Evolution of a Species
▶ Assortive Mating: The diversity of lifeforms on our planet is central to evolution. But how do new species form? A key step is assortive mating, when individuals use physical or vocal cues to choose mates that resemble themselves. Perhaps natural selection favors offspring from similar matings. Eventually, the populations diverge genetically to the extent that the hybrids are unfit, and separate species emerge.
▶ Caught in the act? Take the curious case of the Australian Gouldian finch (Erythrura gouldiae). There are black and red head color morphs (see image) that prefer to mate with like types. This preference is genetic, as chicks reared by foster parents of different type still prefer to mate with their own head color morph. In fact, the head color and mating preference are tightly linked on the sex chromosome Z (males are ZZ and females are ZW in birds). This lack of “sexual imprinting” is unusual, since most birds get their cues from rearing parents.
▶ Hybrid drama: Both head color types coexist in the same geographical area. Shrinking and unequal populations mean that mates of the same type can be hard to find (the bird is endangered). The birds seem to “make the best of a bad situation” and breed with different head color morphs anyway. But there is a steep price to pay : more than a third of the offspring die. The mortality rate is worse in female chicks, nearly half fail to survive. Curiously, the mothers seem to control for this by producing broods with more males. So, if they are tricked into thinking that their mate is of a different head color (using bird make-up!) they produce biased broods! All of this suggests that the Gouldian finch may be in the process of splitting into species, unless it becomes extinct before then 😦
▪ Images (National Aquarium): http://aqua.org/explore/animals/gouldian-finch
▪ H/T Mindy Weisberger whose post on the phosphorescence beads marking the gouldian finch chick’s mouth (http://goo.gl/Zw8tv) set me off on this evolutionary hunt!
▪ Further readings by Sarah R. Pryke ▶ http://goo.gl/Tngj1