What would you expect to occur in a captive population of guppies collected from Trinidad if you allowed them to breed for many generations in an aquarium without any predators?

Answer 1

The girl guppies are going to prefer the most brightly colored boy guppies. The plain color dull boy guppies will reproduce less. These two facts will continue as the guppies continue to reproduce. At the same time, those brightly colored boy guppies won't be happy with other brightly colored boy guppies, so there will be biting and bickering until some of the weakest brightly colored boy guppies will get injured and die off. The genes of non-related plain dull boy guppies won't get passed on, because they won't get a chance to breed; their gene lines will end as they die off.

Assuming the tank and fish get enough filtration, air, and food, that tank will quickly fill up because guppies do like to breed. But over-crowding will kill off the weakest. Even the extremely plainest girls might not get picked by the brightest-colored boys so the girl population could suffer, too.

And if more male guppies are born than female guppies, soon you'll have a tank of really grouchy guys who all want to be God-Guppy of the tank. Any girls left might even go into hiding to get away from the fighting.

Eventually, there won't be reproduction, because either girls have died off too or the girls try to stay away from aggressive boys, and the most aggressive boys will kill off the weakest guys. Plus, guppies do eat their tiniest babies.

So you'd end up with a tank of grumpy guppy males too scared of the God-guppy to even swim much..... making a very unhappy guppy life in a tank...