Tuesday, March 23, 2010

Pass it on, or fail in life?

One of the primary reasons why an organism invests so much of their time and energy in reproducing is in order to pass on the genes to the next generations. And example experiment in explained in page 152-153 of Coyne's book. However, the question "If males with thiry-inch tails won more females, why haven't widowbirds evolved tails that long in the first place?" One of the answers to this question is longetivity. By investing energy in more survival, one many have find more partners. However, birds, such as peacocks, have many beings that go unmated to their deathbeds. Then comes the quesiton, why didn't the force of natural selection promote the elimination of organisms with such features? If organisms, such as peacocks, went unmated, shouldn't evolution have eliminated "unmating" genes? Why do such beings exist today?


  1. The main question guiding the introduction of traits that could increase an individual's chances of reproductive success is the balance of energy spent on survival and reproduction. Many times, the introduction of traits or genes designed to increase chances of reproduction also have a negative effect as well on the life span of the individual; in fact, an example of this appears in our own species, since "longevity requires investments in somatic maintenance that reduce the resources available for reproduction." (Pub Med http://www.ncbi.nlm.nih.gov/pubmed/9874369) In other words, to increase the age of an individual might result in a decreased reproductive ability, and vice versa. Thus, an equilibrium must be reached between the two competing factors, and sometimes the viability of using so much energy to produce far more attention-grabbing sexual traits to increase one's chances of reproduction is in question. In the case of some of these dramatically expanded traits, like that of the widowbirds, Coyne explains on page 154 that "it's likely that having tails that long would reduce a male's longetivity more than they would increase his ability to get mates". Surviving longer would increase the number of chances a male has to mate with a female to reproduce, and thus would also be a factor in choosing whether or not a flashy crest or feather would be beneficial or not. The seemingly weaker (reproductively speaking) individuals, then, might in fact have such traits since they lead to a higher reproductive output measured over a lifetime than other individuals with more ornate sexual traits.
    This explains why many of these seemingly weaker genes propagating less salient sexual characteristics continue to exist in species today. However, other individuals continue to use very ornate crests or feathers perhaps because their overall reproductive output is similar to less ornately decorated males. Thus, perhaps the process of natural selection is still slowly panning out over these specific characteristics, getting rid of those individuals with too much imbalanced focus on either longetivity or sexual characteristics but still leaving some diversification to ensure that the maximum reproductive output can be achieved

  2. Sexual selection is a very powerful force in natural selection. Females only choose mates with certain physical characteristics or those with the most "attractive" characteristics. These could be a longer tail as in widowbirds, or more color as in peacocks. More of these characteristics lead to a greater chance of being "selected" by the female to mate. This causes these traits to be passed on to the offspring. However, many of these traits can be detrimental to the overall longevity of the individual. If the widowbird's tail is too long, it is more easily preyed on by a predator and cannot mate and pass on his traits due to the fact that he will be dead. Therefore, an equilibrium must be reached. The selective positives of a certain trait must outweigh, or at least equal, the possible decrease in fitness. The reason many sexual dimorphic species are still around is because the extraordinary physical traits increase the likelihood of finding a mate as much as or more than it decreases the individual's fitness. This is evident in nearly all species of birds of paradise.

    Another thing to note is that sexual dimorphism may differ in extent depending on the specific population's environment. Birds in one area may be more ornate than birds of the same species in a different area if the second area has more predators. This is evident in fish. Guppies, for example, can differ in color in different parts of the same river. Upstream, where predators are less abundant, male guppies can be more colorful in order to attract more mates. Downstream, where predators are more abundant, male guppies cannot be as ornate because they attract too many predators and their overall fitness is decreased. Therefore, they must find their equilibrium within the certain area and population.




  3. Evolution helps species more aptly survive and/or reproduce in their environment. However, reproducing has a very high cost of energy. Often a trait that may have adapted in an organism to benefit them in reproducing or finding a mate impairs their ability to survive. Evolution finds that balance between surviving and reproducing and it creates stability. The example of widowbirds’ tails is a very basic example of this. They don’t have thirty-inch tails because it would make it harder to survive; a really long tail would make it difficult to be very mobile and it would also make it easier for predators to catch the widowbirds. So since the length of the tail is a factor in mating, evolution found a balance between the benefit of the length in mating with the harm in surviving.
    According to evolutionary theory, animals should die of old age once they reach the end of their reproductive period, which for humans would be around 55. Yet humans on average live into their 70s. This is mainly because of the few men that reproduce with younger women. Men that reproduce while in their 50’s and 60’s don’t receive any personal benefit, but their pattern of reproducing at an older age affects the entire population. Their genes that help them have a longer life-span are then passed off to their offspring, males and females, which is why both sexes live into their 70s and not just men. This could be very similar to the peacocks. Many peacocks go unmated to their deaths which would be a disadvantage, but there are still few with similar traits that do reproduce. Those that do reproduce are numerous enough and produce enough offspring to maintain the population of peacocks with those traits because just like many men don’t reproduce after 55, those few that do sustain the trait of longevity in their life-span.