0
$\begingroup$

The handicap principle is used as an explanation for some genetic traits, such as bright tails in male peacocks. However, it seems that this principle could be used to justify sexual selection of almost any trait. Consider the following examples justifying selection of two opposite traits:

Assuming everything else equal, an individual with a larger body has higher nutritional requirements, which are harder to meet. Therefore individuals with larger body sizes were selected because that signals higher fitness.

Assuming everything else equal, an individual with a smaller body can't protect itself as effectively. Therefore individuals with smaller body sizes were selected because that signals higher fitness.

Does this discredit individual applications of the handicap principle? Or is there a way to measure the extent to which the handicap principle has affected selection of any given trait?

$\endgroup$
7
  • 1
    $\begingroup$ I don't have any expertise in this realm, but I will agree that your example does indicate that at the least, one should be rather careful applying this kind of theory to real data, due to the risk of building a just-so story. I think that a more nuanced view of the principle could argue against your second case, considering e.g. conspicuous consumption, so a strong case would need to engage with that IMO (just as one example). $\endgroup$ Sep 23, 2023 at 22:17
  • $\begingroup$ @MaximilianPress This is more about how one would support the idea that the trait has evolved due to the handicap principle and not something else. Would that require an exhaustive argument considering all possible alternatives? $\endgroup$
    – rus9384
    Sep 24, 2023 at 11:12
  • 1
    $\begingroup$ @rus9384 Yes, more or less. None of these principles occur in isolation so to get the full picture, you need to consider all the relevant factors. Compare "island gigantism" with "island dwarfism" for examples; both are things observed in animal populations on islands - why one way and not the other? $\endgroup$
    – bob1
    Sep 24, 2023 at 20:39
  • $\begingroup$ @rus9384 I agree with bob1. If you are arguing for or against a specific instance, you need to include detail from natural history, consider alternate hypotheses, and evaluate each argument in light of available data. The kinds of statements in your question are very difficult to evaluate when isolated from data; there isn't a purely logical solution. You could probably write down some fancy differential equations arguing either way with equal elegance. $\endgroup$ Sep 25, 2023 at 18:40
  • $\begingroup$ @MaximilianPress It's more so that even if, say, handicap principle explains peacock tails, it's not clear if it explains it 100% or maybe there are other factors. So, the expanded question would be "Is there a way to estimate how big of a factor the handicap principle is for a given selected trait?" $\endgroup$
    – rus9384
    Sep 27, 2023 at 10:23

1 Answer 1

0
$\begingroup$

Let me give two separate answers.

First, for your example specifically, you could design a test to see whether body size has any selective effect from your proposed mechanisms. There are 4 tests, you'd need to do to establish the hypothesis. First you have to test whether the opposite sex finds your proposed trait attractive. For example, inject half the animals with growth hormone so they become large and see if the opposite sex prefers them or not. Second, stunt the growth of half the animals so they become small and again test to see whether the opposite sex actually finds them attractive or not. It's possible the opposite sex won't even notice the difference much less make mating choices based on them. You need two more experiments to establish the cost of the trait in natural selection. Test whether larger animals have more trouble surviving harsh winter by measuring the amount of food they each eat and seeing if animals that eat less food actually suffer health consequences. Maybe they just hibernate an extra week and everything is fine. Similarly with the small animals, test to see if small animals really do get caught and eaten by predators more often. Only once you have shown that the trait of interest has a negative effect on the animal's survival but a positive effect on its mating chances, does the handicap principle come into consideration as a possibility.

Unfortunately, before you can come close to concluding that's the reason, you have to look at all the other possibilities. For example, maybe the reason larger animals have better mating success isn't because they signal being healthy enough to compensate for their size disadvantage. Instead, if animals grow large BECAUSE they eat lot's of food, then perhaps the opposite sex will select them for mating because the animal sends a signal that they are good at finding food and thus will find a lot of food for the children they have together. There is often a strong incentive to find a mate that can find enough food for your children. There could also be genetic linkage between traits that control size and traits that control some other gene of interest. For example, if the size genes are next to the genes for pheromone emissions, then maybe it's completely by luck that the larger animals also have genes that emit more mating pheromones to attract mates, and actually the opposite sex is selecting on that. There could also be linkage between the gene for body size and gene for mate preference. I don't want to write a super long explanation, so look up "runaway sexual selection." And of course, there are many, many other possibilities. For example, the sexy sons hypothesis posits that females will select sexy males not because they find the males attractive, but because other females do. And they want their own male offspring to be found attractive, so therefore they decide to mate with a male that is considered handsome so that her sons which carry her genes will be able to mate a lot. And there's a lot of complexity with any specific real world example.

So this brings me to my second answer which is that while it's fun to think about these concepts, and its certainly useful to study these things in terms of being able to explain the natural world. That is, find evidence for the hypotheses, try to find examples, etc. At the end of the day, I wouldn't put too much stock into these types of considerations. It takes a lot of work than can actually be done in most cases to prove one specific reason is why a species has its mating habits, and once you do prove it, so what? It was all just random to begin with, and other times, other random things will happen, so its not too impressive to say "hey, this time the random thing that happened is the handicap principle because of all these interesting reasons, circumstances, and coincidences." Biology is a very unsophisticated science, and evolution (my area of expertise) is probably the least sophisticated of all the fields within biology, so as long as you don't buy into these theories hook line and sinker, you can have a lot of fun bullshitting about what might or might not be and even trying to actually test these things when possible (which it usually isn't; I mean who's cruel enough to inject animals with growth hormone just to see if the opposite sex finds them more attractive?)

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .