>>12150625What are you talking about? These categories like "fish" and "mammal" aren't like, laws or something; there's nothing keeping them in thier groups they just have a common ancestor and similar traits.
If a fish gains a helpful trait through birth over time that lets it become extremely successful,
if it become successful it will spread and multiply,
if it spreads and multiplies it will both reach new ecosystems with new challenges and resources and have to compete with others in thier own group,
if there's an environmental pressure like that then the animals of that species that gain traits that help them survive the environment or get more food they'll become successful,
if those become different enough they stop being able to easily breed with the species they used to be,
eventually there's multiple different species from the same original species,
if these are unique enough we label them under thier own branch.
It's pretty simple.
