Here’s a project for a playful biology grad student with some time on his or her hands. Take chapter 2 of the Origin of Species, ‘Variation Under Nature’, and modernize the language. Toss in a few figures and some contemporary citations. Give the result a title like ‘A routemap for biodiversity research 200 years after Darwin’, put your name on it and submit to Trends in Ecology and Evolution. I’m not promising anything, but you might get lucky.
In his first chapter on domestic species, Darwin spent a lot of time worrying about questions that, thanks especially to molecular genetics, we now have an idea how to answer. In this chapter, his concerns and uncertainties are alive, well, and bothering researchers.
In retrospect that first chapter, with its pigeon-fancying and classical sources, looks old-fashioned — I felt like I’d gone from 1859 to 1959 in the turn of a page. Besides staking out the ground for population genetics, Darwin, in half-a-dozen dense pages at the end of the chapter, outlines many of the patterns in the diversity, abundance and distribution of living things that ecologists are still trying to understand.
It turns out that when Darwin talks about variation, he means two things. He uses the word in the sense that biologists use it today, to mean variability between the members of a population or species.
Naturalists, he says, see such variability as an irrelevance and an irritation, because if your goal is to define a species, variation is the last thing you want: “These individual differences generally affect what naturalists consider unimportant parts … It should be remembered that systematists are far from pleased at finding variability in important characters.”
Darwin argues that important parts — for example, the “branching of the main nerves close to the central ganglion of an insect” — do vary. And that this variation is signal, not noise: “These individual differences are important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions.”
But as well as being the raw material of natural selection, Darwin uses ‘variation’ to mean the product of natural selection: the variation between species and varieties whose evolutionary history has already diverged. This is something closer to what we know think of as diversity.
This chapter’s lodestone is the idea there is no clear line between these sorts of variation: “I look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history.” And so on, to sub-species and species.
To explain the origin of species, Darwin first undermines the idea of the species. In his first mention of the Galapagos, he says that the island’s birds made him realize “how entirely vague and arbitrary is the distinction between species and varieties”.
The name species “is arbitrarily given for the sake of convenience to a set of individuals closely resembling each other”. He quotes many expert disagreements about what deserves to be called a species and what is a mere variety. This fluidity of species in space underpins his argument that species are also fluid in time.
The idea of the species is still slippery. Best known is Ernst Mayr‘s biological species concept: that you can assign organisms to the same species if they will breed together “in nature” to produce fertile offspring.
But it’s hard to test this for fossils. Also, most of the world’s genetic, biochemical and ecological diversity is contained in all those microbes that don’t have sex as we know it. You could go on appearances — the phenetic species concept — but that seems like a backward step, and it’s not much use for metagenomics, which fishes DNA out of the environment and measures diversity by sequence similarity.
More useful in such cases is the phylogenetic species concept — a species is a group of populations that shares a common ancestor, and is distinct from any other similar group. Although, again, seeing as we all share a common ancestor sooner or later, it’s tricky to know where you draw the line between groups. The microbial ecologist Jessica Green once pointed out to me that microbiologists typically put two cells in the same species if their ribosomal DNA is 97% identical. Applying the same criterion to primates, she says, and you’d be sharing a species with the ring-tailed lemur.
There are lots of other species concepts, but Darwin still seems spot on when he says: “No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.”
Here’s some advice for anyone agonizing over creating a perfect, all-encompassing species concept: chill out. Pretty much any biological category you care to think of has fuzzy boundaries. Genomics is making the concept of the gene more problematic. Colonial, clonal and modular organisms, such as slime moulds, aspen, and the vast underground mycelia of some fungi, make the concept of the individual tricky to pin down. The giant mimivirus blurs the line between viruses and cellular life. There’s been a long debate over whether viruses themselves should be classed as living things. And it’s proved impossible to come up with a list of properties that unambiguously define life.
This does not mean that terms such as species, gene, individual or life are useless. In many or most cases it’ll be obvious whether the thing or group of things you’re looking at belongs in a particular category or not.
But it’s not a lack of ingenuity that has left us without perfect definitions for these things, it’s that such definitions don’t exist. What Darwin shows in this chapter is that if nature doesn’t fit into our boxes, the task is not to build a better box, it’s to work out what the uncertainty is telling us.
On Friday, things turn nasty, in the ‘Struggle for Existence’.