A UC Riverside-led research team has found that as some populations of an organism evolve a longer lifespan, they do so by increasing only that segment of the lifespan that contributes to “fitness” – the relative ability of an individual to contribute offspring to the next generation. Study results appear Dec. 27 in the online edition of the Public Library of Science – Biology.
The study supports the controversial hypothesis that natural selection – the process in nature by which only organisms best adapted to their environment tend to survive and pass on their genetic characters in increasing numbers to succeeding generations – introduces changes in only a specific segment of an organism’s lifespan.
The researchers conducted their experiments by comparing life-history traits in 240 guppies they retrieved from high- and low-predation streams in mountains in Trinidad. In their analysis, they divided the life history into three non-overlapping segments: the age at maturity (birth to first reproduction), the reproductive lifespan (first to last reproduction) and the post-reproductive lifespan (last reproduction to death). They also devised a statistical criterion for evaluating whether or not guppies had a post-reproductive lifespan, that is, did guppies live significantly past the end of their capacity to reproduce?
“We were exploring whether or not fish have the equivalent of mammalian menopause,” Reznick said. “We found that 60 percent of the fish had a significant post-reproductive lifespan, indicating that, yes, fish do have menopause. Indeed, their patterns of growing old are similar to those of mammals.”
The researchers’ statistical analysis also showed that regardless of which environments the guppies lived in, there were no differences among their populations in the probability of having a post-reproductive lifespan or in its duration.
“This is just what one might predict because these fish provide no care for their young,” explained Reznick. “The older fish, after they stop reproducing, do not contribute to the fitness of young fish. As a result, the post-reproductive period is not influenced by natural selection. This result could be of interest to those who study menopause in humans and who have argued that post-reproductive humans can increase their own fitness by contributing to the fitness of their grandchildren and that the prolonged post-reproductive lifespan of humans is, therefore, the product of natural selection.
“But such arguments are difficult to prove by working on a single population or species. Nevertheless, our results show how it would be possible to evaluate whether or not menopause in humans has been shaped by natural selection. Appropriate comparisons, such as those between humans and apes, would help.”
This story has been adapted from a news release issued by University of California – Riverside.