Biologists at UCSB have found that it is not only possible for females to be too attractive, but that the evolutionary prospects of fruit flies, or Drosophila melanogaster, are being harmed by this problem.
A recent study has found that the persistence of male fruit flies in courting only the most attractive females had a profound negative impact on the process of natural selection and, therefore, the biological fitness of the species.
William Rice, a biology professor at UCSB who worked on the study, said that ramifications of female selectiveness in choosing a mate has been explored a great deal by biologists, but this study paid special attention to the effects of male selectiveness.
The competition for reproductive success is the fundamental mechanism that natural selection is dependent upon. According to Rice, an individual fruit fly is considered to be successfully reproducing when it passes its genes onto the next generation.
“We gauge reproductive success of a fruit fly in terms of lifetime offspring produced,” Rice said. “How many copies of their genome they’ve placed into the gene pool. Suppose there was a new mutation that females can fly more efficiently. You would expect that mutation would accumulate.”
However, the study revealed that the persistence of fruit flies in choosing only the most attractive females was severe to the point where it sometimes amounted to harassment. This behavior hinders the reproductive success of not only the females, but the males as well, who in this persistence let other viable mates fall to the wayside, preventing positive mutations from being transmitted and disrupting the process of natural selection.
“When males are too choosy, [a beneficial mutation] accumulates more slowly or not at all,” Rice said. “Thus, because the males have such antagonistic persistence in mating, the population doesn’t get the full advantage of this new mutation.”
The excessive courtships of the most attractive females in the population was found to be harmful because it interfered with other activities the flies needed to engage in. Constantly being surrounded by males performing their courtship rituals hindered their ability to forage for food effectively.
Furthermore, Rice said the excess of semen itself in any individual female can have negative physiological effects.
“Sometimes the females do mate again, and the excessive seminal fluid can damage her endocrine and reproductive systems,” Rice said.
Due to the complications frequent mating causes for the females, it is more desirable for males to mate excessively, as males have much more to gain from frequent mating, while females can be harmed by it.
“After a female mates, she stores enough sperm for her lifetime,” Rice said, “However, males have enough sperm to mate many times. [Because of this,] the reproductive strategy best for the female is not the one best for the male. It’s in his best evolutionary interest to produce as many offspring now.”
The problems arising from the reproductive strategy ultimately damages the evolutionary fitness of the species by making the female fruit flies mate more frequently at the cost of the females’ long-term fertility.
“Males want to maximize the short fecundity of the female [they are] mating with, and this will damage her long-term fecundity,” Rice said. “There is a fundamental conflict of interest.”