What is a species?

Looks?

We can easily identify a sheep from a cow but how do you feel about these?

Salamanders vs gecko
Mudskipper vs Newts
Whales and whale shark
Rabbit vs hare
Fox vs wolf
Slow worm vs snake
Alligator vs crocodile
Chevrotain vs Agouti
Turtles vs tortoises
Salamanders are amphibians
Geckos are reptiles
Whales are mammals
Whale shark is a fish
Chevrotain is a deer
Agouti is a rodent
Turtles have flippers, tortoises don't

Did intuition and observation prove reliable? So we need a better way...

One way biologists identify species is by which ones can mate and reproduce

Horses and donkeys can reproduce. Their babies are called mules. Lions and tigers can reproduce. This how we get ligers. However, these offspring are sterile and can't reproduce themselves.

So most biologists use the general definition of any two animals that can reproduce fertile offspring.

However, there's a vast array of animals you might assume can reproduce but can't. Such as an alligator and a crocodile. A hare and a rabbit. A fox and a wolf. According to this definition there's currently over a million species on earth right now with millions more extinct. It's estimated there's roughly 5,000 species of mammal alone.

Conclusion

If looks don't work and breeding isn't perfect, how do we know? The label of species is just a useful tool we use. If all life is connected you'd expect kinds and species to be blurred.

Species is a human construct and very difficult to pin down within nature. It's easy to identify cows and sheep as different but there are plenty of examples that aren't so easy. Whether is convergent evolution (like flying squirrels and flying lemur) difficult taxonomy (like rabbits and hares, geckos and salamanders)  sexual dimorphism (males vs female differences) or the question of whether an organisms needs reproduce successfully or reproduce successfully with fertile offspring. 

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