This is probally outside your expertise but Ive been having trouble finding an answer. What is the evolutionary advantage of 'male' and 'female'? (sperm/ equivalent distributor and egg bearer or whatever the definition is) Why is hermaphroditism not the norm cus wouldnt it be more optimal if you can breed with any mature member of your species instead of just (at best) half? Esp for loner animals that rarely meet. Most of all itd double the offspring if both parties fertilize each other
It’s not exactly my expertise, but it is the sort of thing the LSB and I might discuss as pillow talk. We were bio nerds. Technically still are, I guess.
The facts are that nobody probably knows for certain, but having two sexes (dioecy) predominates in the animal kingdom. It’s not the case in plants, fungi, or single celled organisms. Also once Dioecy has evolved, or hermaphrodititsm has been lost, in an evolutionary history, it’s difficult for a population to go back.
It should also go without saying, but I wills say it anyway because this is Tumblr, that when discussing hermaphrodites in this context it’s not about humans or intersex individuals, but rather about species in general.
We can assume based on the scientific knowledge already available, that the first cells weren’t divided into male and female, they were just single cells, doing their thing. And at some point they evolved into multi-cellular organisms to better gather nutrients and avoid becoming dead. Not all the cells within a multi-cellular organism reproduce, only a few go on to become what we know as gametes (the sex cells) and zygotes (the combination of two compatible sex cells).
Different species may also have different shapes and strategies for those gametes, too, not everything looks like a sperm, but a large, stationary, nutrient rich one (receiver) and a highly mobile one (donor) is a common and effective strategy, but it’s not the only one. You can get isogamous fungi species (only one type of gamete, and it’s all the same).
So if we’re talking about organisms, and you can assume I’m talking about animals from this point on as plants are not really my thing,
So if we have male and female gametes and their associated support systems, seems like a reasonable default to have both systems in an individual. And embryology of mammals suggests this, sometimes you get remnant cysts. There are duct systems for both male and female reproductive tracts in embryos, Wolffian and Mullerian, but one typically regresses during development. So that implies that at least mammal species (I just don’t know enough embryology for other groups) have hermaphroditism in their evolutionary history. So why lose it?
Hermaphroditism is an advantage for species which don’t encounter each other with much frequency, either because they don’t move very far or very much, and don’t put a lot of energy into raising their young. So for some species, like snails and earthworms, there’s an advantage. But it comes with disadvantages too.
It takes extra energy to maintain two reproductive tracts and you need to stop self-fertilization or you’re going to end up with marked inbreeding and a loss of genetic diversity fairly quickly. Some fish solve this with sequential hermaphroditism, where they are both male and female, but not at the same time. Others are just one or the other.
Now let’s say, hypothetically, you have a hermaphrodite species and one individual has a mutation which renders the female tract non-functional. It can now spend that energy it would have spent on the female reproductive tract on other ways, and if it’s something that permits it to encounter more mates (mobility, increased lifespan, better immune system, etc) then it will potentially be at a reproductive advantage and that mutation will propagate through the population.
And when you have a population that consists of hermaphrodites and males, there is now an advantage to having only a functional female reproductive tract, as you have plenty of mates around, but it is more difficult to compete with males for other mating opportunities. So it may be more efficient to have organisms that are either male or female as far as reproductive strategies go. This also allows more room for evolution of diverse form and function, and behavior.
So it’s about competing with members of the same species, and this strategy becomes more viable with a larger population, and even more with increased energy investments in offspring. Basically, if you want to think of it this way, something ‘cheated’ the hermaphrodite system and became male, and them female developed as a response to that ‘cheating’.
It’s a kind of game theory. New strategies develop to adapt to the dominant strategy in play. If everyone’s putting dragonites in Pokemon Go gyms, you level up your ice types to take them down.
Though my interest in evolution is more ‘casually enthused scientist’ these days rather than professional, an actual evolutionary biologist or embryologist might have more to say.
