Figure 13.17
A random but major catastrophe which causes a sudden, severe reduction in population can lead to a bottleneck effect. The reduction in gene pool size - and often diversity - leaves populations subject to genetic drift; chance variations can cause extinction or accelerated evolution leading to recovery. Even if recovery occurs, genetic diversity remains low.
During the 19th century, overhunting reduced the worldwide population of Northern Elephant Seals (Figure below) to fewer than 100 individuals. Because an alpha bull typically mates with a “harem” of 30-100 females, it is possible that just a single male fathered all the seals which exist today! After legal protection, their numbers have rebounded to 100,000. However, the effects of the past bottleneck - significant loss of genetic variability – remain. Reduced genetic diversity means that today’s seals are more susceptible to disease and weather. Effects of genetic drift on the gene pool may have contributed to the loss of 80% of pups during the El Nino year of 1997-98.
Figure 13.18
The Northern Elephant Seal population fell to fewer than 100 individuals due to overhunting during the 19 century. Although their numbers have recovered, the bottleneck effect of reduced genetic variation limits their potential to adapt to future environmental changes.
Although the exact cause is unknown, a bottleneck for South African Cheetahs (Figure below) during the last ice age about 10,000 years ago has apparently led to extremely low genetic variability. Genetic variation among cheetahs has been compared to that of highly inbred varieties of laboratory mice; skin grafts between unrelated individuals are not rejected. These animals also suffer from low sperm counts. Like many endangered species, cheetahs are threatened not only by habitat loss, but also by reduced genetic diversity, which reduces their potential to adapt to changing environments.
Figure 13.19
Although the cause is unknown, South African Cheetahs apparently experienced a population bottleneck 10,000 years ago. Their current genetic uniformity is remarkable; skin grafts between even unrelated individuals do not elicit rejection responses.
We humans may have experienced a population bottleneck between 70,000 and 75,000 years ago, when supervolcano Mount Toba exploded with category 8 (“megacolossal!”) force in Sumatra. According to anthropologist Stanley Ambrose’s theory, global temperature dropped as much as 5 degrees Celsius for several years, possibly leading to an ice age. Ambrose believes that the environmental effects (“six years of relentless volcanic winter”) reduced the total human population to less than 10,000, and that isolated individual populations would have experienced genetic drift and rapid evolution or extinction.
The Founder Effect
Whereas a drastic reduction in population size causes the bottleneck effect, a form of population expansion leads to the founder effect. If a small group of individuals (the founders) breaks off from a larger population to colonize a distant area, they will probably carry with them only a limited amount of the genetic diversity of the original population (Figure below). For this reason, the new population they establish may differ significantly in genotype and phenotype. Inevitably, it will also be small and therefore subject to genetic drift.
Figure 13.20
In this diagram, the red squares and blue dots represent individuals with different alleles. Small groups of individuals (see the right three circles) which leave a larger population (left) to colonize a new area carry with them smaller gene pools with allele frequencies which may differ significantly from that of the parent population due to chance. The effects of chance in these new populations become more important; genetic drift may result in extinction or rapid evolution.
On newly formed islands, such as the Galapagos, Hawaii, and more recently Surtsey, Iceland, founder populations are often the only source of life on the island. Many founder populations probably become extinct, but others evolve rapidly, due to genetic drift. Some may diverge rapidly to occupy many available ecological niches – a process known as adaptive radiation. As we have discussed in past chapters, Galapagos finches and Hawaiian honeycreepers probably each evolved from small populations of a single ancestral finch-like species (see the previous chapter on Evolutionary Theory).
Historically and even today, human populations have experienced founder effects. In some cases, migration and colonization are the cause. Quebec was founded by a group of no more than 2,600 people, ancestors of today’s more than 7 million Quebecois, who show remarkable genetic similarity and a number of heritable diseases, well studied by geneticists.
Cultural isolation, as well as colonization, can result in founder effects. Amish populations in the United States have grown from an initial group of about 200 immigrants, dating back to the mid-1700s. Because they have remained culturally and reproductively isolated from non-Amish Americans, they show considerable uniformity. The Amish today are often studied for their genetic uniformity, as well as certain recessive conditions. Geneticists believe that just one or two of the initial 200 Amish carried a recessive allele for Ellis-van Creveld syndrome (short limbs, extra fingers, and heart anomalies), yet through genetic drift, the isolated Amish population now has the highest incidence of this syndrome in the world (Figure below).
Figure 13.21
Polydactyly (extra digits) and short limbs are characteristics of Ellis-van Creveld syndrome, a genetic disease which is rare worldwide but more common in the Amish population. Because the Amish population began with just 200 immigrants and has remained isolated, their high incidence of this syndrome may be a result of the founder effect.
Natural Selection
While genetic drift, including the bottleneck and founder effects, can cause microevolution (generational change in allele frequencies), its effects are mostly random. The results of genetic drift may include enhanced capabilities, but more often, they are neutral or deleterious. Natural selection depends not on chance, but on differential survival determined by an individual’s traits. Even though the variations are due to chance, the products of natural selection are usually organisms well-suited to their environment.
Recall that Hardy-Weinberg equilibrium requires that all individuals in a population are equal in their ability to survive and successfully reproduce. As Darwin noted, however, overproduction of offspring and variation among individuals often lead to differential survival and reproduction – in other words, natural selection (Figure below). We discussed natural selection as a part of Darwin’s theory of evolution in the last chapter, but in this section, we will go deeper than Darwin could. We will explore natural selection at the level of populations – in terms of allele, genotype, and phenotype frequencies.
Figure 13.22
Natural selection involves (1) heritable variation (here, giraffe neck length); (2) overproduction of offspring (3 giraffes born, not all can survive); (3) differential survival and reproduction (not enough food for all giraffes; those with shorter necks starve); and (4) gradual change in traits in the population (long-necked giraffes survive and reproduce, so their genes for long necks increase in frequency in the next generation).
Acting on an Organism's Phenotype
Natural selection acts on an organism’s phenotype (appearance), which is a product of genotype and any environmental influences on gene expression. By selecting for alleles which improve survival and/or reproduction and selecting against harmful alleles, natural selection changes the proportion of alleles from one generation to the next – causing microevolution. Let’s return once more to our rabbit population. If a predator such as a hawk can see white rabbits (genotype bb) more easily than brown rabbits (BB and Bb), brown rabbits are more likely than white rabbits to survive hawk predation. Because more brown rabbits will survive to reproduce, the next generation will probably contain a higher frequency of B alleles. Note, however, that the recessive b alleles are unlikely to disappear completely, because they can “hide” from the hawks in heterozygous brown rabbits. This is a good reminder that natural selection acts on phenot
ypes, rather than genotypes. The hawk - or natural selection - is unable to distinguish a BB rabbit from a Bb rabbit. Natural selection - and the hawk - is only able to distinguish a brown rabbit from a white rabbit, demonstrating how natural selection acts on the phenotype rather than the genotype of an organism.
Consider a different example, which emphasizes reproduction rather than survival: If both brown and white rabbits preferred to mate with white rabbits, the next generation’s gene pool would probably show an increase in the frequency of the b allele, because white rabbits would be more likely to reproduce successfully.
Although some traits are determined by a single gene, many are influenced by more than one gene (polygenic). The result of polygenic inheritance is a continuum of phenotypic values which often show a bell curve pattern of variation. Figure below shows the effect of three genes, each having two alleles, on human skin color; the result is a normal distribution ranging from very dark to very light, with a peak near the middle. You can demonstrate polygenic inheritance (probably with some environmental influence) for height, ear length, or handspan by measuring your classmates and graphing the data in a similar fashion. Some curves will be flat, and others sharp – but most will resemble the normal “bell” shape.
Given this pattern of phenotypic variation, natural selection can take three forms (Figure below). We will use the theoretical human skin color distribution Figure below to illustrate the three types of selection. Directional selection shifts the frequency curve away from the average by favoring individuals with an extreme form of the variation. The skin of early humans living in sun-rich Africa received high levels of UV radiation, which destroys vitamin B (folate) and leads to severe birth defects such as spina bifida. Selection, then, favored darker-skinned individuals, and the frequency of the darker alleles increased. After several generations, the curve would still be bell-shaped, but it would have shifted to the right, in the direction of the darker alleles. The average individual would have had darker skin as result of this microevolutionary change.
Figure 13.23
Three types of selection can alter allele frequencies, causing microevolution. The effect of stabilizing selection (1) is to reduce variation. Disruptive selection (2) results in two different populations, which may eventually be isolated from one another. Directional selection (3) enhances or reduces a single characteristic, such as trunk or snout length in the above example.
Natural Selection and Human Migration
As humans migrated into the northern hemisphere, excessive UV radiation was no longer a problem, but the relative lack of sunlight led to lower levels of vitamin D3, normally synthesized in the skin and necessary for calcium absorption and bone growth. Thus, selection in the north favored lighter-skinned individuals – by itself another example of directional selection. However, if we consider the human population as a whole at that time, disruptive selection would describe the microevolution taking place. In northern climates, alleles for light skin would be favored, and in southern climates would select for alleles for dark skin, resulting in two distinct peaks in the distribution of skin color phenotypes and their corresponding genotypes. Keep in mind that the “three gene – dark/light” model is an oversimplification of the genetics underlying skin color, but the adaptive values are real, and the model allows us to illustrate how microevolution works. Note that a map of human skin colors supports this type of selection to some extent (Figure below).
Figure 13.24
The distribution of skin colors at least partially supports disruptive selection for lighter skin in the north, to allow sunlight to form vitamin D in the skin, and for darker skin toward the equator to prevent UV radiation from breaking down vitamin B-folate.
Today, extensive migration, mobility, and intermarriage in the human population may be changing selective pressures on skin color once again. For the sake of argument, let’s make the somewhat unrealistic assumption that mixing becomes complete and that all people will be sufficiently mobile that they experience intermediate levels of sunlight. These conditions would select against both extremely dark skin (too little vitamin D3) and extremely light skin (too little vitamin B-folate). The result would be a taller, narrower distribution – less diversity - about the same mean, a phenomenon known as stabilizing selection. Although our example is perhaps unrealistic, stabilizing selection is probably the most common form of natural selection, preventing form and function from straying away from a “proven” norm.
Stabilizing Selection
Stabilizing selection can lead to the preservation of harmful alleles. A famous example, which we considered in earlier lessons, is sickle-cell anemia. The gene for Beta-hemoglobin - half of the oxygen-carrying protein in our blood - has two alleles, which we will call Hgb-A and Hgb-S. Individuals having two copies of the Hgb-S allele suffer from sickle-cell anemia, a potentially lethal disease in which sickled cells clog capillaries and cannot carry oxygen efficiently. In equatorial regions, individuals with two copies of Hgb-A become infected with Plasmodium parasites and often die from malaria. However, individuals with one copy of each allele (the heterozygous genotype) escape both causes of death; although they may experience slight sickling at high altitudes, they do not suffer from full-blown anemia, and malaria parasites cannot infect their red blood cells. Stabilizing selection has maintained the frequencies of both alleles, even though each is potentially lethal in the homozygous state.
Figure 13.25
The distribution of malaria (top) correlates closely with the distribution of the sickle-cell allele (bottom). Because the heterozygous genotype confers immunity to malaria, this allele which is lethal in the homozygous condition persists in environments where malaria is common. Thus, natural selection can occasionally result in persistence of harmful alleles.
Selection for a particular trait may also select for other traits which do not directly affect fitness – if, for example, genes are linked, or if a single gene influences several different traits.
Fitness
Another way to look at natural selection is in terms of fitness - the ability of an organism with a certain genotype to reproduce. Fitness can be measured as the proportion of that organism’s genes in all of the next generation’s genes. When differences in individual genotypes affect fitness, the genotypes with higher fitness become more common. This change in genotype frequencies is natural selection.
Kin Selection
An intriguing corollary of genotype selection is kin selection. Behaviors which sacrifice reproductive success or even survival can actually increase fitness if they promote the survival and reproduction of close relatives who share a significant proportion of the same genes. Examples include subordinate male turkeys, who help their dominant brothers display to potential mates (Figure below) and honeybee workers, who spend their lives collecting pollen and raising young to ensure that their mother, the queen, reproduces successfully (Figure below).
Figure 13.26
Wild turkeys display in groups of closely related individuals, but only alpha males eventually mate. Subordinate males sacrifice their chance to reproduce, even chasing away other males to promote their dominant brothers success, because this behavior increases the chance that the genes they share will be represented in the next generation. This means of increasing gene frequency is kin selection.
Figure 13.27
Many social insects also illustrate kin selection. These honeybee workers are sterile. They spend their lives collecting pollen, feeding larvae, and cleaning and defending the hive. With no chance of reproductive success of their own, they dedicate their lives to the reproductive success of the hives queen their mother who shares 50% of her genes with each of them.
We have looked carefully at equilibrium populations and at possible disruptions of equilibrium which cause microevolution – a generational change in a population’s allele frequencies:
Mutation, which together with sexual reproduction is the ultimate source of variation, and is an important ca
use of microevolution in microorganisms
Gene flow, which can accelerate microevolution by importing new, already successful alleles
Genetic drift, which can increase the effect of chance variations in small populations
Natural selection, which can be directional, disruptive, or stabilizing
Specialized types of selection, such as mate selection and kin selection
Evolutionary biologists are not yet in agreement regarding the relative importance of each type of selection to the history of life, although most would agree that natural selection is the primary force in microevolution. In the next lesson, we will apply our understanding of microevolutionary processes to that “mystery of mysteries,” as Darwin and Herschel called it: the origin of species.
CK-12 Biology I - Honors Page 59