If they found a mutation in about 50 percent of an offspring’s DNA, they concluded that it was probably a germline mutation, one inherited through the mother’s egg or father’s sperm. Natural selection can act directly on such a mutation. Less frequent mutations were considered to have occurred spontaneously in non-germline tissues; they were less relevant to evolution because they would not be passed on.
(Surprisingly, mismatches in family trios often told researchers that the parents listed by zoos were not related to the babies. Zoo representatives often shrugged at this news, saying there could be two males in the cage. “Yeah, well, the other one is the winner,” Bergeron joked.)
In the end, the researchers had 151 usable trios, representing species as physically, metabolically, and behaviorally diverse as massive killer whales, diminutive Siamese fighting fish, Texas banded geckos, and humans. They then compared the species’ mutation rates with what we know about behaviors and characteristics called its life history. They also considered a statistical measure for each species called effective population size, which roughly corresponds to how many individuals are needed to represent genetic diversity. (For example, although the current human population is 8 billion, scientists generally estimate our effective population size to be around 10,000 or less.) Bergeron and his colleagues looked for patterns of associations in the numbers.
The most surprising finding to emerge from the data was the wide range of germline mutation rates. When the researchers measured how often mutations occurred per generation, the species varied only about 40 times, which Bergeron said seemed pretty small compared to differences in body size, longevity and other traits. But when they looked at mutation rates per year instead of per generation, the range increased to about 120-fold, which was larger than previous studies had suggested.
The sources of variation
The study authors found that the larger the average effective population size of a species, the lower its mutation rate. That provided good evidence for the “drift barrier hypothesis”, which Lynch devised a little over a decade ago. “Selection is relentlessly trying to reduce the mutation rate because most mutations are deleterious,” Lynch explained. But in species with smaller effective population sizes, natural selection is weakened because genetic drift, the effect of pure chance in the spread of a mutation, is strengthened. That allows the mutation rate to increase.
The findings also support another idea in the scientific literature, the hypothesis of evolution driven by men, which proposes that males may contribute more mutations to the evolution of some species than females. Bergeron and his colleagues found that germline mutation rates tended to be higher for males than females, at least in mammals and birds, though not in reptiles and fish.
The authors pointed to a possible reason for those differences: Because males of all species constantly copy their DNA to produce sperm, they face endless opportunities for mutations to occur. Female fish and reptiles also produce eggs throughout their lives, so they are at similar risk of genetic error. But female mammals and birds are born with essentially all the eggs they will ever produce, so their germ lines are more protected.