18.2.2 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.
Fig. 18.2. Charles Darwin
The salient features of Darwin’s theory of evolution are:
- Overproduction
- Heritable variations
- Competition for Survival
- Natural selection
1. 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.
2. 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.
3. 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.
4. 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.