Or any other species for that matter?
In:
Recessive gene disorders take generations to show up. When that first pair with the beneficial mutation that will set them on the path to a new species start in the niche, they have it all to themselves. They exploit that niche by having many offspring. Sure, their offspring interbreed, but that gives you an expanding pool of individuals in the same place. Bad pairs will happen, and those genes will die out, but an open niche gives lots of opportunities.
It’s not like what you see in endangered species today. Those sspecies see some other pressure, mostly habitat loss, that makes it hard for their population to explode as it would in an open niche.
Because most inbreeding is actually fine. Even if you get two carriers for a recessive genetic disease, there’s only a 25% chance of the offspring actually *having* the disease, and only a 50% chance of them carrying it (ie still a 25% chance of them being completely unaffected).
Also, species don’t usually actually start with just 2 members. Speciation (the process that creates a new species) is a long one and it happens across many, many generations. There isn’t just a point where suddenly the generation is a new species, there’s a gradient, and it just so happens that the intermediary generations die out eventually. In species that *do* have a single common ancestor, such as humans, this occurs due to population issues, not as the actual origin of the species – ie, over time, other lineages either die our or merge into the dominant lineage.
In humans, we have a single common ancestor, called Mitochondrial Eve – every single human on earth, at least to our current knowledge, is descended from this human, who lived around 150,000 years ago. This was significantly later than the speciation of humans, however – she would have been living alongside many other humans, including other women, she just happened to be unusually prolific.
Contrast this with Y-chromosome Adam, our other single common ancestor, from whom all current male humans are descended – female humans aren’t due to the lack of any Y chromosome at all. This man is thought to have lived anywhere between 120,000 and 156,000 years ago – not at the same time as mitochondrial eve. Ie, Mitochondrial Eve and Y-chromosome Adam were not a couple, and almost certainly never even met.
It’s also worth noting that both mitochondrial eve and y-chromosome adam can change over time as matriarchal and patriarchal lineages respectively die out.
A lot of us did die out. We only understand a tiny fraction of our own genetic code and the implications for what some genes and genetic markers mean, but sequence the entire genome of any person on the planet and you’ll find everyone’s probably got a few recessive alleles that are less than ideal. Not all recessive disorders are completely incompatible with life. The thing so many people get wrong about biology is that nothing evolves to the height of perfection but rather, evolves until it’s “good enough.”
Latest Answers