Sexual selection

Nonlin.org has a burning desire to discuss sexual selection. So, let’s.

The topic was tangential (though not entirely unrelated) to the evolutionary psychology thread. Nonlin was unable to restrain his contempt for his interlocutors, and so his final missive was guanoed. The opportunity to repost was declined, as guanoing is ‘censorship’ (in plain sight!) and a ‘white flag’ allowing nonlin to declare victory and strut around like … well … a peacock.

Now, we can’t expect too much from nonlin; they have no grounding in genetics, a distaste for the entire concept of selection, and indeed insist that genetics and evolution bear no relation to one another(!). But, I’m a compulsive responder. I realise not everyone finds these exchanges edifying.

Selection occurs when variants in a population are sorted by an environmental factor, such that one variant increases in frequency (and hence all others decrease) due to their relative performance in the presence of said factor. At the limit, this process results in fixation of a single variant: all other variants are eliminated and the trait becomes a characteristic of the entire species.

It is evident from the above that, if the environment changes in some way, such that there is no longer a factor maintaining the trait, it may itself be eliminated, since there is no longer a reward (in the currency of survival and reproduction) for its possession.

Now, ‘the environment’ here is not simply the inanimate portion. Other species form part of the environment of any given species, such that, for example, predators and prey may each undergo adaptations tuned by the presence of the other. Remove the predators and birds may become flightless, or camouflage become unnecessary.

In the specific case of sexual selection, the ‘environment’ is provided by members of the same species. Given that, in obligate outcrossing sexual species, mating partners are a vital part of the reproductive chain, the potential arises for a feature to be maintained by mate preference. Assuming this ‘preference’ itself to be under genetic control, then both preference and trait are passed into the genomes of both sons and daughters. The genes are only expressed in one gender, but carried by both. Mothers may pass the preference for large-tailed mates to their daughters, but also, cryptically, to her sons and hence to their daughters too. Likewise, her mate’s large tail genes may pass to both her sons and her daughters’ sons.

Now, a potential conflict arises with other modes of selection. The feature – say, the elaborate tails of male peacocks – may hinder the animal in the more general challenges of survival. Nonlin astutely observes that, without the layer of sexual selection, the feature would not exist: it is maladaptive. This is hardly fatal to the theory, however. It is entirely in accord with it; selection depends on environment, which includes both predators and mates. Change that environment – remove female preference – and losses to predation would likely see elimination of the feature (amusingly, nonlin here is making an argument for selection, not against it). Nonetheless, given coexistence of feature and preference, it requires only that losses to predation are offset by greater gains in mating success. If big-tail males get more action despite the cumbersome tail, that is sufficient. Indeed, it can be seen as a mark of general health and quality; the passing of various tests before a suitor can win the hand (or wing) of fair lady is a classic of literature! To complete the analogy, we need only imagine the story littered with the corpses of the failures …

Here, shorn of most of its bile, is the guanoed quote:

Me: That ‘preference’ [of females for certain male qualities] generates a competition.

Nonlin: False. The peacock is born the way he is. There’s nothing more he can do.

Nonetheless, there is a competition between different males, ‘born the way they are’, for mates.

Furthermore, that’s why we have definitions (as bad as they are), so people like you don’t start making up stuff. Look up “sexual selection”.

Nothing to say: I just left that in as a classic example of reflexive internet blowhardery. ‘Look up X’. I saw an exchange where someone tried to mansplain the Good Friday Agreement in similar fashion. Her classic response was to post a picture of her grinning, holding up her book entitled “The Good Friday Agreement”.

Me: The genes involved in mate preference are not the same genes as those involved in generating elaborate tails.

Nonlin: The genome is a package. So yes, it selects itself which is self referential and stupid. Even “natural selection” is not that stupid in this limited sense (fully stupid otherwise).

There is no conflict between ‘the genome is a package’ and ‘one part of the genome acts upon another’. That action can be intracellular – for example, DNA polymerisation involves precisely that, so I suppose replication is self-referential and stupid. But it can also involve actions at a distance, between different instances of the genome. The genomes of male and female are indeed separate genomic ‘packages’.

ETA – it’s worth mentioning the rival, Design, explanation in this context. We are presumably to suppose that female ‘choice’, and the chosen feature, were both designed. Why? Firstly, we’d have to asked if Designed Big Tails do better in mating, despite their costs. If they don’t, they’re just a whim – a presumed costly feature added for funzies. If they do, the objection to net selection disappears, and we have the extra question of why females were ‘designed’ to have that filtering preference. It seems an odd thing to do, pitting design against design for no general benefit, assuming (as is generally the Assumption in our Christian-dominated halls) a single designer.

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27 thoughts on “Sexual selection

  1. Selection occurs when variants in a population are sorted by an environmental factor, such that one variant increases in frequency (and hence all others decrease) due to their relative performance in the presence of said factor.

    Presumably, the total adds up to 100% irrespective of population size?

    😉

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  2. Alan Fox: Presumably, the total adds up to 100% irrespective of population size?

    Heh. Making a sly reference to that very statistical fact did cross my mind as I wrote that!

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  3. Selection occurs when variants in a population are sorted by an environmental factor, such that one variant increases in frequency (and hence all others decrease) due to their relative performance in the presence of said factor. At the limit, this process results in fixation of a single variant: all other variants are eliminated and the trait becomes a characteristic of the entire species.

    And, presumably, once a population has lost that genetic variation, it is vulnerable to extinction if change in the environment ensues that renders the trait maladaptive.

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  4. Alan Fox: And, presumably, once a population has lost that genetic variation, it is vulnerable to extinction if change in the environment ensues that renders the trait maladaptive.

    To some degree (using ‘maladaptive’ at population, rather than variant, level). But rescue is potentially available in the form of back-mutation, or change in other parts of the genome, favoured in light of the ‘new normal’.

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  5. Change that environment – remove female preference – and losses to predation would likely see elimination of the feature (amusingly, nonlin here is making an argument for selection, not against it).

    I see that bird species where males are contributors to raising young tend not to have marked sexual dimorphism. Doing one or other is advantageous but doing both is exhausting.

    ETA

    https://en.wikipedia.org/wiki/Sexual_dimorphism#Birds

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  6. Alan Fox: I see that bird species where males are contributors to raising young tend not to have marked sexual dimorphism. Doing one or other is advantageous but doing both is exhausting.

    In cuckoos, though, it may be that dimorphism is driven by parasite-host interactions. For example.

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  7. You’re having a nice conversation, I’m just fearing that some self-unaware and obtuse individual will miss each and every point, yet come to make a mess out of the whole thing while insulting everybody involved.

    Not thinking of anyone in particular.

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  8. Entropy:
    You’re having a nice conversation, I’m just fearing that some self-unaware and obtuse individual will miss each and every point, yet come to make a mess out of the whole thing while insulting everybody involved.

    I’m banking on it!

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  9. Hi! I have two dumb questions, but I do mean them sincerely, so I’d appreciate thoughtful responses as people’s time permits.

    1. Is sexual selection a hypothesized mechanism or is there good evidence for it? I was under the impression that sexual selection was hypothesized for cases where natural selection didn’t seem to make sense (e.g. the peacock’s tail), but has it been confirmed?

    2. My understanding is that sexual selection works insofar as organisms can take phenotypic traits of their prospective mating partners as reliable indicators of increased fitness. Is that the case? Or can sexual selection be genuinely antagonistic to fitness?

    Sorry for my dumb questions!

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  10. Kantian Naturalist: 1. Is sexual selection a hypothesized mechanism or is there good evidence for it?

    There are many examples of sexual dimorphism and sexual selection is an evolutionary explanation for what is observed. The Irish elk is an iconic example.

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  11. Kantian Naturalist:
    Hi! I have two dumb questions, but I do mean them sincerely, so I’d appreciate thoughtful responses as people’s time permits.

    1. Is sexual selection a hypothesized mechanism or is there good evidence for it? I was under the impression that sexual selection was hypothesized for cases where natural selection didn’t seem to make sense (e.g. the peacock’s tail), but has it been confirmed?

    It has been confirmed in numerous experimental settings. This doesn’t of course mean that it is definitively the cause of every sexually dimorphic character. Other potential causes exist. But it’s a pretty good bet as a starting hypothesis.

    2. My understanding is that sexual selection works insofar as organisms can take phenotypic traits of their prospective mating partners as reliable indicators of increased fitness. Is that the case?

    This is but one possibility (and may vary in different situations). But yes, a vigorous and cumbersome feature may be a proxy for general ‘genetic quality’. However, the possibility exists that the feature is a simple consequence of a random preference that has become common in females. There is also the issue of species identification to consider. Where similar species overlap, fruitless matings may be avoided if female preference and male distinction are bimodal, independently correlated in each separate species.

    Or can sexual selection be genuinely antagonistic to fitness?

    Fitness is the resultant of survival and reproduction (slightly at odds with ‘survival of the fittest’, which ignores fecundity). A feature under sexual selection can often reduce survival, but persist due to increase in matings. Being brightly coloured is not a great idea when predators are about, but if females like it, that can maintain it in the population. Females are limiting, usually. Males are sperm factories. The species can maintain itself adequately with drab females and brightly coloured males, because even though more of the latter die, the remainder provide enough sperm to cover female requirements.

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  12. The only thing for this YEC to point out is that the extreme degrees to which creatures go to get the best mates for sex shows the fallen world. The world is in decay and so nature desperately struggles to keep ahead of the decay curve. They are fanatical. However it is just to keep ahead of the curve and does not change bodyplans ever. Or prove it. So there is no sexual selection where the result is a bodyplan change. so no evolution. just holding the fort. Darwin and friends were too quick to conclude the real attention led to selection when it only led to health within species.

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  13. Robert Byers: So there is no sexual selection where the result is a bodyplan change. so no evolution. just holding the fort.

    I really wonder what would constitute a “bodyplan change”. Is the evolution of the peacock’s tail or the Irish Elk’s antlers enough to count as bodyplan changes? Or would we need to see, like, the addition of extra legs, or wings, or heads? Or the loss of any of these?

    So help me out, Robert. What sort of change would you consider sufficient to constitute a new body plan?

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  14. Robert Byers,

    It does not require a ‘new body plan’ to count as evolution. Also: are you suggesting everyone was a bit ‘easy’ prior to the Fall? 🤣

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  15. Robert Byers: So there is no sexual selection where the result is a bodyplan change.

    I see you’ve already been asked what you mean by a body-plan change. The animal kingdom (in mainstream biology) is categorized into two groups, protostomes and deuterostomes (which includes us humans). What defines the deuterostomes is how the embryo develops from the zygote (the first cell formed from union of parent egg and sperm) into a ball of cells that develop two openings joined by an inner tube of cells that becomes the gut. Basically, topologically, deuterostomes all have the same doughnut-shaped body plan: an outer layer of cells that form the skin, an inner layer that forms the gut and a middle layer that forms the organs. There is no great body-plan leap between species of deuterostomes. We are all doughnuts.

    ETA

    I may have mentioned this to Robert before!

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  16. Flint: I really wonder what would constitute a “bodyplan change”. Is the evolution of the peacock’s tail or the Irish Elk’s antlers enough to count as bodyplan changes? Or would we need to see, like, the addition of extra legs, or wings, or heads? Or the loss of any of these?

    So help me out, Robert. What sort of change would you consider sufficient to constitute a new body plan?

    Okay. And my favorite is the walrus. Where the male is high but the female is small. A excellewnt case where the dominant male, this from size etc, gets all the girls and so the better genes.
    Yet I would not see it as a bodyplan change but just within the spectrum of that species of how big they can get. Even the girls could get that big or have the big antlers. Hmmm. The big antlers is defining but its still within what that species can do. So its just healthy. not a selection has evolved new bodyplan. I see it still as within its boundaries of the species. no reason to see evolution going on. how would one know?

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  17. Allan Miller:
    Robert Byers,

    It does not require a ‘new body plan’ to count as evolution. Also: are you suggesting everyone was a bit ‘easy’ prior to the Fall?

    I don’t see how evolution goes on if there is no new bodyplan? if you mean hidden genetic differences WELL even thats a new plan however hidden.

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  18. Alan Fox: I see you’ve already been asked what you mean by a body-plan change. The animal kingdom (in mainstream biology) is categorized into two groups, protostomes and deuterostomes (which includes us humans). What defines the deuterostomes is how the embryo develops from the zygote (the first cell formed from union of parent egg and sperm) into a ball of cells that develop two openings joined by an inner tube of cells that becomes the gut. Basically, topologically, deuterostomes all have the same doughnut-shaped body plan: an outer layer of cells that form the skin, an inner layer that forms the gut and a middle layer that forms the organs. There is no great body-plan leap between species of deuterostomes. We are all doughnuts.

    ETA

    I may have mentioned this to Robert before!

    These classification systems are wrong and probably based on evolutionist presumptions. I don’t see yopur point here anyways relative to rejection of sexual selection as a evolutionary tool. I see creationism should see the mating choices as just desperation to maintain health in biology due to a decaying world. keep ahead of the curve. however this real agenda must not be said to be evidence for bodyplan changes from evolution from sexual selection. thats just guessing and extrapolation without good evidence. another hunch that convinces people without investigation.

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  19. Kantian Naturalist: My understanding is that sexual selection works insofar as organisms can take phenotypic traits of their prospective mating partners as reliable indicators of increased fitness. Is that the case?

    What you describe is known as the “Handicap Principle” as proposed by Amotz Zahavi. However, the trait need not necessarily be a fitness indicator. It has been shown in theoretical models that an initial female preference for some conspicious trait in males can start a runaway process in which the male ornaments will keep getting more and more elaborate until the mating benefit is counterbalanced by negative selection against the ornament.

    To understand this process you must see what the benefit is for both the male ornament and the female preference. For the males it is easy: more conspicious ornaments incur a mating advantage compared to dull competitors, but why does the female preference spread? The solution is that since choosy females preferentially mate with ornamented males, they tend to get sexy sons! In other words, the female preference hitchhikes on the success it is itself responsible for. Since neither the preference nor the ornament are associated with any survival or reproductive benefit, the process tends to be bad for population fitness. It is just like fashion.

    This is called a “Fisherian runaway process“, called after the famous Ronald Fisher, who first conceived of this scenario.

    Kantian Naturalist: Or can sexual selection be genuinely antagonistic to fitness?

    Absolutely! An important aspect of sexual selection is that there is potentially a conflict in the interests of both sexes, as highlighted in the female choice example above. This conflict can manifest in bizarre behaviour. A famous example is traumatic insemination in bedbugs, where the male bypasses the female screening process by simply delivering its semen through piercing the abdomen wall of the female.

    Less anecdotical, Drosophila male seminal fluids are known to contain a number of proteins that benefit male fertilization success but are toxic to females. The Bill Rice lab did some seminal experiments in Drosophila demonstrating that removal of sexual conflict by enforcing monogamy resulted in the evolution of reduced male-inflicted harm and a greater net reproductive rate.

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  20. Robert Byers: The only thing for this YEC to point out is that the extreme degrees to which creatures go to get the best mates for sex shows the fallen world.

    Not to mention those naughty plants dangling their genitals around. Filthy!

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  21. Robert Byers: I don’t see how evolution goes on if there is no new bodyplan? if you mean hidden genetic differences WELL even thats a new plan however hidden.

    If you want to call, for example, a single genetic difference a ‘new bodyplan’, that is your prerogative. No-one is likely to follow you in this usage.

    In the context of sexual selection, is a longer tail in the male a ‘new bodyplan’? More bizarrely, what would we call the genetically-based preference for such tails in the female? ‘Bodyplan’ does have legitimate traction as a term in biology, but its misuse in Creationist circles is good for little but a laugh.

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