Just to give a few quick examples;
Australia was dominated by marsupials until only very recently in geologic time, and the general idea of why they flourish there but are rare or absent across the rest of the planet is that they were able to colonize Australia via migrating across Antarctica (warm at this time period) from South America. However, Australia was not directly connected by land, and the ‘placental’ mammals were also present in the same region at the same time period. Thus the suggestion is that the marsupials just happened to exploit an opportunity at a very brief window of time. Such a brief window suggests, at least to me, that the total founding member count cannot of have been very high. Not million of individuals, and even hundreds feels generous.
Monkeys reached South America from Africa by ‘rafting’. Not as in, building a physical raft, but being blown across in storms in either large trees, mats of vegetation, or chunks of landmass that were dislodged. For the monkeys to have bred they would have had to be closely related species (so a baboon and a chimpanzee wouldn’t be able to start a founding population even if both were blown over there), and given the sheer amount of distance and how closely groups would have to have arrived to meet and breed, this also seems like it only happened once. I don’t imagine you could fit hundreds of monkeys on even a large tree.
Lastly, all the species of true crocodiles in the Americas are not only descended from african crocodiles, either blown in or having swum the enormous amount of distance, but genetics seem to suggest that all, at some point in relatively recent history, were descended from the same female individual. Whether a single pregnant female made the journey and led to various descendance through inbreeding, or several crocodiles were blown over but only one female survived, the result is the same.
In all cases it seems that enormous variety has arisen from very small pockets of closely related animals. How have these animals ‘beaten’ the issues that arise from inbreeding?
In: Biology
There’s a really fun book called Tears of the Cheetah that you may really like, because at some points it talks about “bottle-neck” events where a population got decimated and bounced back. Part of the reason a population seems fine is that any severe recessive genes that would kill an animal would not get passed down because the animal would die young. But also there are huge downsides to the inbreeding; a lot of these populations are vulnerable to new diseases. Without genetic variety, if the new disease can get one of the species it can often wipe out a huge swath of the population before they adapt
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