The hypothesis is that they were strongly selected for living in huge flocks, which is why they all died off instead of surviving in small groups.
I’m not sure that the strong immune systems which make sense for large flocks would make it hard to survive in small groups, but there might be something else.
This caught my eye:
“When a beneficial mutation spreads through a population, it carries along with it adjacent stretches of DNA, so subsequent generations carry not only the good mutation but entire sections of identical DNA. These regions of low diversity can be broken up by recombination, the process in which paired chromosomes exchange sections of DNA during the formation of eggs and sperm (which explains why parents don’t pass on exact copies of their chromosomes to their offspring).
“Recombination tends to happen less frequently in the middle of chromosomes than at the ends, a tendency that is especially pronounced in birds. In the passenger pigeon genome, the researchers found that areas of low genetic diversity were in the middle of chromosomes, while higher diversity regions were at the ends.”
I’d assumed that genes are selected for individually, but apparently not.
This entry was posted at https://nancylebov.dreamwidth.org/1095617.html. Comments are welcome here or there. comments so far on that entry.