Evolution

Evolution

CONCEPT OF EVOLUTION

Evolution is the alteration in the heritable characters ,traits or qualities of biological populations over chronological filial generations. These traits are the expressions of one or more allelic forms of genes i.e combination of dominant and recessive alleles which code for such traits that are passed on from ancestors to progenies during the course of reproduction.

MECHANISM OF EVOLUTION

Various traits abound within group of species which came into existence through intrisinc and extrisinc factors like mutation, genetic recombination and other sources of genetic variation. Evolution takes place when mechanisms like natural selection ( sexual selection inclusive) ,genetic drift,gene flow and mutation bias act on this alleles, leading to skewed distribution of traits with varying geno-type and pheno-type within a given population ,where some characters become frequent, rare and even extinct.It is the combination of these processes that gave rise to biodiversity at every level of biological organisation.

1. MUTATION BIAS

Mutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of developmental bias.

(Haldane,1927) and (Fisher,1930) argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. This opposing-pressures argument was long used to dismiss the possibility of internal tendencies in evolution,(Yamplosky et.al,2001). Until the molecular era prompted renewed interest in neutral evolution.

2. GENE DRIFT

Genetic drift is the unpredictable rise or fall in the allelic frequencies within a population of species of biological organisms under investigation from one generation to another.When selective forces are non-existence or weak, allele frequencies have the tendency to simultaneously skew in the direction of dominant allele arising from the drift, at each successive generation because the allelic recombination during reproductive division is random given rise to random population,where some of this alleles become dominant and others extinct. This drift stops when an allelic population stablizes, either by their total disappearance from the population or they completely replace other alleles in the said population. The elimination of alleles from a population may be indiscriminate but not selective. Even in the absence of selective forces, genetic drift may triggers spontaneous division of two(2) genetically identical populations to become two divergent populations with different sets of alleles.

3.GENE FLOW

Gene flow encompasses the intra species, intra and extra population exchange of allelic forms of genes. The existence or absence of gene flow pertinently influences the path way of evolution. As a result of genotypic complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral and non-selective processes, as in the Bateson-Dobzhansky-Muller model, even if both populations remain essentially identical in terms of their adaptation to the environment.

REFERENCES

(a) Haldane, J.B.S. (July 1927). "A Mathematical Theory of Natural and Artificial Selection, Part V: Selection and Mutation". Proceedings of the Cambridge Philosophical Society.26(7):838 844.Bibcode:1927PCPS...23..838H. doi:10.1017/S0305004100015644.

(b) Fisher 1930

(c) Yampolsky, Lev Y. Stoltzfus, Arlin (December 20, 2001). "Bias in the introduction of variation as an orienting factor in evolution". Evolution Development. 3 (2): 73 83. doi:10.1046/j.1525-142x.2001.003002073.x. PMID 11341676. S2CID 26956345