Chapter 8: Hybridism

George Bernard Shaw, according to a comment left on a previous post, thought that many people gave up on reading the Origin because, convinced of Darwin's argument, they wearied of him making his points over and over again.

But I disagree. It's not seeing Darwin restate his case that's tiring. It's seeing him return, like a dog to its vomit, to questions that he admitted in chapters one and two couldn't be answered.

When I read these earlier chapters, I was struck by how skilfully Darwin skirted uncertainties, and even how he used them in his arguments. I thought of giving this a fancy name -- I'd have gone for 'leveraging ignorance', had not leverage become synonymous with catastrophic idiocy.

Little did I know. We've already had a whole chapter on 'Laws of Variation'. This wasn't really about laws of variation, it was about the mechanism of evolution, which is just as well, as no one at the time knew what caused variation.

Now we get chapter 8, on Hybridism. Unless you've got a jones for Victorian horticulture, you could skip this one and miss nothing. Reading it is like listening to someone describe, one point at a time, an extremely large scatter plot that shows no correlation.

So species rarely hybridize. Except when they do. The offspring of such hybrid matings are rarely fertile. Except when they are. "[I]t is most difficult to say where perfect fertility ends and sterility begins." Sometimes physical similarity is a guide to whether two species will hybridize. Except when it isn't. Sometimes the males of species A hybridize more successfully with the females of species B than vice versa. Sometimes it's the other way round. Sometimes it seems not to matter. Get the picture? Me neither.

To cap it all, Darwin has the cheek to talk about hybrid sterility being governed by "several curious and complex laws". I'm sure philosophers of science argue about the correct definition of a law. But I think we can safely exclude 'extremely long-winded description of an extremely variable phenomenon, the cause of which is unknown'. Otherwise I'd have a Nobel Prize.

This is the kind of stuff that inspired physicist Freeman Dyson's crack about a biologist's goal being to leave the world a more complicated place than he or she found it.

OK, gather. What can we learn from this chapter? For starters that when Ernst Mayr (pictured above, image taken from his Harvard obit) came up with biological species concept in the 1940s (which I talked about back in chapter 2) he was either pushing at an open door or virtually transcribing the Origin, depending on how you look at it. Here's Darwin:

"The fertility of varieties ...when intercrossed...is, on my theory of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species"

Darwin, I'm discovering, makes most of the other bigwigs of evolutionary biology look like borderline plagiarists.

And partly, it's like GBS said: Darwin is using all this variation as evidence that species are fluid, and so not specially created:

"Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties."

That's the last sentence in the chapter. It makes the preceding seem like a lot of effort to expend arriving at a conclusion we'd already reached. So why bother?

What I think Darwin is doing in this chapter -- and in other parts of the book where he seems to get bogged down in data, such as the second half of chapter 5 -- is testing the limits of generalization in his science.

He had discovered the grand unifying theory that provides an ultimate answer to pretty much all of biology's 'why?' questions. You could forgive him for thinking that lots of other generalities might flow from this insight; that if he looked at the data hard enough, or did enough experiments, patterns would pop out, and if they didn't it was just a matter of more data and harder staring.

But neither Darwin nor anyone else in the 150 years since the Origin was published has had much luck in erecting all-encompassing biological generalizations to set alongside evolution by natural selection. There are patterns, like the latitudinal gradient in species diversity (more at the equator, fewer at the tropics), which was discovered before Darwin was born, and the (rather more controversial) power-law relationship between body mass, metabolic rate, and all sorts of other things.

But these are not hard and fast. Living things are adaptive, and reactive, and flexible, and every pattern shows lots of variation and exceptions, to the point where lots of biologists don't believe in grand laws and don't bother looking for them. And the various theories concocted to explain these patterns (more than 200 for the diversity gradient, for example) are rarely mutually exclusive, so one hypothesis seldom kills another. (If you'll forgive a brief plug, my book goes into all this in much, much more detail.)

So 'law' and 'rule' are quite soft terms in biology. To give an example, biologists have been arguing for decades about whether animals within a species are bigger in the parts of their range furthest from the equator (i.e. colder). This is called Bergmann's rule/law. Ernst Mayr suggested that we should treat the rule as sound if it held for more than half of species. This feeble compromise doesn't seem to have cleared things up.

One thing Darwin doesn't discuss in this chapter is hybridization as a force in evolution. Plants in particular seem relatively willing to hybridize, and some biologists see this as an important source of evolutionary novelties, and an important engine of species creation. But -- surprise -- not everyone agrees. Here's a quote from a paper by Loren Rieseberg and colleagues, who study the role of hybridization in creating new species and adaptations in sunflowers:

The role of hybridization in evolution has been debated for more than a century. Two highly polarized viewpoints have emerged. At one extreme, hybridization is considered to be a potent evolutionary force that creates opportunities for adaptive evolution and speciation. In this view, the increased genetic variation and new gene combinations resulting from hybridization promote the development and acquisition of novel adaptations. The contrasting position accords little evolutionary importance to hybridization (aside from allopolyploidy), viewing it as a primarily local phenomenon with only transient effects--a kind of "evolutionary noise". Unfortunately, definitive support for either viewpoint is lacking.

And to quote James Eckenwalder's review of Mike Arnold's 1996 book Natural Hybridization and Evolution (a snip at $170), "Considering that biologists have been studying natural hybridization for over two centuries, the disparity of viewpoints would seem to argue for a highly polymorphic phenomenon, resistant to tidy categorization."

Amen to that.

Next, 'On the Imperfection of the Geological Record'. Aargh! Not more unleveraged ignorance? Whatever, I'll put that post up over the weekend, and then we'll yank this puppy back on track at the beginning of next week.