CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
12. Diversity Within Species and Population Genetics
12.4. Why Genetically Distinct Populations Exist
Many species have wide geographic distribution with reasonably distinct subspecies. There are four reasons that these subspecies developed: adaptation to local environmental conditions, the founder effect, a genetic bottleneck, and barriers to movement.
Adaptation to Local Environmental Conditions
Because organisms within a population are not genetically identical, some individuals may possess genetic combinations that are valuable for survival in the local environment. As a result, some individuals find the environment less hostile than do others. The individuals with unfavorable genetic combinations leave the population more often, either by death or migration, and remove their genes from the population. Therefore, local populations that occupy sites that differ greatly from conditions at other locations would be expected to consist of individuals having gene combinations suited to local conditions. For example, White Sands National Monument in New Mexico has extensive dunes of white gypsum sand. Several of the animals that live there, such as lizards and mice, have very light coloring, which allows them to blend in with their surroundings. Other populations of the same species that do not live in such a white environment do not have the light coloring.
Many kinds of animals that live in caves lack pigments and eyes, including salamanders, flatworms, shrimp, fish, beetles, and crayfish. A blind Texas salamander (Eurycea rathbuni) is at a severe disadvantage when venturing above ground. A blind Texas salamander living in a cave where there is no light, however, is not at the same disadvantage. Thus, these two environments might allow or encourage characteristics to be present in the two populations at different frequencies. Because it takes energy to produce eyes and pigment, individual cave-dwelling Texas blind salamanders that do not produce these characteristics may have an advantage over the individuals that continue to spend the energy to produce these features. (figure 12.7).
FIGURE 12.7. Specialized Local Populations
Many populations of animals that live in caves where there is no light lack pigment and functional eyes. If, in the past, the genetic material that directed the production of these traits was lost or mutated, it would not have negatively affected the organism. In fact, it may have been an advantage to not spend the energy to produce such unneeded traits. Hence, in many cave populations, there is a low frequency of genes for the production of eyes and pigment. This is a Texas blind salamander, an endangered species found only in the Edwards Aquifer underneath and near the city of San Marcos, Texas.
The Founder Effect
The second mechanism that creates genetically distinct populations with unique allele frequencies is the founding of a new population and, so, is called the founder effect. The founder effect is a form of genetic drift, in which a genetically distinct local population is established by a few colonizing individuals carrying with them alleles that differ in frequencies from those in their original population. The collection of alleles of a small founding population is likely to be different from that present in the larger parent population from which it came. After all, a few pioneering individuals leaving a population would be unlikely to carry copies of all the alleles found within the original population. They may even carry an unrepresentative mixture of alleles. For example, the water snake Nerodia sepidon is found throughout the eastern part of the United States and extends into Canada. The northern water snake subspecies is widespread in the north-central United States and adjacent Canada and is generally brown with light diamond-shaped patches. The Lake Erie water snake subspecies is limited to the islands in the western section of Lake Erie. It is generally a solid color without the lighter patches. The difference in color patterns is related to different allele frequencies for color pattern. The Lake Erie water snake may have been founded by a small number of individuals from the mainland that had a high frequency of alleles for solid coloration rather than the more typical banded pattern. (It is even possible that the island populations could have been founded by one fertilized female.) Once a small founding population establishes itself, it tends to maintain its collection of alleles, because the organisms mate only among themselves. This results in a reshuffling of alleles from generation to generation but does not introduce new genetic information into the population.
The third cause of local, genetically distinct populations relates to the history of the population. A genetic bottleneck is a form of genetic drift, in which there is a sharp reduction in population size due to a chance event that results in a reduction in genetic diversity in subsequent generations. When the size of a population is greatly reduced, some alleles will probably be lost from the population. Any subsequent increase in the size of the population by reproduction among the remaining members of the population will not replace the lost genetic diversity. For example, in the late 1800s the whooping crane population was estimated to be 1300 birds (figure 12.8). Their numbers decreased because of hunting and industrial development. By 1941 there were only 22 wild whooping cranes left in North America. Today there are over 400 birds. However, their lack of genetic diversity puts them at risk of extinction. Thousands of other species are currently undergoing genetic bottlenecks. Although some endangered species were always rare, most have experienced recent reductions in their populations and a reduction in their genetic diversity, which is a consequence of severely reduced population size.
FIGURE 12.8. Whooping Crane
Today, the only remaining natural, self-sustaining flock of whooping cranes is in Wood Buffalo National Park, Canada and winters in Aransas National Wildlife Refuge, Texas. This flock had only 16 birds during the winter of 1941-1942, and numbered under 35 birds for the next two decades.
Barriers to Movement
The fourth factor that tends to encourage the maintenance of genetically distinct populations is the presence of barriers to free movement. Animals and plants that live in lakes tend to be divided into small, separate populations by barriers of land. Whenever such barriers exist, there will very likely be differences in the allele frequencies from lake to lake, because each lake was colonized separately and the lakes’ environments are not identical. Other species of organisms, such as migratory birds (e.g., Canadian geese, mallard ducks), experience few barriers; therefore, subspecies are not as common.
12.4. CONCEPT REVIEW
10. List four processes that can lead to local, genetically distinct populations.
11. In what way are the founder effect and a genetic bottleneck similar in their effect on the genetic diversity of a local population?