evolution Dynamics of genetic changescientific theory

Genetic variation is present throughout natural populations of organisms. This variation is sorted out in new ways in each generation by the process of sexual reproduction, which recombines the chromosomes inherited from the two parents during the formation of the gametes that produce the following generation. But heredity by itself does not change gene frequencies. This principle is stated by the Hardy-Weinberg law, so called because it was independently discovered in 1908 by the English mathematician G.H. Hardy and the German physician Wilhelm Weinberg.

The Hardy-Weinberg law describes the genetic equilibrium in a population by means of an algebraic equation. It states that genotypes, the genetic constitution of individual organisms, exist in certain frequencies that are a simple function of the allelic frequencies—namely, the square expansion of the sum of the allelic frequencies.

If there are two alleles, A and a, at a gene locus, three genotypes will be possible: AA, Aa, and aa. If the frequencies of the alleles A and a are p and q, respectively, the equilibrium frequencies of the three genotypes will be given by (p + q)² = p² + 2pq + q² for AA, Aa, and aa, respectively. The genotype equilibrium frequencies for any number of alleles are derived in the same way. If there are three alleles, A₁, A₂, and A₃, with frequencies p, q, and r, the equilibrium frequencies corresponding to the six possible genotypes (shown in parentheses) will be calculated as follows:

The figure shows how the law operates in a situation with just two alleles. Across the top and down the left side are the frequencies in the parental generation of the two alleles, p for A and q for a. As shown in the lower right of the figure, the probabilities of the three possible genotypes in the following generation are products of the probabilities of the corresponding alleles in the parents. The probability of genotype AA among the progeny is the probability p that allele A will be present in the paternal gamete multiplied by the probability p that allele A will be present in the maternal gamete, or p². Similarly, the probability of the genotype aa is q². The genotype Aa can arise when A from the father combines with a from the mother, which will occur with a frequency pq, or when a from the father combines with A from the mother, which also has a probability of pq; the result is a total probability of 2pq for the frequency of the Aa genotype in the progeny.

There is no change in the allele equilibrium frequencies from one generation to the next. The frequency of the A allele among the offspring is the frequency of the AA genotype (because all alleles in these individuals are A alleles) plus half the frequency of the Aa genotype (because half the alleles in these individuals are A alleles), or p² + pq = p(p + q) = p (because p + q = 1). Similarly, the frequency of the a allele among the offspring is given by q² + pq = q(q + p) = q. These are precisely the frequencies of the alleles in the parents.

The genotype equilibrium frequencies are obtained by the Hardy-Weinberg law on the assumption that there is random mating—that is, the probability of a particular kind of mating is the same as the frequency of the genotypes of the two mating individuals. For example, the probability of an AA female mating with an aa male must be p² (the frequency of AA) times q² (the frequency of aa). Random mating can occur with respect to most gene loci even though mates may be chosen according to particular characteristics. People, for example, choose their spouses according to all sorts of preferences concerning looks, personality, and the like. But concerning the majority of genes, people’s marriages are essentially random.

Assortative, or selective, mating takes place when the choice of mates is not random. Marriages in the United States, for example, are assortative with respect to many social factors, so that members of any one social group tend to marry members of their own group more often, and people from a different group less often, than would be expected from random mating. Consider the sensitive social issue of interracial marriage in a hypothetical community in which 80 percent of the population is white and 20 percent is black. With random mating, 32 percent (2 × 0.80 × 0.20 = 0.32) of all marriages would be interracial, whereas only 4 percent (0.20 × 0.20 = 0.04) would be marriages between two blacks. These statistical expectations depart from typical observations even in modern society, as a result of persistent social customs that for evolutionists are examples of assortative mating. The most extreme form of assortative mating is self-fertilization, which occurs rarely in animals but is a common form of reproduction in many plant groups.

The Hardy-Weinberg law assumes that gene frequencies remain constant from generation to generation—that there is no gene mutation or natural selection and that populations are very large. But these assumptions are not correct; indeed, if they were, evolution could not occur. Why, then, is the law significant if its assumptions do not hold true in nature? The answer is that it plays in evolutionary studies a role similar to that of Newton’s first law of motion in mechanics. Newton’s first law says that a body not acted upon by a net external force remains at rest or maintains a constant velocity. In fact, there are always external forces acting upon physical objects, but the first law provides the starting point for the application of other laws. Similarly, organisms are subject to mutation, selection, and other processes that change gene frequencies, but the effects of these processes can be calculated by using the Hardy-Weinberg law as the starting point.

The allelic variations that make evolution possible are generated by the process of mutation, but new mutations change gene frequencies very slowly, because mutation rates are low. Assume that the gene allele A₁ mutates to allele A₂ at a rate m per generation and that at a given time the frequency of A₁ is p. In the next generation, a fraction m of all A₁ alleles become A₂ alleles. The frequency of A₁ in the next generation will then be reduced by the fraction of mutated alleles (pm), or p₁ = p − pm = p(1 − m). After t generations the frequency of A₁ will be p_t = p(1 − m)^t.

If the mutations continue, the frequency of A₁ alleles will gradually decrease, because a fraction of them change every generation to A₂. If the process continues indefinitely, the A₁ allele will eventually disappear, although the process is slow. If the mutation rate is 10⁻⁵ (1 in 100,000) per gene per generation, about 2,000 generations will be required for the frequency of A₁ to change from 0.50 to 0.49 and about 10,000 generations for it to change from 0.10 to 0.09.

Moreover, gene mutations are reversible: the allele A₂ may also mutate to A₁. Assume that A₁ mutates to A₂ at a rate m, as before, and that A₂ mutates to A₁ at a rate n per generation. If at a certain time the frequencies of A₁ and A₂ are p and q, respectively, after one generation the frequency of A₁ will be p₁ = p − pm + qn. A fraction pm of allele A₁ changes to A₂, but a fraction qn of the A₂ alleles changes to A₁. The conditions for equilibrium occur when pm = qn, or p = n/(m + n). Suppose that the mutation rates are m = 10⁻⁵ and n = 10⁻⁶; then, at equilibrium, p = 10⁻⁶/(10⁻⁵ + 10⁻⁶) = 1/(10 + 1) = 0.09, and q = 0.91.

Changes in gene frequencies due to mutation occur, therefore, at rates even slower than was suggested above, because forward and backward mutations counteract each other. In any case, allelic frequencies usually are not in mutational equilibrium, because some alleles are favoured over others by natural selection. The equilibrium frequencies are then decided by the interaction between mutation and selection, with selection usually having the greater consequence.

Gene flow, or gene migration, takes place when individuals migrate from one population to another and interbreed with its members. Gene frequencies are not changed for the species as a whole, but they change locally whenever different populations have different allele frequencies. In general, the greater the difference in allele frequencies between the resident and the migrant individuals, and the larger the number of migrants, the greater effect the migrants have in changing the genetic constitution of the resident population.

Suppose that a proportion of all reproducing individuals in a population are migrants and that the frequency of allele A₁ is p in the population but p_m among the migrants. The change in gene frequency, Δp, in the next generation will be Δp = m(p_m − p). If the migration rate persists for a number t of generations, the frequency of A₁ will be given by p_t = (1 −m)^t(p − p_m) + p_m.

Gene frequencies can change from one generation to another by a process of pure chance known as genetic drift. This occurs because the number of individuals in any population is finite, and thus the frequency of a gene may change in the following generation by accidents of sampling, just as it is possible to get more or fewer than 50 “heads” in 100 throws of a coin simply by chance.

The magnitude of the gene frequency changes due to genetic drift is inversely related to the size of the population—the larger the number of reproducing individuals, the smaller the effects of genetic drift. This inverse relationship between sample size and magnitude of sampling errors can be illustrated by referring again to tossing a coin. When a penny is tossed twice, two heads are not surprising. But it will be surprising, and suspicious, if 20 tosses all yield heads. The proportion of heads obtained in a series of throws approaches closer to 0.5 as the number of throws grows larger.

The relationship is the same in populations, although the important value here is not the actual number of individuals in the population but the “effective” population size. This is the number of individuals that produce offspring, because only reproducing individuals transmit their genes to the following generation. It is not unusual, in plants as well as animals, for some individuals to have large numbers of progeny while others have none. In marine seals, antelopes, baboons, and many other mammals, for example, a dominant male may keep a large harem of females at the expense of many other males who can find no mates. It often happens that the effective population size is substantially smaller than the number of individuals in any one generation.

The effects of genetic drift in changing gene frequencies from one generation to the next are quite small in most natural populations, which generally consist of thousands of reproducing individuals. The effects over many generations are more important. Indeed, in the absence of other processes of change (such as natural selection and mutation), populations would eventually become fixed, having one allele at each locus after the gradual elimination of all others. With genetic drift as the only force in operation, the probability of a given allele’s eventually reaching a frequency of 1 would be precisely the frequency of the allele—that is, an allele with a frequency of 0.8 would have an 80 percent chance of ultimately becoming the only allele present in the population. The process would, however, take a long time, because increases and decreases are likely to alternate with equal probability. More important, natural selection and other processes change gene frequencies in ways not governed by pure chance, so that no allele has an opportunity to become fixed as a consequence of genetic drift alone.

Genetic drift can have important evolutionary consequences when a new population becomes established by only a few individuals—a phenomenon known as the founder principle. Islands, lakes, and other isolated ecological sites are often colonized by one or very few seeds or animals of a species, which are transported there passively by wind, in the fur of larger animals, or in some other way. The allelic frequencies present in these few colonizers are likely to differ at many loci from those in the population they left, and those differences have a lasting impact on the evolution of the new population. The founder principle is one reason that species in neighbouring islands, such as those in the Hawaiian archipelago, are often more heterogeneous than species in comparable continental areas adjacent to one another.

Climatic or other conditions, if unfavourable, may on occasion drastically reduce the number of individuals in a population and even threaten it with extinction. Such occasional reductions are called population bottlenecks. The populations may later recover their typical size, but the allelic frequencies may have been considerably altered and thereby affect the future evolution of the species. Bottlenecks are more likely in relatively large animals and plants than in smaller ones, because populations of large organisms typically consist of fewer individuals. Primitive human populations of the past were subdivided into many small tribes that were time and again decimated by disease, war, and other disasters. Differences among current human populations in the allele frequencies of many genes—such as those determining the ABO and other blood groups—may have arisen at least in part as a consequence of bottlenecks in ancestral populations. Persistent population bottlenecks may reduce the overall genetic variation so greatly as to alter future evolution and endanger the survival of the species. A well-authenticated case is that of the cheetah, where no allelic variation whatsoever has been found among the many scores of gene loci studied.