john@nmt.edu (John Shipman) (09/15/90)
Daan Sandee (sandee@sun16.scri.fsu.edu) writes: +-- | ... the A.O.U. uses interbreeding as the criterion for | species determination +-- Then Andrew Taylor (andrewt@cluster.cs.su.oz) replied: +-- | It seems to me this only helps in the easy case. If the ranges | of two populations overlap but they never interbreed then you | call them separate species. +-- This case is called `sympatry', from the roots `sym', meaning same, and `patry', meaning native land. +-- | But what does the A.O.U do if the ranges don't overlap (common | in Australia)... +-- When the ranges don't overlap, it is called `allopatry', from the root `allo-' meaning `other'. This is indeed a tough case. There are various definitions of species. The evolutionist definition (cf. Ernst Mayr) defines a species as a ``reproductively isolated population.'' In sympatric populations, typical isolating mechanisms may be: (1) Behavior. The Clark's and Western Grebes were split because studies of their mating behavior showed that there was a difference in the first display used by the male to attract a mate. (2) Habitat. The ranges might generally overlap, but one species might nest at a different elevation or in a different type of forest. (3) Genetics. The Wood Duck of the New World and the Mandarin Duck of the Old World are very closely related, but hybrids are impossible due to a lethal gene. +-- | ...and surely it doesn't declare two populations to be the same | species just because interbreeding has been recorded? +-- This is a matter of some controversy. For sympatric populations, the usual approach is to study what happens in the `contact zone' where the ranges overlap. If there is general interbreeding, the forms are lumped (e.g., Northern Flicker, Northen Oriole). But if there is only a small amount of interbreeding, it becomes a value judgement (or a political decision). I once saw someone state that the threshold should be 5%---if fewer than 5% of young are hybrids, the forms are good species. This seems quite arbitrary to me. For allopatric populations, things are much tougher. You have to try to predict what would happen if the ranges of the populations did overlap. One approach that has been used is the Specific Mate Recognition System (SMRS): it is necessary to understand the behavioral mechanisms used by individuals to form pair bonds. The real problem here is that ornithology is a poor science. There just isn't enough money to support sufficient research to answer all the hard questions. -- John Shipman/Zoological Data Processing/Socorro, NM/john@jupiter.nmt.edu ``Let's go outside and commiserate with nature.'' --Dave Farber