LITMIR.BIZ

Скачать книгу

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 of Sciences, USA)

      Environments change through space, as well as time, and a species with greater genetic diversity is more likely to colonize a wider range of environments than a species with limited genetic diversity. For example, a survey of the heterozygosity and polymorphism of 189 species of amphibians indicated that genetic diversity was greatest in amphibians that lived in the most heterogeneous environments (e.g. forests) and least in homogeneous environments (e.g. aquatic ecosystems and underground) (Nevo and Beiles 1991). A similar pattern has been shown for plants (Gray 1996). More disturbed environments also tend to support species with lower genetic diversity (Banks et al. 2013).

      Loss of Fitness

Populations that lack genetic diversity may also experience problems (e.g. low fertility and high mortality among offspring, etc.) even in environments that are not changing (Fig. 5.7). A loss of fitness associated with genetic uniformity often develops from breeding between closely related individuals, an outcome known as “inbreeding depression.” It is a well‐known phenomenon in zoos, where populations of captive animals are often small, and mating among close relatives can be unavoidable (Ralls et al. 1988) (Fig. 5.8). Inbreeding affects traits important for fitness. Reproductive biology is particularly sensitive. For example, female marmoset monkeys inbred in captive situations develop fused genital labia and cannot copulate but are otherwise reproductively healthy (Isachenko et al. 2002). Plant and animal breeders who breed individuals that are genetically similar to one another to promote desirable characteristics that they share, such as a preferred color or resistance to a certain disease, can also promote inbreeding indirectly.

Figure 5.7 Relationships between reproductive fitness and genetic diversity summarized across many studies by Reed and Frankham (2003). The strength of the relationship is measured by the correlation coefficient, which ranges from −1 when higher fitness is associated with lower genetic diversity (and vice versa) to +1 when higher fitness is associated with higher genetic variation (and vice versa). If there was no relationship then most studies would report correlations between fitness and genetic diversity around zero but as this figure clearly indicates relationships tend to be quite positive (averaging about 0.4).

      (Reed and Frankham 2003/John Wiley & Sons)

Figure 5.8 Juvenile mortality in 44 species of mammals (16 ungulates, 16 primates, 10 rodents, one marsupial, and an elephant shrew) bred in captivity. Open bars represent mortality rates with inbred parents; black bars represent mortality rates from matings between unrelated parents. Species are arranged from left to right by increasing mortality from unrelated parents. Numbers on the tops of the bars are the sample size. (

      Data from Ralls and Ballou 1983)

There are three general explanations for relatively low fitness in genetically uniform populations. First, there is more homozygosity in genetically uniform populations, and this may lead to the expression of deleterious alleles that are usually recessive and suppressed in heterozygous individuals. Hip dysplasia in purebred dogs is a widely known current example. At one time, inbreeding within the royal families of Europe resulted in many family members having a split upper lip. An example of inbreeding that increases the expression of a maladaptive trait in a wild species is provided in Fig. 5.9. These traits are widespread in many populations (for example, as many as 1 in 20 people carry the gene for cystic fibrosis, or 5%) but the two recessives rarely combine at random to produce the condition (p² = 0.05 × 0.05 = 0.0025 or 2.5 in 1000 offspring). The problem is small

Скачать книгу