By Vishnu Apr 2, 2005, 23:50 GMT
Sexual behavior is time consuming and exhausting in terms of having to hunt for a mating partner, and running a risk of unfavorable gene combinations, so why are human so sexually active? Sexual reproduction seems to have many disadvantages over the asexual variety.
However, In New Zealand, scientists have demonstrated why humans resorted to sexual reproduction, using strains of yeast.
Yeast cells are normally asexual. But when stressed they undergo a process that produces four spores equivalent to the male and female gametes. This is because when faced with a scarcity of nutrients, the yeast is stimulated to produce spores. The spores are produced through meiosis, a cell-division process that halves the number of chromosomes: a sexual activity.
A team from the University of Auckland, led by Matthew Goddard, conducted a study with two strains of yeast, only one of which was able to reproduce sexually, the other being a mutant strain of yeast that was unable to generate sexual spores under any conditions.
In a stress-free environment, both strains were seen reproducing at about the same rate. But when higher temperature and a harsher environment was imposed, the growth rate over 300 generations, increased by 94 percent in the sexually reproducing yeast, but only to 80 percent in the asexual yeast.
Another conclusion that was drawn from the experiment was that the genetic shuffling from sex allows a greater chance that a useful suite of genes will come together.
August Weismann, for the first time in the 19th century, had explained that sex allowed evolution to progress at a faster rate, and kept the species healthier and fitter. He proposed that sexual reproduction helps speed up natural selection by allowing good genes to spread more quickly through a population, and bad genes to disappear faster. This is the first time that Weismann’s theory has been scientifically proved.
One of the questions that remain unanswered is regarding the "twofold cost" of sex. The concept of "twofold cost" of sex applies to organisms with unequal male/female differences. According to this concept the contribution of the male sperm to an offspring is tiny compared with that of the female egg. Consequently, natural selection is expected to turn females into asexual beings, producing only daughters. However, this has not happened. Since yeast has no male female distinction no conclusion can be drawn from the findings regarding this concept. Another doubt raised is why is it that adaptation in the sexual case is faster than in the asexual case?
Though further study is required in this field, such experiments are, tough to conduct. According to Goddard, "In an experiment, one needs to hold all else constant, apart from the aspect of interest. This means that no higher organisms can be used, since they have to have sex to reproduce and therefore provide no asexual control."
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