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