8.9 Understanding Hermaphroditism

If we exclude insects, then 33% of all animal species and about 94% of flowering plants are hermaphroditic. Recall how common the purple color was in the phylogenetic tree in Figure 8.21 in Section 8.5. If hermaphroditism is so common, then there are likely evolutionary benefits to this strategy.

You can probably imagine a few benefits of hermaphroditism (having the ability to produce both eggs and sperm). For one, any individual could mate with any other individual in the population (this is called outcrossing), expanding the pool of potential mates and minimizing the effort needed to find an individual that you can mate with. Also, if a hermaphrodite could not find any mate, it could fertilize its own eggs (this is called selfing). Furthermore, selfing hermaphrodites save a lot of energy that would otherwise be spent seeking and acquiring a mate.

So why don’t more animal species have a hermaphrodite strategy for sex determination? You could answer this question by focusing on the downside of hermaphroditism or on the positives for a system with separate sexes.

An important takeaway, for any given species, is that evolution by natural selection results in organisms that are more fit for their environment. In other words, the strategy that leads to the most offspring is the strategy that will prevail for a given species in a given environment. If the environment changes then the strategy that is best suited for maximum offspring may be different.

Biology is Sexy

Penis Fencing Flatworms

The above video shows two ocean-dwelling hermaphroditic flatworms fighting to inseminate the other by stabbing the other flatworm with their penises (called penis fencing). In the end, the one who gets stabbed is now pregnant and has to find food – while the other gets to go in search of another mate.

 

^[1]

We have discussed some costs to hermaphroditism, but how would a hermaphroditic species subsequently evolve to have distinct sexes? You may recall that the cells of animals and plants have organelles, small compartments inside a cell that have a function. Some of these organelles, mitochondria found in plants and animals and chloroplast found in plants, contain DNA. Organelles are inherited almost entirely from the egg. As a result, there are genes in mitochondria and chloroplasts that can suppress sperm production in favor of egg production so that those genes continue to replicate.

Imagine a population where this has occurred and there are many individuals who are only producing eggs, because sperm production has been suppressed by the mitochondria. This means an individual that can make sperm will be at a reproductive advantage; all the offspring in this population would carry this individual’s genes. If this is the case, spending energy only on sperm production would increase this individual’s fitness. This is an example of negative frequency-dependent selection, which is when being a rare type in a population is an advantage. In this case, if there is an excess of egg-producing individuals, any organism that disproportionately produces sperm will be at an advantage. This is one hypothesis for the evolution of distinct sexes ^[2].