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James is likely referring to the concept of the "minimal viable population" that can allow a species to persist over time, and geneticist Ian Franklin's 50/500 rule, according to which "genetic effective population size should not be less than 50 in a short term and 500 in a long term" if the population is to persist without inbreeding depression.

But these are likely underestimates of the effects of inbreeding because researchers didn’t collect data from the orcas’ earliest life stages, when inbreeding depression is often the strongest.

Because of the resultant reduced biological fitness known as “inbreeding depression,” white tiger cubs experience a high rate of neonatal mortality.

“When you don’t have a lot of genetic diversity, you can get what’s called inbreeding depression because you’re basically breeding closely related plants with each other,” said Susan Pell, deputy executive director of the United States Botanic Garden in Washington, D.C.

Hybrid vigour is the “mirror image” of inbreeding depression, Nicholas writes, weakening of certain features as a result of excessive breeding with close relatives.