Chapter 4: Natural Selection

Mathematicians and physicists speak of a result 'falling out of the equations', implying that if you set things up properly, the rest takes care of itself. Chapter 4 of the Origin, 'Natural Selection', is where evolution falls out of the machinery that Darwin has spent the three previous chapters assembling.

And I hate to say it, but it's a bit of an anticlimax.

In retrospect, it's difficult to see how it could be otherwise. Darwin has manoeuvred us into position so carefully, showing the power of artificial selection, the mutability of species and nature's cutthroat struggle, that we're practically ahead of him by this stage. Who needs convincing?

Not Darwin: in the first paragraph of the section "Illustrations of the action of Natural Selection" he uses the phrases "I can ... see no reason to doubt" and "I can see no more reason to doubt this..." within the space of a few sentences, and he's only writing about a hypothetical wolf chasing a hypothetical deer.

So instead of the big reveal, which actually came in the previous chapter, he trowels on the detail. And then he trowels on a bit more. Perhaps this is why some people see him as a plodder (something we can come back to at the book's end).

'Charles', I would have said, had I been his editor (which, for all our sakes, we can be glad that I was not). 'Didn't you put enough on pollination in chapter 3? All this stuff about humble-bees [sic] is going to lose you readers. Tell you what: this three-volume work on natural selection you've got planned? Put the rest in there -- save it for the box set. '

'Listen,' I imagine him replying. 'I've done all these bloody experiments on plants. It's taken bloody years. And now I'm going to bloody well write them up, and you're going to bloody well read about it.'

I mean, I don't want to seem shallow, but do we really need sentences like this?

Of the eight descendants from (A) the three marked a14, q14, p14, will be nearly related from having recently branched off from a14; b14 and f14, from having diverged at an earlier period from a5, will be in some degree distinct from the three first-named species; and lastly, o14, e14, and m14, will be nearly related one to the other, but from having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or even a distinct genus.


That's not to say there's nothing nourishing in this chapter. You'd expect some good stuff, what with natural selection being one of the three or four greatest ideas in the history of thought. It's just you need to chew a bit to extract it -- after the martini of chapter three, this is the nut loaf.

All the things that Darwin got right leap out -- I'm continually amazed by the completeness of his vision, and by his ability to ask the right question (another thing to discuss in more detail at the end). There's been a lot said about sexual selection in the past 150 years, but as a first word Darwin's is still as good as anyone's. Another eye-catching moment is where he notes in passing that the more plant species live in a spot, the greater mass of life you will find there. This relationship between diversity and productivity is something that ecologists are still trying to understand.

But just as interesting are the bits where his ground doesn't seem quite so firm. I was struck by what this chapter seems to show about how Darwin thought new species are created. And I wonder if it contributed to the unpopularity of natural selection in the first fifty-odd years of the Origin's life.

After the Origin, the idea that evolution had taken place and that species were related to one another caught on quickly. But the idea that natural selection was the mechanism of evolution was much less popular, and Darwin's reputation took a dip. The most interesting example of this I know of is a remark in a George Orwell essay, 'Lear, Tolstoy and the Fool', where Orwell quotes a 1903 pamphlet by Leo Tolstoy in which the author of War and Peace wrote that Darwinism was a fad that was "beginning to be forgotten". (For a more comprehensive treatment of this topic, check out Michael Ruse's Darwinism and its Discontents.)

In the summary of this chapter, Darwin writes that species diverge when they adapt to use a previously unexploited resource in their environments. So let's say you're a fruit-eating species, and then one of you comes up with an enzyme to digest cellulose. They start eating leaves, and their descendants and the fruit eaters' will eventually go their separate ways.

Darwin understands that isolation is important, and that crossing works against speciation. Earlier in the chapter, for example,he notes that mobile species such as birds "varieties will generally be confined to separate countries", whereas in sedentary animals, "a new and improved variety will be quickly formed on any one spot".

But the key word there seems to be "improved". In Darwin's vision, species don't just drift apart from one another because they've stopped interbreeding. They change because of some variation that gives them an edge in the struggle. Adaptation and speciation are inseparable in his mind, and natural selection on its own, without isolation, is enough to split one lineage into two.


Here, Darwin is describing something we now call sympatric speciation. Demonstrating that this type of divergence-without-isolation can happen has been one of the toughest recent nuts in evolutionary biology. The problem is gene flow -- if there's nothing stopping the members of a population interbreeding, it's hard for selection to begin moving apart.

After much effort by theorists to imagine the circumstances in which such a form of speciation is possible, and by field researchers to find evidence that it happens in the wild, it seems that Darwin was right, and sympatric speciation is possible.

But the idea that a species splits in two because sea, mountain or desert prevents its members interbreeding is much, much easier to grasp. Once you split populations, you don't even need adaptation to drive them apart. In the 1920s, Sewall Wright showed that the genetic makeup of populations can change without adaptation, through a process called genetic drift.

So what I'm asking is: did natural selection fail to catch on partly because Darwin, understandably dazzled by his insight, made the idea carry too much weight?

On Wednesday: Laws of Variation.


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So what I'm asking is: did natural selection fail to catch on partly because Darwin, understandably dazzled by his insight, made the idea carry too much weight?

Quite possible. On the other hand, you (and I) are reading the Origin with a modern knowledge-base. We know sympatric speciation is rare and hard to prove, that allopatric speciation is common as crabgrass, and that genetic drift can produce divergence and eventually speciation. Darwin knew none of these things. He didn't know about genes, much less genetic drift. He didn't know anything about population biology. All that came later, after people had accepted evolution and were looking for its mechanisms. He may well have spent so much time on explaining and defending sympatric speciation because it never occurred to him that speciation could occur any other way.

By wolfwalker (not verified) on 19 Jan 2009 #permalink

Darwin has manoeuvred us into position so carefully, showing the power of artificial selection, the mutability of species and nature's cutthroat struggle, that we're practically ahead of him by this stage. Who needs convincing?

To me this is the beauty of Darwin's work, the evidence he presents is so meticulously presented and overwhelming you have no choice but to come to the same conclusions he does. This will become evident even more when we come to the sections on the problems Darwin saw for his theory, like the imperfection of the fossil record. I won't give the plot away about that but suffice it to say that Darwin not only gives evidence for his theory but also acknowledges problematic points where needed and then dismantles them ruthlessly.

If very few of us have read The Origin of Species from end to end, it is not because it overtaxes our mind, but because we take in the whole case and are prepared to accept it long before we have come to the end of the innumerable instances and illustrations of which the book mainly consists. Darwin becomes tedious in the manner of a man who insists on continuing to prove his innocence after he has been acquitted. You assure him that there is not a stain on his character, and beg him to leave the court; but he will not be content with enough evidence; he will have you listen to all the evidence that exists in the world.

George Bernard Shaw, Preface to Back to Methuselah ca. 1920.

Not that anything else in the Methuselah cycle made any sense, mind you ...

We know sympatric speciation is rare and hard to prove, that allopatric speciation is common as crabgrass, and that genetic drift can produce divergence and eventually speciation

We do not in fact know any of that. Until recently the evidence for *any* speciation was thin, and in the absence of evidence, people liked to imagine speciation by physical separation, in large part due the rhetoric of Gould, who argued that absence of evidence was evidence.

The research on three spined sticklebacks, shows that for three spined sticklebacks, sympatric speciation is extremely common, and allopatric speciation is insignificant in the sense that: if two physically separated groups of three spined sticklebacks adapt to similar environments, they become similar, and can and will interbreed if given the opportunity, but if they adapt to different environments, then, whether separated or not, they become different, and disinclined to interbreed if given the opportunity.

Science. 2000 Jan 14;287(5451):306-8

"Populations of sticklebacks that evolved under different ecological conditions show strong reproductive isolation, whereas populations that evolved independently under similar ecological conditions lack isolation."

Our most complete record on speciation events is that for foraminifera, because we have a humungous number of fossils neatly stacked in layers at the sea bottom.
For foraminifera, all observed speciation events that have been observed in sufficient detail are sympatric.

Gould wanted to discredit sympatric speciation, and argued for the primacy of allopatric speciation, because if adaption, rather than separation, is primary, then this has disturbing implications for our own species. From the supposed primacy of separation over adaption, Gould argued in his essay "human equality is a contingent fact of history" that the races of man must be equal in mean and distribution of abilities, thus the primacy of adaptive speciation over allopatric speciation that we do in fact observe today is as disturbingly politically incorrect as Darwin's comments on the races of man.

Never the less, I think most animal systematists still think allopatric speciation is the more usual event. I think, for example, based on distribution and relationship, that Austrofundulus species are the result of allopatric speciation events. If pressed about some of their relatives, I would have to admit to being less sure.

By Jim Thomerson (not verified) on 21 Jan 2009 #permalink

based on distribution and relationship, that Austrofundulus species are the result of allopatric speciation events.

Geographically separated kinds will usually encounter different environments, resulting in differential selection. The question then is what is usually causative, what usually causes a species difference - differential selection (Darwin's story), or separation (Gould's story)?

To answer this question, one should look at closely related kinds in both similar and different environments - which was the focus of recent experiments and studies on three spined sticklebacks.

Updated Life's Manifest May 2009

Recapitulation of some earlier notes on the
Scientific Comprehension Of The Origin, Drive, Nature And Purpose Of Life

A. Uniqueness Of science among human artifacts

ALL aspects of our culture are, of course, anthropoartifacts, including science. Yet among those artifacts science has a distinct uniqueness for us.

During the recent several centuries in the course of human history humans have been developing science at an accelerating rate as a provider of convincing, ever closer approaching, approximate models of the real world.

B. Origin and nature of life

Astronomically there are two "physics", a "classical physics" system of and between galactic clusters, and a "quantum physics" system WITHIN the galactic clusters.

The onset of big-bang's inflation started gravity, followed by formation of galactic clusters that behave as Newtonian bodies while continuously reconverting their shares of pre-inflation masses back to energy, and of endless intertwined evolutions WITHIN the clusters in attempts to resist this reconversion.

As mass is just another face of energy it is commonsensible to regard not only life, but mass in general, as a format of temporarily constrained energy.

It therefore ensues that whereas the in-space expanding cosmic constructs, the galaxies clusters, are - overall - continuously converting their original pre-inflation mass back to energy, the overall evolution WITHIN them, within the clusters, is in the opposite direction, temporarily constrained energy packages such as black holes and biospheres and other energy-storing mass-formats are precariuosly forming and "doing best" to survive as long as "possible"...

C. The drive and nature of Earth life

Earth life Genesis, formation of the first genes, was a phenomenon of serendipitous occurrence, in a supportive environment, of 'favourably-coursed' energy potential between in-coming sun's radiation and polymerizing-precipitating RNA-related oligomeric configuration.

The drive of Earth life and of its evolution is to enhance the functionality and survivability of Earth's genes, in order to maintain and enhance Earth-biosphere's temporary constrained energy storage and to maintain the biosphere BIO as long as possible.

It is the genes, life's prime strata organisms, that evolve, and the evolution of genomes, the 2nd stratum of life, and of the 3rd life stratum cellular organisms, is an interenhancing consequence of their genes' evolution.

D. The formation of Earth life

Earth Life: 1. a format of temporarily constrained energy, retained in temporary constrained genetic energy packages in forms of genes, genomes and organisms 2. a real virtual affair that pops in and out of existence in its matrix, which is the energy constrained in Earth's biosphere.

Earth organism: a temporary self-replicable constrained-energy genetic system that supports and maintains Earth's biosphere by proliferating and maintenance of genes.

Gene: the primal Earth's organism. (1st stratum organism)

Genome: a multigenes organism consisting of a cooperative commune of its member genes. (2nd stratum organism)

Cellular organisms: mono- or multi-celled Earth organisms. (3rd stratum organism)

E. Update of underlying life sciences conception is thus feasible

- First were independent individual genes, Earth's primal organisms.

- Genes aggregated cooperatively into genomes, multigenes organisms, with genomes' organs.

- Simultaneously or consequently genomes evolved protective-functional membranes, organs.

- Then followed cellular organisms, with a variety of outer-cell membrane shapes and

This conception is a scientific, NOT TECHNICAL, life-science innovation.

It is tomorrow's comprehension of life and of its evolution.



F. The purpose of OUR, human, life

The purpose of OUR life and of its promotion is ours to formulate and set. It derives solely from our cognition.


Dov Henis
(Comments from 22nd century)

By Dov Henis (not verified) on 18 May 2009 #permalink