If we consider land areas that have been largely isolated from each other from a human point of view, but with a similar climate – for example, a part of Europe and a part of Americas with a similar climate before the time of Columbus – generally how different can one expect the flora and fauna to be between the places?

I realize there can be quite different approaches to measuring the biological differences. Not being a biologist, I probably miss some quite important metrics. Some ideas that come to my mind:

  • How distinct are the species in each environment? For example, can we expect to see the same ant species? Same grass species? Same bacteria?
  • DNA difference based approaches: How much of the DNA material in each environment does not exist in the other one? How large a proportion of organisms in one environment does not have a close relative on the other, given some threshold closeness value?
  • Historical approach (probably overlaps the DNA approach quite a bit): How long ago (alternatively: how many generations ago) did the common ancestor of similar organisms in each environment live? Do the organisms share this number, i.e. is the common ancestor of, say, humans in Europe and America as ancient as that of ants and grasses?
  • $\begingroup$ You might narrow this down a bit. There are q number of questions, and to answer them all, the answer would need to be quite long. $\endgroup$ – anongoodnurse Jul 19 '16 at 23:34

The concept that you hint at, has been studied at the example of the island of Australia, where marsupials often look quite similar as their counterparts in the same ecological niche, which live outside Australia and have a placenta, and are less related on the DNA level (e.g.: squirrel-like animals, wolf-like, and the now everywhere extinct saber-toothed tiger-like...).

If you are interested to learn more, I would highly recommend to read Jerry Coyne's "Why evolution is True" ( http://jerrycoyne.uchicago.edu/index.html )

The specific example of Europe and Americas (which does not fully capture the complexity of your question), also is quite interesting - although it is very short on an evolutionary time-scale. E.g.: American squirrels, which lack predators in Europe, and only arrived in Europe because of human journeys following the rediscovery of Americas, are partially outcompeting the beloved red squirrels ( https://en.wikipedia.org/wiki/Eastern_grey_squirrels_in_Europe )

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