chapter three: The Modern Synthesis
Chapter Review
Population Genetics
Scientists’ early interpretations of Mendelian genetics focused on the discrete changes produced by the underlying genetic structures involved. During the early 1900s, this perspective actually weakened earlier acceptance of evolution by natural selection. If change acted primarily through discrete mechanisms, then, as many scientists believed, there would be 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 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) reflect successful evolutionary phenotypes. Researchers in the field of population genetics examine populations in terms of differing proportions of particular genotypes to determine what, if any, evolutionary forces are active in that population.
Hardy-Weinberg Equilibrium
One of the theoretical tools used by population geneticists today was developed independently 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 on five factors, all of which must be in effect:
- No mutation.
- An infinitely large population.
- Random mating.
- No migration.
- No genetic drift.
If all parameters are met, the genotype frequencies should emerge in constant proportions based on the individual gene frequencies. This is because these parameters work to keep the probability of a particular gene being passed on to the next generation at a statistically predictable level. By calculating the probability of each possible genotype based on the contribution of each parent, the frequency (freq) of each genotype can be predicted. Suppose two alleles—A and a—are present in a population; the possible genotypes should occur in the proportions shown in Table 3-1.
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| Table 3.1 Hypothetical
Allele Distribution (for alleles A and a) |
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The proportions of the three possible genotypes—AA, Aa, and aa—are 1:2:1
The Hardy-Weinberg equilibrium can, therefore, be expressed by the following equation:
p2 + 2pq + q2 = 1
where p = freq(A) and q = freq(a).
It is, however, unlikely that the five parameters of the Hardy-Weinberg equilibrium will all be fulfilled within any given natural population. Yet, 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 the idealized or theorized behavior.
An Experimental Example
Let's imagine that in a population of lab-bred flies, a gene controlling eye color is discovered. The R allele produces regular colored eye pigment, and 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 whether this population is in Hardy-Weinberg equilibrium!
Step 1: Determine the Gene Frequencies
Given the information in Table 3-2, calculating the allele frequencies is simply a matter of counting up all of the alleles. Remember:
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| Table 3-2 Hypothetical Genotypes and Phenotypes in a Group of Lab-Bred Flies |
- Each parent carries two alleles, so the total number of alleles twice the population.
- Heterozygous individuals carry one of each allele.
Taking these two factors into account,
freq(R) = [(90 × 2) + 45] / 300 = 225/300 = 0.75
freq(r) = [(15 × 2) + 45] / 300 = 75/300 = 0.25
Step 2: Determine the 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:
freq(RR) = p2 = freq(R) × freq(R) = 0.5625
freq(rr) = q2 = freq(r) × freq(r) = 0.0625
freq(Rr) = 2pq = 2 × freq(R) × freq(r) = 0.375
Multiplying each of these genotype frequencies by the total population number (150), we find that there should be
- 84 normal-eyed flies (RR)
- 9 red-eyed flies (rr)
- 56 pink-eyed flies (Rr)
Because "partial" individuals do not exist, the numbers are rounded off.
Step 3: Compare with the Original Population Numbers
By comparing the expected numbers with the actual numbers of each phenotype, population geneticists can determine whether a population either is in equilibrium (or very close to it) or is experiencing disequilibrium of some sort (Table 3-3). 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.
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| Table 3-3. Expected and Observed Genotypes in the Hypothetical Fly Population |
Disequilibrium
Disequilibrium refers to a difference between the observed and the expected ratios of genotypes within a given population, as we saw in our fly example. 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). In addition, disequilibrium can arise in situations of nonrandom mating. The factors that need to be interpreted are the relative proportions of each genotype (and their corresponding phenotypes), how these compare to expected values, and the elements of the environment that could promulgate evolutionary changes.
For instance, returning to the fruit fly example, the lower than expected incidence of pink-eyed flies suggests that there may be selective forces acting against their survival, or forces 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 in each generation.
Hidden Variation
One of the phenomena 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 wolf-like ancestor. All of the genetic variation necessary to produce the range of phenotypes from the Chihuahua to the St. Bernard can be found in the wolf genome. This hidden variation is the result of the polygenic. effects of genes, by which 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: Most recessive alleles (lethal or not) are not expressed in every generation; instead, they are stored by heterozygous individuals.
This is also why inbreeding carries with it an increased likelihood of the expression of a deleterious or lethal allele. It is 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 are quite low. Outbreeding (or exogamy) significantly reduces the probability of a chance mating between two individuals who carry the same deleterious alleles; however, inbreeding significantly increases this probability on the basis of degree of relatedness.
Constraints on Adaptation
Though natural selection is the only mechanism that explains adaptation, there are a few reasons that evolution does not always lead to the optimal phenotype for a given environment. These are correlated characters, disequilibrium, genetic drift, and the laws of physics and chemistry. Traits that influence the presence of other traits are correlated, either positively or negatively. If selection acts on a trait that is correlated with another, it is possible that the second trait may become maladaptive. Genetic drift, or the changing of gene frequencies by chance alone, is especially effective in small, isolated populations. Last, basic principles of physics and chemistry put constraints on adaptations. Large animals, who bear a significant amount of weight, will not likely stand on long, spindly legs and will tend to move more slowly than smaller animals (consider the elephant, for example).
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