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Curious Kids: what is a species?

Generations of giraffes. Shutterstock.

Curious Kids is a series for children of all ages, where The Conversation asks experts to answer questions from kids. All questions are welcome: find out how to enter at the bottom of this article.

What is a species? – Finlay, age four, London, UK

Thanks for the question, Finlay. In the past, it seemed like a sensible and simple idea to put living creatures – including animals, plants, fungi, bacteria and so on – into different categories called “species”.

Scientists mostly told different species apart from the way they looked, or where they could be found. But sometimes that proved very tricky indeed.

For example, it’s clear that giraffes and mice are very different groups of creatures, and that you can easily tell them apart; in other words, they are two different species. But what about those two little brown birds, which look so similar?

Spot the difference? The mistle thrush and the song thrush look alike, but they’re different species. Shutterstock.

It was also easy to say that emus and ostriches were probably different species, even though they look a bit similar, because they live on different continents, so they must be different groups of birds.

As time passed and scientists got to study more and more creatures, they realised that some creatures could be quite different, but still be part of the same group.

Have you seen a peacock and a peahen next to each other? In the animal kingdom, mums and dads can look quite different, but they should definitely be part of the same species.

Shake your tail feathers! Shutterstock.

So, by the end of the 1800s, the scientists realised that you can’t always decide if creatures belong to the same species, just by how they look.

Around this time, a man called Charles Darwin started to convince other scientists with his idea of evolution: he showed how creatures change over time, to become better at living in their environment.

One example is how the peppered moths in the UK changed from light to dark colour, after pollution from factories darkened the tree trunks where the moths like to hide. Light coloured moths became easy to spot and got eaten by birds, while the dark coloured moths survived and had more dark coloured babies.

By the beginning of the 1900s, scientists also started to understand that a baby looks like their mum and dad because some kind of information is passed between parents and their children, inside their bodies. Nowadays, we know that this information is called DNA.

With this knowledge, scientists decided that it’s better to define a species as a group of living things that can exchange DNA, by creating “viable offspring”. “Viable offspring” means a baby that can survive, and make babies of its own later on.

This is important, because some species can make babies together, like a zebra and a horse. But this baby – called a zorse – is sterile; it cannot have babies of its own.

Carly the zorse can’t have babies of her own. Kumana @ Wild Equines/Wikimedia Commons., CC BY

This “viable offspring” definition of species is useful, and it’s the one that scientists rely on most often today.

But if you want to have some fun and see a scientist get very hot under the collar, you could mention that sometimes it’s possible to bring together, which would never meet without the help of humans, and that they can produce a viable offspring.

This is the case with tigers and lions. They do not exist in the same location, but humans have bred them together to produce the “liger”. Now what? Are they the same or different species?

As you can see, defining species can get tricky… But I think most scientists will agree that if these two groups wouldn’t meet and have babies without humans getting involved, they are probably two different species.

So how can we define a species? Well, in most cases the “viable offspring” test will work just fine.

You just need to remember that groups of creatures are constantly evolving, so sometimes the differences between species might become a bit blurry.

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