Population Genetics

Population genetics is concerned with analyzing the frequencies of the alleles, or forms, of genes in a population of individuals within a species. It examines how gene frequencies change across generations in response to external forces, such as mutation, natural selection, migration, or genetic drift.

The natural process that eliminates individuals of low fitness and advances those of high fitness is termed natural selection. Natural selection causes changes in allele frequency in a population, which is a process called evolution.

Thus, population genetics combines Charles Darwin’s ideas of natural selection and evolution with the basic principles of genetics set forth by Gregor Mendel.

The study of population genetics has many practical uses: It can help reveal how new diseases arise or why diseases persist in living organisms by examining genetic variation within populations.

Population genetics principles also can be useful as guidelines for devising strategies for improving crop plants. Hardy-Weinberg equilibrium provides the framework for population genetics.

Hardy-Weinberg Theorem

The starting point for population genetics is the Hardy-Weinberg theorem. Derived from Mendelian principles, it states that in the absence of mutation, selection, migration, or genetic drift, the allele frequencies and genotype frequencies remain constant from generation to generation.

This law applies to populations in a state of Hardy-Weinberg equilibrium, meaning large populations that are random-mating. If a population is not in this state, one generation of random mating restores equilibrium.

George H. Hardy, an English mathematician, and Wilhelm Weinberg, a German biologist and physician, independently developed this concept in 1908.

In 1903 Harvard University professor William E. Castle also had shown that in the absence of selection, the composition of the randomly bred descendants of a population would remain constant thereafter. Castle did not, however, generalize the concept.

More specifically, Hardy-Weinberg equilibrium ismaintained in a population as long as the following assumptions are true.

First, there are a large (virtually infinite) number of individuals in the population. Second, there is randommating among individuals. Third, there are no new mutations. Fourth, there is nomigration (in or out of the population).

Fifth, there are no genotype-dependent differences for survival to reproductive age and transmission of genes to the next generation. This fifth assumption is often stated more succinctly as: There is no natural selection.

The Hardy-Weinberg theorem can also be expressed mathematically in the form of an equation for calculating genotype and phenotype frequencies.

If two alleles at a locus, A and a, occur in a population with frequencies p and q, respectively, then (p + q) = 1. The proportion of individuals resulting from random matings will occur with the following frequencies:

AA = p2, Aa = 2pq, and aa = q2

Putting these terms together results in the Hardy-Weinberg equation:

p2 + 2pq + q2 = 1

This simple equation can be modified in a variety ofways to mathematically model what happens when one or more of the Hardy Weinberg assumptions are violated. Violation of one or more of these assumptions will result in changes in allele frequencies, and thus, evolution.

Changes in allele frequencies across a limited number of generations is often referred to as microevolution. Extending such changes across thousands of generations or more results in more extensive change and is often called macro evolution.

The Gene Pool

The Hardy-Weinberg equation and its modifications are used by population geneticists to describe changes in allele frequencies in gene pools.

A gene pool represents all the genes carried and shared by individuals of an interbreeding population, with the assumption that each parent contributes equally to a large pool of game tes (eggs and pollen or sperm). Another assumption is that the parent and offspring generations are distinct, or nonoverlapping.

Species and Speciation

The gene pool of a population, when divided or isolated for a prolonged period of time, may form distinct subgroups. If, through isolating mechanisms such as genetic changes or geographical separation, the subpopulations are kept from interbreeding, new species may eventually arise after many generations when the isolated sub populations evolve in different directions.

The entire process of the splitting of a population into two or more reproductively isolated populations is termed speciation.

Thus, a species, according to the commonly accepted biological species concept, is a group of interbreeding individuals that are capable of producing fertile offspring but are unable to do so with other such populations.

Sometimes hybridization between two species can result in a fertile hybrid, thus exposing one of the weaknesses of the biological species concept. Speciation can be the result of either geographic separation or reproductive isolation at the cellular and molecular levels.

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