In an attempt to circumvent some of the problems that may be associated with taxonomy, conservation biologists sometimes concentrate on management units
No. of monophyletic groups classified as subspecies
Figure 7.4 Number of monophyletic mitochondrial lineages per species compared with the number of these lineages that currently match subspecies classifications. The size of each circle is proportional to the number of comparisons in each category. The diagonal line indicates where the circles would be located if the monophyletic mitochondrial lineages in each species were in complete agreement with designated subspecies. Because all circles are above this diagonal line, all species contain monophyletic groups that are not classified as subspecies. Adapted from Zink (2004) and references therein
(MU) and evolutionarily significant units (ESU). An MU is 'any population that exchanges so few migrants with others as to be genetically distinct from them' (Avise, 2000), and is analogous to the stocks that are identified in fisheries. Distinct MUs can be identified on the basis of significant differences in allele frequencies at multiple neutral loci. An ESU consists of one or more populations that have been reproductively isolated for a considerable period of time, during which they have been following separate evolutionary pathways. Examples of this may include lineages that diverged in alternate refugia during glacial periods (Chapter 5). The ESUs are typically characterized by reciprocal monophyly in mtDNA and significant allele frequency differences at neutral nuclear loci (Moritz, 1994). Conservation strategies need to balance the desire to maintain as many MUs and ESUs as possible with the ever-present logistical constraints such as limited finances and a shortage of suitable habitat.
The preservation of distinct MUs and ESUs is generally seen as desirable because each unit contributes to a species' genetic diversity. Conservation of hybrids, on the other hand, is a much more controversial issue. The US Endangered Species Act (ESA), for example, originally proposed that hybrids would not be protected. This clause has since been revoked, although a proposed replacement policy on 'intercrosses' (avoiding the sometimes pejorative term 'hybrids') has yet to be officially integrated into the ESA. This lack of resolution is partly attributable to the different categories of hybrids (Allendorf et al., 2001). On the one hand, narrow hybrid zones that have been stable for many years are often adaptive (Chapter 5) and therefore may be considered ESUs. On the other hand, invasive species may threaten the genetic integrity of endemic species through hybridization, in which case the desirability of these hybrids becomes a matter for debate. In New Zealand, introduced mallard ducks (Anas platyrhynchos) have hybridized extensively with the native grey duck (Anas superciliosa superciliosa), and as a result there may no longer be any 'pure' grey duck populations remaining (Rhymer, Williams and Braun, 1994). In cases such as this, one option may be to eliminate populations of the invasive species and its hybrids; if this is unrealistic, the protection of hybrids may be the only way to preserve any of the threatened species' alleles.
Despite a number of unanswered questions regarding taxonomy and conservation, it is fair to say that molecular data provide us with an important window into the evolutionary history and genetic differentiation of species, and this may help us to make informed decisions about which populations constitute a conservation priority. There are some cases in which species boundaries have been altered solely on the basis of molecular data, for example in morphologically simple taxa such as the Cyanidiales, a group of asexual unicellular red algae (Ciniglia et al., 2004), or in numerous other marine species for which ecological data are difficult to acquire. Substantial sequence differences between the ITS region of ribosomal DNA in Australian and South African populations of the marine green alga Caulerpa filiformis, for example, suggest that these are in fact two cryptic species (Pillmann et al., 1997), whereas the negligible divergence between ribosomal genes of the seaweeds Enteromorpha muscoides and E. clathrata suggests that these should in fact be merged into a single species (Blomster, Maggs and Stanhope, 1999).
At other times the contributions of molecular data to taxonomy seem to have raised as many questions as they have answered. For example, all individuals are genetically unique but just how much genetic dissimilarity should we tolerate within a single MU? How much genetic divergence is required before ESUs are designated distinct species? How can molecular taxonomy accommodate incomplete lineage sorting and hybridization? Our inability to answer these questions to everyone's satisfaction does not mean that identifying the most appropriate units for conservation is an impossible task, although we need to remain aware of the limitations and assumptions that surround many taxonomic decisions. For the rest of this chapter we will, for the most part, be talking about species and populations as unambiguous entities, but we must keep in mind the possibility that species and population boundaries will be redrawn some time in the future.
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