Figure 5.11 Ankole watusi cattle raised as beef on a dry pasture in Malawi versus Holstein Friesian cattle on lush pasture of a dairy farm in Belgium cannot effectively cannot trade places or purpose.
(Matthew Bellemare/Flickr/CC BY 2.0 [top] and Tobias Nordhausen/Flickr/CC BY 2.0 [bottom])
The genetic diversity of some wild populations is also important to plant and animal breeders because wild relatives of domestic species are a significant source of genetic material. For example, when scientists at the International Rice Research Institute in the Philippines set out to develop a variety of rice that would be resistant to a major disease, grassy stunt virus, they screened over 6000 varieties of rice and found only one variety that was resistant to the disease. That variety, a wild species of rice called Oryza nivara, was represented in their collection by only 30 kernels, of which only three showed resistance (Hoyt 1988). Returning to the area in north‐central India where the rice sample had been collected, they could find no new material; the original collection site had been inundated by a dam. Fortunately, this story still had a happy ending because they were able to use the genetic information in these three kernels to develop a new variety of rice, IR36, that is resistant to this virus and is planted across millions of hectares in Asia (Ma et al. 2016).
Differences within a species can be of strategic value to conservation because they provide a clear justification for protecting a species across its entire geographic range, including all subspecies and major populations. For example, the Plymouth red‐bellied turtle is protected in the US state of Massachusetts. The one population that occurs in Massachusetts is slightly differentiated from other red‐bellied turtles (cooters) (Browne et al. 1996) so protecting it helps protect the entire suite of genetic variation found in the species. Protecting it also increases the entire species range by 30–40%, and doing so benefits a wide variety of other biota that also depend on the turtles’ wetland habitat. We will return to this issue in Chapter 13, “Managing Populations.”
Postscript
Careful readers may wonder why we have departed from the taxonomy of values used for species and ecosystems: intrinsic, instrumental, and uniqueness. We could squeeze genes into this classification, but it seems a bit contrived to talk about intrinsic value and uniqueness of molecules. Although a DNA helix is conceptually a beautiful and inherently intriguing structure, the value of genes lies in what they do, rather than what they are, and in this sense all of their value is instrumental. The classification used here distinguishes between values that are important to the species itself (evolutionary potential and loss of fitness) versus those that are important to people and other species (utilitarian values).
Processes That Diminish Genetic Diversity
To better understand the relationship between reduction in genetic diversity and loss of fitness, we will now consider the processes that diminish genetic diversity, especially in small populations: genetic bottlenecks, random genetic drift, and inbreeding.
Bottlenecks and Drift
Some populations are quite large: thousands of individuals are loosely connected through a web of interbreeding that ensures gene flow throughout the population. On the other hand, some populations are quite small, perhaps because they are confined to tiny, isolated patches of habitat and have limited dispersal abilities. In this section we are primarily concerned with what can happen to the genetic diversity of small populations, especially among species that typically occur in large populations but have been forced into small numbers. Profound changes occur in reduced populations and thus management of small‐population phenomena is a major focus for conservation biology.
Populations become small for a variety of reasons. Sometimes, large populations experience a catastrophe such as a hurricane and collapse to a few remnant individuals. Sometimes, a few individuals arrive in a new area and establish a new population that is inevitably small at first; this is called a founder event. When a population collapses or a new population is established, the genetic diversity of the original larger population is likely to be reduced because only a sample of the original gene pool will be retained. Returning to the bison example, if you start with a population of 1000 bison with 2000 alleles for MDH‐1 and reduce it to 50 bison, only 100 alleles will remain. Moreover, the remaining sample is not likely to be representative of the original population. This phenomenon is called a genetic bottleneck. Passing through a genetic bottleneck can create two problems: (1) a loss of certain alleles, especially rare alleles; and (2) a reduction in the overall amount of genetic variation. For example, a population that ranged across a continuum from very dark individuals to very light individuals might, after a bottleneck, have only intermediate colored individuals or only dark or only light individuals (Frankel and Soulé 1981), posing problems for future fitness.
The proportion of genetic variation and number of alleles likely to be retained after a bottleneck can be estimated using the formulas presented in Table 5.2. From this table we can see that most of the genetic variation is retained even in a tight bottleneck, 95% with just 10 individuals. The situation is worse, however, for retention of uncommon alleles. In this example, 10 individuals are likely to retain only two of four alleles if three of the alleles were uncommon (2% each of all the alleles). This figure improves to an estimate of 3.63 alleles retained if the alleles are more common, 10% of the total in this example. Genetic data from the whooping crane illustrate this phenomenon: six genotypes were detected in a sample of old museum specimens, but only one of these persists in the modern population after a 1938 bottleneck in which only 14 adults survived (Glenn et al. 1999). A study by Bouzat et al. (1998) of greater prairie chicken microsatellite variation provides another example. Birds in Illinois, which remain only in very small populations, have just two‐thirds as many alleles as those from neighboring states with much larger populations, and many fewer than museum specimens collected before 1960 when the severe population decline began.
Table 5.2 Proportion of genetic variation remaining after a genetic bottleneck.
Based on Frankel and Soulé 1981
| Average number of alleles retained from an original set (m) of 4 | |||
|---|---|---|---|
| Sample size (N) after bottleneck | Proportion of heterozygosity retained | p 1 = 0.70, p2 = p 3 = p 4 = 0.10 | p 1 = 0.94, p 2 = p 3 = p 4 = 0.02 |
| 1 | 0.50 | 1.48 | 1.12 |
| 2 | 0.75 | 2.02 | 1.23 |
| 6 | 0.917 | 3.15 | 1.64 |
| 10 |
0.95*
|