The Sparrow With 4 Sexes

Has a chance genetic mutation changed the evolutionary course of a species of sparrow?

Husband and wife and dedicated research partners, Rusty Gosner and Elaina Tuttle have spent, nearly, the last 30 years researching and understanding the weird genetics of a species of sparrow, the white-throated sparrow (Zonotrichia albicollis), that has been observed to have 2 different morphs, with both male and female, and very different behaviors between the 2. The first morph is a white-striped sparrow (like in the picture above) that sings exceptionally well, is promiscuous, aggressive, and provides little parental care. The other is a tan striped sparrow that sings poorly, is monogamous, protective, and provides good parental care.

What occurred was and inversion of a section of chromosome 2, which left it unable to pair with it's partner chromosome or homologous chromosome. This inverted region is therefore unable to exchange genetic information and no crossing over occurs, thus mutations form. From these mutations, 4 sexes emerge that can only mate with the other species, so white-striped males can only mate with tan-striped females and tan males can only mate with white females (think of it like opposites attract).

It was thought that these birds were evolving a second set of sex chromosome. To prove this, the duo would have to demonstrate that the genes in the inversion form mutations faster than other mutations on other chromosomes. By sequencing the genomes of 50 birds, the duo did just that and showed how chromosome 2 was evolving like a sex chromosome.