Ryder  proposed the evolutionarily significant unit (ESU)5 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 worthy of 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.''6 Pennock and Dimmick  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 the lower 48, 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  argues that use of ESUs to define population segments in fish7 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.  respond by arguing that ``any unit of conservation defined solely in terms of adaptation is likely to underestimate biological diversity.
Questions for discussion
- 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.