How Does Evolution Expalin The Change Of Birds

So chief theories of evolution are:

(I) Lamarckism or Theory of Inheritance of Caused characters.

(2) Darwinism or Theory of Natural Choice.

(Iii) Mutation theory of De Vries.

(IV) Neo-Darwinism or Modern concept or Synthetic theory of evolution.

I. Lamarckism :

It is also called "Theory of inheritance of caused characters" and was proposed by a keen French naturalist, Jean Baptiste de Lamarck (Fig. seven.34) in 1809 A.D. in his famous book "Philosphic Zoologique". This theory is based on the comparison between the gimmicky species of his time to fossil records.

His theory is based on the inheritance of acquired characters which are defined as the changes (variations) developed in the body of an organism from normal characters, in response to the changes in environment, or in the operation (use and disuse) of organs, in their own life fourth dimension, to fulfill their new needs. Thus Lamarck stressed on accommodation as means of evolutionary modification.

A. Postulates of Lamarckism:

Lamarckism is based on following 4 postulates:

ane. New needs:

Every living organism is plant in some kind of environment. The changes in the environmental factors similar calorie-free, temperature, medium, food, air etc. or migration of animal atomic number 82 to the origin of new needs in the living organisms, particularly animals. To fulfill these new needs, the living organisms accept to exert special efforts like the changes in habits or behaviour.

2. Use and disuse of organs:

The new habits involve the greater employ of certain organs to encounter new needs, and the disuse or lesser utilise of certain other organs which are of no use in new conditions. This apply and decay of organs greatly affect the class, structure and functioning of the organs.

Continuous and extra utilize of organs brand them more efficient while the continued disuse of some other organs lead to their degeneration and ultimate disappearance. And so, Lamarckism is also called "Theory of employ and disuse of organs."

So the organism acquires sure new characters due to direct or indirect environmental effects during its own life bridge and are chosen Caused or adaptive characters.

3. Inheritance of acquired characters:

Lamarck believed that caused characters are inheritable and are transmitted to the offsprings so that these are born fit to face up the inverse ecology conditions and the chances of their survival are increased.

4. Speciation:

Lamarck believed that in every generation, new characters are acquired and transmitted to next generation, so that new characters accumulate generation subsequently generation. Subsequently a number of generations, a new species is formed.

So according to Lamarck, an existing private is the sum full of the characters caused by a number of previous generations and the speciation is a gradual process.

Summary of 4 postulates of Lamarckism:

1. Living organisms or their component parts tend to increase in size.

2. Product of new organ is resulted from a new need.

3. Continued use of an organ makes it more developed, while decay of an organ results in degeneration.

4. Acquired characters (or modifications) developed by individuals during their own lifetime are inheritable and accumulate over a period of time resulting a new species.

B. Evidences in favour of Lamarckism:

1. Phylogenetic studies of equus caballus, elephant and other animals show that all these increase in their evolution from simple to complex forms.

two. Giraffe (Fig. vii.35):

Development of present twenty-four hours long-necked and long fore-necked giraffe from deer-like ancestor past the gradual elongation of cervix and forelimbs in response to deficiency of food on the barren ground in dry deserts of Africa. These body parts were elongated so as to swallow the leaves on the tree branches. This is an case of effect of extra use and elongation of certain organs.

three. Snakes:

Development of present twenty-four hour period limbless snakes with long slender body from the limbed ancestors due to connected disuse of limbs and stretching of their body to conform their creeping style of locomotion and fossorial way of living out of fear of larger and more powerful mammals. It is an example of disuse and degeneration of certain organs.

iv. Aquatic birds:

Development of aquatic birds like ducks, geese etc. from their terrestrial ancestors past the acquired characters similar reduction of wings due to their continued decay, development of webs between their toes for wading purposes.

These changes were induced due to deficiency of food on land and severe competition. It is an example of both extra apply (skin between the toes) and disuse (wings) of organs.

five. Flightless birds:

Development of flightless birds like ostrich from flying ancestors due to connected disuse of wings as these were plant in well protected areas with plenty of food.

vi. Horse:

The ancestors of modem horse (Equus caballus) used to live in the areas with soft ground and were short legged with more number of functional digits (e.thou. 4 functional fingers and 3 functional toes in Dawn horse-Eohippus). These gradually took to live in areas with dry ground. This modify in habit was accompanied by increase in length of legs and decrease in functional digits for fast running over hard ground.

C. Criticism of Lamarckism:

A hard blow to Lamarckism came from a German biologist, August Weismann who proposed the "Theory of continuity of germplasm" in 1892 A.D. This theory states that ecology factors exercise touch on only somatic cells and not the germ cells.

As the link betwixt the generations is merely through the germ cells and the somatic cells are non transmitted to the side by side generation and then the acquired characters must be lost with the expiry of an organism so these should take no office in development. He suggested that germplasm is with special particles chosen "ids" which control the development of parental characters in offsprings.

Weismann mutilated the tails of mice for about 22 generations and allowed them to brood, but tailless mice were never born. Pavlov, a Russian physiologist, trained mice to come for food on hearing a bell. He reported that this training is not inherited and was necessary in every generation. Mendel's laws of inheritance also object the postulate of inheritance of acquired characters of Lamarckism.

Similarly, boring of pinna of external ear and nose in Indian women; tight waist, of European ladies circumcising (removal of prepuce) in certain people; modest sized feet of Chinese women etc are non transmitted from one generation to another generator.

Eyes which are being used continuously and constantly develop defects instead of being improved. Similarly, heart size does non increment generation afterward generation though it is used continuously.

Presence of weak muscles in the son of a wrestler was besides not explained past Lamarck. Finally, in that location are a number of examples in which there is reduction in the size of organs e.g. amidst Angiosperms, shrubs and herbs take evolved from the trees.

So, Lamarckism was rejected.

D. Significance:

1. Information technology was first comprehensive theory of biological development.

2. It stressed on accommodation to the environment as a primary product of evolution.

Neo-Lamarckism :

Long forgotten Lamarckism has been revived every bit Neo-Lamarckism, in the low-cal of contempo findings in the field of genetics which confirm that surround does affect the grade, structure; colour, size etc. and these characters are inheritable.

Primary scientists who contributed in the development of Neo-Lamarckism are: French Giard, American Cope, T.H. Morgan, Spencer, Packard, Bonner, Tower, Naegali, Mc Dougal, etc. Term neo-Lamarckism was coined by Alphaeus S. Packard.

Neo-Lamarckism states:

1. Germ cells may be formed from the somatic cells indicating similar nature of chromosomes and factor make upward in two cell lines east.g.

(a) Regeneration in earthworms.

(b) Vegetative propagation in plants similar Bryophyllum (with foliar buds).

(c) A part of zygote (equipotential egg) of human female can develop into a complete baby (Driesch).

2. Effect of surroundings on germ cells through the somatic cells e.g. Heslop Harrison found that a pale variety of moth, Selenia bilunaria, when fed on manganese coated nutrient, a truthful breeding melanic variety of moth is produced.

3. Result of environment directly on germ cells. Tower exposed the immature ones of some potato beetles to temperature fluctuation and found that though beetles remained unaffected with no somatic alter but next generation had marked changes in body colouration.

Muller confirmed the mutagenic role of 10-rays on Drosophila while C. Auerbach et., al. confirmed the chemical mutagens (mustard gas vapours) causing mutation in Drosophila melanogaster, so neo-Lamarckism proved:

(a) Germ cells are not immune from the event of environs.

(b) Germ cells can carry somatic changes to next progeny (Harrison's experiment).

(c) Germ cells may be directly affected by the environmental factors (Tower's experiment).

2. Darwinism (Theory of Natural Selection):

A. Introduction:

Charles Darwin (Fig. 7.36) (1809- 1882 A.D.), an English naturalist, was the most dominant figure amidst the biologists of the 19th century. He made an all-encompassing study of nature for over 20 years, especially in 1831-1836 when he went on a voyage on the famous ship "H.M.S. Beagle" (Fig. 7.37) and explored S America, the Galapagos Islands and other islands.

He nerveless the observations on animal distribution and the relationship betwixt living and extinct animals. He found that existing living forms share similarities to varying degrees not just amid themselves but as well with the life forms that existed millions of years ago, some of which accept become extinct.

He stated that every population has built in variations in their characters. From the analysis of his data of drove and from Malthus's Essay on Population, he got the idea of struggle for beingness within all the populations due to continued reproductive pressure level and express resources and that all organisms, including humans, are modified descendents of previously existing forms of life.

In 1858 A.D., Darwin was highly influenced past a short essay entitled "On the Tendency of Varieties to Depart Indefinitely from the Original Type" written past some other naturalist, Alfred Russel Wallace (1812-1913) who studied biodiversity on Malayan archipelago and came to similar conclusions.

Darwin and Wallace'south views nigh evolution were presented in the meeting of Linnean Gild of London past Lyell and Hooker on July 1, 1858. Darwin's and Wallace'southward work was jointly published in "Proceedings of Linnean Society of London" in 1859. So it is also called Darwin-Wallace theory.

Darwin explained his theory of evolution in a volume entitled "On the Origin of Species by ways of Natural Choice". It was published on 24th Nov., 1859. In this theory, Charles Darwin proposed the concept of natural selection equally the mechanism of evolution.

B. Postulates of Darwinism:

Master postulates of Darwinism are:

1. Geometric increase.

2. Limited food and space.

iii. Struggle for beingness.

iv. Variations.

5. Natural selection or Survival of the fittest.

six. Inheritance of useful variations.

7. Speciation.

one. Geometric increment:

Co-ordinate to Darwinism, the populations tend to multiply geometrically and the reproductive powers of living organisms (biotic potential) are much more required to maintain their number due east.g.,

Paramecium divides three times by binary fission in 24 hours during favourable atmospheric condition. At this rate, a Paramecium tin produce a clone of most 280 million Paramecia in merely one calendar month and in five years, can produce Paramecia having mass equal to 10,000 times than the size of the earth.

Other apace multiplying organisms are: Cod (ane million eggs per twelvemonth); Oyster (114 1000000 eggs in i spawning); Ascaris (70, 00,000 eggs in 24 hours); housefly (120 eggs in ane laying and laying eggs six times in a summer season); a rabbit (produces 6 young ones in a litter and four litters in a yr and young ones start breeding at the age of six months).

Similarly, the plants as well reproduce very rapidly due east.thousand., a single evening primrose plant produces nigh 1, 18,000 seeds and single fern plant produces a few meg spores.

Even irksome breeding organisms reproduce at a rate which is much higher than required e.chiliad., an elephant becomes sexually mature at 30 years of age and during its life span of 90 years, produces only six offsprings. At this rate, if all elephants survive then a single pair of elephants can produce about 19 million elephants in 750 years.

These examples confirm that every species tin can increase manifold within a few generations and occupy all the bachelor space on the earth, provided all survive and echo the procedure. Then the number of a species will be much more can be supported on the globe.

2. Limited nutrient and space:

Darwinism states that though a population tends to increase geometrically, the food increases only arithmetically. So two master limiting factors on the tremendous increment of a population are: limited nutrient and space which together form the major role of conveying capacity of environment. These do not allow a population to grow indefinitely which are almost stable in size except for seasonal fluctuation.

3. Struggle for beingness:

Due to rapid multiplication of populations but limited nutrient and space, there starts an everlasting contest betwixt individuals having similar requirements. In this competition, every living organism desires to have an upper paw over others.

This competition between living organisms for the bones needs of life like food, space, mate etc., is chosen struggle for existence which is of 3 types:

(a) Intraspecific:

Betwixt the members of same species e.g. two dogs struggling for a piece of meat.

(b) Interspecific:

Betwixt the members of different species e.g. between predator and prey.

(c) Ecology or Extra specific:

Between living organisms and adverse environmental factors like heat, cold, drought, overflowing, earthquakes, calorie-free etc.

Out of these three forms of struggle, the intraspecific struggle is the strongest type of struggle as the needs of the individuals of aforementioned species are almost similar e.g., sexual option in which a cock with a more beautiful rummage and plumage has meliorate chances to win a hen than a cock with less developed comb.

Similarly, cannabilism is another example of intraspecific competition as in this; individuals eat upon the members of same species.

In this death and life struggle, the majority of individuals dice before reaching the sexual maturity and only a few individuals survive and reach the reproductive phase. So struggle for existence acts every bit an effective check on an ever-increasing population of each species.

The nature appears saying, "They are weighed in the balance and are found wanting." So the number of offsprings of each species remains nearly constant over long period of fourth dimension.

4. Variations:

Variation is the law of nature. Co-ordinate to this law of nature, no ii individuals except identical (monozygotic) twins are identical. This everlasting competition amidst the organisms has compelled them to modify co-ordinate to the conditions to utilize the natural resource and can survive successfully.

Darwin stated that the variations are generally of 2 types—continuous variations or fluctuations and discontinuous variations. On the basis of their consequence on the survival chances of living organisms, the variations may exist neutral, harmful and useful.

Darwin proposed that living organisms tend to suit to changing environs due to useful continuous variations {e.chiliad., increased speed in the prey; increased water conservation in plants; etc.), as these volition accept a competitive advantage.

5. Natural selection or Survival of the fittest:

Darwin stated that as many selects the individuals with desired characters in artificial selection; nature selects only those individuals out of the population which are with useful continuous variations and are all-time adapted to the environment while the less fit or unfit individuals are rejected by it.

Darwin stated that if the human can produce such a large number of new species/varieties with limited resources and in short menses of time by bogus option, then natural selection could account for this large biodiversity past considerable modifications of species with the help of unlimited resource available over long span of time.

Darwin stated that discontinuous variations appear suddenly and will mostly be harmful, so are not selected past nature. He chosen them "sports". So the natural selection is an automatic and self going procedure and keeps a cheque on the fauna population.

This sorting out of the individuals with useful variations from a heterogeneous population past the nature was called Natural selection past Darwin and Survival of the fittest by Wallace. And then natural selection acts every bit a restrictive force and not a creative forcefulness.

6. Inheritance of useful variations:

Darwin believed that the selected individuals laissez passer their useful continuous variations to their offsprings then that they are born fit to the inverse surroundings.

7. Speciation:

According to Darwinism, useful variations announced in every generation and are inherited from one generation to another. So the useful variations go on accumulating and later a number of generations, the variations go so prominent that the individual turns into a new species. So according to Darwinism, development is a gradual process and speciation occurs past gradual changes in the existing species.

Thus the 2 key concepts of Darwinian Theory of Evolution are:

1. Branching Descent, and ii. Natural Selection.

C. Evidences in favour of Darwinism:

1. There is a close parallelism between natural option and bogus choice.

2. The remarkable cases of resemblance eastward.g. mimicry and protective colouration can be achieved just past gradual changes occurring simultaneously both in the model and the mimic.

3. Correlation between position of nectaries in the flowers and length of the proboscis of the pollinating insect.

D. Evidences against Darwinism:

Darwinism is not able to explicate:

i. The inheritance of pocket-size variations in those organs which can be of use only when fully formed due east.g. wing of a bird. Such organs volition be of no use in incipient or underdeveloped phase.

2. Inheritance of vestigial organs.

3. Inheritance of over-specialised organs due east.g. antlers in deer and tusks in elephants.

4. Presence of neuter flowers and sterility of hybrids.

five. Did not differentiate between somatic and germinal variations.

6. He did not explain the causes of the variations and the style of manual of variations.

seven. Information technology was also refuted by Mendel's laws of inheritance which land that inheritance is particulate.

So this theory explains only the survival of the fittest just does not explain the arrival of the fittest so Darwin himself confessed, "natural choice has been main but not the sectional means of modification."

Principle of Natural Selection (Table 7.7) :

It was proposed by Ernst Mayer in 1982. It stems from five of import observations and three inferences as shown in Table 7.vii. This principle demonstrates that natural selection is the differential success in reproduction and enables the organisms to adapt them to their surroundings by development of small and useful variations.

These favourable Variations accumulate over generation after generation and atomic number 82 to speciation. So natural selection operates through interactions between the environment and inherent variability in the population.

III. Mutation Theory of Development :

The mutation theory of evolution was proposed past a Dutch botanist, Hugo de Vries (1848-1935 A.D.) (Fig. 7.38) in 1901 A.D. in his book entitled "Species and Varieties, Their Origin by Mutation". He worked on evening primrose (Oenothera lamarckiana).

A. Experiment:

Hugo de Vries cultured O. lamarckiana in botanical gardens at Amsterdam. The plants were, allowed to cocky pollinate and adjacent generation was obtained. The plants of next generation were again subjected to self pollination to obtain second generation. Process was repeated for a number of generations.

B. Observations:

Majority of plants of beginning generation were found to be like the parental type and showed only minor variations but 837 out of 54,343 members were establish to exist very unlike in characters like blossom size, shape and arrangement of buds, size of seeds etc. These markedly different plants were called primary or elementary species.

A few plants of second generation were found to be still more different. Finally, a new type, much longer than the original blazon, called O. gigas, was produced. He as well found the numerical chromosomal changes in the variants (e.g. with chromosome numbers 16, 20, 22, 24, 28 and 30) upto 30 (Normal diploid number is xiv).

C. Conclusion:

one. The evolution is a discontinuous process and occurs by mutations (L. mutate = to change; sudden and inheritable large differences from the normal and are not connected to normal past intermediate forms). Individuals with mutations are called mutants.

2. Elementary species are produced in big number to increase chances of pick by nature.

iii. Mutations are recurring so that the same mutants appear once again and over again. This increases the chances of their selection past nature.

4. Mutations occur in all directions so may cause proceeds or loss of any character.

5. Mutability is fundamentally unlike from fluctuations (pocket-size and directional changes).

So according to mutation theory, evolution is a discontinuous and jerky process in which there is a leap from one species to another so that new species arises from pre-existing species in a unmarried generation (macrogenesis or saltation) and not a gradual process every bit proposed by Lamarck and Darwin.

D. Evidences in favour of Mutation theory:

1. Advent of a brusk-legged sheep variety, Ancon sheep (Fig. seven.39), from long-legged parents in a single generation in 1791 A.D. It was first noticed in a ram (male sheep) by an American farmer, Seth Wright.

two. Appearance of polled Hereford cattle from horned parents in a single generation in 1889.

3. De Vries observations have been experimentally confirmed by McDougal and Shull in America and Gates in England.

iv. Mutation theory can explain the origin of new varieties or species by a single gene mutation e.g. Cicer gigas, Nuval orangish. Red sunflower, hairless cats, double- toed cats, etc.

v. It tin can explicate the inheritance of vestigial and over-specialized organs.

six. It can explain progressive likewise as retrogressive development.

East. Evidences against Mutation theory:

1. It is not able to explicate the phenomena of mimicry and protective colouration.

ii. Charge per unit of mutation is very low, i.eastward. one per million or one per several one thousand thousand genes.

3. Oenothera lamarckiana is a hybrid plant and contains anamolous blazon of chromosome behaviour.

4. Chromosomal numerical changes as reported past de Vries are unstable.

5. Mutations are incapable of introducing new genes and alleles into a gene pool.

IV. Neo-Darwinism or Modern Concept or Constructed Theory of Evolution:

The detailed studies of Lamarckism, Darwinism and Mutation theory of evolution showed that no unmarried theory is fully satisfactory. Neo-Darwinism is a modified version of theory of Natural Selection and is a sort of reconciliation between Darwin'southward and de Vries theories.

Modern or synthetic theory of development was designated by Huxley (1942). Information technology emphasises the importance of populations as the units of evolution and the central part of natural selection as the most important mechanism of development.

The scientists who contributed to the outcome of Neo-Darwinism were: J.S. Huxley, R.A. Fischer and J.B.Due south. Haldane of England; and S. Wright, Ford, H.J. Muller and T. Dobzhansky of America.

A. Postulates of Neo-Darwinism :

1. Genetic Variability:

Variability is an opposing force to heredity and is essential for development as the variations grade the raw material for development. The studies showed that the units of both heredity and mutations are genes which are located in a linear style on the chromosomes.

Diverse sources of genetic variability in a genetic pool are:

(i) Mutations:

These are sudden, large and inheritable changes in the genetic material. On the footing of corporeality of genetic material involved, mutations are of 3 types:

(a) Chromosomal aberrations:

These include the morphological changes in the chromosomes without affecting the number of chromosomes. These effect changes either in the number of genes (deletion and duplication) or in the position of genes (inversion).

These are of 4 types:

1. Deletion (Deficiency) involves the loss of a cistron block from the chromosome and may exist terminal or intercalary.

2. Duplication involves the presence of some genes more than than once, called the repeat. It may be tandem or reverse duplication.

3. Translocation involves transfer of a gene block from one chromosome to a non-homologous chromosome and may exist unproblematic or reciprocal type.

4. Inversion involves the rotation of an intercalary cistron block through 180° and may exist paracentric or pericentric.

(b) Numerical chromosomal mutations:

These include changes in the number of chromosomes. These may be euploidy (gain or loss of one or more than genomes) or aneuploidy (gain or loss of one or ii chromosomes). Euploidy may be haploidy or polyploidy.

Among polyploidy, tetraploidy is about mutual. Polyploidy provides greater genetic fabric for mutations and variability. In haploids, recessive genes express in the same generation.

Aneuploidy may be hypoploidy or hyperploidyl Hypoploidy may be monosomy (loss of one chromosome) or nullisomy (loss of 2 chromosomes). Hyperploidy may exist trisomy (gain of one chromosome) or tetrasomy (proceeds of ii chromosomes).

(c) Gene mutations (Point mutations):

These are invisible changes in chemic nature (DNA) of a gene and are of three types:

1. Deletion involves loss of i or more nucleotide pairs.

2. Addition involves gain of one or more nucleotide pairs.

iii. Substitution involves replacement of 1 or more nucleotide pairs by other base pairs. These may be transition or transversion type.

These changes in DNA cause the changes in the sequence of amino acids and then changing the nature of proteins and the phenotype.

(2) Recombination of genes:

Thousands of new combinations of genes are produced due to crossing over, chance organisation of bivalents at the equator during metaphase – I and gamble fusion of gametes during fertilization.

(iii) Hybridization:

It involves the interbreeding of 2 genetically different individuals to produce 'hybrids'.

(iv) Physical mutagens (e.m. radiations, temperature etc.) and chemical mutagens (e.chiliad. nitrous acid, colchicine, nitrogen mustard etc.).

(v) Genetic drift:

It is the elimination of the genes of some original characteristics of a species past extreme reduction in a population due to epidemics or migration or Sewell Wright effect.

The chances of variations are also increased by non-random mating.

2. Natural Selection:

Natural choice of Neo- Darwinism differs from that of Darwinism that it does not operate through "survival of the fittest" but operates through differential reproduction and comparative reproductive success.

Differential reproduction states that those members, which are best adapted to the environs, reproduce at a higher charge per unit and produce more offsprings than those which are less adapted. So these contribute proportionately greater percentage of genes to the genetic pool of adjacent generation while less adjusted individuals produce fewer offsprings.

If the differential reproduction continues for a number of generations, so the genes of those individuals which produce more offsprings will become predominant in the gene pool of the population as shown in Fig. seven.40.

Due to sexual communication, there is free flow of genes so that the genetic variability which appears in certain individuals, gradually spreads from 1 deme to another deme, from deme to population and then on neighbouring sister populations and finally on most of the members of a species. And then natural selection causes progressive changes in gene frequencies, 'i.east. the frequency of some genes increases while the frequency of some other genes decreases.

Which individuals produce more offsprings?

(i) By and large those individuals which are best adjusted to the environs.

(two) Whose sum of the positive pick pressure due to useful genetic variability is more the sum of negative choice pressure due to harmful genetic variability?

(iii) Which accept better chances of sexual selection due to development of some brilliant coloured spots on their trunk e.g. in many male birds and fish.

(iv) Those who are able to overcome the physical and biological environmental factors to successfully reach the sexual maturity.

So natural option of Neo-Darwinism acts as a creative force and operates through comparative reproductive success. Accumulation of a number of such variations leads to the origin of a new species.

3. Reproductive isolation:

Any cistron which reduces the chances of interbreeding between the related groups of living organisms is called an isolating machinery. Reproductive isolation is must so as to allow the accumulation of variations leading to speciation by preventing hybridization.

In the absence of reproductive isolation, these variants freely interbreed which lead to intermixing of their genotypes, dilution of their peculiarities and disappearance of differences between them. And so, reproductive isolation helps in evolutionary divergence.

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