Darwinism

Several theories had already been proposed to explain organic evolution before Darwin's own work — including Lamarck's 1809 theory of the inheritance of acquired characters, which was ultimately ruled out (as described above). Charles Darwin (1809-1882; the book prints his portrait as Fig. 5.4) arrived at his own theory only after years of direct observation: as a naturalist, he travelled extensively around the world from 27 December 1831 to October 1836, and drawing on the living and fossil material he collected on that voyage, he eventually published 'On the Origin of Species by Means of Natural Selection' in 1859.

Before publishing, Darwin was influenced by several important sources. The geologist Charles Lyell had argued that the natural forces shaping the Earth in the past are the very same forces still acting today (uniformitarianism), an idea Darwin saw borne out first-hand when he observed changes in the Earth's crust along the coast of South America caused by ongoing natural forces. In the Galapagos Islands and the surrounding archipelago, Darwin also observed a striking variety of plants and animals — most famously variation among the islands' tortoises and finches — observations that Alfred Russel Wallace independently made as well. Darwin was further influenced by the economist Thomas Malthus, who had pointed out that the reproductive potential of the human population is greater than the corresponding growth in food supply, leading inevitably to competition and a struggle for existence — and by plant and animal breeders, whose use of artificial selection to alter the traits of cultivated plants and domestic animals showed Darwin that selection really could reshape a species over time.

Darwinism (Theory of origin of species by Natural Selection). Darwinism is based on five main postulates:

1. Overproduction (prodigality of nature). Every species has a natural tendency to produce far more progeny than can possibly survive, in a roughly geometric ratio, in order to perpetuate itself — Darwin noted, for instance, that a single Salmon can produce about 28 lakh eggs in one season, and that a single pair of elephants, left unchecked, could theoretically give rise to 19,000,000 elephants over 750 years. Despite this reproductive potential, the actual population size of a given species in a given area stays relatively constant, because of seasonal and other fluctuations that keep numbers in check.

2. Struggle for existence. This natural tendency toward overproduction inevitably leads to a struggle for existence among members of the population — a struggle over limited food, over space and mates, against adverse environmental conditions, and to escape from enemies.

3. Organic variations. Populations show all kinds of differences in morphology, physiology, nutrition, habit and behaviour among their members — differences that occur both within a single species and between different species. Darwin recognised these variations as the essential raw material that evolution works on.

4. Natural selection. In the ongoing struggle for existence, organisms carrying favourable variations are better adapted to survive under existing environmental conditions and are therefore selected by nature, while those with unfavourable variations tend to perish. Darwin called the principle by which nature preserves useful variations 'natural selection'; the philosopher Herbert Spencer later gave the same idea its more familiar name, 'survival of the fittest'.

5. Origin of new species (speciation). As favourable variations are passed down from one generation to the next, successive generations become progressively better adapted to their environment. Over time, these accumulated adaptations, along with a few further modifications, become permanently fixed in the life cycle of the population, eventually giving rise to a distinct new species.

Evidences cited for Darwinism include the evolution of the long-necked giraffe, whose lengthened neck let it reach and eat leaves from tall trees and woody climbers — an adaptation that helped tall-necked individuals survive in famine-prone, sparsely vegetated areas and pass the trait on to their offspring; the gradual evolution of a new, dark-coloured variety among the peppered moths; and the spread of DDT resistance among mosquitoes, where intensive DDT spraying killed most mosquitoes but a resistant few survived, reproduced, and gave rise to increasingly DDT-resistant offspring.

Drawbacks and objections to Darwinism. -

a. Darwin treated minute, fluctuating variations as a principal factor in evolution, without distinguishing between variations caused by the environment and those that are truly hereditary. …