Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 41

When we try to classify the natural world, it seems relatively easy to recognize different species – peregrines and redwoods are readily distinguished from other birds and trees, especially compared to drawing the lines between different kinds of ecosystems and genes. Nevertheless, the question “What is a species?” is more complex than most people realize. One widely used definition, often called the biological definition, is based on reproductive isolation: “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” (Mayr 1942). For example, mammalogists classify brown bears in Eurasia and North America as the same species, even though they have been separated by the Bering Strait for about 10,000 years, because they would interbreed given the opportunity. On the other hand, American black bears and brown bears are considered separate species because they do not interbreed despite having overlapping ranges for thousands of years. Occasionally, interbreeding does occur between two apparently distinct species, and the offspring are considered hybrids. Here some difficult questions arise (Stronen and Paquet 2013; Fitzpatrick et al. 2015). How much hybridization can occur before we decide that the two parent species are really just one species? And what if the hybrid offspring form self‐perpetuating populations? These issues have come to the fore as biologists work to determine if North America is inhabited by two, three, or four species of the genus Canis (gray wolves, coyotes, red wolves, and eastern timber wolves) and various hybrids (Stronen et al. 2012; Wilson et al. 2012; vonHoldt et al. 2016).

Questions about hybrids are more familiar to botanists than to zoologists (Mallet 2007). Look through any comprehensive list of plant species, and you will find many listings such as Typha angustifolia × latifolia, indicating that hybrids of the narrow‐leaved cattail (angustifolia) and the broad‐leaved cattail (latifolia) occur routinely. However, this is only the tip of the iceberg; many species of angiosperms (flowering plants), perhaps over 70%, owe their origins to hybridization (Arnold 1992; Soltis and Soltis 2009). Plant species are also harder to define in terms of reproductive isolation than animal species because they frequently use asexual reproduction, self‐fertilization, polyploidy (multiple sets of chromosomes), and other variants of what we usually consider “normal” reproduction. Similarly, most microorganisms reproduce asexually, thus confounding the idea of reproductive isolation. Their extremely rapid reproduction and thus evolution adds another complexity: is the bacterium that embarks on a transoceanic voyage with a ship’s crew the same species when it returns to shore weeks later? Note, too, that species definitions do not adequately represent some of life’s odder forms, such as viruses, which reproduce by invading other cells and commandeering the cellular machinery, and prions, which are infectious self‐reproducing proteins (and of great concern because of their impact on deer populations) (Zabel and Ortega 2017).

Evolutionary biologists and taxonomists are wrestling with these issues and have proposed many other species definitions – phylogenetic, evolutionary, genotypic, cohesion, morphological, and more (see Coyne and Orr 2004 and Hausdorf 2011 for reviews). The differences among definitions would be an academic issue except that species distinguished by different definitions do not always correspond to one another. For example, an African antelope, the klipspringer, may be one species or 11 depending on which definition you use (Heller et al. 2013). Different definitions serve different purposes, and no one of them is “best” or “correct.” That said, a group of eminent conservation biologists has made a strong case for why the classic, biological definition of a species (i.e. reproductive isolation) works well in a conservation context (Frankham et al. 2012), for reasons that we consider in subsequent chapters.

The bottom line is that conservation biologists should be aware of the important implications of how we define species (Agapow et al. 2004; Mace 2004) but not allow ourselves to be paralyzed by uncertainty. Fortunately, any ambiguity about species definitions helps to highlight the importance of considering genetic diversity. In particular, sometimes conservation biologists can sidestep the definition of species and use terms such as “evolutionarily significant units,” or “taxa,” in their efforts to conserve both species and intraspecific groups such as subspecies, races, varieties, or even populations (Fraser and Bernatchez 2001). As we will see in Chapter 5, “Genetic Diversity,” all of these units of biological organization merit some attention from conservationists.