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).

       Quantitative Variation

So‐called “qualitative variation,” such as allele frequencies in populations, tells us much about how species are organized and their history, but the key traits that most determine fitness are in fact “quantitative” characters, such as height, weight, litter size, seed set, survival rate, etc. (Frankham et al. 2009). Such traits vary continuously because they are polygenic (controlled by many genes) and affected by the environment as well. Adaptive evolution results from changes in quantitative traits, so scientists study quantitative traits because they tell us much about the capacity of a population to evolve in response to environmental change (Storfer 1996). As an example, consider coral reefs, which are threatened by increasing ocean temperatures. Acute temperature increases are stressful for corals, but gradual temperature changes can result in adaptation depending on how heat tolerant a coral is. Heat tolerance is a “quantitative” trait because it varies gradually, relatively higher or lower from individual to individual (versus if it were “qualitative” = heat tolerant or heat intolerant) and is an inherited trait. This natural and quantitative variation in temperature tolerance may facilitate rapid adaptation among corals as our oceans warm (Dixon et al. 2015) with some researchers even suggesting transplanting more heat tolerant coral individuals to reefs that are warming rapidly. As another example, the endangered, annual plant known as the spinster’s blue‐eyed Mary, grows on and off serpentine soils, which have high concentrations of some toxic minerals and low concentrations of some essential nutrients. Plants from serpentine habitats grow best in serpentine habitats, and plants from nonserpentine habitats do best in nonserpentine habitats (Fig. 5.5) even though they are from populations as little as 100 m apart. Restoration efforts need to focus on collecting locally adapted seeds, as using nonserpentine seed sources in serpentine areas may lead to failure (Wright et al. 2006).

Figure 5.5 The native annual plant, Collinsia sparsiflora, grows on [“S”] and off [“NS”] serpentine soils with plants from serpentine habitats growing best in serpentine habitats, and plants from nonserpentine habitats doing best in nonserpentine habitats.

      (Adapted from Wright et al. 2006 [left]; Don Loarie [right])

The Importance of Genetic Diversity

      We have touched on the importance of genetic diversity already. To really understand the importance of genetic diversity, it is useful to think of genes as units of information rather than tangible things. As tiny aggregations of carbon, hydrogen, oxygen, nitrogen, and some other common elements, genes have little value in and of themselves. Indeed, when DNA is extracted from any organism and concentrated in the bottom of a test tube it appears as a small, sticky, gray, and rather unattractive blob. As sources of information, however,

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