Why are different ways to be born (Like being mammal or born through eggs)


Excuse my english please.
So I was wondering, why some animals are born in different ways, I would understand for example that some fishes are born through egg because of the water, it would be too hard to swim with a fetus, and then you have the whales. So in general, why are there so many ways of reproduction?

In: Biology

Different envioromental niches select for different traits, including reproductive traits. If you’re a simple fish, making lots of eggs but not caring for them much might be the best trait. If you’re a super smart mammal who needs years to educate your youlg, like an elephant, you can’t make as many.

Evolution is the simple answer.
When the first animals moved to the land the first ones had to return to the water with their eggs – like amphibians still do.

Animals that moved into drier niches needed eggs more resistant to drying out – resulting in eggs with leathery and sometimes hard eggs – like the ones birds have.

Some animals evolved to do more of the growth of their young inside their bodies – marsupials give birth to very immature young that need to migrate to the pouch and nipple their to finish their development.

While others evolved to gestate their young longer – giving live birth to (relatively) well developed young.

Some mammals moved back to aquatic environments. Some Ancient relatives of dogs and cats evolved into seals while whales are more closely related to hippos.

In both cases their descendants continue to give live birth.

The short answer is many forms of reproduction work and are successful and so evolution has explored these and found some are better or at least good enough for some animals in some environments with some lifestyles.