Species

The idea of “species” is clear and simple in our everyday folk biology. In the Hebrew bible, to save the animals from the flood, Noah brought “two from all” (“shnaim mi-col” = שְׁנַיִם מִכֹּל), male and female onto the ark. (An “amah” = אַמָּה – translated today as a “cubit” – must have been quite a bit bigger than we think, given what we now know about the “all”, even just of all animals.) We have a pretty good idea what is intended by the story. Everyone can tell the difference between a tiger and a lion, even my three-year-old granddaughter. (But what’s a “liger” or a “tiglon”?) Darwin’s On the Origin of Species caused a furor, in part because he suggested that species were malleable.

But from the point of view of a physicist – especially one like me who has studied a little cog sci – the idea of “species” looks more to me like the standard categorization that our brain does. We define two things as the same if they are similar – equivalent for whatever limited purpose we might have in mind at the time. (See “One and the Same”.) Categorization seems to be based largely on our early experience, cultural environment, and practical considerations. It does not seem as if things have a “true essence”, a la Plato.

In physics, over the past century we have built up pretty good evidence that the particles of which atoms are made (and photons) do need to be thought of as identical because of the structure of quantum mechanics (about which more in a later post). But when it comes to living organisms, it’s pretty clear that while some are similar, they are not “identical” in the sense that if you exchanged them there would be no experiment you could do that would tell you that the exchange has taken place.

So if all we really have is a lot of distinct individuals, what do we mean by a “species”?  One thing we like to do in physics, especially when we are getting started in studying a field and don’t have a strong theoretical framework, is to define things operationally – by some measurement process. That’s effectively how a species is defined in biology today – as a breeding population. Saying two organisms are of the same species doesn’t mean that two individuals can breed – they may be of the same sex, for example – but it means that two individuals are of the same species if there is a chain of individuals that can breed and produce fertile young. So Scooby-Doo (a Great Dane) and Ren (a Chihuahua) are of the same species even though a Chihuahua female could not carry a Great Dane’s pup to term and survive. Note that the production of a viable zygote, fetus, or even adult is not sufficient (viz. the liger or tiglon). It has to be able to survive a natural birth and be able to grow to adulthood and be fertile. You can “measure” whether two organisms are of the same species by doing a series of breeding experiments.

Many animals in biology are considered to be different species because different behaviors or choice of habitat makes natural breeding impossible. For example, two species of fly seem to differ only by the sounds they make in courting. (Of course there is a hidden assumption here – that we are talking about beings with two sexes. Things get all messed up when an organism can basically clone itself by reproducing by parthenogenesis, or when, like bacteria, they may exchange genetic material with very different organisms via viruses. But we’ll leave that discussion for another time.)

The definition of species as a breeding population is interesting since it means that you might not be able to tell if two individuals are members of the same species by experiments on them alone. (You might … if they could actually breed directly.) It might seem the opposite might be more obvious – that we could show that they are not members of the same species, but this raises interesting questions.

In The Ancestor’sTale: A Pilgrimage to the Dawn of Evolution, Richard Dawkins tells the interesting tale of a “ring species.” (I gather from conversations with biologist friends that this is a standard example in intro bio classes.) Apparently there are two species of salamanders that live together amiably in a region in California near the south end of a lake. Let’s call them groups A and B. They are clearly different species since they look different and don’t interbreed. But … if you travel up along the side of the lake on one side, you will find apparent variants of salamander A that do breed successfully with it. Similarly, if you travel up along the other side of the lake you find variants of salamander B that breed with it. And at the north side of the lake? You find salamanders that breed with both variants establishing the chain that prove that salamanders A and B are actually of the same species! Although this is called a “ring species” in biology, I prefer to call it a horseshoe species – because it’s open at the bottom.

This is neat – but a bit creepy. Suppose Harry was a salamander of group A, and Sally was a salamander of group B. Since there is a chain of friends through whom they can exchange genes they are of the same species. (To get in the right frame of mind, listen to Tom Lehrer’s, “I got it from Agnes”.) But now suppose a mining company was granted mineral rights on the north side of the lake by an eco-unfriendly administration. They destroy the habitat on the north side of the lake and all of the northern salamanders are wiped out. The chain is broken. Harry and Sally can no longer share genes and therefore they are now members of different species – despite the fact that they haven’t done anything or changed in any way!

Now here’s where it gets interesting. Physicists like myself who have studied relativity and quantum field theory are used to space and time being considered variations of the same thing. To see the implications of doing this, let’s consider a map of the lake as a 2-D graph of the different members of our horseshoe species of salamanders. But that’s at a particular instant of time. If we want, we can stack up 2-D maps of space, each one representing a later (or earlier) time. This makes our 2-D maps into a 3-D space-time graph, with the vertical axis (perpendicular to the maps) representing time. Let’s now imagine rotating our 2-D horseshoe downward, keeping Harry and Sally fixed in place, but taking our north-side salamanders down into an earlier time. The joining point of the two legs of the horseshoe now represents the common ancestor of the two groups of salamanders A and B. To see the implications of this for the concept of species, let’s switch to dogs where it’s easier for most of us to get a picture.

Analogous statements are true of Scooby-Doo and Ren. A plague wiping out all dogs except Great Danes and Chihuahuas would have the same effect on them. We would no longer consider them to be of the same species than we would tigers and lions to be the same species.

Let’s start with a “generic” dog – something like a Tamaskan (picture below) that looks a lot like a wolf – and imagine it as the common ancestor of all the dogs today. Clearly this is an oversimplification, since we should think of a breeding population as the ancestor of a population, not a single animal. (Even in the Bible, Seth, the third son of Adam and Eve, had to find a wife.) The descendants of that population were selected for by people from the natural variation the original breeding population showed – and variations that developed later (by recombination and mutation). The result is the variety of current dogs we see today. Now the ancestors of various breeds produced a range of variations in their pups from which the ones with desired characteristics were selected.

(Source, Wikipedia Commons)

Suppose now that all of the ancestors had not only pups with the variations that eventually led to Chihuahuas and Great Danes, but also ones that bred true. We’d then have not just a continuous breeding population of current dogs connecting our two extremes, but a breeding population connecting all current dogs to dogs who are identical genetically to all their ancestors. 

So this suggests that what looks to us to be a “species” is not defined by the individuals involved, but by the gaps – which ancestors and intermediates happened to die out before today. If the right ancestors had “bred true and through”, we would not separate lions from tigers nor horses from donkeys (nor humans from chimps). These are all pretty close, though. What about more significant gaps – like lizards from birds?

This is what makes The Ancestor’s Tale so interesting. Dawkins traces our ancestry back, considering close relatives and showing what our common ancestors might have looked like. He goes further and further back and attaching to our family tree species that seem farther and farther from us.

So if I take the time-rotated picture seriously, it suggests to me that we are all part of one grand continuum of life, with our definition of species being (certainly) a convenience to us, but one that we might view as accidental if we had a more complete historical record. It’s a definition that depends on which intermediates happened to die, and not on anything intrinsic to the individuals we are considering.