This theory posited the concept of constant and gradual change as characteristic of all life. To this, geneticists contributed the finding that characteristics favoured by selection are passed on from generation to generation according to Mundelein laws of particulate inheritance, thus preserving genetic variation in a population.
Further, individuals do not evolve, populations do. That is, the genetic make-up (genotype) of an individual remains constant throughout life, except for special situations, while the types and numbers of gene flow in a population of creatures can vary. The field of population genetics deals with questions of gene flow among individuals in interbreeding group.
Populations comprise groups of individuals in one locale or within defined geographic boundaries capable of freely interbreeding. All of the genes of these individuals form a single gene pool and the way in which each allele is represented in the gene pool determines a population’s variation or gene frequencies (A frequency is the ratio of actual number of individuals falling in a single class to the total number of individuals).
Variation is both phenotypic and genotypic, the range of both ultimately determined by the individual genotype. Phenotypic variation represents adaptation by the individual, whereas genotypic variation represents adaptation by the population, enabling it to change over time.
Genotypic variations is both continuous and regular'(e.g., quantitative or polygenic traits such as body size and skin pigmentation), and discontinuous or sharp (e.g., qualitative traits such as form, structure, etc) (see Verma and Agarwal, 1982 for more details). When individuals within a population differ strikingly from each other in one or more qualitative traits the population is considered polymorphic.
Variation originates in mutation and recombination, and is affected by natural selection, migration (introgression), and genetic drift (random changes in gene frequencies). In the absence of these factors it will remain constant from one generation to the next, provided the population is randomly mating.
This principle is known as the Hardy-Weinberg principle because it was developed independently in 1908 by a British mathematician, G.H. Hardy and a German physician, W. Weinberg. In mathematical terms, this principle is stated as follows:
(p + q)2 = 1
Which car, be written in expanded form as
(p + q) ? (p + q) = l
Or, p2 + 2pq + q2 = l
Where, p =frequency of the dominant gene; q = frequency of the recessive gene, p2 = frequency of the homozygous dominant genotype, 2pq = frequency of heterozygous genotype, q2 = frequency of the homozygous recessive genotype, and 1 = a population in genetic equilibrium.
The Hardy-Weinberg principle, thus, enables us to determine equilibrium genotypic frequencies in future generations from any given gene frequencies. The degree to which actual genotypic frequencies depart from equilibrium frequencies indicates the rate of evolution, for evolution can be considered a change in a population’s gene frequencies.
Sexual reproduction permits recombination of genes in instances of crossing-over at meiosis in gamete formation and in the pairing of parental chromosomes at zygote formation. Mutations take the form of changes in the gene’s DNA (thereby called point mutations) and changes in chromosome number and changes in chromosome structure (both are called gross or chromosomal mutations). Recombination and mutation are regarded basic source of evolutionary change.
Migration operates as a force of evolution when it occurs between relatively isolated populations with distinct gene pools, as immigrants from one population to another can contribute totally new alleles. This exchange of genes is known as gene flow and functions to increase variation in a population.
Operating upon genetic variation are the forces of selection and genetic drift. Natural selection occurs in nature, whereas artificial “election is engineered in laboratory conditions; both, however, result in differential reproduction of the genotypes of a population, election acts upon the individual phenotypic to increase or decrease the corresponding genotype in a population following three types of selection have been categorized by population geneticists: (i) Directional selection describes the change that occurs when a population shows a particular trend through time.
An example of this change is seen in the evolution of horse, in which the average size has increased through time, presumably because there are selective advantages for being large. The case of industrial melanoma in peppered moth (Bistort betularia) represents another popular example of directional selection; (ii) Stabilizing selection describes the change when extreme individuals are eliminated from the population.
The result of this process is a reduced variability in the population. Most selection that occurs in populations is stabilizing and homeostatic because it tends to maintain the status quo. (iii) Disruptive selection describes the process where certain types of a character have a high survival value, whereas intermediate types are not advantageous.
This would be the case if big horses and small horses of a population had a high survival rate, while middle-sized horses were not favoured. Directional selection, combined with stabilizing selection, is probably most significant in terms of evolution.
Genetic drift is most likely to become an evolutionary factor in a small population, especially one characterized by a high degree of inbreeding. Due to more frequent pairing, homozygous recessives may appear in relatively high frequencies. On the other hand, some genes may be carried by so few members that they become lost by chance. When such changes occur as a result of a non-representative original population, they are ascribed to founder effect.
Founder effect describes the presence of one or more deleterious allele in relatively high frequencies, resulting from a non-reprehensive sample in the seed stock, or founders of a population. For example, founder effect may apply to the surviving members of a population considerably reduced by plague or famine or some other catastrophe.
If as this small population expands by inbreeding and it remains isolated, due to geographical inaccessibility or general environmental hardship, a harmful recessive gene which may have been insignificant in the parent population (before the disaster) because of the small chances of its combining with another such gene, may reappear more frequently (Goldsby, 1976).
Variation is protected or maintained in any population by the combination of a deleterious recessive gene in heterozygous form and selection favors modifying genes that render the heterozygote phenotypic ally similar to the dominant homozygotes.
Moreover, standard patterns or canalization of development are imposed upon genetic variation for traits essential to survival. A demonstrable influence on continued variation is heterocyst or heterozygote superiority. This adaptive superiority may be due to the fact that the heterozygote has the benefit of two kinds of metabolites, one produced by each allele. Since it has the capabilities of both homozygotes, it is better adapted, in certain conditions, than either.
The most frequently quoted example of heterocyst is the human variant known as sickle-cell anemia, in which the hemoglobin structure is modified so that a homozygote for the condition usually dies of anemia in childhood.
However, the heterozygote’s of this abnormality are found to have considerable tolerance for attacks of malaria (Boughey, 1968). Greater fitness of the heterozygotes ensures the maintenance of both alleles in populations operated upon by opposing selective forces.
This results in balanced polymorphism, a stable proportion of two or more forms; any imbalance in either direction will be disadvantageous for the population as a whole, but selection against the homozygote will tend to restore equilibrium.
Polymorphism is particularly useful to species living in a variable environment, for it enables one or another form to thrive under various local conditions. Finally, variation is assured by sexual reproduction, and selection has favoured the evolution of some form of out breeding even in self-fertilizing or asexual organisms.