Interacting evolutionary constraints in pelvic reduction of threespine sticklebacks, Gasterosteus aculeatus (Pisces, Gasterosteidae)

Abstract

The pelvic girdle of Gasterosteus aculeatus is normally a complex and robust structure that varies moderately among and within populations, but in several populations it occurs as a vestigial series. Based on 14 Recent and fossil populations from North America and Scotland, three general patterns of intrapopulation variation of vestigial pelvic structures can be recognized. In most cases of pelvic reduction, the size and complexity of the pelvic girdle are reduced mostly by progressive truncation of distal structures, but two patterns of distal truncation occur. Paedomorphosis appears to play a role in all cases of pelvic reduction, but in a fossil assemblage and one extant population it accounts for most of the structural simplification. Pelvic reduction in populations of Gasterosteus depends on an interaction between natural selection, which appears to determine whether or not pelvic reduction occurs, and phylogenetic constraint (especially developmental constraint), which strongly influences the form of pelvic vestiges. The empirical difficulty of discriminating between phylogenetic constraint and effects of natural selection, and the potential for phylogenetic constraint to produce homoplasy are discussed. Pelvic reduction apparently has been a recurrent phenomenon in Gasterosteus since at least the early Pliocene, but it has not resulted in evolution of a separate lineage of ‘pelvicless’ sticklebacks because such populations are restricted to lakes, which are ephemeral over evolutionary time. Rapid phyletic pelvic reduction coupled with selective extinction of populations with vestigial pelvic structures has resulted in phylogenetic stasis.