Many organisms dedicate their entire adult lives to finding a mate and producing offspring. The rhythms of sex govern the actions and choices of so many animals that it seems to be a rule of biology: Sex is important.
But life’s multifariousness yields some exceptions. A small percentage of animals reproduce asexually, though many of these also resort to sex now and then. These asexual outliers have different techniques for reproducing: Some small invertebrates create offspring by budding, growing small versions of themselves that eventually detach; others, like some corals, can divide themselves in two.
Some fish species need males around only because they require sperm to kickstart their reproductive process, even though they only rarely incorporate any genetic material from those males.
And then there are the parthenogenetic lizards: entirely female species that produce eggs with no males required. These unusual lizards – there are a few dozen such species – avoid many of the pitfalls of sex.
But asexual reproduction comes with its own problems, as evolutionary biologist Sonal Singhal of California State University, Dominguez Hills, and her colleagues describe in an article about parthenogenesis in the 2020 Annual Review of Ecology, Evolution and Systematics.
By studying these lizards, the researchers hope to understand how parthenogenetic lizards evolved the ability to reproduce asexually, and to uncover hidden truths about biology and sex itself. Singhal spoke with Knowable Magazine about what these exceptions to the rule can teach us. This conversation has been edited for length and clarity.
Why study these lizards that don’t have sex?
Most animals reproduce sexually. If we want to understand how sexual reproduction works and why it’s important, it’s hard to study it by looking at animals generally, because everyone’s doing it. So we have to look at the outliers, the organisms that opted out and are doing something different. And, in particular, parthenogenesis is in some ways the most extreme form of asexuality in animals.
Biologists have puzzled over the existence of sexual reproduction for generations. Why? Is there something wrong with sex?
Sex is inefficient, quite simply. From a practical standpoint, you have to find a mate; it takes resources to find a mate. Finding a mate can be costly, because often, when you’re mating, you’re exposing yourself to predators. And it can also be costly because doing all the things to attract a mate can be expensive. You have to invest all that energy into making a peacock’s tail, or whatever other trait you might be using to attract a mate. And then, of course, there’s a risk of catching disease while you’re mating.
On top of that, it’s also inefficient because the rate of population growth is just slower. Let’s say we start out with one female, and let’s assume she has two offspring. In an asexual population, they’re both going to be female. Each of those offspring will have two female offspring. You go from one, to two, to four. So you have this exponential population growth in asexual populations. Whereas in sexual populations, the female produces a male and a female. In your second generation, the male and female will mate, and again, only the female will reproduce. Under those conditions, the sexual population stays the same size.
Of course, this is more of a thought experiment, but it shows why sexual reproduction can be such a downer.
All else being equal, asexual species should reproduce much faster than sexual ones. That’s because in an asexual species, every individual is female and can bear young, while about half of most sexual species’ offspring are males. Considering how common sex is, there must also be some significant benefits to it, evolutionarily speaking.
The main reason why we think sex is good is that it creates variation. If environmental conditions change, sex allows organisms to quickly respond to that change because they have the genetic variation to do so. So that is, on a very fundamental level, why we think sex is good.
Another reason is on a much more microscopic level. Let’s imagine a case where you have a good mutation. And it’s sitting right next to a bad mutation on a chromosome. If there’s no sex, those two mutations are always going to travel together, because they’re on the same chromosome. Mom is going to pass on both the good mutation and the bad mutation to her kid, and so on.
But because of sex, you can actually break apart those two mutations. And that’s through this process called recombination, where the mutations can, through sex, get broken apart, and we can then get a new individual that only has the good mutation.
Natural selection will weed out the individuals with the bad mutation, and then the good mutation can start to increase. Before, they were trapped by each other. When you separate them, they can do their own thing.
- A Knowable Magazine report