Theories of Evolution

Last Updated on Thursday, 16 December 2010 01:16 Written by Sandesh Monday, 24 May 2010 04:16

Theories of Evolution

History of evolutionary ideas Jean-Baptiste de Lamarck (August 1, 1744 – December 18, 1829) was a French soldier, naturalist, academic and an early proponent of the idea that evolution occurred and proceeded in accordance with natural laws. Details of his theories of organic evolution were published in his book titled Philosophic Zoologique in the year 1809.

Charles Darwin (1809-1882) laid down the foundation for the concept of organic evolution. He published his great book titled on the Origin of Species by Natural Selection in the year 1959.

Alferd Russel Wallace also arrived at the same theory of organic evolution by natural selection.

Hugo de Vries, a Dutch botanist proposed that sudden changes in genes responsible for variations. He called such sudden changes in genes as mutations and his theory is popularly known as Mutation theory.

Lamarckism

Jean Baptist de Lamarck, a French biologist formulated the first theory of organic evolution.

Proposition of Lamarckism:

1. Tendency to grow: According to Lamarck, living organism and their component parts tend continually to increased in size

2. Formation of new organ: Lamarck believed that new organs are formed in our body as a result of need or want in order to adapt in the environment.

3. Use and disuse of organs: According to Lamarck, continued use of an organ makes it strong, efficient, active and leads to an increase in its size. On the other hand, disuse of organ makes it weak, inefficient and result in a reduction of its size, ultimately leading to its degeneration. This view of Lamarck is popularly known as the theory of use and disuse. He explained this part of his theory by giving the examples of giraffe, water birds and snakes.

a. Use: A classic example cited by Lamarck in favour of this view is the elongation of neck and limbs in giraffe. He presumed that ancestor giraffe were shot-necked and short-legged. They lived in plains and grazed on grass. When environmental conditions changed and grass was not available, they had to feed on the leaves of the trees. Thus to do this the ancient giraffe had to stretch their neck and limbs in order to reach leaves of higher branches. The continuous stretching of neck and limbs for several generations gave rise to giraffes having long neck and limbs, like those of today. Similarly, water birds such as ducks developed their webbed feet by constant stretching of its toes apart to give more pushing during swimming.

b. Disuse: Lamarck gave examples of snakes to show how an organ degenerates when not used. Ancestors of modern snakes were four-limbed like lizard. The lizard-like ancestor snakes felt insecure of mammals and stared to hide themselves in bushes and burrows. To accommodate their body in such narrow spaces they could not use their hind-limbs for locomotion. This disuse of limbs resulted in their degeneration.

4. Inheritance of acquired characters: All changes or modification in the body of an organism developed due to the influence of the environment or due to use and disuse of an organ are acquired characters. Lamarck believed that such acquired characters are preserved and passed on the offsprings. this view of Lamarck is popularly known as ”inheritance of acquired characters”. The concept of the theory is that an organism inherits the bodily changes acquired by its parents and passes it on to the next generation.

Criticisms of Lamarckism:

• Constant and continuous use of eyes develops defects instead of being more improved.
• If the limb is damaged or cut in parents, it doesn’t disappear in offsprings.
• August Weismann (1890), a German scientist, conducted some experiments to test if characters may disappear due to disuse. This he did by cutting the tails of white mice for more than twenty generations to see if this has any effect on tail of coming generation. The measuring of tail length of the offsprings of twenty succession generations revealed that on average, the tails were not shorter. It means that acquired characters are not inherited.

Neo-Lamarckism: The inheritance of acquired characteristics was rejected by August Weismann. he corrected this theory by developing a new theory of inheritance known as Germplasm Theory. In this theory “germ-plasm” (the hereditary material passed from parents to offspring) remained separate and distinct from “somaplasm” (the material composing the body of an organism). He concluded that the changes in somatic cells are not inherited where as changes in germplasm are inherited.

Darwinism (Theory of Natural Selection)

Darwinism doesn’t mean what evolution is, but explains how evolution might have occurred.

Principles of Darwinism:

1. Enormous power of fertility or over productions of offsprings or prodigality of production: According to the Darwin’s concept of natural selection there is the tendency in all organisms to produce more offsprings than can survive in geometric ration. For example, a female elephant which is considered to be the slowest breeder becomes sexually mature at the 30 and can reproduce on an average, 6 young ones during its fertile period. If all these young ones survived and reproduced in their turn, there would be as may as 19 million elephants in about 750 years of time. Similarly, the fruit fly, Drosophila takes 10 to 14 to give rise to a new generation and a single female lays about 200 eggs at a time. If all of them hatch and survive, the offsprings of a pair of fruit flies would be about 20 million within a matter of about 40 to 50 days. However, all the above figures are theoretical. Populations never reach such numbers. All the offsprings do not survive because number of factors such as natural calamities, shortage of food, accidents etc. In spite of enormous fertility in living beings, the number remains constant.

2. Struggle for existence: Darwin observed that overproduction results in competition among the newly produced organisms for food, shelter and mating. This competition is called struggle for existence which may be following types:

Intraspecific- It is the competition among the individuals of same species. This type of competition is more intense because the needs and approach of competing individuals is precisely the same. Interspecific- It is the competition between the organisms of different species. Struggle with environment- The struggle of living things against extreme temperatures, excess moisture, drought, lightening, storms, earthquakes, volcanic eruptions etc., is called struggle with the environment. In this struggle the individuals die without reproducing, others reproduce less and only a few survive. The struggle for existence acts as natural check against increase in the number of individuals on a large scale. With such everlasting competition to live, some individuals develop adaptive variations for better survival.

3. Variations and heredity: During competition every individuals try to become better adapted than the other by utilizing the resources, to survive successfully. For this, variations start arising in the organisms. But all variations that arise are not significant from evolutionary point of view. Thus variations may be harmful, useless or useful. Harmful variations make organisms unfit in struggle for existence and cause their elimination. Useful variations are quite significant and make the organisms fit in the struggle for existence. Such variations are inherited in the progeny of those organisms in which they arouse, and therefore progeny have better chances of survival.

4. Survival of the fittest or the natural selection: In the struggle for existence only those organisms having favourable adaptive variations will survive. Those with unfavourable variations stand no chance in the struggle for existence and therefore perish. Thus Darwin concluded that nature selects only those organisms which carry good variations. Organisms selected in this way are adapted to their respective environments and can and can transmit the variations to their offsprings. In this way, a better adapted species replaces the unfit one. Darwin explained the survival of the fittest and the natural selection by giving the examples of Lamarck’s giraffe. According to Darwin, there were two varieties of giraffe, one having long-neck and long limbs and other having short-neck and short limbs. When the grass was scarce on land, they had to eat the leaves of tall trees. Naturally, the giraffes with long necks and legs had an advantage over those with shorter necks and shorter legs because they could get food more easily and had better chances of survival. These forms fed, reproduced and became abundant. On the contrary, giraffes with shorter necks and shorter legs starved and gradually became extinct. Thus to Darwin nature plays an important role in selection.

5. Origin of new species: Darwin concluded that as a result for struggle of existence, variability and inheritance successive generations tend to become better adapted to their environment. These adaptations are preserved and accumulated in the individuals of the species and ultimately lead to origin of species. As natural selection continues, the differences become more pronounced to mark off the successive generations as separate species. Thus new species of organism arise by natural selection over many generations during which the offsprings become markedly distinct from their ancestors.

Criticism to Darwinism: 1. According to Darwin only useful variations are inherited, but sometimes small variations which are not significant form evolutionary point of view are also transmitted to the next generations.

2. Darwin could not explain the cause of variations in living organisms.

3. Darwin in fact considered that all variations are heritable. Today we know that only those variations are inherited which are germinal and not somatic.

4. The theory of natural selection cannot explain the presence of vestigial organs in some organisms, generation after generation.

5. Sometimes the over-specialization of some organs becomes the cause of extinction. Huge antlers of Irish elks and large teeth of saber-toothed tigers led to the extinction of these species. Darwin’s theory could not explain the cause of over-specialization of some organs.

6. Darwin’s theory only explains the survival of the fittest, but it is unable to explain the arrival of the fittest.

7. The theory of natural selection also cannot explain degeneration of certain organs in animals.

8. The theory also fails to explain the cause of sudden change in the body. The main drawback of Darwinism is the lack of knowledge of heredity.

9. Darwin theory is not able to explain the evolution of terrestrial animals from aquatic animals.

10. The main emphasis of Darwin’s theory is on the slow and gradual changes in the organs but it does not explain how these organs were formed in the first place.

Neo-Darwinism: (Modern Synthetic Theory of Evolution)

It is referred to as Genetic Theory of Natural Selection or Synthetic Theory of Evolution. It is the modern concept of organic evolution which has been developed by renowned scientists Sewall Wright, R.A. Fischer, T. Dobzhansky, H.J. Muller, J. Huxley, J.B.S. Halden, G.L. Stebbins and others. It is a new approach to organic evolution in the light of new branches of biology like Genetics, Cell Biology, Molecular Biology etc.

Neo-Darwinism accepts following major causes of evolutionary changes:

1. Mutation

2. Genetic recombination

3. Genetic drift of Sewell Right Effect

4. Natural selection

5. Isolation

1. Mutation: Mutation is sudden heritable changes in genetic makeup of an organism. Mutations can effect individual genes, individual chromosomes or the entire set of chromosomes. The factors causing mutations are called mutagens. It is a source of genetic variation in a gene pool. Such variations are the raw-materials for evolution. The term mutation was first used by Dutch Botanist, Hugo de Vries and his theory is popularly known as the Mutation theory.

2. Genetic recombination: In all sexually reproducing populations, gametes are formed as result of meiosis. During Prophase I of meiosis, exchange of genetic material between non-sister chromatids of bivalent chromosome occurs. Such an exchange of genetic material provides new combinations of gene which bring about new characters in the off springs.

3. Genetic drift of Sewell Wright Effect: The term genetic drift refers to the elimination of the genes of certain traits when a section of population migrates or dies of natural calamity. It is defined as the change or variation in gene frequency purely as a matter of chance. Smaller populations have greater chances for genetic drift than larger populations. Sewell Wright was the first to recognize that genetic drift could cause changes in gene frequency.

4. Natural Selection: Natural selection ensures the continuation of favourable genes and elimination of unfit genes. Natural selection works on members of reproductive age. Examples of Natural Selection are as follows:

i. Industrial Melanism in Peppered Moths: Industrial melanism in, Biston betularia (peppered moth) is one of the best evidence for Natural Selection. This curios instance of natural Selection was recorded by H.B.D Kettlewell in 1959. There are two varieties of Biston betularia, light coloured and black coloured. In the early part of 19th century, only light coloured peppered moths were dominant in England. These moths inhibited upon the lichens that grew heavily on the trees trunks. The light coloured moths resting upon light coloured lichen were practically invisible to the predator birds. Later there was steady rise of industrialization. The industrial pollution not only killed the lichens growing on these trees but also made the tree trunks turn black with the deposition of industrial soot. As a result the light coloured moths became quite vulnerable to the predator birds and thus their population started declining. Soon they got replaced by black coloured moths. Black moths resting upon dark tree trunks were not easily visible to the predator birds. Thus they had better chance of survival than the light coloured ones in the areas which were under the influence of industrial pollution. However, in the unpolluted areas like country sides, the light coloured peppered moths were still dominant. Here the environmental factor that has selected the dark varieties against the light forms is bird predation. The natural selection has operated in the direction of eliminating the gene for light colour and the gradual increase of gene for dark colour.

ii. DDT resistance in mosquitoes: A remarkable evidence of natural selection has come from the work involving the use of DDT on mosquitoes and other insects. During the first year of spraying of DDT, nearly all the mosquitoes were killed. However there were a few mosquitoes which were not killed by DDT. This is because of their genetic makeup. They had the unique quality of resistance to DDT. When DDT was sprayed, those mosquitoes lacking the gene to resist DDT got killed while those few ones having DDT resistance gene survived. These resistant mosquitoes, which were originally present in small numbers, now got chance to multiply rapidly. As a result there were more resistant types later on in the locality where DDT had been used. Hence DDT became less effective against mosquitoes in following years. Eventually, wherever DDT was sprayed for many years, only DDT resistant mosquitoes were left, making the use of DDT ineffective. Here DDT does the work of natural selection by eliminating mosquitoes lacking DDT resistant gene and favouring those having DDT resistance genes.

5. Isolation: Separation of a single population into its subunits is called isolation. It is also an important factor for evolution. Isolation may be geographic, biotic, reproductive or genetic. Individuals of such separated groups do not interbreed. Each group may develop a set of new characters which lead to evolutionary changes towards the origin of new species.