THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
14.9. Agents of Evolution
Many factors can alter allele frequencies. But only five alter the proportions of homozygotes and heterozygotes enough to produce significant deviations from the proportions predicted by the Hardy-Weinberg rule:
2. Nonrandom mating
3. Genetic drift
A mutation is a change in a nucleotide sequence in DNA. For example, a T nucleotide could undergo a mutation and be replaced with an A nucleotide. Mutation from one allele to another obviously can change the proportions of particular alleles in a population. But mutation rates are generally too low to significantly alter Hardy-Weinberg proportions of common alleles. Many genes mutate 1 to 10 times per 100,000 cell divisions. Some of these mutations are harmful, while others are neutral or, even rarer, beneficial. Also, the mutations must affect the DNA of the germ cells (egg and sperm), or the mutation will not be passed on to offspring. The mutation rate is so slow that few populations are around long enough to accumulate significant numbers of mutations. However, no matter how rare, mutation is the ultimate source of genetic variation in a population.
Individuals with certain genotypes sometimes mate with one another either more or less commonly than would be expected on a random basis, a phenomenon known as nonrandom mating. One type of nonrandom mating is sexual selection, choosing a mate often based on certain physical characteristics. Another type of nonrandom mating is inbreeding, or mating with relatives, such as in the self-fertilization of a flower. Inbreeding increases the proportions of individuals that are homozygous because no individuals mate with any genotype but their own. As a result, inbred populations contain more homozygous individuals than predicted by the Hardy-Weinberg rule. For this reason, populations of self-fertilizing plants consist primarily of homozygous individuals, whereas outcrossing plants, which interbreed with individuals different from themselves, have a higher proportion of heterozygous individuals. Nonrandom mating alters genotype frequencies but not allele frequencies. The allele frequencies remain the same—the alleles are just distributed differently among the offspring.
In small populations, the frequencies of particular alleles may be changed drastically by chance alone. In an extreme case, individual alleles of a given gene may all be represented in few individuals, and some of the alleles may be accidentally lost if those individuals fail to reproduce or die. This loss of individuals and their alleles is due to random events rather than the fitness of the individuals carrying those alleles. This is not to say that alleles are always lost with genetic drift, but allele frequencies appear to change randomly, as if the frequencies were drifting; thus, random changes in allele frequencies is known as genetic drift. A series of small populations that are isolated from one another may come to differ strongly as a result of genetic drift.
When one or a few individuals migrate and become the founders of a new, isolated population at some distance from their place of origin, the alleles that they carry are of special significance in the new population. Even if these alleles are rare in the source population, they will become a significant fraction of the new population’s genetic endowment. This is called the founder effect. As a result of the founder effect, rare alleles and combinations often become more common in new, isolated populations. The founder effect is particularly important in the evolution of organisms that occur on oceanic islands, such as the Galapagos Islands that Darwin visited. Most of the kinds of organisms that occur in such areas were probably derived from one or a few initial founders. In a similar way, isolated human populations are often dominated by the genetic features that were characteristic of their founders, particularly if only a few individuals were involved initially (figure 14.22).
Figure 14.22. The founder effect.
This Amish woman is holding her child, who has Ellis-van Creveld syndrome. The characteristic symptoms are short limbs, dwarfed stature, and extra fingers. This disorder was introduced in the Amish community by one of its founders in the 18 th century and persists to this day because of reproductive isolation.
Even if organisms do not move from place to place, occasionally their populations may be drastically reduced in size. This may result from flooding, drought, earthquakes, and other natural forces or from progressive changes in the environment. The surviving individuals constitute a random genetic sample of the original population. Such a restriction in genetic variability has been termed the bottleneck effect (figure 14.23). The very low levels of genetic variability seen in African cheetahs today is thought to reflect a near-extinction event in the past.
Figure 14.23. Genetic drift: a bottleneck effect.
The parent population contains roughly equal numbers of green and yellow individuals and a small number of red individuals. By chance, the few remaining individuals that contribute to the next generation are mostly green. The bottleneck occurs because so few individuals form the next generation, as might happen after an epidemic or a catastrophic storm.
Migration is defined in genetic terms as the movement of individuals between populations. It can be a powerful force, upsetting the genetic stability of natural populations. Migration includes movement of individuals into a population, called immigration, or the movement of individuals out of a population, called emigration. If the characteristics of the newly arrived individuals differ from those already there, and if the newly arrived individuals adapt to survive in the new area and mate successfully, then the genetic composition of the receiving population may be altered.
Sometimes migration is not obvious. Subtle movements include the drifting of gametes of plants, or of the immature stages of marine organisms, from one place to another. For example, a bee can carry pollen from a flower in one population to a flower in another population. By doing this, the bee may be introducing new alleles into a population. However it occurs, migration can alter the genetic characteristics of populations and cause a population to be out of Hardy-Weinberg equilibrium. Thus, migration can cause evolutionary change. The magnitude of effects of migration is based on two factors: (1) the proportion of migrants in the population, and (2) the difference in allele frequencies between the migrants and the original population. The actual evolutionary impact of migration is difficult to assess, and depends heavily on the selective forces prevailing at the different places where the populations occur.
As Darwin pointed out, some individuals leave behind more progeny than others, and the likelihood they will do so is affected by their inherited characteristics. The result of this process is called selection and was familiar even in Darwin’s day to breeders of horses and farm animals. In so-called artificial selection, the breeder selects for the desired characteristics. For example, mating larger animals with each other produces offspring that are larger. In natural selection, Darwin suggested the environment plays this role, with conditions in nature determining which kinds of individuals in a population are the most fit (meaning individuals that are best suited to their environment; see section 14.3) and so affecting the proportions of genes among individuals of future populations. The environment imposes the conditions that determine the results of selection and, thus, the direction of evolution (figure 14.24).
Figure 14.24. Selection for coat color in mice.
In the American southwest, ancient lava flows have produced black rock formations that contrast starkly with the surrounding light-colored desert sand. Populations of many species of animals occurring on these rocks are dark in color, whereas sand-dwelling populations are much lighter. For example, in pocket mice, selection favors coat color that matches their surroundings. The close match between coat color and background color camouflages the mice and provides protection from avian predators. These mice are very visible when placed in the opposite habitats.
Forms of Selection
Selection operates in natural populations of a species as skill does in a football game. In any individual game, it can be difficult to predict the winner because chance can play an important role in the outcome. But over a long season, the teams with the most skillful players usually win the most games. In nature, those individuals best suited to their environments tend to win the evolutionary game by leaving the most offspring, although chance can play a major role in the life of any one individual. While you cannot predict the fate of any one individual, or any one coin toss, it is possible to predict which kind of individual will tend to become more common in populations of a species, as it is possible to predict the proportion of heads after many coin tosses.
In nature, many traits, perhaps most, are affected by more than one gene. The interactions between genes are typically complex, as you saw in chapter 10. For example, alleles of many different genes play a role in determining human height (see figure 10.12). In such cases, selection operates on all the genes, influencing most strongly those that make the greatest contribution to the phenotype. How selection changes the population depends on which genotypes are favored. Three types of natural selection have been identified: stabilizing selection, disruptive selection, and directional selection.
When selection acts to eliminate both extremes from an array of phenotypes—for example, eliminating larger and smaller body sizes—the result is an increase in the frequency of the already common intermediate phenotype (such as a midsized body). This is called stabilizing selection:
In a classic study carried out after an “uncommonly severe storm of snow, rain, and sleet” on February 1, 1898, 136 starving English sparrows were collected and brought to the laboratory of H. C. Bumpus at Brown University in Providence, Rhode Island. Of these, 64 died and 72 survived. Bumpus took standard measurements on all the birds. He found that among males, the surviving birds tended to be bigger, as one might expect from the action of directional selection (discussed later). However, among females, the birds that survived were those that were more average in size. Among the female birds that perished were many more individuals that had extreme measurements, either very large or very small.
In Bumpus’ quaint phrasing, “The process of selective elimination is most severe with extremely varying individuals no matter in what directions the variation may occur. It is quite as dangerous to be conspicuously above a certain standard of organic excellence as it is to be conspicuously below the standard. It is the type that nature favors.”
In the Bumpus study, selection had acted most strongly against these “extreme-sized” female birds. Stabilizing selection does not change which phenotype is the most common of the population—the average-sized birds were already the most common phenotype—but rather makes it even more common by eliminating extremes. In effect, selection is operating to prevent change away from the middle range of values.
Many examples similar to Bumpus’s female sparrows are known. For example, in humans, infants with intermediate weight at birth have the highest survival rate:
More specifically, the death rate among human babies is lowest at an intermediate birth weight between 7 and 8 pounds indicated by the red line in the graph above, which consists of data compiled from U.S. birth records over many years. The intermediate weights are also the most common in the population, indicated by the blue area. Larger and smaller babies both occur less frequently and have a greater tendency to die at or near birth. In a similar way, chickens eggs of intermediate weight have the highest hatching success.
In some situations, selection acts to eliminate the intermediate type, resulting in the two more extreme phenotypes becoming more common in the population. This type of selection is called disruptive selection:
A clear example is the different beak sizes of the African black-bellied seedcracker finch Pyrenestes ostrinus. Populations of these birds contain individuals with large and small beaks, but very few individuals with intermediate-sized beaks. As their name implies, these birds feed on seeds, and the available seeds fall into two size categories: large and small. Only large-beaked birds, like the one on the left in the figure below, can open the tough shells of large seeds, whereas birds with the smallest beaks, like the one on the right, are more adept at handling small seeds. Birds with intermediate-sized beaks are at a disadvantage with both seed types: unable to open large seeds and too clumsy to efficiently process small seeds. Consequently, selection acts to eliminate the intermediate phenotypes, in effect partitioning the population into two phenotypically distinct groups.
In other situations, selection acts to eliminate one extreme from an array of phenotypes, resulting in the other extreme phenotype becoming more common in the population. This form of selection is called directional selection:
For example, in the experiment below, flies (Drosophila) that moved toward light were eliminated from the population, and only flies that moved away from light were used as parents for the next generation. After 20 generations of selected matings, flies that flew toward light were far less frequent in the population.
In the time since Darwin suggested the pivotal role of natural selection in evolution, many examples have been found in which natural selection is clearly acting to change the genetic makeup of species, just as Darwin predicted. Here we will examine three examples.
Key Learning Outcome 14.9. Five evolutionary forces have the potential to significantly alter allele and genotype frequencies in populations: mutation, nonrandom mating, genetic drift, migration, and selection. Selection can favor intermediate values, or one or both extremes.