If they found a mutation in around 50% of an offspring\u2019s DNA, they concluded that it was likely a germline mutation \u2014 one inherited through either the mother\u2019s egg or the father\u2019s sperm. Natural selection can act directly on such a mutation. Less frequent mutations were deemed to have happened spontaneously in tissues outside the germline; they were less relevant to evolution because they wouldn\u2019t get passed on.<\/p>\n

(Surprisingly often, mismatches in the family trios told the researchers that the fathers listed by the zoos were unrelated to the babies. Zoo representatives would often shrug at this news and say there might have been two males in the cage. \u201cYeah, well, the other one is the winner,\u201d Bergeron would joke.)<\/p>\n

In the end, the researchers had 151 usable trios, representing species as physically, metabolically and behaviorally diverse as massive killer whales, tiny Siamese fighting fish, Texas banded geckos and humans. They then compared the species\u2019 mutation rates with what we know about the behaviors and characteristics called their life history. They also considered a statistical measure for each species called the effective population size, which roughly corresponds to how many individuals are needed to represent the genetic diversity. (For example, although the human population today is 8 billion, scientists usually estimate our effective population size to be around 10,000 or fewer.) Bergeron and her colleagues looked for patterns of associations in the numbers.<\/p>\n

The most surprising finding that emerged from the data was the wide range of germline mutation rates. When the researchers measured how often the mutations occurred per generation, the species varied by only about fortyfold, which Bergeron said seemed quite small compared to the differences in body size, longevity and other traits. But when they looked at the mutation rates per year rather than per generation, the range increased to about 120-fold, which was larger than previous studies had suggested.<\/p>\n