Yes, natural selection would take place in a fish tank, especially if the fish are overcrowded, under predatory threat or the resources for their survival are overstretched. Natural selection is an evolutionary mechanism, but it doesn’t have to result in the development of new species.
Natural selection would in some instances lead to a variation of different features of the population within the same species. The developed change may only be in numbers depending on member attributes and not result in the development of a new species with a better adaptation to the environment.
Natural selection should occur when:
• More members are born than can survive
• The different members of the species or the fish have variations
• The variation among the fish is inherited
• Reproduction of the fish is based on the inherited variation
An experiment on guppies by American zoologist John Endler confirmed that natural selection can occur in an aquarium setting. In his experiment, male guppies existing in a tank together with a predator had fewer spots compared to guppies in a tank with no predator. The spots are a hereditary feature which are important for attracting the female guppies.