Evolution and natural selection across 10 bird species

Comparative genomics based on massive parallel transcriptome sequencing reveals patterns of substitution and selection across 10 bird species.

Künstner et al* performed brain transcriptome sequencing on 10 species of birds, with the intent of studying natural selection between bird species. All of the bird species were primarily tested against the coding sequences of the zebra finch (whose genome sequence was already available), as it appeared most comparable to the other species (the chicken was initially used as well, but was found to be less useful.) 

By studying the synonymous substitution rate (ds) in comparisons between the zebra finch and 10 bird species and estimating the mean ds for each chromosome, Künstner et al. reached the conclusion that small chromosomes have an elevated mutation rate. This suggests that small chromosomes, being more prone to “slightly deleterious mutations,” have less input in natural selection, supporting the Hill-Robertson effect. 

The Hill-Robertson effect is the idea that recombination of genes makes evolution go more quickly. In populations of fixed sides that are undergoing natural selection, genetic drift and gene mutations can slow evolution down. Suppose that there are two competing advantageous mutations, A and B. With recombination, organisms could have both A and B, making them stronger and more able to thrive. However, without recombination, a mutation would have to occur for both A and B to exist in the same organism. Waiting for this mutation could take a long time and delay evolution. 

Further study of chromosomal mutations suggested that the male mutation rate was at least twice that of the female rate, although the meaningfulness of the incidence was debatable. Künstner et al. tried to correct for statistical measurements they found to be misleading by more carefully analyzing the relationship between mean ds and mean ɷ (the sum of the rate of non-synonymous selection divided by ds.) 

Their analysis again supported the importance of chromosome size and the Hill-Robertson effect. 

*_{Kunstner et al (2010).  Comparative genomics based on massive parallel transcriptome sequencing reveals patterns of substitution and selection across 10 bird species.Molecular Ecology 19: 266–276. doi: 10.1111/j.1365-294X.2009.04487.x}