Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 98

Another index, called heterozygosity (usually abbreviated H), is defined in two ways. The most typical is the proportion or percentage of genes at which the average individual is heterozygous. The second is the proportion of individuals in a population heterozygous for a particular gene (Hartl and Clark 1997). In the bison example, two out of five individuals were heterozygous at the MDH‐1 locus, so heterozygosity 2/5 = 0.4 for this gene. We can calculate H by averaging the heterozygosity of each gene across all 24 genes. In this case,

Three uses of heterozygosity measurements merit mentioning. First, geneticists often compare the heterozygosity that they measure – the observed H, or H_o – with the heterozygosity they would expect to find, H_e, given the relative or “background” frequency of alleles. The expected heterozygosity is calculated by using the middle component (2pq) of the Hardy–Weinberg equation, p ² + 2pq + q ² = 1. In this example, given a frequency of p = 0.6 for the Y allele and q = 0.4 for the X allele (see Table 5.1), the Hardy–Weinberg equation is

Consequently, H_e for MDH‐1 is 0.48 (2 × 0.6 × 0.4 = 0.48). The H_e based on all genes for these five bison is 0.48/24 = 0.02, which is close to the H_o of 0.017. So in other words, the bison population is likely operating as if most individuals were reproducing and interacting with one another, as you would hope for in a healthy herd. But what if only a few females are producing most of the calves? Or just one male? H_o would depart strongly from H_e. In a small population this can be problematic as generally we seek genetic contributions from most individuals to preserve what genetic variation remains. Departures of H_o from H_e can therefore provide a useful and quick glimpse into a population’s reproductive dynamics, and point to management actions to address problems revealed. We expand on this issue further on in this chapter.

Another use of the heterozygosity index relates to its other interpretation: average diversity within an individual. Generally speaking it “helps” to be a heterozygote, that is, to have different versions of each gene present in your genome. You are a good example: your long developmental trajectory from an embryo in your mother’s womb (during a short period of which you even had gills and appeared much like a larval fish) to the bipedal adult ape you now are is a long one during which you faced many different physiological challenges. Having different gene forms (alleles) to “call on” as you developed through these phases (some alleles are more useful than others at different developmental stages) helped buffer your development from environmental stresses along the way. Fish present another good example. A large body of research on rainbow trout (Allendorf et al. 2015) has shown a positive correlation between heterozygosity and developmental stability (particularly how symmetrical the trouts’ bodies are – important in fish for moving through water in a streamlined manner). The reasons for these relationships are still not well understood (Miklasevskaja and Packer 2015) but correlations between fitness of individuals, symmetry, heterozygosity, and stress seem widespread in many organisms (de Anna et al. 2013), including humans (Trivers et al. 2013), but not always (e.g. in frogs: Eterovick et al. 2016).

Last, geneticists often use the heterozygosity index to estimate how much of a species’ total genetic diversity (H_t) is due to genetic diversity within the populations that compose the species (H_s) versus how much is due to variability among those populations (D_st) (Nei and Kumar 2000). Mathematically, this can be expressed as H_t = H_s + D_st. (This concept is often expressed with different but related formulas, but the basic idea is the same: partitioning the overall variability that exists in a species within and between the populations that comprise it.) This may seem an arcane concept but it is quite useful. If a species has a relatively high D_st, then it is necessary to maintain many different populations to maintain the species’ overall genetic diversity. For example, many salamanders have extraordinarily high values of D_st, which emphasizes the importance of preserving many populations that comprise a species to “capture” the genetic diversity that makes up the species (Tilley 2016). Alternatively, if most of the species’ genetic diversity exists within every population (i.e. H_s is relatively high), then it is less critical to maintain many different populations (Fig. 5.4). This is the case, for example, for many (but not all) birds, which fly and disperse well, intermixing their populations and thereby lowering D_st (e.g. for endangered piping plovers, Haig and Oring 1988). These considerations are often important for people who manage populations of endangered species, and we will return to in Chapter 13, “Managing Populations.”

Figure 5.4 Genetic diversity is partitioned within versus among populations to varying degrees with important implications for conservation strategies. First we tackle this conceptually (a). In the first case (“between”), the two alleles present (“W” or “w”) are each sequestered into different populations. Here conserving genetic diversity can be accomplished only by protecting both populations. In the second case (“within”), each population has both alleles present, and protecting a single population captures all the diversity present. In more practical terms (b), desert fishes living in isolated springs (left) will likely have higher genetic variability among populations (higher D_st) than desert fishes in which populations are connected by streams through which the fishes can disperse (right).