due to the synthesis of protoplasmic structure (hypertrophic) 

2. increase in the number of cells by cell division ( hyperplasia )

3. Increase in the amount of apoplastic structure like the fibres, material, etc.

Different cells may grow differently in humans for examples

1. Lens cells grow by multiplication.

2. Cardiac and skeleton muscle cells grow by the increase in volume.

3. Newron growth by extension and growth of the axons and dendrites.

4. Cartilage and bone cell growth by the secretion of extracellular Matrix.

Pattern of growth

On the basis of changes in body proportions, there are two types of growth patterns.

(1) Isometric growth It is a type of growth in which all the parts grow at the same rate and same time Thus, there is no change in the form and body proportion, e.g. fish, locust, and grasshopper.

(2) Allometric growth It is a type of growth, in which different organs grow at a different rate and at different time. Thus, the external form and body proportion change which the stage of growth, e.g. mammals.

Types of growth

The different types of growth in the bodies of a living organism are 

(1) Auxetic growth It is the growth due to the increase in the size of cells (not a number), e.g. nematodes, rotifers, and tunicates (early chordates).

(2) Multiplicative growth It is the growth of the body due to the increase in the no of cells, e.g. embryonic development of higher vertebrates.

(3) Accretionary growth   It is the growth in which undifferentiated reserve cells get differentiated to perform a specific function, e.g. formation of erythrocytes from erythropoietic tissue, replacement of worn-out cells of the skin, secretion of extracellular matrix by chondrocytes and bone osteocytes.

Growth curve

It is a graphic representation of growth against time. The sigmoid curve or S-shaped growth curve is the characteristic growth curve of all higher animals including humans. In the initial phase, The curve rise at a very slow rate.

In the middle part, the curve rises steeply indicating a very fast rate of growth (acceleration phase). Finally, in the last part, the rate of growth again slows down till the curve becomes horizontal (Plateau phase).

It signifies that no further growth is taking place and the individual is simply itself. Hence, animals show a differential growth rate.

An absolute growth rate curve shows how much the rate of growth changes with time, whereas a relative growth rate curve is taken into account the existing size.

Positive and negative growth 

Positive growth occurs when the synthesis (anabolism) of the material exceeds the crackdown (catabolism) of the material. in some cases, continuous positive growth is non-vertebrates, fish, and certain some reptiles. in such cases, growth may continue to rise slightly until the organism dies.

Negative growth occurs when catabolism exceeds anabolism. It is during the germination of seeds. Germination of seeds includes a period of negative growth, which becomes positive when seedlings start to photosynthesis and make their own food. In certain cnidarians, the growth curve flattens out, indicating zero growth.

Growth in human

the human body undergoes the following pre-birth phase of growth.

                                                    

1. cellular growth

The unicellular organism grows only through cell growth. In a multicellular organism, growth is the sum total of growth shown by their cells. The steps involved in cellular growth initiate with cell division following by cell enlargement and cell differentiation finally leading to maturation. cell growth occurs during the post-mitotic phase and interphase.

2. Embryonic growth

In humans, the early embryonic development stage constitutes transitional growth. During cleavage and blastula formation, the cells exhibit little growth.

During gastrulation (early embryonic state), rudiments of primary organs develop, but they are in their functional state. with further development, the rudimentary organs grow, differentiate, and become functional.

The overall growth of the fetus occurs mainly in the period after the basic morphological plane is established, i.e. 

Post-embryonic growth

East animals have a specific rate and rhythm of growth. There is no uniformity in the rate of growth. Animals grow at different rates at different periods of life. The human embryo is about 150-micro meters at the time of implantation, which grow about 50 cm over the 9 months of gestation. embryos can not grow considerably during the first 2 month after implantation as it is a time for the formation of basics human body structure. after four months, the embryo grows 10 cm/ month and during the first year, 2 cm/month and it stops after 22-33 yrs of age. 

Control of growth

Growth depends on growth factors and hormones. The various growth factors involved in the control of growth in the human body are discussed in afters.

ape man || Homo habilis, Homo erectus discovery and characteristics

 Australopithecus africanus

                  Discovery

                                                  Prof. Raymond Dark (1924), a South African anthropologist discovered the skull of a 5-6-year-old baby, about 5 million years old near the Tuang river in Africa. This baby fossil is termed Australopithecus africanus ( African ape-man or Tuang baby),

                                                The 3.8 million-year-old fossils of a new species Australopithecus Africanus was first discovered by Mary Leakey from the volcanic ash of Lateral and later on Donald Johanson and Timothy D white established a new species.

                                               Zojanthropus (the old vain toolmaker) was discovered by Mary Leakey and LBS Leakey (1959) from Tanzania (East Africa) which has been named Australopithecus boisei by Leakey. 

                                            A. Africanus (African ape-man or southern ape-man or Tuang baby), A. boisei (Olduvai tool man), A. Afarensis (Lucy), Aramids, A. aethiopicus, and A. robustus are six known species of Australopithecus.

                 Characteristics

                                        Its height was 1.5 m and both human and ape's characters.

                         

                                       It was a fully bipedal hominid.

                                      It lived from 4-1.5 million years ago in caves during the Pleistocene period.

                                     It has an erect posture and an omnivorous diet.

                                     Its cranial capacity volume (brain size) was 500-700 cc,i.e. equivalent to a modern gorilla.

                                     Brown ridges projected over the eyes and lack chin.

                                   A distinct lumbar curve was present in the vertebral column.

                                 Its thigh and hips were adapted for erect standing, walking, and running.

                                  Its arms were somewhat less mobile than ours and pelvis were broad.

                                 Its ankle bone was intermediate between human and ape.

                              

                                 It had protruding jaws and a modern man like teeth.

                        

                               Its face was prognathous with larger jaws and teeth than modern man.

                               Australopithecus africanus gave rise to Homo habilis approximately 2 million years ago, also a separate line gave rise to Australopithecus robustus and Australopithecus boisei.

Homo habilis

                       Discovery

                                                Louis SB Leaky and his wife Mary Leakey (1961) obtained the fossils of homo habilis Pleistocene rocks of Olduvai Gorge in East Africa.

                                              Richrd Leakey (1972) also obtain fossils of Homo habilis from the east side of Lake Turkana in Kenya.

                      Characteristics

                                           Homo habilis is called the first toolmaker because he had developed the most primitive stone tools.

                                         Homo habilis man was about 1.2-1.5 m tall.

                                        Its cranial capacity was 700-800 cc, which lived in Africa about 2 million years ago.

                                    Homo habilis was carnivorous and begun hunting for meat.

                                  Homo habilis lived in small communities or groups in caves.

                                 Perhaps they show the sexual division of labor and communication with visual signals and simple audible sounds.

                                                                 

Homo erectus

                       Discovery

                                             Fossils of Homo erectus were obtained from diverse sites from Olduvai Gorge in Africa java, Algeria, Germany, Hungary, and china. 

                                              Fossils were 800000 to 30000 years ago.

                                             Homo erectus is considered the direct ancestor of modern man. It evolved from evolution.

                    Characteristics

                                         They were the oldest known early human to have modern human-like body proportions.

                                        There were the first human species to have a fleshy nose. they had a flat skull with prominent ridges over the brow.

                                      They had short arms and long legs. The long legs depict that they are better suited for long-distance migration

                                  They were the first ones to walk upright and stood erect thus, named so. They are also known as Homo register.  

Pithecanthropus erectus

                  Discovery

                                   In 1891 Eugene Dubois obtained fossils (some teeth, skull cap, and femur bone) from Pleistocene deposits (500000-1500000 year ago) in Central java (an island of Indonesia).

                             It was named Pithecanthropus erectus (ap man that can walk erect) by Eugene Dubois and Homo erectus by Mayer (1950)

    Characteristics

                             Java man was more than 1.65-1.75 m tall weight about 70 kg.

                             Its legs were long and erect, but its body slightly bent during movement.

                            The face of the Java man was prognathous, Whereas the forehead was low and slanting.

                          Its Java was large and heavy, but quite similar to those of modern man, except large canine of the lower jaw.

                         Its nose was flattened and chin inconspicuous along with thick and protruding lips.

                       Java man was omnivorous and cannibal.

                     Java man was the first prehistoric man, who began the use of fire for cooking, defense, and hunting.

                     Its crinal cavity was 940mcc, which is about intergraded between Australopithecus (600-700 cc) and modern man (1400-1600 cc). 

Sinanthrops

         Discovery

                       The fossil (skulls, jaws, and postcranial bone fragments) of parking man were discovered by W C PAI (1924) from the limestone caves of Choukoutien near Peking (Peking is the former name of china"s capital Beijing).

WC PAI named java man Sinanthropus, whereas Davision Black (1927) named it Sinanthropus Pekinensis, and Mayer (1950) called it HOMO Erectus Pekinesis.

          These fossils of Peking man was about six lakh year old.                                                                                                                                          

    

   Characteristics

     Peking man was1.55-1.60 m tall, i.e. slightly shorter, lighter, and weaker than java, man.

The cranial cavity of Peaking man was 850-1200 cc than is more than java man.

The peaking ape-man was omnivorous and cannibal.

  clear evidence has been obtained from Hungary and China in support of the use of fire by peaking man. These were hunters and food gatherers. 

Like java man, Peaking man also lived in caves in small groups or tribes or communities.

The peaking man was first to bigger with the uses of clothes ( made from animal skins), rudimentary languages, and tools made with woodman's bones.

Morphology was similar to a java man with heavy bony eyebrow ridges, a low slanting forehead, chinless face.                                       

 

human evolution // Ramapithecus, Dryopithecus discovery and cheracteristics.

human evolution

It is the aspect of evolution which is concerned with the emergence of human as distinct species. Various evidence support that humans and apes have descended from common ancestors.

The beginning of primates' evolution is presumed in the Eocene of the Tertiary Period (75-60 million year ago) in evergreen forest and later in Oligocene (25-30 million year ago). Its returns of the ground from the tree due to replacement of evergreen forest into dried savanna-Greenland. thus 

The place of origin of humans is a grate controversial matter. The fossil of ancestral human forms is obtained from the widely diverse regions of Africa, Asia, and Central Asia, China, Java, and India (Shivalik Hills).

Many fossils have been discovered from East Africa (Olduvai Gorge, Lake CIctoria, and Lake Natron in Tanzania and Lake Turkana in Kenya) which indicate a significant role of Africa in regard to the site of origin of human.

Changes During human evolution 

Some changes that took place during human evolution are given below 

(1) Skull and Brain increase in size, complexity, and intelligence.

(2) bipedal locomotion

due to bipedal locomotion forelimbs become free 

(3) Forelimb 

it is modified into an organ of manipulation and lengthening of hindlimbs and shortening of forelimbs.

(4) Thumb

the perfection of thumb opposability in forelimbs.

( 5) Toe 

loss of opposability of great toe in hindlimbs.

(6) Upright posture

becomes of development of the lumbar curve.

(7) Jaw

Power reduction

(8) Teeth

due to omnivorous food habits, the size of incisors and canines reduced.

(9) Chin

development of the chin 

(10) Pelvic girdle

development and broadening of iliac bones of the pelvic girdle.

(11) Social organization

and cultural evolution 

History of human evolution 

The study of living and fossil primates (including monkeys and man) shows that they have descended from a common ancestor and separated from the main stock ( of a homogenization) during the Oligocene period.

The evolutionary history of man can be divided into the following three staps only for the convenience of study 

1. prior to the ape man

2. Apeman 

3. True man (including living modern man)

                            Prior to the ape man 

(1) Propliopothecus 

            *  Discovery : 

                                     fossil obtain from Faiyum deposits of Egypt.

                                    Fossils have Jaws and teeth.

                                    

                                    It lived about 35-30 million yrs ago in the Oligocene epoch.

            Characteristics 

                                  It was more or less ape-like.

                                  These were short-statured with monkey-like teeth.

                                   Their dental formula was 2,1,2,3, i.e. both jaws have 8 incisors, 4 canines, 8                                              premolars, and 12 molars like modern man. 

                                  Their incisor teen was vertical rather than directed forward.

                                 Their molar has 5 cups each.

                                 There are two views regarding ancestry. One says Propliopithecus directly gave rise toRamapithecus, while by another view Propliopithecus gives rise to Dryopithecus which in turn evolved into apes and humans.

(2) Aeggyptithecus 

                  Discovery: 

                                        fossil found from Faiyum province of Egypt in 1980 by E Simon

                     Characteristics: 

                               These were similar to Propliopithecus.

                              These live between 35-33 million yrs ago in the early part of the Oligocene epoch.

                             The predates divergence between hominoids (apes and humans) and old-world monkeys.

Limnopithecus and pliopithecus

                 Discovery

                             Fossils found from Faiyum deposits of Africa.

               Characteristics

                     This existed in Miocene and Pliocene periods.

                    They had a short tail and 7 lumbar vertebrae.

                   They represent altogether a different line of evolution which diverged from the hominid line quite early and become extinct during the late Pliocene.

Proconsul (intermediate between apes and man) 

              Discovery

                  Fossil of proconsul Africanus was described by Louis SB Leakey(1948).

                 The fossils were obtained from the rock of East Africa near Victoria Lake in Kenya. It lives in the early Miocene

                Characteristics

                  The face was prognathous.

  

                 Proconsul walked on its four legs.

             

                   Proconsul had long, point ape-like canines.

            

                   Proconsul has a man like a forehead.

                  Proconsul is not considered as the base ancestor of man because it walked on its four legs.

                 About four million years ago, during Proconsul, proconsul gave rise to the ancestors of chimpanzees and gorillas.

                 Diversion of chimpanzee and gorilla took place during the Pleistocene epoch about 2.3 million years ago.

                                                                   

Dryopithecus

        Discovery

              Fossil of Dryopithecus Africanus was obtained from rock of Africa and Europe.

              

             Recently, a lower jaw of Dryopithecus Africanus was found from Haritalyanga in Bilaspur district of Himachal Pradesh, India.

          Several members of Dryopithecus Africanus has been discovered from Shivalik hills as well.

    Characteristics

        It lived about 15 million years ago during the Miocene.

      

       Dryopithecus Africanus shows close similarity to Chimpanzee.

     Dryopithecus is considered to be a common ancestor of man and apes.

Dryopithecus Africanus was the direct forerunner of man.

   Dryopithecus Africanus is apes with the same length of arm and legs.

  

   Legs and heels in its feet indicate that Dryopithecus Africanus was of a semi-erect posture and Knuckle walker.

    Dryopithecus Africanus was arboreal and herbivorous, which ate soft fruits and leaves.

 

  Dryopithecus Africanus has large canines and incisors.

                                                           

                                                             

 Shivapithecus

      Discovery

                       Fossile of Shivapithecus was discovered from the middle and late Pliocene rock of Shivalik hills of India.     

   Characteristics

                 Its forelimb, Skull, and brain resembled the monkeys.   

                  Its face, jaws, and teeth had a resemblance with those of apes. 

Ramapithecus

     Discovery

                 Edward Lewis (1932) obtain the fossil of Ramapithecus from rocks of Shivalik hills of India.

                 Fossile was represented by a few teeth and some fragments of the jaw.

               Kenyapithecus (Kenyapithecus wicker) similar to Ramapithecus was discovered by LSB Leakey (1955) from Pliocene rocks of Kenya in Africa, but Ramapithecus was older than Kenyapithecus.

       Characteristics

Ramapithecus survived about 14-15 million years ago during the Late Miocene to Pliocene.

Ramapithecus walked erect on its hind legs.

It was similar to ape, which lived on the treetops but also walked on the ground.

It eats hard nuts and seeds like modern man. Its jaw and teeth were similar to humans.

Its canine teeth were small but molar teeth were broad and flattened with thicker enamel.

Dryopithecus gave rise to Ramapithecus , which is on the direct line of human evolution.

Ramapithecus became extinct about 7-8 million years back.

There is a fossil gap of about 9-10 million years in Ramapithecus and Australopithecus.

The fossil record of man's ancestor from the Pliocene are completely blank and once again reappear in the Pleistocene.

# what is the mutation ???????

# example of mutation???????                         

what is mutation? / what is gene flow? / what is non-random mating? define

MUTATION 

It is a sudden and heritable change in an organism. It is generally due to a change in the base sequence of nucleic acid in the genome of an organism. Mutations are considered to be the ultimate source of variation. The mutation may produce a new allele that was not present earlier in the population.

It can also change the frequency of existing alleles. Virtually, any characteristic of an organism can be changed by mutation. However, some mutation is selectable (i.e. conferring them), while others are on-selectable. A selectable mutation confers a clear advantage of the mutant strain under certain environmental conditions. So, the progeny of the mutation cell is able to outgrow and replace the parent cells.

REPLICA PLATE EXPERIMENT TO DEMONSTRATE MUTATION

 The adaptive significance of mutation was demonstrated by Joshua Lederberg and Esther Lederberg (1952) by replica plant experiments. in the experiment, the genetic basis of drug-resistant mutation in the bacteria Escherichia coli was tested.

from a culture of Escherichia coli derived from a single cell, they spread cell onto a 'master agar plate' each cell gives rise to a distant colony.

They place a velvet cloth onto a wooden block and pressed it gently on the master plate. As a result plate of master agar plate was made. then later touched this replica plate to a new plate containing a medium with antibiotic penicillin.

As a result of this most colonies found on the master, plate died due to penicillin. forever, a few colonies appeared on the replica plate. They were penicillin-resistant. Later, all the colonies on the master plate were tested for penicillin resistance, only those colonies that had been the source of penicillin resistance cell on the replica plate display resistance.

This given experiment indicated that some bacterial cells had acquired drug-resistant mutation even before they were exposed to penicillin. Natural selection has selected and supported the progeny carrying the beneficial mutation.

RECOMBINATION

The non-parental arrangement of alleles in the progeny is called recombination. It can result from the independent assortment and crossing over. recombination can occur not only between genes but also within genes resulting in the formation of new alleles in a population.

GENE FLOW

It is the physical movement of alleles into and out of a population. it can be a powerful agent variation because the members of two different populations may exchange genetic material.

some time gene flow is obvious when an animal moves from one place to another. if the characteristics of newly arrived animals differ from those of the animals already there, and if the newcomer is adapted well enough to the new area to survive. Gene flow also keeps separated populations genetically similar.

NON-RANDOM MATING 

in this process, the organisms are mate with each other randomly this phenomenon is known as non-random mating. inbreeding (mating with relatives) is a type of non-random mating that caused the frequencies of particular genotypes to differ greatly from those predicted by the Hardy-Weinberg equilibrium.

Inbreeding does not change the frequency of the alleles, but rather increase the proportion of homozygous individuals. However, the relatives are likely to be genetically similar and thus, produce offspring with two copies of the same allele.

The population of the self-fertilizing plants consists primarily of homozygous individuals, whereas outcrossing plants, (which interbreed with individuals different from themselves) have a higher proportion of heterozygous individuals.

By increase homozygosity in a population, inbreeding tends to increase the expression of recessive alleles. Due to this marriages between close relatives are discouraged and to the same degree outlawed. 

Inbreeding may also increase the possibility of producing children homozygous for an allele associated with one or more receives genetics disorders. 

RANDOM GENETIC DRIFT

It was discovered by American geneticist Sewall Wright. Hence, also called Sewall wright effects. in a small population, frequencies of particular alleles may change drastically by change alone. such changes in alleles frequencies occur randomly as if the frequencies are drifting. This is known as genetic drift. it is usually rare in the case of a large population. Thus, a population must be large to be in Hardy-Weinberg equilibrium.

    Genetic Drift is known to occur when by changes only certain members of the population reproduce and pass on their genes to the next generation. 

Genetic drift fixes new alleles, genes that arise by mutation from time to time, and eliminates the original gene, thereby changing the genetic make of a small population.

 

evidence of connective link // example and detail explanation

 EVIDENCE FROM CONNECTIVE LINKS

A connective link demonstrates the characteristics of more than one group of organisms (taxonomically). Such organisms establish continuity in the series by proving that one group has evolved from the other.

VIRUSES

A connective link between non-living and living is any virus-like bacteriophage (the virus that infects bacteria)

the genetic material of the virus is basically RNA ( ribonucleic acid) 

viruses are non-living organism 

                                      

Affinity with the non-living organism 

(1) Theas can be crystallized like inorganic and organic compounds.

(2) These are unable to respond to external stimuli.

(3) Cell organelles are absent. 

(4) Lack of growth and metabolism.

Affinity with a living organism

(1) These under mutation. 

(2) These reproduce in the living cells. 

(3) These show recombination and heritability.

Euglena

A connecting link between the animals and plant,

reason-based to the connective link between the animals and plant a euglena present the chloroplast basically animals has heterotropic hence animals do not present the chloroplast, but euglena present the chloroplast hence, euglena called the connective link between the animals and plant.

                                               

Affinities with animals

(1) Body is covered by a pellicle.

(2) reproduction is like animals.

(3) some characters are matched with animals.

Affinities with plants

(1) It has chloroplast containing chlorophyll.

(2) Nutrition is holotypic or autotrophic.

(3) It may synthesize food through photosynthesis.

proterosongia

It shares the characters of the group protozoa and Porifera.

                                

Affinities with protozoan and sponges

(1) protozoan characters: Colonial behaviour.

(2) Spong's characters:  Its cells are similar to choanocytes or collar cells of sponges.

peripatus

A connective link between the Annelida and Arthropoda.

                                                

Affinity with annelids

(1) Presence of thick cuticle.

(2) Present of simple eyes.

(3) Body is pseudosegmented and worm-like appearance.

(4) Excretory organ is nephridia.

Affinities with arthropods

(1) Presence of antennae.

(2) Respiratory organ is the trachea.

(3) Presence of clawed and pseudosegmented walking legs.

Neopilina

A connecting link between Annelida and Mollusca. 

                                                       

Affinities with annelids:

(1) Presence of trochophore larva.

(2) Egg divides by spiral cleavage as in polychaetes.

(3) presence of 5-6 pairs of ctenidia, 6 pairs of nephridia, 8 pairs of retracted muscles and two pairs of auricles.

Affinities with molluscs:

(1) Presence of shell and mantle.

(2) Presence of flat muscular fool as found in a chiton.

(3) Presence of soft and dorsoventrally flattened body.

Balanoglossus

A connective link between non-chordates and chordates.

                                                   

Affinities with chordates.

(1) Presence of notochord.

(2) Presence of a dorsal hollow nerve cord.

(3) Presence of pharyngeal gill slits.

Affinities with non-chordates.

(1) Burrowing habit and tubicolous life.

(2) Dorsal position of the heart.

(3) Nervous system is poorly developed and forms epidermal nerve plexus.

Chimaera

A connecting link between bony fishes and cartilaginous fishes.

                                                           

                             

Affinities with bony fish.

(1) Presence of four pairs of gills.

(2) Presence of smallmouth with flesh lips.

(3) cloaca remains absent.

(4) Anus and urinogenital apertures are separate.

 (5) Presence of operculum.

Affinities with cartilaginous fish

(1) Presence of ventral mouth.

(2) Presence of cartilaginous endoskeleton.

(3) Pelvic claspers presence in males.

(4) Presence of 2 dorsal fins.

(5) Placoid scales presence in young ones.

Lung fishes

Lung fishes are the connective link between the fishes and amphibian.

                                                          

Affinities with fish

(1) Fish like appearance.

(2) Presence of paired fins.

(3) Presence of dermal scales.

(4) Presence of gills.

(5) Presence of lateral line sense organ.

(6) Ear is represented by internal air only.

Affinities with amphibian

(1) Presence of internal nostrils.

(2) Presence of lung, i.e. modified air bladder.

(3) Heart with imperfectly divided auricle.

Prototheria

Echidna (spiny anteater) and Ornithorhynchus (platypus) are connecting link between reptile and mammals.

                                                              

Affinities with reptiles.

(1) Presence of cloaca.

(2) They lay eggs, which are reptile in structure and development.

(3) Egg contain enough yolk to nourish in structure and development.

(4) Embryonic development is similar to reptiles.

Affinities with mammals.

(1) Presence of hairs.

(2) They produce milk and nourish their children.

(3) They have mammary glands.

Archaeopteryx

It is a connective link between reptiles and birds. Archaeopteryx is also considered as a missing link because it is a fossil organism. it was found in the rock of cretaceous period by Andreas Wagner (1861) in Germany.

Some reptile characters of Archaeopteryx are given below

(1) Jaw are provided with homodont (similar) teeth.

(2) 

(3) Bone are not pneumatic.

(4) Cervical vertebrae are fewer, 9-19.

(5) Amphicoelus vertebrae.

(6) Cervical and abdominal ribs are present.

(7) Sternum is weak or absent.

(8) Scales are present.

(9) Carpals and metacarpal are free.

(10) Pelvic girdle has an elongated ilium and backwardly directed pubis.

Some avian characters of Archaeopteryx are given below 

(1) Presence of feathers.

(2) Forelimbs are modified as wings.

(3) Rounded braincase.

(4) Beaks are present.

(5) bones in the skull are intimately fused.

(6) Bones of limb and girdle are bird-like.

(7) Tibia and fibula are separate.

(8) v-shaped furcula is present. 

theory of Darwinism

Darwinism

Lamarckism the theory of evolution

natural selection of darwin

embryological evidence of evolution

Hardy-Weinberg equilibrium || polymorphism/ Defination, example.

 specialized supportive tools in speciation 

(1) Hardy-Weinberg equilibrium 

The Hardy-Weinberg law equation theorem is the fundamental law of population genetics.it provides the basis for studying the mendelian population. this law was independently developed by GH Hardy, an English mathematical and  G Weinberg, a German physician in 1908.' the Hardy-Weinberg law state that the gene and genetic frequency in Mendalin population remain constant generation after generation if there is no selection, mutation, migration or random genetic drift.'

Thus, the original proportions of the genotype in a population will remain constant from generation in a population will remain constant from generation to generation, as long as the following assumptions are met

* The population size is very large.

* Random mating occurs. 

* No mutation takes place.

* No gene is input from other sources, i.e. no immigration take place

* No selection occurs. 

Hardy-Weinberg principle gives the generation a tool to determine whether the evolution is occurring or not. population geneticists use the Hardy-Weinberg principle to calculate allele frequency at a starting point and then compare it with the frequencies measured at some feature time. Mathematically, it can be interpreted as a binomial expression p2 + 2pq +q2 to calculate the genotypic and allele frequencies of a population where, 

                                                        p2 = % homozygous dominant individuals

                                                       p = frequency of dominant allele

                                                      q2 = % homozygous recessive individuals 

                                                      q = frequency of recessive allele

                                                     2pq = % heterozygous individuals 

            Realize that (p + q )2 = 1 ( there are only two allele )

p2 + 2pq + q2 = 1 (there are the only genotype) 

Hardy-Weinberg principle says that the sum total of all the allelic frequencies of a gene is 1. Any disturbance in Hardy-Weinberg equilibrium, i.e. change in frequency of alleles would be interpreted as resulting in evolution. 

Example:  An investigator has determined by the inspection that 16% of a human population has a recessive trait. Using this information, we can calculate all the genotype and allele frequencies for the population, provide the condition for Hardy-Weinberg equilibrium are met. 

Given: q2 =16%= 0.4= frequency of recessive allele

p = 1.0 -0.4 = 0.6 = frequency of domiant allele 

p2 = (0.6) (0.6) = 0.36= 36% are homozygous domain individual 

2pq= 2 (0.6) (0.4) = 0.48 =48% are heterozygous individual 

p2+ 2pq+ q2 = 1.00-0.52=0.48

note: Though many other factors can alter frequencies, the above five can alter the proportions of homozygotes and heterozygotes predicted by hardy-Weinberg principle.

only selection produce adaptive evolution changs because only in selection, the result depent on the nature of the envirnment. mutaion, gene flow, genetic drift, etc., operate relatively independent of the envirnment. so. the chang they produce are not shaped by the envirnment demands.

2 POLYMORPHISM   

It is defined as the existence of two or more from or phenotypes of the species within the same population. Polymorphism can apply to biochemical, morphological, and behavioral characteristics. it also plays a significant role in the process of natural selection. 

Polymorphism in a population may develop due to the following reasons 

* Changes in the environment. 

* Superiority of heterozygotes. 

* mutation pressure, which may introduce a variety of alleles.

* frequency-dependent selection. 

polymorphism is of following two types

TRANSIENT POLYMORPHISM

This arises when the difference from or morphs, exist in a population undergoing a strong selection pressure. The frequency of the intensity of the selection pressure, such as the melanic and the non-melanic forms of the peppered month. the distribution of insular, i.e. an intermediate variation of the peppered moth. Transient polymorphism usually applies in the following situation polymorphism usually applies the following situation 

(1) When one form is gradually being replaced by another.

(2) A newly arose mutation is found to be advantageous and is favored by selection. it spread through the population and become more abundant, while the population wild type is becoming rare due to negative selection. 

(3) In a changing environment, a previously rare form may become advantageous and is favored by selection. it spread throughout the population.

BALANCE (STABLE) POLYMORPHISM 

This occurs when different forms coexist in the same population in a stable environment. Like the existence of the two sexes in animals and plants. These genotypic frequencies of the various polymorphism forms exhibit equilibrium since each form has a selective advantage of equal intensity. some example is

(1) ABO blood groups

in humans, the existence of the a, b, ab, o, blood group of an example of balanced polymorphism. however, the genotypic frequencies within a different population may vary, they remain constant from generation to generation within that population. This is because none of them have a selection advantage over the other. statistics reveal that the white men of blood group o have a greater life expectancy than those of other blood groups. However, interestingly they also have an increased risk of developing duodenal ulcers, which may perforation and lead to death.

(2)  sickle- cell anemia in some regions of the world (e.g. part of Africa) where malaria is caused by plasmodium falciparum, natural selection favors heterogonous allelic people over both homozygotes.

(3) Tay-Sachs disease 

it is common in children of Jewish heritage because one out of 40 Jewish heritage person is a heterozygote, where is one out of 380 non-Jewish people is a heterozygous carrier. this is because the grandchildren of a person suffering from Tay-Sachs disease were resistant to pulmonary tuberculosis in Bethesda Maryland. The incidence of Jewish tuberculosis patients from Eastern Europe is relatively high. This finding indicates that heterozygous carriers of Tay-such disease are resistant to pulmonary tuberculosis.

                                                                         

other examples: red-green color blindness in humans, the existence of workers, drones, and queen in social pin eyed and thrum-eyed from in primrose, etc. 

#Hardy-Weinberg equilibrium important 

#HarHardy-Weinberg equilibrium equation

# Hardy-Weinberg equilibrium example

#Hardy-Weinberg equilibrium  information 

 

natural selection/ definition, characters, industrial melanism, and example

 NATURAL SELECTION :

some genetic variability is always presence in a population in a population. such alleles make organism better adapted to the environment leading to their survival and continuous reproduction. 

The frequency of such alleles in a population, thus increase gradually. this process in called selection. if the breeders selection for the desired characteristics it is Artificial selection, and if the environment condition operate operating the natural population it is said to be natural selection.

condition for natural selection :

for natural selection to occur and result in evolutionary charge, three condition must be met 

(1) variation must exist among individual in a population. natural selection works by favouring individual with some traits over individual with alternative traits. if .

(2) variation among individual result in difference in the number of offspring surviving in the next generation. this is the essence of natural selection. it means that some individual are more successful than other in producing offspring and thus, passing their genes on the next generation because of their phenotype or behaviour. 

(3) variation must be genetically inherited for natural selection to result in evolutionary changes . even genetically identical individual may be phenotypically quite distinctive, if they grow up in different environment.

therefore, when phenotypically difference individual do not differ genetically, then differences in the number of their offspring will not alter the genetic composition of the population in the next generation. hence, no evolutionary charge will occur. 

types of natural selection :

there are following three kind of natural  selection 

(1) stabilising / normalising selection :

this occur when the environment does not change and favours the individual possessing the value of the mean characters. thus, natural selection produces value. thus , natural selection process change in favour of those individuals which posses an intermediate form of trait, while elimination extremes form the population.               the stabilising selection pressure does not promoting evolutionary charge but tend to maintain phenotypic stabilising within the population from generation to generation. fossil evidences suggest that many species remain unchanged for long periods of geological time.                                                                                                  for example : coelacanth (a fish species) was assumed to have been extinct for 70 million years until a living specimen was found in a trawler net of south Africa in 1938 . so, this species has not changed in all the time. 

another example of stabilising selection can be be seen in the birth weights of human. the heaviest and lightest babies have the highest mortality and are less likely to survive to reproduce and pass on their alleles. 

(2) directional selection 

this occurs whenever the environment change in a particular direction. thus, natural selection production change in a fever of one extreme while elimination the extreme. therefore, thare is a selective pressure for species to change in response to that environment change, e.g. industrial melanism.

 industrial melanism : 

the term refers to the evolution of dark body colour in animals species that live in habitats blackened by industrial soot. the occurrence of industrial melanism is closely associate with the progress of the industrial revolution in a grate Britain during 19th century. it has occur in several species of moth. of this, peppered moth ( Biston betularia) is the most intensive study. thes on existed in 2 variety, i.e. light coloured (white) and dark coloured (black). before the emergence of industrialisation, the with moth were numerous than dark moth in Britain. 

                                                            

however, after industrialisation , i.e. in 1920 the condition was reversed. this was due to the fact that, white moth were easily spotted by predation (birds) on soon covers tree trunk, so there population decrease and the population of black moth increase. we can also say that black moths got naturally selected. Disruptive/ Diverging selection. 

this occurs where an environment change may produce selection pressure that favours two extreme characteristics. 

some of the examples of disruptive selection are given below grass plant of welsh copper mine grew in soil. 

contaminated soil by copper which was lethal to the normal grass plant. however, a chance mutation allowed one plant to grow. this plant prospered and reproduced, but only on the contaminated soil. on normal soil, it grows more slowly than the normal plant and was easily out-competed. so, now there are two varieties of grass plant growing close together. 

sickle- cell anaemia:

people who are homozygous for this recessive allele, usually die before reproduction. their red blood cell contains abnormal haemoglobin, which makes them become a sickle-shaped and stick in their capillaries. people heterozygous for the allele are at a disadvantage because their red cells becomes sickle-shaped during exercise due to lack of oxygen. 

the allele should therefore, be selected again and rare, however, its frequency is a high is a part of the world, where malaria is common. people heterozygous for sickle cell anaemia are more resistant to malaria than people homozygous for a normal allele because the malarial parasite is not able to survive to sickle-shaped RBCs, hence, where malaria is found, people with heterozygous alleles are at advantage. 

# natural selection 

# natural selection importance

#natural selection example

Darwinism or Darwin's theory of evolution /postulates of Darwin || definition , natural selection ,speciation , example.

DARWINISM  OR DARWIN'S THEORY OF EVOLUTION :

after collection a vast amount of information, Darwin began to realised that under the intention competition of members in a population, any variation, which favour survival in a particular environment would increase the individual ability to reproduce and leave fertile offspring. a less favourable variation would be at disadvantage and organism possessing them would, therefore, have decreased chance of their successful reproduction. 

this data provide Darwin with the framework to formulate, a theory of evolution by natural selection in 1839. he did not publish his finding at that time. indeed, Darwin greatest contribution to science was not so much to show that evolution occurs but how it might occur .

in the meantime, another naturalist Alford Russel Wallace, came to the same conclusion as Darwin regarding natural selection . in 1858, Wallace wrote a 20 pages essay outlining his theory and send it Darwin. Darwin and Wallace presented paper on their ideas at a meeting of the Linnea Society in London

in November 1859 Darwin publish his thought on evolution in 'Origin of Species ' by the name of natural Selection.

                                        

POSTULATE OF DARWINSM :

the main postulate, which formed the basics of Darwin theory of natural selection are as follows

overproduction (rapid multiplication) 

  All organism possess enormous fertility. they multiply in a geometric proportion with some organism producing very large number of species. Despite of this high rate of reproduction of a species, its number remain constant under fairly stable environment. the production of more offspring by some organism and fewer by other is termed as differential reproduction.

limited resource:

the resource like food, space, etc. remain limited inspite of rapid multiplication of the individual of all the species.

struggle for existence : 

the limited resource present in an environment are the main cause of struggle for existence, which may occy at following three levels. 

intraspecific struggle :

it is the struggle among the individual of same species for their common requirement like food, shelter, mate, breeding, places, etc. intraspecific struggle is very severe, e.g. young tree in a forest, cannibalism, (eating individual of its own species), human wars, etc. 

interspecific struggle : it is the struggle between the individual of different species for their similar requirement like food and space. for example, a frog feed on insect and it prayed upon by snake while kite feed on both as well as snakes. 

environmental struggle : it is the struggle of living from against the environment condition like extreme heat, cold, drought, earthquakes, storms, disease, volcanic eruption, etc. 

4 appearance of variation :  

all individuals are dissimilar in some of their characters except the identical twins. this dissimilarity is mainly due to the variation . 

those are the small or large difference among the individual . variation allow some individual to better adjust with their environment. 

variation can be categorised into following type 

somatic variation : these variation affect the somatic cell of an organism. they are also called modifications or acquired characters because they are caused by various environment factors, use and disuse of organs and conscious efforts, etc.

germinal variation :  these are inheritable variation recognised by Darwin . they are formed mostly in germinal cell . they are further two type 

(a) continuous variation : these are fluctuation variation, which oscillate due to race, variety and species .

(b)discontinuous variation : these appear suddenly and show no spots gradation . this variation were termed as' spots ' by Darwin and Mutation by Hugo de Vries . Darwin regarded continuous variation to be more important because the discontinuous variation being mostly harmful would not be selected again. 

the organism which are provide with favourable variation would survive, which the unfit are destroyed. originally it was an idea of Herbert spencer (  1820-1903) who used the phrase ' survival of the fittest ' first time Darwin named it natural selection. 

survival of the fittest refers to the idea that the natural selection tents to favore those organism that are most fit to survive with the reproductive age in a particular environment and to produce offspring. 

for example, according to the theory of Darwin , the long necked giraffe evolved due to natural selection. 

                                                   

                                                              

he started that initially both long neck and legs and short neck and leg containing giraffes were present , but due to the scarcity of grass on land ,they had to feed on the leaves of tall trees. 

Now giraffe with long neck and longer legs could get food more easily and better change of survival . as a result, long neck and legs containing giraffes become abundant  and giraffes with short neck legs become extinct . 

the giraffes which short necked and limb soon died due to starvation when climate change due to their natural selection .

evidences of natural selection :

  it has the following evidence 

(1) rate of reproductive , limitation of resources , competition and struggle for existence for existence can be seen in natural normally.

(2) abundance of variation is evidenced by the absence of two similar individual except monozygotic twins , which also show some environmental variation 

(3)  preproduction of new varieties of plant and animals through artificial selection . these show evidence that the natural having vast resources can easily that the natural having vast resources can easily produced new species through the natural selection . 

(4) pedigrees of some animals (horse, camel, elephant), mimicry and protective colouration also supports natural selection. 

formation of new species (speciation) 

Darwin considered that as a result of struggle for existence, variability and inheritance, species become better adapted  to their environment . this beneficial adaptation are preserved and accumulation in the individual of species generation after generation . this result into the origin of species or speciation.