for example elephant isn’t the same size as a sheep
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Evolution is guided by natural selection. An elephant’s environment has lots of plant-based food, but also lots of large, dangerous predators. It turns out that being absolutely massive is a viable niche to fill because they tend to get enough food, and their size and strength are defense against predators.
Some other animals find being small to be advantageous. A mouse eats very little, and being small and quick allow it to evade predators either by speed and agility, or hiding where they can’t go.
Finally, there’s no reason why everything should be the same size. Some would conclude that life has not gone according to a grand plan, but the life forms that emerge do so just because that’s what worked first.
There are a lot of factors. Both the environment, the amount of food, sexual selection and other factors all influence size of species.
If there’s not a lot of food, being smaller means needing less food. On an island with little food, it’s better to be small, that trait is selected for.
Species evolve to be bigger to not be vulnerable to predictors, while others evolve to be smaller, to hide.
Some animals fight physically and some kill off each others young. Being big and strong gets selected for, it gives an animal a better chance at winning fights, getting mates and being protect ones children. Think lions.
They exploit different ecological niches. A mosquito’s biology is going to be starkly different from a blue whale’s because they exist in different environments, eat different things, face different predation, etc. Creatures grow big and small as a species over time based on the evolutionary pressures imposed upon them. In the face of predation, being bigger tends to aid survival. In times of scarcity, being smaller tends to aid survival as you need fewer resources, unless of course you can use your size advantage and take resources from other creatures. Apply these pressures constantly over tens or hundreds of millions of years, and you’ll end up with lots of biological diversity, including physical size.
Like you’re 5? That’s hard.
I guess there’s no benefit in evolving like that.
Adaptations are varied to best benefit the species.
After hundreds of thousands of reproductive generations, in one biome, smallish size and wool coats led what became sheep to consistently not die before reproducing, whereas in another biome, being massive and stompy, and having tusks and a trunk led what eventually became elephants to consistently not die before reproducing.
There’s a very important concept in evolutionary biology, called niche [“nish”].
A niche is a set of things that can serve as food, home and other needs of a living being. For example a an alga and an underwater cave can be a niche for an alga-eating cave fish. A rabbit can be a niche for a little bug that lives in the fur of the rabbit and eats the blood of the same rabbit.
There are some rules in evolution that we can observe. One is that if you have two species that need the same niche, they will compete and one will win.
The other rule is that of there is an empty niche, sooner or later some living being will evolve and take that niche. Actually an empty niche is one main driver for evolution.
But here’s the thing: different niches require different ways to live there. You cannot be a cave fish if you can’t fit in the cave.
As you see, if there’s an empty niche for cave fish, there will be evolution making that size of fish. But then there will be ant sized things living in the small cracks. There must be bacteria living in the guts of the ant.
Every animal question on this sub is an evolution question. Do students seriously not learn evolution anymore?
Animals are different because the exact circumstances they live in will differ. Certain properties are a better fir to the environment than others, so animals with those characteristics will pass on their genes more than those who don’t have those traits.
A creature might be smaller to better suit the nutrition and energy required to maintain their mass. A creature with more access to food and more opportunity to eat may eventually lead onto larger sizes later in its evolution, especially if the larger size benefits them in their environment, such as being harder to kill by predators. A small elephant is easily picked off, which means the genes of larger elephants tends to carry on.
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