(Theory of Natural Selection by Charles Darwin)
(By Dr. Girish Chandra)
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.
(Synthetic theory of natural selection)
(Modern theory of evolution)
(By Dr. Girish Chandra)
Darwin’s theory lacked an input of modern concepts of genetics and the mechanisms how characters appear and persist in a population. Darwinism, although basically sound, now needed improvements in several aspects of the theory. Neo-Darwinism is originally associated with Weismann (1834-1914), who tried to explain inheritance of characters by his theory of germplasm. He divided body into two parts: somatoplasm and germplasm and explained that only those characters that are in germplasm are heritable. Synthetic theory emerged by the synthesis of the original idea given by Charles Darwin and addition of new knowledge of genetics, population dynamics, statistics, and heredity to the theory. This is the most modern theory of evolution and has been constantly improved during 20th century by the contribution of the following scientists: R.A. Fischer, J.B.S. Haldane, Ernst Mayr, Julian Huxley, G.G. Simpson contributed with their studies on population dynamics. T. Dobzhansky, H.J. Muller, H. DeVries, G.L. Stebbins added information on genetics and mutation. G.H. Hardy, W. Weinberg, Sewall Wright did extensive work on population genetics and statistics, which helped to understand the mechanism of heredity.
The modern synthetic theory in its present improved form can be outlined as follows:
1. Overproduction: This point of Darwinism that says that organisms have a tendency to reproduce at much higher rate than required is retained in this theory without any change.
2. Limited space and food: Earth as well as all its ecosystems has limited space, which cannot be stretched to accommodate unlimited number of animals. Similarly food supply that any ecosystem can provide is also limited.
3. High death rate: All offspring produced do not reach maturity but only a small number of them manage to gain adulthood and reproduce. A large number die due to limitations of space and food, which imposes struggle for existence among them.
4. Variations: Variations are differences among the closely related individuals. According to the new theory, only heritable variations are important in evolution, which are caused by haphazard mutations, chromosomal aberrations, aneuploidy, polyploidy, hybridization etc. Somatic variations are short-lived and therefore have no value in natural selection. For details see chapter on Variations.
5. Net reproductive differential: Reproductive differential is the ability of a species to leave more progeny than the others in the next generation. It is different from reproductive capacity, which simply means producing more offspring. In reproductive differential, the offspring must also survive, grow and produce the next generation. Therefore, leaving more individuals in the next generation is more important for the competing species. All beneficial characters help in reproductive differential, which may be physical, behavioral, physiological or morphological. Dice’s experiment with mice explains this phenomenon more clearly. He placed equal number of white and brown mice in a large cage that had natural terrain, with bushes rocks etc. There was plenty of food and their reproductive rate was almost same. Dice released a pair of owls in the cage and allowed the mice to breed for several generations. There were no limiting factors in the cage except that the brown mice could camouflage against rocks and bushes whereas white mice were prominent and could be easily spotted by the owls. Dice found that white mice died out gradually, while brown mice survived and flourished.
6. Speciation due to isolation: Speciation is the origin of one to several species from the ancestral ones, due to isolation. When species are split into two populations due to geographical isolation, they separately accumulate several inversions, translocations and mutations and become reproductively isolated in due course. Isolation is the primary requirement for the formation of species. For details see chapters on Isolation and Speciation.
Examples of natural selection
1. The industrial melanic moth: Biston betularia, the industrial melanic moth, is a gray colored moth that perfectly camouflages on tree trunks covered with lichen in England and escapes predation by birds. With industrial revolution in England in the middle of 19th century, lichens on tree trunks got killed due to smoke belching out of factories. Tree trunks were now bare and dark and made the light gray moth prominent to the predatory birds. Now natural selection favoured dark coloured moths, which could camouflage on bare tree trunks. Since the moth has only one generation in a year, in less than 50 generations, the natural selection replaced gray population with black population.
2. Resistance in mosquitoes and houseflies: DDT was used extensively, sometimes by airplanes over large areas. Initially it killed 99% of mosquito population but at the same time put a lot of pressure on the surviving individuals to mutate. Mutant resistant strains survived DDT application and became the parents of the next generation. Natural selection preserved the resistant populations and eliminated the susceptible ones. This can be called an artificial selection by man, due to which today not only mosquito and housefly but also many agricultural pests have become resistant to most of the available insecticides.
3. Liederberg’s replica plating experiment: Liederberg (1952) conducted experiment on Escherichia coli by exposing the susceptible strains to penicillin repeatedly. As the generation time of the bacterium is 20-30 minutes only, hundreds of generations could be cultured and exposed to penicillin within a short time. He found that mutations for resistance appeared instantly and quickly replaced the susceptible populations by natural selection.
4. Fluctuation test experiment: Salvador Luria & Max Delbruck (1943) cultured a population of E. coli in one flask along with bacteriophage viruses. He then cultured samples from the flask on agar plates and found similar growth on all agar plates. He found that in some flasks instant mutations had appeared for resistance against viruses while in others susceptible strains died out. This experiment proved that in populations exposed to environmental extremes, either the mutants appear or hidden recessive mutations express and get exposed to natural selection and save the population from the possible extinction.
Natural selection is a phenomenon that forces the species to keep improving generation after generation so that they remain in the fittest state to survive in a particular environment. Random genetic changes provide raw material that causes variations and gives natural selection a chance to operate.
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