What has natural selection favored in the male populations and female populations (think investment)? Also, how does this scenario affect whether an organism evolves to be monogamous (2 percent) or polygamous? Can these strategies provide answers to questions related to the phylogeny of organisms? Use information from the text and outside resources to explain.
Wednesday, March 10, 2010
Different Mating Strategies Among The Two Sexes
From pages 155-159, Coyne explains how males and females have developed different mating strategies. "A male can produce large quantities of sperm, and so can potentially father a huge number of offspring, limited only by the number of females he can attract and the competitive ability of his sperm." On the other hand, females have the bigger investment (pregnancy) of child development and therefore are unlike males in the sense that if they mate many times, it still would not increase their number of offspring. Coyne then goes into details from the Guinness Book of World Records and describes although the record number of children one mother gave birth to was 69, Mulai Ismail, a male- emperor of Morocco, was to have fathered "at least 342 daughters and 525 sons...".
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Sex has become the most common method among a variety of species to reproduce and pass on genes to future generations. Although it is not yet clear, sex must have a huge evolutionary advantage that outweighs its cost. Sexual reproduction provides variation and the random shuffling of genes creating new combinations of genes in offspring. By combining several different favorable traits, sex may promote faster evolution to deal with aspects of the environment that are constantly changing. "Sex could also purge bad genes from a species by recombining them together into one severely disadvantaged individual, a genetic scapegoat." (pg. 156) It is still up for question among biologists, however, as to whether or not sex outweighs the twofold cost. It is interesting to look at though why the two sexes that mate together have different gametes and traits. Two sexes is the most stable and robust compared to three or more sexes, which is why there are no organisms that have more sexes than just two. Natural selection has favored one sex to have lots of gametes, pollen or sperm, and one sex to have fewer larger gametes. This asymmetry of gametes causes the two sexes to develop different mating strategies. The evolutionary difference between males and females is investment. Females must invest more time during pregnancy, taking care of eggs because they are more valuable, and investing in parental care of the offspring in many species in which the male leaves. For males, mating does not require as much investment. They only use a little sperm. Females must produce a large, nutrient rich egg and expend a lot of energy and time in order to reproduce. Males have little to lose if they mate with a female that is not the "best quality" because they can easily mate and repeatedly reproduce. Selection favors genes that make men promiscuous, relentlessly trying to mate with nearly any female because it betters their chances of reproduction and the passing on of their genes. Females must be more picky because of all the energy they expend during reproduction. They must make each opportunity count. The good males, either more attractive or vigorous, will often secure a large number of mates because they will be preferred by more females and the substandard males will go unmated. Females on the other hand will all eventually find mates because since every male is competing for them, their distribution of mating success will be more even. It is seen that males mating success is higher than females. In the red deer, the variation among males in how many offspring they leave during their lifetime is three times higher than that of females. Due to the competition for mates, males tend to be more colorful, or louder, or stronger in their attraction for mates. Female preference for certain traits favors the evolution of longer tails, more vigorous displays, and louder songs in males. Evolution may favor monogamy because it ensures a greater chance that the offspring of the male survives; however, it is uncommon and is seen in only two percent of mammal species. Polygamy is more common because the male can spend more time reproducing and passing on genes and taking the chances that some offspring may die because they were not around to protect them. Queen bees are also polygamous and mate with many different males; in one study, "the results suggested that social insect queens may benefit from mating with multiple males by making their colonies more genetically diverse and therefore more resistant to disease," said Hughes. (http://www.abc.net.au/science/news/stories/s1204826.htm)
ReplyDeleteMany organisms are similar in mating strategies, reproduction, and other various traits. These common characteristics among species has a lot to do with the phylogeny of organisms. "Evidence from morphological, biochemical, and gene sequence data suggests that all organisms on Earth are genetically related, and the genealogical relationships of living things can be represented by a vast evolutionary tree." (http://tolweb.org/tree/learn/concepts/whatisphylogeny.html) Many species are similar in their mating methods because it is effective. All species want to reproduce and survive and pass on their offspring to further generations. Natural selection favors methods of reproduction that enhances their chances in doing so. Therefore, it is not surprising that many species have similar methods of reproduction or other various traits as well.
ReplyDeleteThe response above does a great job of discussing the differences between the sexes in mating strategies, including adaptations between the sexes with regard to gamete size, investment in development and energy required to reproduce, and the social roles of the sexes (whether they are promiscuous or “picky”). One key element that was not mentioned above is the idea that “natural selection for reduced competition between members of the two sexes could lead them to evolve differences in body size” (159). This thought is interesting because it appears to go against the idea of natural selection, that is, why would it favor any sort of adaptation that is directly dependent on the opposite sex? Given that (usually) two are required to reproduce – as previously stated, “two sexes is the most stable and robust strategy” (156) – it may be understandable why the two sexes may evolve with one another, and rely on each other in order to continue to reproduce. Sexual selection is understandable, and necessary, given natural selection, since, generally, females invest more into reproduction, they need to carefully select the males with the most favorable genes so that their offspring will survive to pass on their genes. Reducing competition between the sexes increases the chance that each will be able to reproduce successfully with another of the opposite sex, allowing for the most efficient system for reproduction; if males and females had to compete for the same scarce resource, because they take on different roles, one sex may have an advantage over the other, therefore potentially destroying the other sex, which probably would not be conducive towards reproduction. At least, I think that both a male and female need to be present for sexual reproduction, but then again, maybe I'm wrong.
ReplyDeleteI would like to focus more on the mating systems within populations (monogamy, polygamy, etc). First, Coyne, on page 158, glides over the exceptions to the general idea of gender roles described above (a generalization, as he calls it); what about monogamous relationships happened to be more advantageous in these populations? Going to the textbook, I think it is important to note that mating systems evolve alongside different modes of development (1134). If the young need extended parental care or a food supply that they cannot obtain for themselves, then you might expect the male to stay with a single mate to help in development (since he has a vested interest in seeing that his offspring may stay alive to pass on his genes). For other organisms, the young may be able to nourish themselves immediately, in which case, the males are free to go and “maximize their reproductive success by seeking other mates” (1135). The book also speculates that paternity influences mating behavior; that is, in species with external fertilization, where there is a lesser certainty of paternity, males may invest less (or no) time towards parental care, whereas in those species with internal fertilization, where the certainty of paternity is higher, the males may tend to stay more (and more monogamous relationships are noted in this instance). Going further into this topic, we see over evolutionary time changes in internal and external fertilization and development (most obviously seen in our survey of the different classes of organisms), all of which may be grouped together (using general trends) with other characteristics of reproduction such as number of offspring (which may be higher in external fertilization and external development – consider spawning). Given all these relationships, we may even be able to draw connections with such ideas as k-selection and r-selection in noting general trends in the mating strategies with organisms that maximize their reproductive success given this wide variety of variables; its not surprising we see many different mating strategies that appear to go against the idea of natural selection driven evolution, when, in fact, they are exactly what you would expect as a product of evolution.