Natural Selection


(Synthetic theory of natural selection)

(Modern theory of evolution)

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.