Survival of the prettiest: Sexual selection can be inferred from the fossil record

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Detecting sexual selection in the fossil record is not impossible, according to scientists writing in Trends in Ecology and Evolution this month, co-authored by Dr Darren Naish of the University of Southampton. The term “sexual selection” refers to the evolutionary pressures that relate to a species’ ability to repel rivals, meet mates and pass on genes. We can observe these processes happening in living animals but how do palaeontologists know that sexual selection operated in fossil ones?

Historically, palaeontologists have thought it challenging, even impossible, to recognise sexual selection in extinct animals. Many fossil animals have elaborate crests, horns, frills and other structures that look like they were used in sexual display but it can be difficult to distinguish these structures from those that might play a role in feeding behaviour, escaping predators, controlling body temperature and so on.

However in their review, the scientists argue that clues in the fossil record can indeed be used to infer sexual selection.

“We see much evidence from the fossil record suggesting that sexual selection played a major role in the evolution of many extinct groups,” says Dr Naish, of the University’s Vertebrate Palaeontology Research Group.

“Using observations of modern animal behaviour we can draw analogies with extinct animals and infer how certain features improve success during courtship and breeding.”

Modern examples of sexual selection, where species have evolved certain behaviours or ornamentation that repel rivals and attract members of the opposite sex, include the male peacock’s display of feathers, and the male moose’s antlers for use in clashes during mating season.

Written By: e! Science News
continue to source article at esciencenews.com

10 COMMENTS

  1. What bothered me about sexual selection was how much it relied on individual whim. I can understand that a male trying to woo a female with certain aesthetic tastes would benefit from the arrangement – because it gives him access to her sexually – but not why it would benefit the female, which it would have to do if it was to evolve when the selection pressure grew high. After all, it doesn’t make much difference whether or not a mate merely looks nice, as opposed to whether he’s good at fighting off rivals, supporting the young, defending them from predators, and so on.

    The Zahavi-Grafen handicap principle makes more sense to me. A male has to prove that he can obtain enough nutrition to support his own body, and what better way to do that than to spend the surplus? After all, if you’ve got the ability to find enough nutrients to grow a healthy body and a large bony crest with, then your offspring are likely to inherit those genes that ensure their own success, and a mother doesn’t want to waste time on low-risk investments. If a rival male makes a fake crest with his smaller surplus, then over generations those who break their rivals’ crests while having solid crests of their own will benefit, hence you get the evolution of the unfakeable signal, or one long arms race between new signals and fakes. In turn, females evolve finer powers of discrimination to avoid being exploited or cheated.

    The beauty of this is that it allows any body part – even behavioural repertoires – to evolve anywhere, so long as they are costly to make and show that the bearer is good enough to find food to support such a lavish lifestyle. Moreover, the more useless the growth is, the more it signals its worth as a handicap, since a useful growth would defeat the purpose of proving one can go above and beyond the normal.

    Of course, it can’t go on indefinitely, either because the most extreme examples overstep the boundary and die, or because the body part has technical limitations.

  2. In reply to #1 by Zeuglodon:

    What bothered me about sexual selection was how much it relied on individual whim. I can understand that a male trying to woo a female with certain aesthetic tastes would benefit from the arrangement – because it gives him access to her sexually – but not why it would benefit the female, which it would have to do if it was to evolve when the selection pressure grew high. After all, it doesn’t make much difference whether or not a mate merely looks nice, as opposed to whether he’s good at fighting off rivals, supporting the young, defending them from predators, and so on.

    As you mention, sexual selection can work to a species’ peril, as was the case of the Irish Elk who’s antlers became so big they could hardly lift their heads. Selecting for colorful display can cost a species camouflage. It doesn’t have to be a wise choice, just as there is no intelligence behind mutation. This has me wonder though, if sexual selection is driven by genes, would that amount to the discarded theory of orthogenesis?

    Females who select attractive mates secure their offsprings’ attractiveness, and thus the future propagation of their genes. If it is a male specific trait (like peacocks), that will ensure the daughters of sons will carry on the genes.

  3. I wonder if there is a study on numbers of children born to women who have undergone breast augmentation ? This is of course not inheritable, but perhaps it says something about the wealth of those who undergo this procedure.

  4. In reply to #3 by rod-the-farmer:

    I wonder if there is a study on numbers of children born to women who have undergone breast augmentation ? This is of course not inheritable, but perhaps it says something about the wealth of those who undergo this procedure.

    I cite Desmond Morris because it’s such a sweeping statement, but he once said humans are the only species with expressed mammary glads outside of lactation periods. Achievement of sexual selection? I think so, and that would make our species even more curious with both sexes engineering the other.

    More to your point, it may select for transhumanist tendencies, their offspring more willing to get the synthetic upgrades needed in the William Gibson dystopic future.

  5. It always amazes me; the lengths that organisms will go to to have sex! When I see a characteristic that is somehow ridiculous (male probiscus monkeys noses, male peacock plumage, courtship dances in birds of paradise….etc…), I get my “sexual selection” radar going.

    It is also echoed throughout the sexually reproducing groups that the male is ridiculously adorned while the female is “in a plain brown wrapper”. To think that it is a collective aesthetic that drives a population to have such patently weird rituals, adornments, dances .. etc…. all to “get a good one” when it comes to a mate.

    Makes me wonder if religion evolved due to sexual selection pressures. It is ridiculous, seems random, has silly dances and rituals, the men are heavily adorned while the women are “in plain brown wrappers…” man, I think I am onto something!!!!

  6. In reply to #2 by This Is Not A Meme:

    As you mention, sexual selection can work to a species’ peril, as was the case of the Irish Elk who’s antlers became so big they could hardly lift their heads.

    I can concede that, but I would also point out that this is an issue of body economy. If a species’ design is such that its members spend more resources on building elaborate tails or antlers, the limiting factors are how much can physically be allocated to them (because tails and antlers can’t grow indefinitely, since there’s not enough matter to go around) and when those virtues in reproductive success start to impinge too heavily on survival success (because energy spent making oneself look pretty is energy not spent making oneself fast or strong). In theory, the elks could have survived as a population that got the balance right, and their extinction could be attributed to the arrival of new predators with new tactics.

    Females who select attractive mates secure their offsprings’ attractiveness, and thus the future propagation of their genes. If it is a male specific trait (like peacocks), that will ensure the daughters of sons will carry on the genes.

    This isn’t what I meant. I understand that males who learn to appeal to her whims give themselves a reproductive advantage, and therefore can come to dominate the population, but if multiple females exist with varying beauty requirements, then males could just as easily evolve multiple “beauty spots” to deal with each one individually, thereby maximizing their investment.

    What I don’t understand is how a female with the “looking for beauty” mutation would come to outcompete or outnumber more pragmatic rivals. Suppose the females in a population all judge males based on how strong or fast or intimidating they prove to be. If a mutant arose that judged males based on whether or not they had a large red patch on their chests, she could just as easily mate with a male who had the patch but not the strength or speed of his rivals. The other males in the next generation crowd out his offspring in the next generation because they’re strong or fast enough to avoid getting killed by predators. The only way the mutation could survive would be if it coincided with stronger or faster bodies, but then it opens itself to exploitation when a mutant male arises with the red patch but none of the pragmatic talents.

    So there doesn’t seem to be any benefit to the female having arbitrary aesthetic tastes. This is why I think the handicap principle is more convincing; because it combines pragmatic survival value (a mate who can find enough food to grow healthy and grow a beautiful tail is likely to be a male with good genes that coded for his successful design) with female aesthetics (because the “whim” of the female is ultimately a test for fitness in natural selection terms). It doesn’t matter too much how the male advertisement arises, so long as it shows signs of costliness and is visible to the female.

    Selecting for colorful display can cost a species camouflage. It doesn’t have to be a wise choice, just as there is no intelligence behind mutation. This has me wonder though, if sexual selection is driven by genes, would that amount to the discarded theory of orthogenesis?

    I don’t think it would. Orthogenesis is the notion of a drive that powers evolution, as though organisms wanted to evolve and progress. Females in sexual selection don’t want to evolve and progress; they just want pretty tails. Also, it’s not to be confused with the notion that the desires of individual organisms over time could result in a Baldwin Effect, which would look a bit like the inheritance of acquired characteristics on the surface.

  7. Makes me wonder if religion evolved due to sexual selection pressures. It is ridiculous, seems random, has silly dances and rituals, the men are heavily adorned while the women are “in plain brown wrappers…” man, I think I am onto something!!!!

    I know you were joking, but it does make sense. Religion is great at hijacking pagan rituals and ceremonies so it really isn’t far-fetched to think that they hijacked community ceremonies. Maybe a bonfire dance with singing and later fertility rituals slowly became “mass.”

  8. Could someone tell me why it seems as if the female of most all species are in “brown paper” while it seems to be reversed for humans? Does it have to do with us being a social species? If the human female does not need to be camouflaged for safety, is that reason for the drive to primp and be drawn to adornment? Most females actually dress up to compete or impress other females socially. I assume females are the “drivers” /selectors of Evolution in most species, does it shift to dual selection with more social species? The male now seeks out the most obvious/attractive alpha female of the group?

  9. The sexual habits of humans are odd from a biological point of view in any case. As Desmond Morris points out, females are not only sexually advertising themselves at all times, but can have sex and conceive virtually all year round instead of during a designated mating season. Humans are also among the few species that have casual sex, and human females are unusual in being able to have orgasms similar to the males.

    That said, not everything a female does to her body is in the service of sexual attraction. Females compete with fads, status symbols, and displays of wealth and power and social ties amongst themselves just as much as the males do. Males in general aren’t nearly as fussy over female partners as you seem to think, though getting a noted hottie is likely to score them points with other males and smacks right into beliefs of social hierarchy. And don’t believe too readily that human males are plain; wealth, fashion sense, culture-savviness, charm, expensive commitment, trust-earning abilities, and social power and rebelliousness are simply the new peacock feathers to a social species like us.

    In reply to #8 by QuestioningKat:

    Could someone tell me why it seems as if the female of most all species are in “brown paper” while it seems to be reversed for humans? Does it have to do with us being a social species? If the human female does not need to be camouflaged for safety, is that reason for the drive to primp and be drawn to adornment? Most females actually dress up to compete or impress other females socially. I assume females are the “drivers” /selectors of Evolution in most species, does it shift to dual selection with more social species? The male now seeks out the most obvious/attractive alpha female of the group?

  10. In reply to #8 by QuestioningKat:

    Could someone tell me why it seems as if the female of most all species are in “brown paper” while it seems to be reversed for humans? Does it have to do with us being a social species? If the human female does not need to be camouflaged for safety, is that reason for the drive to primp and be drawn to adornment? Most females actually dress up to compete or impress other females socially. I assume females are the “drivers” /selectors of Evolution in most species, does it shift to dual selection with more social species? The male now seeks out the most obvious/attractive alpha female of the group?

    I’m not sure selection on the grounds of pure aesthetics works for social species like humans, even when it signals spare resources as Zeuglodon pointed out. Once you go down the large brain, long childhood route you end up with young that are helpless and therefore costly and time consuming to rear and other factors have to come into play with mate selection for both sexes. Passing on your genes requires offspring that are born alive and survive long enough to reproduce and that means both males and females who selected mates mates with the intelligence and social skills to rear needy offspring over a long period of time would be favoured over those selecting purely on aesthetics?

    Hence I think both males and females tend more towards things like intelligence, negotiation skills, humour, compatibilty in long term mate choices above looks as they are the things more likely to ensure survival of any young in difficult circumstances. It would be difficult to rear a needy infant, hunt, gather and whatever else is required without appealing to a partner or wider group and that requires complex social skills. Looks are also fairly transitory, so any male or female relying purely on them as a mating strategy knows there are younger fresher models looking to suplant them at any moment. Plus not many of our primitive ancestors would have had the time required to maintain levels of preening. The fact that what attracts a peahen remains unchanged whilst what attracts humans changes, as Zeuglodon says, with fashions and whims suggests it’s more complex.

    What both you and Zeuglodon say about ‘noted hotties’ and attractive/alpha females is also interesting. Whilst getting the noted hottie does raise a males status with other males it often reduces it with other females as they then see him as a shallow and less attractive mate. If there is an obvious age difference quite often he’ll become a figure of ridicule with other females. I would imagine the same holds true for men?So it seems to be a strange trade off situation. I’d agree with your comment that females often dress to impress other females rather than males. Do males preen for the same reasons or is it to attract females?

    There seem to be too many unknowns in sexual selection in humans and human psychological evolution at this point in time I think. I do think sexual selection in humans has the added complicating factors of longer childhoods which possibly became another driver towards social grouping and increasing intelligence and a measure of dual selection?

    I think joking apart that there may be something in the evolution of religion comment as well. Like Zeuglodon says, traits like peacocks tails signify something deeper like enough resources to waste on something functionally useless. Maybe early religion signified the same. A good enough hunter or gatherer to have time to waste doing something functionally useless.

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