1/5/2024 0 Comments Real passenger pigeon![]() ![]() These findings are consistent with the idea that the passenger pigeon's adaptation to large populations may have become a liability when their population was reduced. Among the 32 genes with strong evidence of adaptive evolution were genes associated with the immune system and stress reduction (large, dense populations tend to have a high burden of disease and social stress) and with the ability to eat lots of certain foods. When the researchers looked at what types of genes showed evidence of adaptive evolution, they found many that could be related to aspects of passenger pigeon ecology and the demands of living in large flocks. "At the ends of the chromosomes, nothing gets dragged along with the beneficial mutation because of the high rate of recombination," Shapiro explained. 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. Recombination tends to happen less frequently in the middle of chromosomes than at the ends, a tendency that is especially pronounced in birds. 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). 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. "That is exactly what you would expect to see if selection is causing the differences in genetic diversity." "When we looked at rates of adaptive evolution and purifying selection in both species, we found evidence that natural selection had resulted in both a faster rate of adaptive evolution in passenger pigeons and a faster purging of deleterious mutations," Murray said. ![]() The researchers did not find the same patterns of genetic diversity across the genome in the closely related band-tailed pigeon, which has a relatively small population of about 2 million native to western North America. The analysis revealed patterns in the passenger pigeon genome indicating that the species' low genetic diversity was the result of natural selection causing the rapid spread of beneficial mutations through the population and the elimination of bad mutations. We found that it wasn't just lower than expected overall, it was also more variable, and we were able to see where those regions of high and low diversity are in the passenger pigeon genome," said first author Gemma Murray, a postdoctoral researcher in Shapiro's Paleogenomics Lab at UC Santa Cruz. "What we did, which the previous study didn't do, was to look at variation in diversity across the genome. But where previous researchers saw evidence of an unstable population that had fluctuated between highs and lows, the new study reached very different conclusions. The researchers confirmed earlier observations of remarkably low genetic diversity in the passenger pigeon population. Shapiro's team looked at the genetic diversity of passenger pigeons, using DNA recovered from museum specimens. ![]() Paradoxically, their enormous population size may have been a factor in their extinction," said corresponding author Beth Shapiro, professor of ecology and evolutionary biology at UC Santa Cruz. "Passenger pigeons did really well for tens of thousands of years, and then suddenly they went extinct. One theory, which is consistent with the findings of a new study published November 17 in Science, suggests that passenger pigeons were well adapted to living in huge flocks, but poorly adapted to living in smaller groups, and the change in population size happened so fast they were unable to adapt. Yet it remains a mystery why the species wasn't able to survive in at least a few small, isolated populations. The passenger pigeon is famous for the enormous size of its historical population in North America (estimated at 3 to 5 billion) and for its rapid extinction in the face of mass slaughter by humans. ![]()
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