This is about those many species where males fight it out for mating rights, in battles which depend mostly on size and strength. You would think that the results of this would be that paternal genetic input would mostly be from the higher end of the size/strength distribution. Even allowing for some regression to the mean, ISTM that the net impact should be a constantly increasing size and strength for the offspring of each succeeding generation, which should manifest itself into significant change over a relatively short time. But I’ve never heard of this being a factor.
It already happened. But there’s a limit to how big/strong you can be and still survive, and such species are pretty close to that limit.
Everything in evolution is a tradeoff, which leads to equilibrium positions. Factors like competition for mates may shift the position of those equilibria, but there will always be an equilibrium.
I suspect it’s to make sex with the females mechanically feasible, so females would necessarily become larger too. This would require more resources that may be restricted; or the species may be put on a disadvantage if they cannot also develop adaptations that also allow them to defend against predators that’d normally not notice most if any of them.
There’s also the square cube law that would require a suite of changes just to get larger than they already are. There might be some barrier that would not allow that degree of sexual dimorphism, at least in vertebrates.
These responses seem to address the issue at a species level, but the mechanism operates on an individual level.
The implication then would seem to be that individuals who are larger/stronger are at a survival/reproduction disadvantage which offsets their advantage in mating competitions. Is this correct?
Keep in mind when discussing these evolutionary situations (or any) - every advantage has a cost. Too big, and you are likely to starve to death during shortages (drought, cold winter), or spend too much time grazing to get around to fighting and fu… reproducing. Fun fact I read, that chimps spend most of their day chewing on the grasses and other low-value plant they consume - we only got to be smart when we started eating meat (eating meat equals smart!) because of the high concentration of protein in a small package; and cooking to remove the need to chew too much accelerated the process. Resource limitations - being trapped on a tiny island - is probably what produced the pygmy elephants of Malta. The big ones die first during bad times.
I knew a fellow who was over 300lb (but not muscle). His children were born in the 11-lb range. His wife was a normal thin lady when she started. By the third one, she was pushing 300lb too. It would seem to me that animals where papa is pushing T-Rex size would need to have matching females thus simply expanding the scale of the species. The size of the species has already stabilized at the best trade-off to available food, need to weather climate, competition and predation, etc. Particularly, a grazing herd species relies on the offspring being able to walk right out of the womb, so unlike a nesting mammal like a bear or human, or even a kangaroo, they cannot simply give birth to relatively tiny immature child and then wait for it to be moderately self-mobile. The female must be capable of birthing a mobile male - so that limits the size of the male. If the male is too grow too fast, it subtracts from milk available to siblings, and also then needs to eat far faster than they do now. Indeed, one of the limiting factors of human brain size is the ability to fit through female hips (but, our brains are plenty big enough considering many don’t use what they have). Female hips have probably reached the limit of how they can be, without presenting too wide a bridge that the spine weight would break the hip cradle.
That’s what I was going to say- there’s probably some sort of cap- more food, not as fast, etc… that keeps males from being larger than they are. With humans, it seems to be not a whole lot taller than 6’. Believe it or not, 6’1" is like the 87th percentile for grown men in the US, and is a tad higher in most other countries.
The answer is basically the same with regard to any other trait where it would seem more would be better. Why don’t cheetahs run even faster than they do, or why aren’t giraffes even taller than they are, etc. There’s always a trade off. You may not be able to find enough food to become bigger, or being bigger means you’re clumsier and at a disadvantage in a fight, or a more tempting target for your predators. Just because the selection affects only one sex doesn’t change the basic principle.
The question was not about why animals are not faster/taller than they are. The question was about the rate of change. Or as the OP put it, that it “should manifest itself into significant change over a relatively short time”. IOW, it’s possible that cheetahs are constantly increasing in speed and giraffes in height etc. but that the survival advantage of being marginally faster or taller is so small that the increase is virtually imperceptible over a noticeable timeline, and also more easily offset by trade-offs. But in the case of mate selection ISTM that the impact of only a relatively small cohort siring the next generation would magnify that advantage based on a reasonable size distribution, which would make the rate of change much faster and also harder to overcome due to trade-offs.
To put it in human terms, where the average height of an adult male human is 5’ 9"; imagine that the average height of those who sired offspring were 6’. Even after allowing for a 50% offset due to regression for the mean and a further 50% offset due to the dilution of maternal height remaining average, that would still imply a 1% increase in average height each generation. Seems like a lot.
I recall reading that exaggerated features selected for in males are often detrimental to survival! Deer don’t need big antlers; peacocks don’t need huge feathers. These exaggerated traits are, in effect, signalling “See? I’m so healthy and robust that I can thrive even with this (beautiful) handicap!”
Sexual selection is often described in that way, but it makes no sense. Mates don’t care about “healthy and robust”; they care about total fitness, and a decrease in fitness is a decrease in fitness.
To the extent that a peacock is sending an articulable message, it’s “See how beautiful I am? If you mate with me, you’ll have beautiful sons who are also capable of attracting the choicest mates, and have many grandchicks.”.
This doesn’t make a lot of sense. Why does the potential rate of change matter? You still run up against the same constraints. Over the long run, it doesn’t matter if you hit the limits in two generations or one hundred. You will still end up the same size or strength.
If the rate of change is extremely small, it’s possible that in fact the species is increasing but that the rate of change is so small that it’s imperceptible. So the question “why don’t we see them changing?” has no basis. But if the process would seem to produce a perceptible rate of change, then the question arises.
[Separate from the above, I had thought there may be a difference in this regard between a survival advantage being offset by a survival disadvantage and a mating advantage being offset by a survival disadvantage. But on further reflection, it’s possible that my thinking was muddled in this regard. I still think the dynamic would be different - in that it would imply that a species could actually be above their optimum size & strength in terms of survival but be at the point of equilibrium where the survival disadvantage is finally big enough to cancel out the mating advantage. But the overall equation could still hold.]
The rate of change is limited by the rate at which beneficial mutations crop up.
Now we’re getting somewhere.
Sexually selected traits could potentially spread throughout a population faster than other traits. However, in most cases such traits (like other traits) are already near the limits imposed by environmental and other constraints. Only occasionally and for a few species will constraints be relaxed (such as obtaining a new food source, or the disappearance of a predator) that a population could shift appreciably. We don’t see such changes because they are rare, and we are not observing populations in evolutionary time.
OK, but the logical implication of this is that for all such species the optimum size for survival is (slightly) below the current average size.
The optimum size for survival may change over time due to environmental and competitive factors. You can’t consider a species in isolation from a specific environment.
I don’t think this is correct. Any such species may or may not currently be at its optimal size for survival. Here we are talking about survival of the species as a whole, which includes all of the things an animal needs to succeed at in the time between birth and mating.
Of course. In fact, it may be much lower. But “survival” is only one component of reproductive success. A big male may have a life expectancy much lower than a little runt. But if he gets all the matings and the little guy gets none, his genes will be passed on and the little guy’s won’t. So it doesn’t matter if he doesn’t live as long.
Bear in mind that in species where males compete for females there may be two alternative mating strategies. Large males may dominate others in direct competition for females. However, in some species a small morph exists (sometimes resembling females) that sneaks in and steals matings with females while the dominant male is occupied. This was dubbed the “sneaky fucker” strategy (I am not making this up) by the respected evolutionary biologist John Maynard Smith.
That’s an important point too - evolution has to be moderately slow to allow everything to catch up. There are people well over 6’0" but the rest of the body has to evolve to handle that- Kobe Bryant, for example, had to quit because his knees couldn’t take it. Our skeletons are scaled for 5’10" not 7’0". Double the size of a person, the area of their knee joint increases by 4x, but their weight increases by 8x. So even a 20% increase in scale can be long-term detrimental.
Similarly, we’ve bred elegant thin domesticated horses from the small ponies originally found wandering the plains of east Europe/Asia. their bones did not scale proportionately (except maybe Clydesdales) so breaking a leg is always a risk.
What happens with natural evolution is that the pieces scale as demanded, through the basic process of “not good enough, you die early”. humans short-circuit this process, by limiting for example, the need for horses to run frequently from predators, by selecting for breeding based on appearance not suitability to avoid fractures (except in extreme cases). Between crutches and laws prohibiting big guys from raping the girlfriends of small guys, there is less evolutionary selection for healthy and larger humans. (and absent armies of eunuchs to guard the harem or the isolation of a Great Salt Lake, most societies don’t have highly polygamous marriage arrangements for the males in general.)
For mother nature, we have examples like elephants, where the bones are appropriately scaled; but again, speed and endurance are limited compared to a cheetah or deer. Except for elephants in the tropics where food is abundant much of the year, there appears to be a size limit on grazers -and note the bigger ones, like cows, oxen, etc. seem to be fighters, not runners - there’s a limit factor to size and speed. (and which defence is dictated by the type of predator).
Perhaps the closest to what the OP suggests would be some seals, where the bull can significantly outweigh the typical cow by a huge amount. Again, there is less demand that the skeleton support a different weight is less of an issue in water, and as for food, your mother will happily tell you there are plenty of fish in the sea. The seal cubs do not have to be as agile-mobile as newborn deer or antelope, so can be born fairly small and grow. Seals tend to beach in areas isolated from land predators, giving the cubs time to grow and get mobile.