Biologists have long believed that it’s adaptive for most species to avoid mate pairings between close kin because of the potential genetic fallout, but a meta-analysis published May 3 in Nature Ecology & Evolution challenges this long-held assumption.
The authors examined nearly 140 experimental studies of inbreeding avoidance conducted on 88 species—everything from fruit flies to humans—and found little evidence that animals on the whole prefer non-relatives.
The inclusion criteria limited the analysis to explicit studies of mate choice, notes Regina Vega-Trejo, an evolutionary biologist at Stockholm University in Sweden and a coauthor of the new paper. Although in the wild, numerous mechanisms can interfere with those choices—such as living in a large, intermingled population where the odds of pairing up with kin are low—the results align with what theoretical models predict: that animals only avoid mating with kin when the costs of inbreeding are high. The finding also bolsters what were previously considered to be unexpected findings of frequent inbreeding or a lack of inbreeding avoidance in some wild populations.
Furthermore, Vega-Trejo and her colleagues found what they consider evidence for publication bias in favor of studies that support kin avoidance, indicating that a distaste for incestuous relationships may be even rarer in animals than their data suggest.
The Scientist spoke with Vega-Trejo about the results.
The Scientist: Why might mating with relatives be an issue for animals?
Regina Vega-Trejo: If you think about how populations are becoming smaller and more fragmented, the fact that animals choose a related mate might mean that the genetic diversity might be lower. . . . Animals mating with a relative, it might not be bad for themselves, but their offspring might be less fertile, or might have a shorter lifespan, for example.
TS: But it really depends, right?
RV-T: Yeah . . . it can depend on whether the animals disperse. If one sex, for example, in a certain population stays, and the other one goes away, then they may be less likely to actually encounter related individuals. There’s the range of conditions that can affect who animals decide to mate with.
TS: How did you decide to approach this overall question of whether animals avoid inbreeding?
RV-T: We knew that there were a lot of studies made on single species, but that didn’t answer, ‘Do animals, overall, avoid inbreeding?’ That’s why we did a meta-analysis. And what that does is that it summarizes a lot of studies so we can actually then answer the bigger question.
TS: And in your data, do they?
RV-T: What we found is that they don’t differentiate. When making a decision to choose between an unrelated and a related individual, they don’t seem to care.
Of course, we’re looking across a lot of studies. I think what’s important to keep in mind is that, in seventy percent of the studies, they didn’t care. Of course, some did avoid inbreeding, and then some preferred inbreeding. But when you do these studies, what you really focus on is the average, and in seventy percent of our studies, they didn’t care… they basically just want to mate.
TS: You said some seem to prefer inbreeding. Why might that be?
RV-T: One of the things to keep in mind is that when you make a decision to mate or to reproduce, what you basically want is to pass on your genes. And half of your genetic material will go to your offspring, but the other half of the genetic material will come from your partner. And if you mate with your brother, for example, you’re actually passing on more genes that belong to you [because he has some of the same genes]. So, that might be one of the things that animals—I mean, they don’t think or consider—but that's one of the advantages [of inbreeding].
TS: Where did people fall on this spectrum?
RV-T: We decided to include humans, but I need to highlight that it’s a very unnatural setting. The only studies we were able to include in our study were those where people have manipulated images. What you do [in these studies is] compare images that look more like yourself, versus those images that are far away from that. So, you have different levels of relatedness. And what we did was compared that setting against animals that use visual signals. What we found was that there, again, was no difference. Humans, as well, fell into that particular category—they didn’t care whether it was related or unrelated.
Of course, we would have loved to have different experiments, but human studies come with their own caveats. There’s a lot of ethical issues. So, of course, there are way more cues that humans use [when choosing a mate]. But it’s a bit more difficult to do those experiments with humans.
In seventy percent of our studies, they didn’t care . . . they basically just want to mate.
TS: In your study, you also mentioned the idea of publication bias. Can you expand on that and how that affected your results?
RV-T: I think publication bias is a massive issue in science in general. And this is because we have preconceived ideas of what we expect to find. I think if you ask almost everyone: ‘do you think animals should avoid mating with a relative?’ The answer is, ‘yes, of course, most likely.’ And when you start doing experiments, or looking at something new, you always have that in the back of your mind: this is the expectation, and everyone thinks we should find this. It’s a bit hard to take that away from our minds when we’re performing a study.
What we found was that small studies that go against the expectations are a bit more rare to find. What we suggest that probably means is that this lack of inbreeding avoidance is probably even more common. This [publication bias] may be because people either can’t publish their studies, or because they don’t think it’s going to be relevant, or, because it’s harder to publish, they keep it in a drawer. There’s a lot of issues associated with publication bias.
TS: What do you think the overall lesson is from your findings?
RV-T: I think the overall lesson is that, against our previous expectations, animals don’t really care when they choose a mate. . . . They don’t really care if they’re going to mate with a related individual, or kin, as we call it, versus an unrelated individual.
If you would ask me if this was unexpected or not, I would say yes and no. Yes, because I think as humans, we think of incest, and we think, well, that shouldn’t happen. But a lot of theoretical studies have actually suggested that inbreeding should be way more common [than we think]. . . . And this aligns with those theoretical expectations.
Editor’s note: This interview was edited for brevity.