Darwinism (Theory of Natural Selection):A. Introduction:Charles Darwin (Fig.7.36) (1809-1882 A.D.), an English naturalist, was the most dominant figureamong the biologists of the 19th century. He made an extensive study of naturefor over 20 years, especially when he joined the expedition on Beagle to SouthAmerica.
(Fig. 7.37) and explored South America, the Galapagos Islands andother islands.He collected the observations on animal distribution and the relationshipbetween living and extinct animals. He found that existing living forms sharesimilarities in many ways, not only among themselves but also with the lifeforms that existed millions of years ago, some of which have become extinct.He stated that every population has built invariations in their characters. He found that all living organisms,including humansare modified descendants of previously existing life forms.
Darwin perceivedthe origin of species and adaptations as closely related processes. By theearly 1840’s,Darwin had worked out the major features of his theory of naturalselection as the mechanism of evolution.In 1884,Darwin wrote a long essay onthe origin of species and natural selection,But before it could be published,Alfred Wallace, a young naturalist,working in East Indies developed a theory ofnatural selection, identical to Darwin’s.Wallace’s paper,along with extractsfrom Darwin’s unpublished 1884 essay, were presented to the Linnaean society ofLondon on July 1,1858. Darwin elaborated his theory of evolution in abook entitled “On the Origin of Species by means of Natural Selection”,published on 24th Nov, 1859. In his book, Darwin developed the two main points:· Descent with Modification · Natural SelectionIn this theory, Charles Darwin proposed theconcept of natural selection as the mechanism of evolution.B. Postulates of Darwinism:Main postulates of Darwinismare: Geometric increase.
Limited food and space. Struggle for existence. Variations. Natural selection or Survival of the fittest. Inheritance of useful variations.
Speciation. Ø Geometric Increase:According to Darwinism, the populations tendto multiply geometrically and the reproductive powers of living organisms(biotic potential) are much more than required to maintain their number e.g.,Paramecium divides thrice by binary fission injust 24 hours under favourable conditions. At this rate, a Paramecium canproduce a clone of about 280 million Paramecia in just one month and in fiveyears, can produce Paramecia having mass equal to 10,000 times than the size ofthe earth.Other rapidly multiplying organisms are:Oyster (114 million eggs in one spawning); Ascaris (70, 00,000 eggs in 24hours); housefly (120 eggs in one laying and laying eggs six times in a summerseason).
Similarly, the plants also reproduce veryrapidly e.g., a single evening primrose plant produces about 1, 18,000 seedsand single fern plant produces a few million spores.
Even slow breeding organisms reproduce at arate which is much higher than required e.g., an elephant becomes sexuallymature at 30 years of age and during its life span of 90 years, produces onlysix offsprings. At this rate, if all elephants survive then a single pair ofelephants can produce about 19 million elephants in 750 years.
These examples confirm that every species canincrease manifold within a few generations and occupy all the available spaceon the earth, provided all survive and repeat the process. So the number of aspecies will be much more than can be supported on the earth.Ø Limited food and space:According to Darwinism,a population tends toincrease geometrically, but the food increases only arithmetically.Therefore,the two main limiting factors on the tremendous increase of apopulation are: limited food and space, which together constitute the majorpart of carrying capacity of environment. These do not allow a population togrow indefinitely which are nearly stable in size except for seasonalfluctuation.
Ø Struggle for existence:Due to tremendous increase in populations butlimited food and space, there starts an everlasting competition and war betweenindividuals having similarities in their features. In this competition, everyliving organism desires to have an upper hand over others.This competition between livingorganisms for the basic needs of life like food, space, mate etc. for theirsurvival is called struggle for existence which further is of three types:(a) Intraspecific:Between the members of same species e.g. twodogs struggling for a piece of meat.(b) Interspecific:Between the members of different species e.g.
between predator and prey.(c) Environmental orExtra specific:Between living organisms and adverseenvironmental factors like heat, cold, drought, flood, earthquakes, light etc.Out of these three forms of struggle, theintraspecific struggle is the strongest type of struggle as the needs of theindividuals of same species are most similar e.g., sexual selection in which acock with a more beautiful comb and plumage has better chances to win a hen thana cock with less developed comb.In this death and life struggle, the majorityof individuals die before reaching the sexual maturity and only a fewindividuals survive and reach the reproductive stage. So struggle for existenceacts as an effective check on an ever-increasing population of each species.
The nature appears saying, “They are weighedin the balance and are found wanting.” So the number of offsprings of eachspecies remains nearly constant over long period of time.Ø Variations:Variation is the law of nature.
According tothis law of nature, no two individuals except identical (monozygotic) twins aresame/identical. This everlasting competition among the organisms has compelledthem to change according to the conditions to utilize the natural resources andcan survive successfully.Darwin stated that the variations aregenerally of two types—continuous variations or discontinuous variations. On the basis oftheir effect on the survival chances of living organisms, the variations may beneutral, harmful and useful.Darwin proposed that living organisms adapt to changing environment due to usefulcontinuous variations {e.g., increased speed in the prey; increased waterconservation in plants; etc.), as these will have a competitive advantage.
Ø Natural selection or Survivalof the fittest:Darwin stated that as humans select theindividuals with desired characters in artificial selection; nature selectsonly those individuals out of the population which are with useful continuousvariations and are best adapted to the environment while the less fit or unfitindividuals are rejected by it. Natural Selection can amplify or diminish onlythose variations that are heritable.It is noteworthy to say that adaptationsthat an organism acquires by it’s own actions are not heritable.The specificsof natural selection are regional and timely; environment factors vary fromplace to place and from time to time.An example of natural selection in actionis the evolution of antibiotic resistance in bacteria. Ø Inheritance of usefulvariations:Darwin believed that the selected individualswith the best features pass their usefulcontinuous variations to their offsprings so that they are born fit to thefluctuated environment.Ø Speciation:According to Darwinism, the unequal ability ofindividuals to survive and reproduce will lead to a gradual change in apopulation, with favorable characteristics accumulating over the generationsthus leading to the evolution of a new species.C.
Evidences in favour of Darwinism:1. There is a close parallelism betweennatural selection and artificial selection.2. The remarkable cases of resemblance e.
g.mimicry and protective colouration can be achieved only by gradual changesoccurring simultaneously both in the model and the mimic.3. Correlation between position of nectariesin the flowers and length of the proboscis of the pollinating insect.D.Evidences against Darwinism:Darwinism is not able toexplain:1. The inheritance of small variations inthose organs which can be of use only when fully formed e.
g. wing of a bird.Such organs will be of no use in incipient or underdeveloped stage.
2. Inheritance of vestigial organs.3. Inheritance of over-specialised organs e.
g.antlers in deer and tusks in elephants.4.
Presence of neuter flowers and sterility ofhybrids.5. Did not differentiate between somatic andgerminal variations.6. He did not explain the causes of thevariations and the mode of transmission of variations.7. It was also refuted by Mendel’s laws ofinheritance which state that inheritance is particulate.
So this theory explains only the survival ofthe fittest but does not explain the arrival of the fittest so Darwin himselfconfessed, “natural selection has been main but not the exclusive means ofmodification.”Principle of Natural Selection(Table 7.7):It was proposed by Ernst Mayer in 1982. Itstems from five important observations and three inferences as shown in Table7.7. This principle demonstrates that natural selection is the differentialsuccess in reproduction and enables the organisms to adapt them to theirenvironment by development of small and useful variations.
These favourable Variations accumulate overgeneration after generation and lead to speciation. So natural selectionoperates through interactions between the environment and inherent variabilityin the population.
