Among animal populations, it has been observed that only a few males are needed to keep the numbers healthy and fertilize all the females. However, a new study shows that they are also important for keeping their genetics healthy and keeping bad mutations away.
This challenges the preconception that the number of males in a population has little effect on the growth of the population. A new study from Uppsala University in Sweden offers new insight on how sexual selection in a population, which depends on the number of possible male sexual partners, affects their genetics in the long run.
Researchers presented their findings in the article "Selection in males purges the mutation load on female fitness," now published in the latest Evolution Letters journal.
The More Partners, The Better
The new study lends credence to the theory that for many animal species, selection that acts on the male members creates an advantage to the population. In Particular, the offspring get healthier genes. Having a stiff competition among the males in a population leads to selective elimination of individuals that have disadvantageous characteristics or even better, those that have deleterious mutations. These traits will no longer be passed down to the next generation since the individuals that carry them have less chances of success in finding a mate. Over the long run, this has positive effects on a sexually reproducing group — its health and survivability.
"When deleterious mutations are purged from a population through rigorous selection in males, resulting in fewer males reproducing, the process can take place with little or no effect on population growth," explains Karl Grieshop, lead author of the study and an evolutionary biologist from the University of Toronto in Canada, in a news release from Uppsala University. Grieshop explains that this is because few males are enough to fertilize all the females in a population. Therefore, the number of males present makes little to no difference to the number of offspring the female population can produce — a particularly existent scenario in cases where the male does not have to look after the offspring. Meanwhile, having the same rigorous selection in females would lead to less females reproducing, decreasing the number of offspring produced. This could lead to a sharp decline in population or worse, lead to extinction.
Testing How Deleterious Mutations Affect Seed Beetle Populations
In their study, researchers used 16 genetic strains of seed beetle (Callosobruchus maculatus), which, according to the Centre for Agriculture and Bioscience International, is a wide species of beetle whose larvae live in and feed on dried seeds. The setup was designed to investigate how their inferred number of deleterious mutations in each strain affected the reproductive ability of both males and females.
By employing an intensive inbreeding of these genetic strains, followed by cross-breeding them, researchers were able to quantify the total effect of each strain's unique combination of mutations. Comparing the inbred and the crossbreeds, the team identified that the deleterious mutations actually harmed both males and females — at almost the same rate. But when researchers observed only the crosses between the strains, which is the more genetically variable setting and is more likely to happen in nature, the deleterious mutations were only more significant toward the male species.
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