Chapter 3: The Modern SynthesisThe early interpretation of Mendelian genetics focused upon the discrete changes which were produced by the underlying genetic structures. During the early 1900's this perspective weakened earlier acceptance of evolution by natural selection. If change primarily acted through discrete mechanisms, there is no basis for the gradual accumulation of change necessary for natural selection to operate. However, it was through the work of three biologists - J. B. S. Haldane, Sewall Wright, and Ronald A. Fisher - that the integration of a theory of inheritance, the maintenance of variation, and a basis for the genetics of continuous variation were applied to the evolutionary paradigm. These ideas were subsequently expanded and refined by later researchers such as geneticist Theodosius Dobzhansky, biologist Ernst Mayr, and paleontologist George Gaylord Simpson.
Evolution can be described in many ways; one way is look at
evolutionary change as a change in genotype frequencies
over time. If organisms are considered evolutionarily successful
if they have more offspring, and offspring are created from
genes, then changes in gene frequencies (or more specifically,
genotype frequencies) will reflect successful evolutionary
phenotypes. Researchers in the field of population
genetics examine populations in terms of differing
proportions of particular genotypes in order to determine what,
if any, evolutionary forces are active in that population.
One of the theoretical tools utilized by population geneticists
today was independently developed in 1908 by a British
mathematician named G. H. Hardy, and a German physician named W.
Weinberg. The Hardy-Weinberg equation expresses an ideal
distribution of genotypes within a population, assuming that the
gene frequencies are known. The validity of the results from
Hardy-Weinberg analysis is contingent upon five factors,
all of which must be in effect :
If all parameters are met, the genotype frequencies should emerge in constant proportions based on the individual gene frequencies. This is because the above parameters work to keep the probability of a particular gene being passed on to the next generation at a very statistically predictable level. Therefore, by calculating the probability of each possible genotype based on the contribution of each parent, the frequency of each genotype can be predicted. Supposing two alleles - A and a - are present in a population where frequency(A) = p and frequency(a) = q, the possible genotypes should occur in the following proportions:
A glance at the above parameters suggests that it is
very unlikely they will all be fulfilled within any given natural
population. However, as with other theoretical notions based on
ideal principles, the main product of these calculations is to
provide a comparison by which a population geneticist can assess
why the sampled population is not conforming to ideal
In a population of lab-bred flies, a gene controlling eye color is discovered. The R allele produces regular colored eye pigment, while the r allele produces red pigment. Individuals that are heterozygous (Rr) have pink eyes. In a population of 150 flies, 15 flies have red eyes, 90 have normal eye color, and 45 have pink eyes. Check if this population is in Hardy-Weinberg equilibrium.
Step 1: Determine gene frequencies
Given this information, calculating the allele frequencies is simply a matter of counting up all of the alleles.
Taking these two factors into account,
Step 2: Determine expected genotype frequencies
Plugging the frequencies of each allele into the Hardy-Weinberg equation, we find the expected numbers of each genotype in the population:
Step 3: Compare with original population numbers
Comparing the expected numbers with the actual numbers of each phenotype, population geneticists can determine if populations are either in equilibrium (or very close to it) or are experiencing disequilibrium of some sort. In this example:
In this example, the population is not in equilibrium since the expected and observed values do not match. Disequilibrium can be attributed to different possible mechanisms, depending on (1) the context of the population, and (2) the manner in which the population is skewed.
Disequilibrium refers to a difference between observed and expected ratios of genotypes within a given population as we saw above. This situation may come about as a result of small population size (which accentuates the effects of genetic drift), migration, and selective forces which favor or hinder particular phenotypes. Additionally, disequilibrium can arise in situations of non-random mating. What needs to interpreted are the relative proportions of each genotype (and their corresponding phenotypes), how these compare with expected values, and the elements of the environment which could promulgate evolutionary changes.
For instance, in the above example of fruit flies, the lower incidence of pink-eyed flies suggests that there may be selective forces acting against their survival, or perhaps encouraging both homozygous conditions over the heterozygous one. Or perhaps the flies are more likely to select mates with the same eye color, leading to a slight reduction of heterozygous individuals each generation.
One of the phenomena which the study of population helps us to understand is the preservation of variation at the genetic level. One example of this is the artificial breeding and selection of all of the modern species of dogs from a wolflike ancestor. All of the genetic variation necessary to produce phenotypes like the Chihuahua and the St. Bernard can be found in the wolf genome. This hidden variation is due to the effects of polygenic effects of genes, where many different loci additively affect a particular trait (for example, height). A similar line of reasoning can apply to the question of why recessive lethal alleles linger in population gene pools. This is due to the fact that most recessive alleles (lethal or not) are not expressed in every generation; instead, they are stored by heterozygous individuals.
This is also the reason why inbreeding carries with it an increased likelihood of the expression of a deleterious or lethal allele. It is very likely that all individuals carry some deleterious alleles which are left unexpressed from generation to generation, primarily because the frequencies of these alleles in the whole population is very low. While outbreeding (or exogamy) will significantly reduce the probability of a chance encounter between two individuals carrying the same deleterious alleles, inbreeding will significantly increase this probability on the basis of degree of relatedness.