I am currently studying the Stickleback Fish which has a tendency to do a mating ritual. One of the steps requires the fish to dance in front of his partner (zig-zag back and forth). Sexual selection implies “By providing more chances to mate, it ensures that genes related to skillful execution of the dance are more represented in the next generation” (Russem et. al 2023).
If this has been occurring for multiple generations, how is it, not a normal standard, wouldn’t this part of the process of selection be redundant? Is their proof that it has changed over time for other animals?
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Sexual selection still works, because no animal or plant is immortal: they/we all still depend on finding mates who’ll say “yes.” For millions of years, lifespans have been measured in years or decades — meaning that the species goes on for millions of years only if mortal short-lived individuals still have sex every few years or decades, and they generally exercise some agency in doing so, thus affecting what the species looks like in the next round and next thousand rounds.
Because most of the mating displays involve traits that are also survival characteristics. A Stickleback fish who is stronger and more agile than another will put on a better display and be more likely to mate. A bird who is a great hunter will have a healthier and more vibrant plumage, making it more likely to mate. Ditto for a Buck, the better it forages the bigger the horns grow, giving it both a survival and sexual advantage.
As to why it never stops? Because we don’t know of any ideal forms yet. Sometimes nature will jump ahead and something will be zoologically stable for millennia, see things like alligators and sharks, but in time even they get overtaken (see us driving the prior into extinction).
And if you want to get really bonkers, consider the evolutionary race of things like bacteria that have generations measured in hours and not decades.
Couple things:
1. In some cases, the bar is always rising and what was good enough several generations ago is now a bottom of the barrel pick. If you’ve ever seen those comparison videos of early Olympic gymnastics routines that took gold vs the routines necessary now to take home the gold… it can be a bit like that.
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2. Animals unable to meet those standards are still born. Sexual selection is only part of what causes gene variance. One pair can mate several times over and their offspring will still have gene variance. The specific genes passed on are randomized and there’s always the chance of gene mutation. Choosing the best mate is an attempt to have successful offspring, but the chance those offspring will be lacking desired traits is still present.
Just because your father was really good at dancing, doesn’t mean you will be too… and to make things worse, someone else comes along with a gene mutation that makes them even better at dancing than your father was!
If there is variability in the trait, then there will always be some individuals who are really bad and some that are excellent– think about it like a bell curve; most individuals are average but there are extreme outliers on either side. Over time, you might expect the entire species to move the average towards the “best” dance. Except that…
There may be a trade-off. Maybe the fish that dance the best are also more likely to be spotted by predators and eaten. Maybe every few years, their food source wanes and the big fancy swimmers waste too much energy dancing and die before mating. Those trade-offs would likely maintain diversity in the gene pool because occasionally it’s good to be a bad dancer.
In addition to the trait being beneficial and detrimental in different environments/circumstances, another reason for variation could be the gene for dancing is linked to another gene where variation is helpful to the species, like age at maturity or metabolic rate. If the genes are highly linked, you won’t see the response you expected on the trait you’re studying unless you consider both genes.
Also, there could be random variation because there is no selection on that trait. Maybe all the fish have equal chances of having offspring regardless of their dancing. There are lots of examples where a trait or behavior looks like it’s important for natural selection, but the nice story doesn’t actually match the biology.
There are still other reasons that variation is maintained, like immigration from other populations or random mutations.
Nature is amazing.
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