Evolution has been described as the process of gradual modification in the plants or animals (living organisms) basically two patterns are distinguished in the process of evolution. The minor changes in the gene pool of a population from one generation to the next may not produce new populations. The newly formed population is not genetically identical with its predecessor. This is called ‘Sequential evolution’. The changes occur in the newly evolved populations, species, families and classes is known as divergent evolution.
The animals of the same group are closely related groups exhibit great divergence in their morphology when they are found in different habitats. Prof. Osborn states that “each isolated region, if large and sufficiently varied in its topography, soil, climate and vegetation will give rise to a diverse fauna. The larger the region and more diverse the conditions, the greater will be varieties of animals found.” Therefore, the divergent evolution in specialized directions, starting from a common and generalised type or the entry of organisms of the original stock to new adaptive zones.
Example (1): The limb structure of placental mammals provides a classical example of divergent evolution. The ancestors of all the present day types of mammals can be traced back to a primitive insect eating five toed, short -legged creature walked with the soles of their flat feet. The pent dactyl limbs were not modified for any particular type of locomotion. These lived on land and formed as ancestors to the modern mammals. Now the modern mammals have occupied five different habitats. Therefore, divergence occurred in five lines for five different habitats with modification In their limb structure.
- The first line lead to “arboreal (climbing) modification “seen tree-dwelling forms like squirrels and primates.
- The second line acquired “aerial (flying) modification”, found in animals adapted for flight (Bat)
- The third line represents “cursorial (running) modification”. This type of mammals are adapted to fast running - Horse, Deer’s, Dogs etc.
- The fourth line acquired ‘fossorlal (burrowing) modification’, seen in moles.
- The last line lead to “aquatic (swimming) modification “found in seals, whales etc.
The incisors for biting, canines for tearing and grasping and the premolars and molars suited for grinding. The premolars and molars exhibit greatest structural modifications for different types of food.
- Insectivorous type Insect feeders - modified for crushing feeble prey.
- Carnivorous types: Meat eaters - modified by having high crowned with complicated cusps-carnasial.
- Herbivorous type modified for succulent vegetation & harsh grasses. Incisors are suited for cuffing the vegetation.
i) Geographical Isolation: Two parts of one population are separated by some geographical barrier and arc prevented from interbreeding. Large bodies of water are barriers for land-dwelling animals. High Mountain ranges, deserts, dense forests and extremes of temperature serve as affective barriers. Such populations are completely ‘out of touch with each other genetically’ so that new mutations, genetic drift and the action of natural selection, in one population have rio effect on the other population. Thus, a new population may be developed.
vi) Hybrid Sterility: Normal vigorous hybrids are formed but they are sterile and further exchange of genes is completely blocked. The Mule is a classical example of hybrid sterility.
This law can be defined as
‘The relative frequencies oi various kinds of genes in a large and randomly mating sexual panmictic population tend to remain constant from generation to generation in the absence of mutation, selection and gene flow”.
If the gametes unite at random, the total number of different genotypes will be.
There is a random union of the gametes with gene ‘A’ and ‘a’ at the Equilibrium state, the population will contain the following frequencies of the genotypes and genes ‘A’ and ‘a’ generation after generation.
- The gent ‘A’ from both this parents will be p c p = p2, ii) for gene ‘a’ will be q x q = q2
- the probability of being heterozygote will be pq + pq = 2 pq.
p +2pq+q =1
- The gene and genotype frequencies of each allele in a population remain at an equilibrium generation after generation.
- In a population, the mating is a completely random manner.
- The equilibrium in the genotype and gene frequencies occurs on in large sized populations. But in small sized population gene frequencies may be un predict table.
- MI the genotypes in a population reproduce equally and successfully,
Therefore, the geno type frequency in the first generation will be TT-25% Tt = 50% and It = 25%. The homgous tasters (Ti’) and heterozygous tasters (Tt) are phenotypic ally alike. So the populations possess 75% tasters and 25% are non-tasters. The same results can be obtained if we consider the union of gametes at the time of fertilization.
There the genotype frequencies according to Hardy-Weinberg’s equation.
p2 ÷2pq+q2 =l (q isfrequency for t )
This law provides a situation, where the genes in the population have reached the equilibrium and the gene pool is constant In such case, there will be no evolution. In nature, the mutations, natural selection, Non- random mating, genetic drifts and differential migration operate to change the genetic equilibrium actually can bring about organic evolution.