Tuesday, April 6, 2010

What About the Females? (Sexual Selection)

Coyne states, in passing, while offering examples about male competition and sexual selection, that “Females, who don't have to fight, are presumably close to their optimal weight for reproduction” (150). It is shown that “direct competition between males” and “female choosiness” (148) are apparent among organisms, and a key to sexual selection. As a result, it is arguable that males would tend to change more than females in order to increase their fitness and opportunity to produce offspring. Coyne already addresses the issue of the tradeoffs between adaptations benefitting survival and those aiding in reproduction in males; though there are many seemingly energy intensive/wasteful traits that males have to “win over the females”, they tend not to become so elaborate that these adaptations would result in the death of the organism before they even have a chance to pass of their genes to their offspring. However, in females, this becomes more of a question; since they tend not to have to fight for males, and do the choosing of their potential male mates, how is it that the females are able to adapt to become more suitable for reproduction? It would seem as if the males would tend to undergo selection “faster” than females do. Also, how far does this go? A deer's antlers can only get so large; with males having the largest, most robust antlers winning more jousting contests and impregnating more females. With this type of selection, wouldn't it be reasonable to presume that the deer's antlers would approach a certain maximum size (consider the tradeoff with survival and reproduction – at some point, the antlers would hinder their ability to survive). Speculate as to what happens in this situation.


  1. "Darwin, when proposing his theory of evolution and sexual selection, suggested that the size of male ungulate antlers and horns may reflect male individual quality, and thereby be used by conspecifics as an honest signal of male sexual vigor, health, strength, hierarchical status, or ability to fight" (http://www.physorg.com/news93522459.html). This makes sense because males who are healthier overall have less need to conserve or take in energy in order to survive and reproduce, and therefore will not be harmed as much by sacrificing part of their energy supply and their mineral supply to make antlers. There will be an equilibrium of antler size created by the tradeoff between energy conservation and effectiveness of displaying sexual prowess, and depending on the environment, this equilibrium could be anything within a certain range of antler sizes. The only reason we would think of this equilibrium as a "maximum" is that, since deer's ancestors originally did not have antlers, antlers must have grown throughout the generations, meaning the equilibrium would most likely be higher than the average antler size within a population if different at all, meaning it will be approached from below, making it a maximum. I don't think speculation is really the right thing to do here, because we can just observe what happens, as most deer are probably evolved enought that they are already at this equilibrium. For example, the Irish elk went extinct supposedly because its antlers became so big that when it encountered a new environment that had less calcium available to make bones and antlers, it was driven extinct because it had evolved antlers that took up a huge amount of energy to maintain and gave it a condition similar to osteoporosis (http://en.wikipedia.org/wiki/Irish_Elk#Evolution_of_antler_size). Most of the time, though, such drastic changes will not occur, and antler sizes should stay at whatever size is best for survival.

  2. Well Nathan, you bring up a really good question about how females evolve their reproduction if they are the organisms that males have to compete for. I mean, males are competing for ANY female in order for them to pass on their genes, so females should always have a mate so long as there are enough males around. However, what would cause females to evolve their reproduction is the efficiency. For example, if one female could one support one offspring at a time, but another female had a mutation that made multiple offspring per fertilization more likely AND that female could provide sufficient care for all of the offspring, then the reproductive efficiency of the second female would be much higher. The gene which caused multiple offspring per fertilization would be passed onto more offspring, and eventually this gene would work through the entire population because they would outcompete the non-mutated individuals. However, if the gene mutates again to cause even more offspring per fertilization and this becomes detrimental to the mother as well as the offsprings, then the genes would die with these organisms. This is similar to the equilibrium that must be reach in males. There is a number of offspring that causes the maximum efficiency for it, just like there is a temperature for enzymes that causes them to catalyze their reactions at a maximum efficiency. This is only one example of how sexually selective females could evolve in order to increase their reproductive efficiency.

    You also asked about males and the potential trade-offs made when developing sexually selective advantages. Eric hit the nail on the head. There needs to be an equilibrium reached. There must be a balance between winning females and staying alive because many of the ornate physical characterstics can be detrimental to their fitness. This is evident in red collared widowbird (Coyne 145-6). These birds have very long tails that are used to attract females. The longer the tail, the more desirable. However, if the tail got too long, it would inhibit their flying and make them more easily preyed upon. Therefore, an optimum length must be reached. Its all about equilibrium between "sexiness" and "ability to not die".

    Coyne, Jerry. "Why Evolution is True." 2009.

  3. I believe that the females just adapt normally for reproduction through natural selection. There is seemingly no reason for them to evolve (even though they do) because the men are the one that carry the sperm to fertilize the egg. As we know it's the survival of the fittest for the creatures in the wild. Those that produce the most offspring would be able to reproduce, and survive predation. They are the ones that would affect the surrounding creatures. That's why the males need to develop adaptations that will help them get picked to reproduce. It is the males job to "look appealing" to the female so they would have offspring that would survive, and have more offspring. So as we can see it is the males responsibility to "look appealing" and develop adaptations. The female is in turn helping to choose the males that will provide a higher chance of their offspring survive. This is called sexual selection (148). The female is there to choose, and even though it does evolve, it does not necessarily need to develop new adaptations. If there was one thing that would help them get more suitable, it would be their sense to perceive which males harvest the better genes, and which males would have the best chance of creating offspring that would survive. The females are the regulators and help regulate the species survival by creating offspring with a better chance of survival.
    This will go as far as it needs to as long as it does not hinder its ability to survive. What I mean is that the species would be creating new adaptations that would go farther than just color, sounds, and tails. Basically they will go further than the visible features that the males have, and they will need to show a female how it flies, strength by bringing food, etc. If this ever occurs, then the females sense of choosing a male would have to intensify. Let's take the example of strength. The female would need to be able to distinguish a male by his ability to bring stuff to her. The males would go through selection "faster" than females, but females would eventually make the judgement call.
    I believe that evolution has gone so far that it is now able to distinguish between what is suitable for a species, and what is not. Grant it there are mutations, but there are very few of them. What I mean is that evolution has led to adaptations that do not hinder the species survival. Natural selection has played a great part in this because it is choosing those adaptations that will help an organism survive better. So if we look at the the deer's antlers, they would reach a certain limit because natural selection would eventually choose the antlers that help that animal survive rather then have huge antlers and die. There would not necessarily be a maximum because survival is more important than the any adaptation. Going back to the point that males undergo selection "faster," I stated that males will eventually have to develop other traits that would appeal to the females. The deer's might move away from the antler and evolve other adaptations to help the females choose them. I agree with Eric that there would be eventually equilibrium, but not between energy conservation and sexual prowess, but between survival and sexual prowess. Energy is not the only thing that an animal needs. It needs to be able use energy and its senses to be able to find danger, and enhance its survival while still having the adaptations that enhance its ability to being picked to reproduce.

    Eric's comment

    Coyne Why Evolution is True page 148