We see in wildlife mules, ligers and tigons as example of genetic dead end offspring as their parents were genetically too distinct
At the same time offspring from genetically too similar parents : siblings cousins etc produces an offspring with bad genes
What is the goldilocks zone genetically that is considered just right? Is it the same or different among species?
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Genetic robustness depends on the environment and the environment changes over time. Darwin’s finches are a good example of this because they deliberately hybridize to survive as the climate shifts cyclically because of ENSO weather cycles (el nino/la nina). Hybrids are more common than people are aware of but they’re not always fit. A lot of animals in zoos aren’t releasable because we unintentionally hybrid bred them. Orangutans are one example where animals from different populations were bred together in zoos and their offspring had health and behavior issues that make them unfit for any repopulation programs for orangutans. Looking at humans, we have around 2% Neanderthal or Denisovan DNA, so only a little bit of hybridization was survivable.
“Hybrid vigor” is probably the genetic robustness you’re thinking of where the offspring of two different species is better than the parents. This isn’t something that’s guaranteed, you can get a bad mix up of genes just as you can get a good one. What scientists think happens in hybrid vigor is that one species contributes a better allele for a gene that winds up being dominant. To use an example gene, this would be like one species having only brown hair while another had only blond hair and when they have offspring, the brown hair gene is dominant to the blonde hair gene so all offspring have brown hair. In theory the brown hair species would have a number of dominant good genes and the blond haired species would have a number of different dominant good genes and when mixed you get an offspring with more good genes than either parent. The problem here is, when those offspring have their own children, the genes are shuffled again so you don’t necessarily wind up with grandchildren that are still robust, they’d only have some of the better genes their parents had. So you only get hybrid vigor once and then it gets diluted again. That’s because that blond gene is still there and available for offspring to inherit, it was just recessive in the first hybrid. Breeding hybrids to other hybrids, there’s a 25% chance of blond offspring.
In terms of inbreeding, what happens is something called Muller’s ratchet. Mutations occur at a predictable rate for each species depending on how quickly the animal breeds. Some of those mutations cause health issues. If we just cloned ourselves repeatedly those mutations would just build up over time until our clone offspring had too many mutations to survive. Genetic recombination through sexual reproduction helps get rid of those mutations because some of the other parent’s DNA gets swapped in for the mutant version so for any offspring with a bad copy, there are a few that have good copies. How much inbreeding any population can tolerate depends on generation timing and how quickly new mutations occur and how big the population is because that dilutes the bad mutation. So in a country with a high, international population like China, even siblings giving birth isn’t a big deal as long as it’s not happening repeatedly. But in a more limited population, like Iceland for instance, siblings having children is a bigger risk.
So the answer is that there’s no fixed spot for most genetically robust offspring. It depends on the environment, the species, and the population size.
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