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If humans all went extinct from war and lack of resources and left no trace except for their big cities, would any species change drastically over the course of 1000 years? Would they have notable mutations in size or form overall? Or would they just stay the same but have much larger quantities of said animals all over the world?

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    $\begingroup$ The word mutation does not mean what you think it means. Are you interested in mutations, or are you interested in phenotypic changes? (And, very obviously, some species will see a population increase, for example volves and lions; others will see a dramatic population decrease, for example mice, rats and cattle; others will go extinct, for example Pediculus humanus...) $\endgroup$
    – AlexP
    Commented May 14, 2021 at 16:34
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    $\begingroup$ There would be vast changes in various micro organisms since they woul have very many generations in 1,000 years. There would be much less change in most small muliticelled animals that have about one generation per year. And large mammals which have generations about as long as humans would change very little. $\endgroup$ Commented May 14, 2021 at 16:52
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    $\begingroup$ Do you want them to change? Because a YEC would say that most of the variety we have today is the result of only 6,000-7,000 years of natural selection. Indeed, even within a century or so we've seen non-trivial changes in some species, particularly ones that are either being selectively bred by humans or whose environment is being rapidly and drastically changed (again, usually by humans). $\endgroup$
    – Matthew
    Commented May 14, 2021 at 22:26
  • $\begingroup$ You've not mentioned a specific environment either. Many species of fish have been reduced in size (and number obviously) by our fishing tactics, that might bounce-back somewhat. $\endgroup$ Commented May 15, 2021 at 15:00
  • $\begingroup$ Welcome to site, jatchiboi. Please note that the Worldbuilding SE is not a What-If site. If you have a specific question about the world you are attempting to build, go ahead and ask that as a new question. If you haven't already, feel free to take the tour and check out our site culture. $\endgroup$
    – Frostfyre
    Commented May 17, 2021 at 12:23

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The most immediate change in the genetics of post-human animal life would be the collapse of the artificial barriers which we have built up to subdivide domesticated species into breeds. In the absence of humans to selectively breed them, the liberated domestics would choose breeding partners without regard for human definitions of beauty or purity. So a thousand years after we are gone, most dogs will be mutts and many combat disadvantaged miniature yippie dog-forms will be approaching extinction.

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  • $\begingroup$ A lot of domesticated species may also see a massive reduction in numbers without humans to feed and/or breed them. $\endgroup$
    – John
    Commented May 14, 2021 at 16:40
  • $\begingroup$ In the Great Plains of US, South America, and Russia, we would see a return to large herds of roaming herbivores. It would be very interesting to see which species dominate. (Up north, cattle do not do as well as bison when sudden cold snaps hit.) $\endgroup$
    – David R
    Commented May 14, 2021 at 22:09
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One obvious subset would be domesticated animals. There are many instances of previously domesticated animals going feral in the world right now: dingos, wild boars, feral horses, and feral cats, for example. The changes to their behavior and even appearance are notable after only a few generations. After 1000 years it would not be unusual to expect further changes and adaptations to their environments as they establish permanent populations in various places around the planet. Interbreeding with local species or among different breeds within their populations might also occur.

You might see also changes in game animals. Hunter activity (as well as unmanaged poaching) affects species from African elephants to whitetail deer in the US (where variations in hunting regulations such as antler size restrictions has effects on the average antler size of species in the wild). I would guess that elephant tusk size would increase in Africa, and average whitetail deer antler size would increase in several US states.

Without human population control of invasive species, existing introductions of non-native species may become permanently established. It's possible that some adaptations in behavior and even minor changes in appearance/abilities could occur particularly in fast-reproducing species that are vulnerable to these non-natives. For example, after living alongside pythons for 1000 years in the everglades, it's likely that at least one of the hundreds of species of birds, rodents, and other affected species would develop a favorable adaptation of some sort that allows them to survive around such predators.

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Humans exterminated most of megafauna which did not have chance to co-evolve with us. This niche simply become vacant. 1k years is somewhat short period, but either existing domesticated animals would try to take it or some wild animals would be slowly evolving bigger to take advantage of it. (idea: slightly bigger, not reaching yet anywhere near equilibrium)

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