Theories of Evolution

The term ‘ evolution ’ has a special place in the study of history of life. In terms of organisms, it may be defined as the development of more advanced and complex organism from preexisting relatively simple (primitive) ancestors over a course of time. Various hypotheses have been developed to account for evolution. The outline of the historical background to the development of history of evolution is given in table 18.1.

Background of Evolution Theory 

From the above table two main theories of evolution have been discussed here to explain the mechanism of organic evolution.

1. Lamarck’s Theory of Evolution (Lamarckism).
2. Darwin’s Theory of Evolution (Darwinism).

Lamarck’s Theory of Evolution (Lamarckism)

In 1809, a French naturalist jean Baptiste be Lamarck presented the ideas of evolution in his book, philosophic zoologique. These ideas about evolution are called as Lamarckism.

There are two important points of Lamarckism.

• Use and disuse of organs.
• Inheritance of acquired characters.

Use and disuse of organs   

According to Lamarck, in organisms certain changes in the body features are developed by use and disuse of body parts due to biological needs arising out of environmental changes. The organ greatly used develops in its size and organ disused tends to become weaker and can eventually disappear. This statement of Lamarck that use or disuse can produce changes in organs seems correct e.g., the development of muscles in athletes and the development of calf muscles in ladies who wear high heels are common observations.

Inheritance of acquired characters

Variations in phenotype due to use or disuse of body are called acquired characters as these characters are developed during the lifetime of an individual. Lamarck believed that these acquired characters are inheritable and are passed onto the offspring from their parents during reproduction.

Lamarckism is illustrated by an example of giraffe’s long neck. Lamarck assumed that long necked giraffes evolved from short- necked ancestors. The short-necked ancestors could graze on grasses. When the grasses became scanty the giraffes tried to stretch their necks to reach the top of trees to feed on leaves.  As a result of these efforts the giraffes’ necks became longer. The same trend continued and the neck went on becoming longer and longer. The giraffes with long necks were able to survive in such an environment. Lamarck believed that acquired characters of long neck in giraffes was heritable and was passed on to the next generation. He believed that gradual accumulation of such changes lead to the evolution of individual which are better suited to their environment.

Regarding disuse of organs, Lamarck-ism can be described with the help of an example of snakes. Ancestors of modern snakes had four legs with which they used to move o land. In order to escape from enemies the snakes started hiding in holes and creeping in narrow cracks in rocks. In this new mode of life the legs were not used, but muscles of their bodies were used more for creeping. As a result of disuse, the legs were completely lost and body of snakes becomes elongated and cylindrical. This is how present day snakes have evolved.

Webbed feet of aquatic birds are another example of use of organs and non-functional wings of kiwi and ostrich are the examples of disuse of organs. In human beings appendix (small sac of unknown functions connected to large intestine) and coccyx (fused tail bones) are the result of disuse of organs, as per Lamarck’s theory of evolution.

Lamarck’s theory of evolution lays emphasis on biological needs, which arise as a result of change in environment. This need produces an urge in the organism to use or disuse certain organs which then develop or degenerate as a consequence there of. As already discussed Lamarck believed that these variations were inheritable. That these changes were inheritable could not be established. To demonstrate this, august Weismann in 1870 cut off the tails of mice over many successive generations.

In each generation the mice were born with normal tails. According to Lamarckism, the acquired character (mice without tail) would be inherited. But this was not the case. Weismann postulated that somatic (body) acquired characteristics resulting in phenotypic changes did not directly affect the germ cells (gametes) which are the means by which characteristics are passed onto the next generation.

Darwin’s Theory of Evolution (Darwinism)

Theories of Evolution – Charles Darwin (1809-1882) while on voyage aboard on a ship (HMS Beagle) made observation about the great variety of birds and reptiles on the Galapagos Islands and their relationship to similar groups on the Ecuadorian mainland.

Charles Darwin was also inspired by the work of his geologist friend Charles Lyell whose major thesis was that geological forces produce a constant changing environment. He also come across an essay on population written by a clergyman and economist Thomas Malthus, who maintained that the human population tend to increase at a much greater rate than does the food supply necessary to sustain the population.

Darwin studies essay outlining a similar theory of evolution by another naturalist Alfred Russel Wallace who observed diversity in plants and animals and their peculiar distribution in South America, Malaya and Indian Archip elago. He was also aware of the practices of plant and animal breeders in producing, in a few generation, the desired traits in domestic animals and plants by artificial selection.

All this knowledge helped Charles Darwin to propose the theory of evolution by natural selection. He presented his theory at a scientific meeting of Linnaean society in London (1858) and published his famous book named ‘’the origin of species by means of natural selection’’ in 1859. It created a storm of controversy but generated a favorable band of supporters.

Charles Darwin

The salient features of Darwin’s theory of evolution are:

1. Overproduction
2. Heritable variations
3. Competition for Survival
4. Natural selection     

Overproduction

All organisms tend to reproduce in geometrical progression (1, 2, 4, 8, 16, 32,…). If all the organisms produced remain alive it would result in millions of organisms of a single species on this earth in a matter of a century even in the case of slow breeders such as elephants.

Heritable Variations

Darwin noticed that not all individuals of a species when born are alike. He realized that some of the variation s was useful to individuals carrying them as these variations helped them to live comfortably in that environment more than the individuals which did not possess these useful variations.

Competition for Survival

The available resources of the environment for example, food, water, shelter, etc, are limited. Food can increase in arithmetic fashion (1, 2, 4, 6, 8, 10, …). We have seen that organism’s population increase in geometric progression. Therefore ratio of organisms to food etc; reach unmanageable proportion and leads to a secure struggle for other species to get their food etc.

Natural selection

In this competition for survival or struggle for existence, the variants that are better adapted to a particular environment are successful and continue to produce offspring with their adaptive characteristics. Over time the characteristics, which provide addictiveness or fitness come to accumulate in the population than those characteristics, which decrease fitness. The greater reproductive success of the better adapted form constitutes the natural selection. This means to say that better adapted individuals (with useful variations) are favored by nature to survive and reproduce, as compared to individuals which are not fit for that environment.

Now question is ‘’How would Darwin have explained the giraffe’s long neck?’’

According to Darwin the original giraffe population had variations in the length of their necks; some individuals were with short necks and some with long necks. The long necked giraffe would eat leaves of the trees as well as grass on the ground. When the grass became scarce, the long necked giraffe could obtain more food form the top of trees than short-necked giraffe and were mare likely to survive and reproduce. This favorable variation of long neck was inherited by the offspring generation after generation. The present long necked giraffe is the result of process of evolution.

Darwin’s theory of natural selection was reasonable and supported by arguments that is way it was accepted by many scientists.
A comparison of Lamarckism and Darwinism with example of giraffe’s neck is given in table 18.2.