degrees with important implications for conservation strategies."/>
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 Dst) 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, genes are clearly essential; they shape the synthesis of the biochemicals that control cellular activity and, ultimately, all biological activity and form. The capacity for genes to encode this information is stupendous; a typical mammal might have 100,000 genetic loci.
Of course, most of this wealth of genetic diversity is encapsulated in the diversity of species and their interspecific genetic differences. The key issue to address here is the distribution and diversity of alleles that characterize a species. Why is it important to maintain different versions of the same gene and, in many circumstances, to have them well distributed in a population dominated by heterozygotes rather than homozygotes? There are three basic answers: evolutionary potential, loss of fitness, and utilitarian values.
Evolutionary Potential
A key requisite for natural selection is genetic‐based variability in the fitness of individuals; that is, some individuals must be more likely to survive and reproduce than others. If every individual were genetically identical and only chance determined which ones left progeny, then populations would change erratically through time, if at all. If they are to persist, however, populations must change as their environment changes, which environments everywhere are now doing rapidly (see Chapter 6). Of course, the physical world has always changed as continents drift around the globe, mountains rise and erode, oceanic currents and jet streams shift paths, and the planet’s orbit around the sun varies. The biological world also changes as species evolve, become extinct, and shift their geographic ranges, coming into contact with new species that may be predators, prey pathogens, or competitors. Changes have been particularly dramatic during the last few decades as human populations and their technological capabilities have grown and profoundly altered the conditions for evolution in most species. To put it more directly: humans are now the central organizing reality around which all nonhuman life will evolve. To some degree, all species must respond to the environmental changes we are wreaking almost everywhere if they are to survive. And they need genetic diversity to do so.
The potential rate of evolution is directly proportional to the amount of variability in a population, or to put it another way, species with greater genetic diversity are more likely to be able to evolve in response to a changing environment than those with less diversity. The gray squirrel situation discussed earlier (see Fig. 5.3) is a good example: if the genetic variation that generates the two color morphs of squirrels were not present 300 years ago the species may well have gone extinct rather than evolved toward the more common gray form we see today. Overharvest can also select for changes in plant morphology as long as there is genetic variation present for selective processes to operate on (Fig. 5.6). A similar story could be told for many species that have rapidly adapted to human‐caused changes in their environments (see Stockwell et al. 2003; Carroll et al. 2014).
Figure 5.6 (a) Species of snowball plants of the genus Saussurea that are used in traditional Tibetan and Chinese medicine have declined in height over the past 100 years (S. laniceps). (b) Another species that is seldom collected, S. medusa, showed no significant decline.
(Courtesy of Law and Salick/National Academy
