What conditions would be necessary to have all of the world's mainland animals be subjected to island gigantism?

Question

Most of the humans on my alternate universe's Earth have gone extinct due to a global war, leaving a handful of survivors and the buildings to serve as a reminder of how great they once were. This severely dropped humans on the food chain, and to make the overall world more interesting, I wanted animals to evolve in both size and ferocity.

The survivors the story focuses on aren't living in the far future, only little over a century after the war. Would evolution in animals be able to happen this fast, and would island gigantism even be possible on continents?

Answer 1

Given sufficient time small animals would develop gigantism, but not on the timescale that you are asking.

Gigantism is a frequent occurence in evolutionary history, not just on islands. Usually what occurs is the dominant large species are wiped out and lineages of smaller, more adaptable species become gigantic and take their place. Generally this happens because bigger species are more specialized and sensitive to environmental changes and generally exist at lower population sizes. This is one form of what is known as a macroevolutionary ratchet.

The most famous example of this kind of thing is the K-T extinction. Big dinosaurs die, and within a few hundred thousand years you have mammals tripling in size and eventually taking their place. This also happens a lot within groups as well. Tyrannosaurs were dog-sized carnivores under foot of allosaurs and megalosaurs in the Jurassic, and modern dogs, cats, and ruminating ungulates were small, inconspicuous creatures until the Miocene, whereas before then those niches were occupied by odd-toed ungulates, bone-crusher dogs, and nimravids, among others.

If you have a mass cataclysm enough to wipe out most of humanity, it's going to take most of the dominant megafauna with them. Depending on how severe it is you may have something on par with the K-T, where only housecat-sized or smaller animals survive, which would mean complete extinction of things like odd-toed ungulates (i.e., horses), most even-toed ungulates (at best maybe some small duikers survive), and most carnivores. At that point you would get the surviving species adapting to fill the empty niches. Maybe shrews, rats, and weasels become large carnivores, and rodents and rabbits become the new herbivorous megafauna.

However, three important things to note...

1. This will not happen within a century. One century from doomsday you are going to have a bunch of small mammals scurrying over the ruins of the modern world. It takes thousands of years for nature to recover from a mass extinction. It took 700,000 years for mammals to become pig or wolf-sized after the K-T. If one century were all it took for gigantism to develop you would see species becoming gigantic every time there was a local collapse of civilization throughout human history.
2. You are not going to have everything become gigantic. There are still going to be small rats and shrews under foot of this new megafauna, waiting for this batch to go extinct so it is their turn. Small members of a clade can die out, but even in that case something is going to fill the niche.
3. What animals do evolve will likely not be identical to scaled up shrews, rats, etc. Big animals have different structural and ecological problems than small ones, and so their anatomy is going to reflect that. They will look shrew or rat-like, but they will have their own specializations and adaptations for large size (or feeding on large prey, etc.) and you cannot simply say "this is a Rattus norvegicus the size of a grizzly". For example shrews lack a zygomatic arch because at their small size they don't need it, but at larger sizes it becomes useful for buttressing the skull when biting. They might end up re-evolving a zygomatic arch similar to golden moles, or develop a complex jugal similar to ground sloths. They won't look like modern shrews scaled up.

Answer 2

GMO hijinks!

Can we talk about triffids?

https://en.wikipedia.org/wiki/The_Day_of_the_Triffids

The protagonist is Bill Masen, a biologist who has made his living working with triffids—tall, venomous, carnivorous plants capable of locomotion. Due to his background, Masen suspects they were bioengineered in the U.S.S.R. and accidentally released into the wild. Because of the excellent industrial quality of an oil produced by and obtained from the triffids, the result is triffid cultivation around the world.

Then - well, I don't want to spoil it. Read the book! Good stuff. In any case humans are set back and triffids are already in existence and ready to capitalize on the situation and claim the world.

In your scenario, before the war there are already giant animals, genetically engineered for various reasons including entertainment, animal husbandry and the like. The kind of people who like to keep ligers like to keep engineered giant animals too. After the fall of humanity, some of these escape (or are released) and begin breeding with other escapees and wild animals, leading to some of the strange things your survivors encounter.

Remember: there are the fierce superligers of course, but there could be other sorts of GMOimal too. Some of the GMO pets might be cuddly and sweet: giant golden retrievers. Some are food or fur animals and not particularly fierce or maybe just the males - like a giant superbison bull presiding over his harem of escaped cattle and buffalo. Some animals are basically art projects. You could have engineered demihumans too, and maybe one or two of your characters are descended from these.

Answer 3

Why make the animals bigger...

...when you can make the humans smaller?

What you're dealing with is basically a severe genetic bottleneck for humanity, where the only survivors are those who have, through artificial or natural means, managed to survive the apocalypse.

It isn't all that unreasonable that these survivors may have tended to be on the smaller side. Smaller people have an easier time finding places to hide, require less food to survive, and are slightly harder to hit with bullets, giving them an advantage in apocalyptic scenarios that do not involve melee fighting with other humans for survival. Smaller survivors have smaller children, causing the human race to shrink. Also, children who grow up with less food tend to have their growth stunted.

Or maybe the government simply tried to save as much people as possible, so they tended to focus more on saving smaller people who would use up less space and food in the shelters.

Maybe the war was biological, and for one reason or another the pathogen tended to kill tall people more easily than short people. Growth hormones and cancers are interlinked; maybe the disease triggered fatal cancers somehow but had a smaller chance of affecting short people.

Any of these scenarios could happen in a single generation, so it could happen in the timescales you're looking for.

Either way, it's a lot easier to come up with a reason for the handful of surviving humans are smaller on average than to find a reason for all the animals to be bigger on average.