Darwin’s observations during the voyage of the Beagle made him think differently and changed his views permanently. He started believing that species change gradually due to the force of natural selection and started writing his ideas. In 1859, Darwin published his famous book, “Origin of species” that gave details of the theory and made Darwin a celebrity. Ever since, the theory is known as Darwinism or Darwin’s theory of Natural Selection.
In his theory, Darwin gave details of his observations in different parts of the world during the voyage of the Beagle, from which he made two deductions and in the end concludes that natural selection takes place.
1. Overproduction: Darwin said that all organisms, without any exception, reproduce at a much higher rate than required. For example, fishes lay millions of eggs during spawning. Each oyster can lay 60-80 million eggs. Darwin calculated that if a pair of houseflies lays all its eggs and if all offsprings survive and reproduce to their full potential, then in one season (March to October) it will leave a progeny of 191,000,000,000,000,000,000 individuals.
Elephant is considered to be the slowest breeder. Its reproductive age is 30-100 years, during which it produces only 6 youngs. Darwin says even this is overproduction because if all offsprings and their offsprings survive and reproduce, then one pair will give rise to 19 million offsprings in 750 years.
2. Number is constant: In spite of overproduction by all organisms, their number in a given area remains more or less constant. All ecosystems have a limited capacity to support a particular species of organisms, e.g. a pond can support a fixed number of fish and a forest can support a fixed number of tigers or deer.
3. Variations: No two individuals are alike, even within a species, race or cohort. There are variations among individuals in appearance, physiology and capacity to starve, run and tide over cold or heat and many other traits. Darwin believed that all variations are heritable and he gave some weightage to Lamarckism because he did not know mechanism of heredity.
4. Struggle for existence: From the first two points, that is, overproduction is going on in nature everywhere but the number of organisms that can be supported by any ecosystem has to be constant, Darwin deduced that large number of offsprings produced will have to struggle among themselves to survive. Large number of individuals will perish in this struggle and few will be able to reach adulthood. Darwin believed that the struggle among the animals is always in the form of physical combat.
5. Survival of the fittest: As the struggle for existence is fierce, according to Darwin only the fittest individuals survive it and reach adulthood to reproduce and leave offspring. As Darwin gave a lot of importance to physical strength in the struggle, he believed that fittest is the individual which is physically strong and is able to fight for food, space, mates and can also escape predators and survive to reproduce to become parent of the next generation.
6. Natural selection: When animals overproduce, nature has a large number of individuals to choose from. Variations and struggle for existence give it the variety from which it selects the fittest individuals that become the parents of the next generation. Therefore, nature selects the cream from generation to generation and unfit and weaker individuals die out. Natural selection is a refining process, which brings improvement in a species from generation after generation, making the species fittest to live in a specific environment. Thus the species constantly changes and evolution goes on forever.
Criticisms of Darwinism
1. Darwinism mentions about survival of the fittest but does not explain about ‘Arrival of the fittest’, which means a character needs a long time to develop before it can become useful or fittest. For example, bird’s wings or electric organs of fish must have taken millions of years to develop fully and during this period of growth they were not useful organs and therefore should have been eliminated by natural selection.
2. Darwin believed that all variations are heritable, which is not true. Only genetic variations are heritable and not the somatic ones. During Darwin’s time mechanisms of heredity were not known and hence he tried to explain the inheritance by his theory of pangenesis.
3. Many useless and non-adaptive characters also persist in many animals and are not eliminated. For example, small tails in giraffe or pig and ear muscles and appendix in man have no selective value.
4. Darwinism does not explain over-specialization, which ultimately led to the extinction of the species. For example, dinosaurs became extinct due to overspecialization in body size. Saber tooth tiger (Smilodon) had oversized canines, which led to its extinction in Africa. Similarly Iris deer (Megaloceros) grew such huge antlers that they interfered in its movement through the forest and ensured its extinction.
5. Struggle for existence is not a physical one as Darwin suggests. Most of the time it is passive as for example many species of insects and other animals camouflaging against the predators do not actually fight physically.
6. Most of the mutations that produce new characters are harmful and cause diseases and therefore not useful in natural selection. Darwin did not know about mutations and explained sudden appearance of characters due to “Spots” or “Sports”.
7. Natural selection does not operate on one character as Darwin thought.
8. Darwin believed in blending inheritance, according to which characters of both parents blend in the offspring. But now according to the Mendel’s laws of heredity, it is known that characters segregate in the second generation.