Criteria for Ascertaining the Existence of Host Races

Abstract

Defs. of host races: Bush (1969:237): "a population of a species living on and showing preference for a host which is different from the host or hosts of other populations of the same species"; but no restriction of gene flow. So Mayr (1970: 414): for host (=biological) races: "noninterbreeding sympatric populations that differ in biological characteristics but not, or scarcely, in morphology. Supposedly prevented from interbreeding by preference for different food plants or other hosts." Addition by Jaenike: host races rather than host-associated sibling species "if gene flow among two or more populations were restricted solely or primarily because of differential host preference ..." "... if this basis for reproductive isolation were not present, host races would fuse into a single panmictic population, whereas sibling species would maintain their separate identities." HA HA; so host races are sibling species because of host choice. Conditions for populations to be host races: 1) Sympatric; 2) Significant genetic differences between them => gene flow not extensive; 3) Genetic difference in (2) must not be directly related to host selection; 4) Genetic difference in (2) not solely a result of selection acting on current generation; 5) If feasible, confinement on a single host, differences should slowly disappear, HW established, linkage disequilibria declines, for polygenic traits bimodal distributions become unimodal. Rhagoletis pomonella: rejected as host race on grounds of lack of genetic differentiation; all differences could be environmental. Laspeyresia pomonella, walnut, apple, plum races do differ genetically in diapause induction and termination, but are not microsympatric, and the sp. has limited dispersal. Neodiprion abietis much better. Sympatric, differences in survival, host preference, development time; F1 and F2 crosses are fertile, but it is not known whether there is any other hybrid breakdown. Various tree-feeding moths and drosophilids do not differ at allozymes, suggesting high rates of gene flow. Only in Alsophila pometaria is there evidence of genetic differences, and in this species the parthenogenesis maintains the genetic dfferences rather than the host choice