Evolution refers to change over time in the genetic composition of natural populations. Natural selection plays a vital role in adding adaptation, accumulation and preservation to the new desired genetic states. The accumulation of small but observable micro evolutionary changes necessary for the development of new species occurs over a long period of time (Haviland, Walrath, Prins, & McBride, 2010). Four forces that affect the course of evolution and variation are selection, migration, mutation, and population size. These four forces of evolution and variation determine the structure and the extent of the genetic variation within the population (Evans, 2001). The interaction of all four factors is a highly complex one in terms of integrating the system and changes in genes.
For selection to have any considerable effect on speciation and the structure of organisms there must be a difference in genotypes. Normally, a genotype refers to an ensemble of alleles in the gene loci that distinguishes it from other genotypes. Such differences in alleles may affect survival and reproduction capabilities, which results in the longer or shorter lifespan of specific genotype (Mayr, 1970). However, differences in genotypes have adverse effects on the overall traits of organisms. If a trait is advantageous, there are better chances for survival and growth of next generations. On the other hand, if a trait is disadvantageous then it will be eliminated sooner or later as it will appear in fewer genotype representatives over time. Selection has a positive effect on the evolutionary potential of populations and species in small increments generation after generation (Goldsmith, 1994). These changes in the traits of the organisms do not reach optimal condition at any time. If alleles have partial dominance in the genotype, the tendency for its replacement might appear and, consequently endanger the trait in the genotype and vice versa (Evans, 2001). For example, if alleles determine adaptation trait in a population dominate, then the whole population will have the trait more often. However, selection of dominating alleles provides that only some traits can develop in future generations while some other traits become endangered.
Mutation provides for a random change in the structure of genes and organisms function in the entire genome. Mutations favor the organism’s survival or can lead to their elimination. Although not all mutations are beneficial to the organisms, nature has its way of maintaining the balance between the new mutants and their rate of elimination (Evans, 2001). Most mutations occur at extremely low rates and, therefore, would not be expected to affect the adaptability of the population except when it occurs in small, isolated populations. Independent mutations, selection and hybridization can generate high genetic variance among populations (Wool, 2006). These genetic variances are the main causes of the differences in physical traits among populations. The new traits then enable the new organisms to adapt in the new environments.
Migration affects the survival of traits and development of the new ones. The exchange of the genetic traits between immigrants can affect the development of well adapted organisms among the general population. On the other hand, migration can lead to separation and, hence, the development of the new adaptation features accelerates. If sufficient time passes between two separated species, then considerable evolutionary changes may occur. In some cases, they may then prove incapability of interbreeding even after their separating barriers vanish. These isolating mechanisms cannot lead to speciation alone but must involve other factors as well (Wool, 2006).
The population size affects the future composition of the population. The chances of retaining the genetic traits are higher when the population is large (Wool, 2006). Population size has strong effect when it comes to competition for food. Adaptability and development of new traits within the population is affected by population’s ability to reproduce, grow and compete with the existing populations and other species (Wool, 2006).
Isolating mechanisms refer to various devices or mechanisms that prevent the interbreeding between species (Haviland, Walrath, Prins, & McBride, 2010). For the evolution to take place, isolating mechanisms in and among the general population must exist. Isolation mechanisms can be reproductive or geographic in nature. They can be further divided into pre-mating mechanisms and post mating mechanisms (Mayr, 1970). In pre-mating isolation, the potential mates may not meet either due to seasonal changes, or due to changes in habitat (Ereshefsky, 1992). Although mates may meet, they may not mate (ethological isolation), but if mating is attempted, the transfer of the sperm may not take place (due to mechanical changes). On the other hand, for post mating, sperm transfer may take place but fertilization may not occur (game tic mortality). The egg might be fertilized but the zygote dies (zygotic mortality) (Mayr, 1970). The zygote produced may be of low viability (hybrid viability) or even if the zygote is viable, it can be partially or entirely senile, which results in gradual isolation of the population (hybrid sterility) (Ereshefsky, 1992). These isolating factors play a pivotal role in the development of the new species as well as in the improvement and continuation of the existing ones.