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Which Came First, the Chicken or the Egg?

Which Came First, the Chicken or the Egg?Jerry Coyne recently started a thread on his website in the entry, The Eternal Question, asking his readers to weigh in on the question, 'Which came first, the chicken or the egg?'. Some of his commenters left answers I largely agreed with, and I even added my own two cents in response to one comment, but I thought I'd leave my full thoughts here.

The short answer is that evolution is a gradual process that occurs among entire populations, and species are such a fuzzy concept, that it doesn't even make sense to think of a first chicken or a first chicken egg*.

One of Coyne's commenters suggested that even if the line is arbitrary, you should be able to draw it somewhere. What if you set up your dividing line such that there was a single last mutation responsible to shift a genome from an almost-but-not-quite-chicken to a true chicken? And that mutation finally appears in one organism, so presto, it's a chicken. But if you think about this threshold a bit more, it doesn't make sense. Imagine that your first chicken grows up, finds a mate, and reproduces. Since the chicks get half of their genetic material from each parent, and the division is random, somewhere around half of the 'first chicken's' offspring will get the mutation that defined it as a chicken, and around half will have the old version of that gene. So only half of its offspring would be 'true chickens' - there would be brothers and sisters that were different species! And then all those chicks would grow up and have their offspring, and on and on, and you'd end up with a breeding population composed of a mix of almost-but-not-quite-chickens and true chickens. That's just silly, and doesn't even meet the biological definition of a species.

In reality, species is a very fuzzy concept. The biological species concept is the one most used for sexually reproducing animals. Wikipedia gives a definition for it as follows.

A biological species is a group of individuals which can breed together (panmixia). However, they cannot breed with other groups. In other words, the group is reproductively isolated from other groups.

So, any organism will always necessarily be the same species as its parent. It's only after generations of reproductive isolation that two groups will gradually change to be sufficiently different that they won't be able to interbreed.

There are some interesting modern examples that show how even the biological definition of species can be difficult. One is polar bears and grizzly bears. They can, in fact, interbreed to create fertile offspring. But they don't usually do so naturally. So, it's not that the reproductive isolation has to be complete. It just has to happen little enough that the gene pools don't do too much mixing. How much is too much? Who knows. That's one of the grey areas.

A very interesting case is what's known as a ring species. These are animals that have a range that encircles some type of barrier. A classic example is the Larus gull, which lives in a band around the Arctic Ocean. If you start with the European Herring Gull which lives mainly in Great Britain, it can mate with the American Herring Gull to the west. And they do this often enough that their gene pools mix, which indicates that they're merely subspecies, not completely different species. And if you go west from there, the American Herring Gull can mate with the East Siberian Herring Gull. And you can keep going west, with the groups being able to interbreed, all the way until you get to Lesser Black-backed Gulls, which live mainly in Europe but which also stray into Great Britain. But guess what, the Lesser Black-backed Gulls in Great Britain don't mate with the European Herring Gulls on the same island. So how do these animals get classified as a species? There's one large interbreeding population right now, which would indicate one species. But what if all of the subspecies were to go extinct except for Lesser Black-backed Gulls and European Herring Gulls? Would they instantly become two new species?

Perhaps a more familiar example is dogs. Everybody knows that dogs can interbreed. That's where muts come from. But what if some super-villain were to come along and kill every breed of dog except for Chihuahuas and great Danes? Now, I know that technically you could probably give them a hand to do the deed and make puppies, and they might even be fertile. But, if left to their own devices, they'd be two pretty effectively isolated breeding populations. (I suspect this page is either a joke or an urban legend, but it still reveals the difficulties that would be involved). So, given the current dog population, chihuahuas and great Danes are part of the same species. But there's no way they would be considered the same species if they were the only dogs left in existence.

Given that species is such a fuzzy concept to begin with, it makes no sense to think of a 'first' of any species. There are gradually changing populations, and there's no point where you can pick one organism as being a different species from its parent.

So, the next time you hear someone ask, 'Which came first, the chicken or the egg?', you can tell them to go learn some biology before asking such a silly question that doesn't have an answer.

Image Source: Brain Pickings

*I'm assuming that the question implies it's a chicken egg. It doesn't make much sense to ask which came first, the chicken or the dinosaur egg, because then there's no conundrum at all.

Updated 2015-02-26: Slightly reworded 3rd paragraph for better flow.

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