The Aftermath of Sex (Or Why Males Exist)


Yesterday I wrote about why sexual reproduction is so pervasive throughout several eukaryotic kingdoms of life (e.g., animals, plants, fungi, protozoans). In essence, sexual reproduction exists because it increases genetic variability, allowing natural selection to produce organisms that can adapt to changing environments, parasites, and purge bad mutations. These evolutionary benefits are worth the energetic expenditure required to find a suitable mate to share your genome with. Unfortunately love and companionships are only proximate by-products.

Sex is a complicated form of reproduction. After all, you can’t just share your genome with anyone. In fact, you can only possibly share your genome with about half of all humans. Obviously this is because there are females and males. But why are there two sexes? Do you need two sexes in order to sexually reproduce? And if not, why have most species dichotomized sexual reproduction? In essence, why are there males?!

First question first. You do not need two sexes to sexually reproduce. For example, some fungi are perfectly happy as isogamous organisms. Isogamous means that they can share their sex cells with any other member of the same species. There are no “males” and “females”. That seems like a more effective system than the male/female system where half of the population is genetically off limits. But then why have so many other animals, plants, and protozoans developed a system with males?

It seems shocking to some, but males only exist because of runaway sexual exploitation. Wait… that’s not actually a shock. But how did it happen?

Sometime, probably between 2 and 1 billion years ago, there were early eukaryotic sexual reproducers. These sexual reproducers were likely isogamous, happily sharing genomes with any other member of the same species. In this world there was no a disproportionate burden on any group within a species to provide food reserves.

Obviously this is not the case with species sexually divided into males and females. The very definition of male and female, at its fundamental level, is defined by the differences between gametes. Females have large and relatively few gametes (e.g., eggs). In contrast, males have small and practically infinite gametes (e.g., sperm). As a result of this difference, females overwhelmingly shoulder the burden of providing the biological sustenance for the next generation. It isn’t fair, but biological evolution doesn’t care about fairness.

This system evolved in a exploitative fashion. Because mutation and recombination produce variation, there were bound to be isogametes that varied in size. Larger isogametes would have provided a little more of their fair share of nutrients than smaller isogametes. One might then think that this would lead to a system where isogametes continually evolved larger sizes in order to equally provide offspring with more and more resources. And if it weren’t for co-evolution, that may have occurred.

Smaller isogametes were not “content” to become relics of evolutionary time. Smaller isogametes could not provide an offspring with more resources, but they could be produced in high quantity and move faster than larger isogametes. This produced the first hints of dichotomized sexuality within species. Some members of a species became specialized at producing large and nutrient rich gametes (e.g., eggs), while other members of a species became specialized at producing small and exploitative gametes (e.g., sperm).

In this new increasingly sexually competitive (rather than cooperative) biological landscape, members of the same species became rivals. Large isogametes were too slow and scarce to ever combine with one another. Small isogametes were too fast and numerous. Sexual selection was born. One group within species were “honest”, investing large amounts of energy into the next generation. The other group within species became “exploitative”, investing very little energy. It is for this reason that males are inherently less valuable than females. Females are the scarce resource, because they have the scarce gametes. To better understand this, let’s use a quick thought experiment.

sn-neandertal.jpg (Image Credit /

Imagine you are a Neanderthal living 30,000 years ago on the Iberian peninsula. Due to small group sizes, slowly evolving technology, and interbreeding with modern humans, there are only 20 individuals of your species remaining. Your species, once widespread throughout western and central Eurasia, is now relegated to a cave overlooking the Atlantic Ocean. Let’s assess two extremely divergent demographic patterns that this group could possess:

(1) 18 males; 2 females

(2) 2 males; 18 females

When a species is faced with extinction, sex ratio really matters. Hypothetical population (1) has no chance of survival. There just aren’t enough eggs to continue producing viable offspring. It doesn’t matter that there are a near infinite number of sperm available. There are just too few eggs. The groups fecundity as a whole makes the group evolutionarily obsolete. Inbreeding depression would set in within a few generations and the population would cease to exist.

In hypothetical population (2) there is actually a small chance of survival. Although inbreeding depression would still be a problem, it would not inevitably lead to extinction. In the first generation as many as 18 offspring could be produced at a time. Even though roughly half of these offspring would have the same father, if A) the fathers were genetically distinct enough and B) they enforced cultural rules preventing offspring with the same father from reproducing, they could in principle stave off extinction.

This may seem like a pointless and extreme example. But considering 99% of all species have gone extinct, it is not. Females are biologically more valuable. They have the scarce resource. Males are genetically expendable.

Like my last post about sex, deep knowledge about the evolution of sex (i.e., why we have sex and why it is dichotomized) forces us to confront some interesting questions about the future of sex. In the 20th century we uncovered the mysteries of biological law. In the 21st century we are going to start playing around with biological law. Will females continue to shoulder the entire biological burden of producing the next generation? Not necessarily. There are technologies that could relieve us of the male/female ; sperm/egg sexual reproduction system.

I am throwing these question out there because they are questions that will become widely discussed in mere decades. What do we want our sexual lives to look like? How do we want to reproduce? What do we like about what biological evolution has programmed within us? And what don’t we like? If we could design our sexual lives intelligently at the biological level, what type of sexuality would we like to engage with?

Discuss this on Hubski or let me know what you think on Twitter!


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