Evolutionarily significant units

Ryder [10] proposed the evolutionarily significant unit (ESU) as the minimal unit of conservation management. It is an attractive idea because it avoids problems associated with species definitions--or at least it seems to. An ESU is simply

• a set of populations that is morphologically and genetically distinct from other similar populations or

• a set of populations with a distinct evolutionary history.

This captures the idea that in most groups of plants and animals there are ``units'' of some sort above the level of individuals and populations that are the appropriate units of conservation concern. But you can probably see the difficulty with this definition already.

• How distinct morphologically or genetically do populations have to be to be regarded as different ESUs?

• How do we tell whether one set of populations has an evolutionary history distinct from another?

Well, the answer to the first question is: ``It depends.''<sup>4</sup> Pennock and Dimmick [8] argue, for example, that many population segments of vertebrates currently protected under the ESA would not be protected if population segments were defined as ESUs. Protected populations of grey wolf and grizzly bear in northern Minnesota, for example, don't seem particularly different either genetically or morphologically from those just across the border in Canada, nor do they seem likely to have an independent evolutionary history. In response, Waples [12] argues that use of ESUs to define population segments in fish<sup>5</sup> is precisely what the ESA intends when it directs that listing decisions be based ``solely on the basis of the best scientific and commercial data availabls'' (§4(b)(1)(A)). Dimmick et al. [5] respond by arguing that ``any unit of conservation defined solely in terms of adaptation is likely to underestimate biological diversity.

• What kind of data would you want to determine whether protected grey wolf and grizzly populations are ESUs?

• Suppose that they aren't ESUs, would you still advocate protection for them under the ESA?

What about determining whether populations are historically distinct from one another? That's a little more straightforward, at least in principle. Consider the case of the dusky seaside sparrow.

• A single popualtion in Brevard County, Florida (see Figure 1)

  Figure 1: Distribution of seaside sparrow (Ammodramus maritimus) subspecies in eastern North America. The dusky seaside sparrow is subspecies nigrescens (from [1], © 1989 American Association for the Advancement of Science).

  

• Avise and Nelson [1] extracted mtDNA from the last surviving male after it died in the late 1980's and compared it with mtDNA extracted from other seaside sparrows collected from the Atlantic and Gulf coasts.

• Many different haplotypes identified.

• Major division between haplotypes on Atlantic and Gulf coasts.

• Dusky seaside sparrow haplotype a small twig on the Atlantic coast branch. (see Figure 2)

  Figure 2: Haplotypes found in analysis of seaside sparrow DNA. The haplotype of the last male dusky seaside sparrow is 1. That haplotype is also found in subspecies maritima and macgillivraii (from [1], © 1989 American Association for the Advancement of Science).

  

• Their conclusion: conservation efforts directed at the dusky seaside sparrow were misguided. The major evolutionary disjunction is between Atlantic and Gulf coasts.

• PROBLEM: mtDNA is maternally inherited. Hybridization between dusky seaside sparrow and other seaside sparrows was documented. A single non-dusky maternal ancestor of the last remaining male would produce this result even if the dusky were actually very different for all or nearly all of its nuclear genes. Multiple haplotypes from within populations would lessen concern about this problem.