If they find a mutation in about 50 percent of a child’s DNA, they conclude that it is likely a germline mutation—one inherited through the mother’s egg or the father’s seed. Natural selection can act directly on such mutations. Less frequent mutations are considered to occur spontaneously in tissues outside the germline; they are less important in evolution because they are not transmitted.
(Surprisingly often, family trio inconsistencies tell researchers that the fathers listed in zoos are not related to the children. Zoo representatives often cling to this news and say that (There are two guys 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 killer whales, tiny Siamese fighting fish, Texas banded geckos, and humans. They then compared the species’ mutation rates to what we know about behaviors and traits called their life history. They also consider a statistical measure for each species called the effective population size, which roughly corresponds to how many individuals are needed to represent genetic diversity. (For example, although the human population is now 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 among number.
The most striking finding that emerged 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-fold, which Bergeron says seems small compared to differences in body size, longevity , and other characteristics. But when they looked at mutation rates per year instead of per generation, the range increased to about 120 times, which is greater than previous studies had suggested.
The Sources of Variation
The authors of the study found that the higher the average effective population size for a species, the lower its mutation rate. That provides good evidence for the “drift-barrier hypothesis,” which Lynch formulated more than a decade ago. “Selection relentlessly tries to reduce the mutation rate because most mutations are deleterious,” Lynch explained. But in species with smaller effective population sizes, natural selection can be weaker because genetic drift – the effect of pure chance on the spread of a mutation – is stronger. That allows the mutation rate to increase.
The findings also support another idea in the scientific literature, the hypothesis of male-driven evolution, which suggests that males may contribute more mutations to the evolution of some species than females. Bergeron and his colleagues found that germline mutation rates are higher for males than females – at least in mammals and birds, though not in reptiles and fish.
The authors noted a possible reason for those differences: Because males in 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 have a similar risk of genetic error. But female mammals and birds are essentially born with all the egg cells they can produce, so their germlines are more protected.