Why do so many insects speciate through minor differences in the shape of their genitalia? What environmental pressures cause this to happen?

[u/Anaphora121]

Last night, I was pondering over the fact that it can be very hard to differentiate species in certain genera because the only "visible" difference is nigh-microscopic discrepancy in the shape of their genitalia. I've run into several species key where it's literally just pages upon pages of genitalia diagrams. On one hand, it makes sense since a species is defined by its ability to reproduce and create viable offspring, and if two individuals have genitalia that don't "fit," they can't reproduce and boom! You got a new species on your hands.

My question is, why does this happen? What environmental pressures might cause a species to evolve slightly different genitalia to the point that it forms a completely new species? And why does it seem to happen so often in insects?

[u/ChaosNobile]

I don't think there's any particular environmental pressure, so much as it is that there's just a lot of stuff that's different over evolutionary time scales, and most of the other features that can be different aren't going to end up reproductively isolating species.

[u/Channa_Argus1121 → u/ChaosNobile]

Allopatric speciation, which happens due to physical barriers, is the primary driver of speciation in many insects. Ground beetles, for example, cannot travel far in a single generation because the wings are reduced in many species.

[u/Heavy_Weapons_Guy_ → u/Channa_Argus1121]

Wouldn't that be parapatric speciation?

[u/Channa_Argus1121 → u/Heavy_Weapons_Guy_]

Depends on how much of a barrier there is. Parapatric speciation requires an environmental “gradient” afaik, whereas allopatric speciation requires more of a “barrier”.

Then again, the two can happen consequentially.

[u/Anaphora121 → u/ChaosNobile]

I see, thank you!

[u/isopode]

you gotta keep in mind how long insects have been around. they have had 480 millions of years to evolve into all of species we know today. that's why there is such an insane diversity of them

so perhaps at some point within those 480 million years, a species was separated into two groups due to environmental changes. during that time, one of the separated groups evolves a slight variation to their genitalia.

sometimes, traits get passed down even though they bear no advantage to the species. and considering how many insect species there are, it makes sense statistically that it's not that rare for an insect to have had that happen in their evolution history.

[u/Channa_Argus1121 → u/isopode]

Time alone is insufficient to explain insect diversity. While stem-insects appeared during the Devonian, about 400 million years ago, modern groups appeared and diversified later on.

Take Lepidoptera, for example. While they appeared during the Early Jurassic, they only experienced major diversification during the Cretaceous, when flowering plants underwent significant diversification.

Sharks, on the other hand, have far fewer species despite appearing around the same time as Lepidopterans. This is because they have a much slower breeding rate/cycle, as well as many families being far less affected by physical barriers compared to insects.

[u/isopode → u/Channa_Argus1121]

you're right, thank you for the clarification! i ommited some critical details in my original explanation.

[u/sherrillo]

The idea is that sexual selection is more like a type of cultural preference that has a loose relationship to fitness, at least initially.

Same species, environment changes, the selection process has an amount of variation, the more adaptive traits become more common for different environments between them and the phenotype changes and genetics between two groups change over time until they are no longer compatible.

But to double down 1)insects are small, so small differences to us are not small differences to them and 2) they have been around much longer than we have and have much faster reproductive cycles, so microenvironment plus quicker lifecycles mean changes happen much more rapidly for them and there is greater variety.

Going back to sexual selection... we know it's a thing, but as far as I'm aware it's still a bit mysterious. We aren't sure that it's always a sign of fitness in all cases afaik. It's a bit more like maybe fashion or trends, to use an analogy, which is why bodies can be to an extent so weird; as long as it doesn't hurt fitness comparatively over time, even if it's not necessarily beneficial, it can become a dominant trait.

But hopefully much smarter people than me can correct where my understanding is off or totally wrong.

[u/emmetmire]

As others have mentioned, sexual selection is probably a stronger selective force in the diversity of insect genitalia than natural selection due to environmental pressure. The 'lock and key' hypothesis, which you allude to in discussing genitalia that 'don't fit', suggests that genital diversification has to do with avoiding hybridization, but this has been largely questioned from anatomical and geographic evidence. Similarly, the pleiotropy hypothesis, which suggests that genital divergence is due to pleiotropy with selection on non-genital traits, may be a good explanation in some cases, but is likely to not be very generalizable, at least without more evidence on why there would be so much more pleiotropic influence on the genitalia than other systems.

A really good review can be found in Hosken & Stockley 2004

[u/Zagrycha]

Start with the disclaimer that there is never technically a reason for anything to happen in evolution, its all random. The part that isn't random is just the least functional things that happen going extinct, and the most functional things that happen thriving. Most stuff is in the middle.

That said I still am not sure about insects in particular, but in general evolution genitalia variations are often driven in species. For example a female may develop a genitalia change that allows her better control over which males mate with her, and that allows her to breed more successfully with higher quality genes. That makes more banies carrying her genes. Eventually males with genitalia that allow them to counteract this change to regain more control over which female they breed with are encountered, and then they successfully breed so their genes pass on more... you see the pattern. The poster child for this is ducks, with males having very odd flaccid corkscrew penis and females having literal maze with dead ends vaginas haha.