Ecomorphs are not species: the case of locally adapted populations of red crossbills

Abstract

The process of speciation remains poorly understood and hence definitions of species continue to be a focus of sharp debate. A key unresolved issue in speciation debates is whether local populations that are somewhat distinct from a parent population in morphology or genotype but that remain connected to the parent population via gene flow should be recognized as species. The species status of the Cassia crossbill Loxia sinesciuris, as well as other distinctive populations in the red crossbill Loxia curvirostra clade including the parrot crossbill L. pytyopsittacus and the Scottish crossbill L. scotica, are test cases for whether the hundreds of other avian ecomorphs should be taxonomically elevated to full species. We argue that these regional populations of red crossbills do not meet stated criteria for species designation under the Biological Species Concept and, regardless of species concept, fail as species simply following logic, parsimony, and consistency. There are no diagnostic morphological characteristics that definitively separate birds in the Cassia crossbill, parrot crossbill, or Scottish crossbill populations from birds in other sympatric crossbill populations. Call type, the behavioral characteristic that is proposed to distinguish Cassia crossbills, parrot crossbills and Scottish crossbills from other populations of crossbills, is learned and can change within the life of an individual. There is evidence of substantial on-going gene flow between Cassia crossbills, parrot crossbills, and Scottish crossbill and other populations of red crossbills, and there is no divergence in mitochondrial genotype. Instead, there exist few fixed genetic differences in nuclear genotype between individuals from these local crossbill populations and individuals from other red crossbill populations. We propose that the recognition of the Cassia crossbill, parrot crossbill and Scottish crossbill as species sets a poor precedent in avian taxonomy and muddles understanding of the process of speciation.