This is an I-Hard podcast.

Guaranteed human.

Some questions that seem straightforward at first become devilishly hard once you start to dig in.

For example, what is a species? In our day-to-day lives, the answers

are pretty obvious. A cat is clearly not a dog. When you call your children by your dog's name,

it's not because you're confused at the species level. It's because your children kept you up

last night and your brain isn't working anymore. But then you go to the grocery store.

And you look at the cabbage, the broccoli, the cauliflower, kale, colerabi, and Brussels sprouts.

Those are all actually the same species.

But they look super different.

And don't get me started on dogs.

How is it possible that Great Danes and teacup chihuahuas are the same species?

Alright, now Brussels sprouts and domestic dogs are examples where human intervention has resulted in a lot of the variation in appearance.

But for a lot of scientific history, we relied on things like appearance to figure out what a species is.

And clearly, nature can produce a lot figure out what a species is.

And clearly, nature can produce a lot of variability at the species level.

So this makes things hard.

But we also rely on other criteria.

For example, many of us have heard that species can be defined as organisms that breed together and produce offspring.

So dogs and cats don't make cogs or dats, so they're definitely different species. But what about polar bears and grizzly bears?

They split evolutionarily not very long ago, so not only do they look pretty genetically similar,

but if you get them together, they can produce offspring.

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