Is inbreeding really that bad?
I'm Anna Rothschild and this is Gross Science.
A few weeks ago I released a video on fig wasps
and in it I mentioned that in at least some species
male fig wasps will often mate with their sisters.
And a bunch of you asked a really sensible question
that I didn't know the answer to.
You wondered why inbreeding is a successful life strategy for these species
when conventional wisdom tells us that mating with your relatives
is a really bad idea.
Now this isn't just something that humans made up.
Lots of organisms avoid inbreeding.
And for good reason.
Inbreeding limits the available gene pool
and can increase the chances that you'll pass on harmful, recessive traits to your kids.
And, just to clarify, a recessive trait is one that's only expressed if you inherit it from both your parents.
You see, in humans and lots of other organisms
we get one set of genes from our mom and one set from our dad.
So if your dad has a harmful, recessive version of a gene and passes it onto you
you won't express it unless your mom gives you a copy of it too.
Consequently, these genes can hide for generations
secretly being passed down, but never expressed.
But if your parents are closely related
the chances that they both carry the harmful version of the gene are way higher.
And that's how you end up with people like the famously inbred Charles the Second of Spain
who was allegedly sickly, infertile, and had a tongue so big that he drooled and could barely speak.
If inbreeding lowers the fitness of a population
scientists say the population is experiencing inbreeding depression.
That being said, mating with partners who aren't genetically similar to you is risky too.
If your traits make you really well-suited to your environment
why mess with a good thing by mating with someone who's genes might be super different.
If instead, you mate with your relative who's genes are similar to yours
you may be ensuring that those good traits get passed on.
And, in nature, when the costs of mating with unrelated partners outweigh the fitness benefits
inbreeding may be a better option.
So what's going on with the fig wasps?
Well, let me first be clear that there are hundreds of different species of these guys
and many of them don't exclusively inbreed.
But I spoke with a scientist and here's what I can tell you, in a general way
about why inbreeding might be a good strategy for some of them
though there may be other factors at play.
Okay, so fig wasps are what are called, haplodiploid
which means that their offspring don't necessarily get a set of genes from each parent.
The females do. They get a set from both their mom and their dad.
But the males only get one set of genes and it comes from their mother.
This is actually true of most members of Hymenoptera
which is the order of insects that includes wasps, bees, and ants.
Because males have only one set of genes
recessive traits really can't hide for long in a population.
Males will outwardly express whatever traits their mom give them
and if those traits make them unhealthy
then it's less likely they'll be able to mate and pass their genes on to the next generation.
In that way, undesirable genes are more easily weeded out of the population in a haplodiploid species
making inbreeding a safer practice.
And one that can preserve the good genes as well.
Now inbreeding is actually not as rare in nature as you'd expect.
Some species simply do it because it's really hard to find a mate
and lots of plants self-fertilize which is basically the ultimate form of inbreeding.
All that said, genetic diversity is a wonderful thing.
It can make populations more resilient in the face of change or hardship.
But expanding your gene pool is not always the best strategy for survival.
As a wise man once said, "Life, uh, finds a way."
And for fig wasps, that way is sometimes inbreeding.
Ew.
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