In the comments of my dinosaur genome size post, Shelley asked:
So do ALL birds have equally small genomes or is there variation among species?
I don’t think she was looking for a trite response along the lines of: “Of course there’s variation among species.” What she was asking, I presume, is how much variation in genome size do we see in birds? As you can see in this phylogeny, all birds (and nearly all theropods) have small genomes. But that tree only presents data from a few species.
To get a better idea of genome size variation within birds, I downloaded C-values (amount of DNA in a haploid cell measured in picograms) from the Animal Genome Size Database (the same database used by the authors of the nature paper). I included all available data from aves (birds), crocodylia (crocodiles and alligators), tuatara (lizard-like reptiles), squamata (lizards and snakes), and testudines (turtles, tortoises, and terrapins). In some cases, multiple estimates were available for a single species; I averaged those values so that I had a single estimate of genome size for each species. There was only a single species sampled from tuatara (there are only two extant species in this taxon) and only five crocodilians, so these taxa were excluded from much of the analysis.
The results of the analysis are shown below.
In the histogram shown below, you can see that birds (N=204, mean C-value=1.45, SD=0.16) tend to have smaller genomes than squamates (N=253, mean C-value=2.11, SD=0.53) and testudines (N=44, mean C-value=2.86, SD=0.73). The variation in testudines is quite large, whereas birds have very little variation.
For those of you interested in such things, an ANOVA for the effect of taxon on genome size yields a very large test statistic (F=235.53, p<0.001). I've also got a box plot for all five taxa because you never can show the same set of data in enough formats.
So, Shelley, there ain’t much variation in genome size among birds. Purifying selection, anyone?