During the dinosaurs time 100-200 million years ago there were giant insects like 1 meter long. An example is here. One of the theories why they could grow this big is that oxygen level was much higher than now. Now we have like 20% oxygen in the air and that time it was 30 or 40.

The question:

If I make a tank/aquarium with 40% oxygen and all the equipment to make this oxygen concentration constant. I put there modern insects, dragon fly, worms, slugs, etc. Will they grow to much bigger size than normal from the 1-2 generations, or I will need to have 100 or 1000 generations before they will be bigger?

  • 1
    $\begingroup$ This has been done in insects at least: jvandenbrooks.wordpress.com/research $\endgroup$
    – kmm
    Sep 19, 2020 at 14:35
  • $\begingroup$ You would need some sort of factor that selects for a larger size. This isn't impossible, since humans do selective breeding for size in animals (e.g. St. Bernards and Chihuahuas) and in plants. I don't know of any selective breeding of insects (maybe honeybees?), but I'm sure it could be done. $\endgroup$
    – jamesqf
    Oct 19, 2020 at 17:01

1 Answer 1


that sounds like a fun experiment! But unfortunately you are right, the size difference is shaped by genetics as well as an environmental condition that might be hard to pin down.

For the genetics, It would indeed take millions of years (or at least 100,000s) for the genetic adaptation to kick in to create that change.

As for the environmental conditions, another source of gigantism is being on an island. As the small number of species expand to fill out all the niches in the tank, they may get larger simply because the environment allows it with no predators.

The tank would have to be enormous to get those niches I think. This paper describes an archipelago of islands and shows that you tend to get smaller animals more easily with smaller islands, and larger species need larger islands. Large here is millions of square meters.

Both of these processes take many generations. This is not my field but this paper studied 200 generations of plants and showed only migration and lifespan changes. Size adaptations that come with changing ecological roles might have to take 1000s of generations. We'd have to freeze ourselves to see the giant slugs. I'm sure there's a better answer out there, but this is what i found so far.

nixon's head

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    $\begingroup$ was richard nixon frozen in time? $\endgroup$
    – gfdsal
    Sep 20, 2020 at 21:39

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