Evolution: Sexual Systems in Fish

Why would a fish change sex? What sort of selective pressures would cause such a trait to evolve? Is there any fitness advantage at all? Many researchers subscribe to some form of the size-advantage model (SAM). This model suggests that sex-change is adaptive when a correlation between size (or age) and reproductive success differ between different sexes. That sounds vague and confusing, so let’s break it down a little more. The SAM would tell us that being able to change sex is advantageous when success at reproducing is greater as one sex when small and the other sex when large. Maybe an example will make this a little clearer: Say a species of fish has large females and small males. This species has a mating strategy of very large dominant females mating with a small male. As the male grows and ages, it doesn’t gain any better chances at mating. If it were born a small female, it also wouldn’t have any better chance at mating because the largest females are the ones with access to mates. But being able to mate as a small male, and then changing sex to female when it is large and able to take over the position of a large dominant fish, would give it the best chances at maximizing reproduction. In fact, this hypothetical fish exists. It’s the clownfish! This example shows the basic concept of the SAM. Evolving the ability to change sex is adaptive in fish where mating chances are heavily tied to size and dominance, and where these strategies differ between the sexes. This sounds like a great explanation, but what about bidirectional hermaphrodites? This model is not very explanatory for the rare cases of fish that can change either direction. Bidirectional hermaphrodites usually have sessile lifestyles and live in habitats spread across a range that is dangerous to navigate. For this strategy, evolving sex change can be advantageous in that it allows any two individuals to breed without venturing out in search for a compatible mate. All said, it is important to consider the mating strategy and life history of any given species when trying to construct an explanation of their particular sex system.

So now we have some ideas of what sorts of pressures may push a species to evolve sequential hermaphroditism. That said, do we have any idea of what really happened? Techniques such as genetic sequencing allows researchers to reconstruct evolutionary histories. A reconstruction of the evolutionary relationships between species is called a phylogeny. This is basically a family tree of organisms! This lets scientists compare traits and try to work out where and when they may have occurred. This process has shown scientists that sequential hermaphroditism is polyphyletic. This means that sex change in fish didn’t evolve once in a past ancestor, but has evolved multiple times in lineages that have already diverged from another. This tells us that there must be something useful about sex change for it to evolve several times. This technique also tells scientists something even stranger: sequential hermaphrodites often revert to gonochorism. It appears that though sequential hermaphroditism is adaptive in the short run (short here still meaning millions of years), it may ultimately be unstable as a sexual system. Evolutionary biologists have shown using this technique of molecular phylogenetics, that lineages that evolve sequential hermaphroditism generally switch back or even evolve into simultaneous hermaphrodites. This means that many of the systems of sequential hermaphroditism we see today, driven to evolve by sex size differences and unique mating strategies, may ultimately be evolutionary snapshots on the road back to gonochorism or on the way to simultaneous hermaphroditism.

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