There are three theories that have been put forward to explain the mode of evolution i.e., origin of species.
1. Lamarck’s Theory of Evolution:
Lamarck proposed that the evolution of life forms had occurred by the use and disuse of organs. He gave the example of Giraffe, which initially did not have long necks.
But, in order to reach the leaves on tall trees, they adapted by elongation of their necks. By passing this acquired character to succeeding generations, over the years, they came to acquire long necks. This theory, also called as Lamarckism is no more accepted now.
2. Darwin’s Theory of Natural Selection:
Charles Darwin made a sea voyage around the world in a sail ship HMS Beagle.
Based on his observations, he concluded the following:
1. There has been gradual evolution of life forms with new forms arising at different periods of history.
2. Varying degrees of similarities can be observed between existing life forms and those existed million of years ago.
3. Population of all organisms have variations in characteristics, which make them adapt to better environment.
4. The characteristics which enable some population or individuals to survive better in natural conditions (foods, climate, physical factors) would outbreed others (survival of the fittest).
This fitness of the individual, according to Darwin, refers ultimately to the reproductive fitness. Such fit individuals leave more fit individuals than others. Thus, those organisms, which are better fit or adapt well will survive more in nature and will be selected by nature. This is called natural selection.
It is based on following observations, which are factual:
(i) Limited natural resources due to which populations are stable in size except for seasonal fluctuations.
(ii) Varying characteristics in members of a population, i.e., no two numbers of a population are identical even though they show similarities.
(iii) Variations are generally inherited and population size will grow exponentially.
(iv) The population size is stable except seasonal fluctuations. It is due to the competition among the individual for resources. Those which are better adapted could survive and will reproduce at the cost of others, i.e., who are less adapted the environment.
Examples of natural selection are as follows:
Industrial melanism is an example of natural selection. There were collection of white moths in England. It was observed that before industrial revolution in England, white-winged moths were more in number than dark-winged moths. After industrialisation, dark-winged moths became more than white-winged moths.
This is because during industrialisation, the tree trunks covered by white lichens became dark due to air pollution (dust and soot particles). Due to this, white-winged moths could be easily eaten by the predators as they failed in camouflaging (hide or disguise the presence of a person, animal or object by means of camouflage).
It is an another example of natural selection. The excess use of herbicides and pesticides leads to resistant varieties of microbes in very less time. Due to this, pathogenic bacteria appeared in a very short period.
3. Mutation Theory of Evolution:
Hugo de Vries carried out experiments on evening primrose plant and proposed the mutation theory of evolution. This theory states that the evolution occurs by the sudden large differences in the population, i.e., he believed that mutation is the only cause of evolution and not the minor variations. He stated that the mutation caused speciation and called it saltation (single step large mutation). Mutations are random and without directions, while Darwinian variations are small and directional.
This principle, states that the allele frequencies in a population are stable and remain constant from generation to generation, i.e., gene pool. This is called genetic equilibrium or Hardy-Weinberg equilibrium. Sum total of all the allelic frequencies is equals to 1, e.g., in a diploid, if p and q represent the frequency of an allele and allele a. The frequency of AA individuals in a population is p2, of aa is q2 and of Aa is 2pq.
Hence, it can be expressed by the following reaction:
P2 + 2pq + q2 = 1.
This is called binomial expansion of (p + q) 2.
The extent of evolutionary change is the difference between the value of frequency measured (frequency obtained) and the value expected. The disturbance in genetic equilibrium (Hardy-Weinberg equilibrium) would result in evolution.
Factors Affecting Hardy-Weinberg Equilibrium:
Certain factors that affect Hardy-Weinberg equilibriums are as follows:
(i) Gene Migration or Gene Flow:
The removal of alleles from one population or addition of alleles into an another population is called gene migration.
New genes or alleles are added to new population and are lost from old population in turn changing the frequencies. Hence, this migration is also called gene flow.
(ii) Genetic Drift:
It refers to the change in the population of alleles in the gene pool, i.e, multiple times of gene flow. It is a random gene frequency and occurs only by chance. At times, the change in allele frequency is so different in the new sample of population that they become a different species. The original drifted population becomes the founder and the changes in the phenotype and the genotype of the progeny, constitute the founder effect.
The sudden appearance or variations are called mutations. They lead to the new phenotypes. Though mutations are random and occur at very slow rates, they are sufficient to create considerable genetic variations for speciation to occur.
(iv) Genetic Recombination:
The alleles of parental linkage groups separate and new associations of alleles are formed due to crossing over in meiosis in the gamete cells. This process is known as genetic recombination.
(v) Natural Selection:
It occurs due to the inheritance of variations. It leads to the survival of those, who best fit in an environment reproduce well and survive Hence, selected by the nature (Survival of the fittest). According to Hardy-Weinberg principle, gene frequencies will remain constant if all the above five conditions are met.
Depending upon the traits favoured, Natural selection can have following three effects:
(a) Stabilization Large number of individuals acquire mean character value, i.e., variation is much reduced.
(b) Directional Change Large number of individuals acquire value other than mean character value.