It is a well known fact that Darwin’s On the Origin of Species contained only one figure: A rather arresting depiction of tree-like descent with modification. What is less well known is that Darwin does not introduce the figure in order to illustrate common descent. Since I’ve come across this misconception a number of times in the past days, I thought it was worth pointing this out.
Look at the figure carefully. Here it is (modified from the PDF of the first edition of The Origin of Species at www.darwin-online.org.uk):
At first glance this certainly looks like a visual representation of common descent: The original species (A) diversifies into eight species, while (I) diversifies into six, with some side branches going extinct in both cases. However, the main message of the diagram is actually about something different.
The vertical axis of the diagram unsurprisingly represents time: There are 14 time steps between the original species (A) to (L) and their descendants a14 to z14. But the horizontal axis is also important. It is supposed to represent divergence between species. The idea is that among (A)’s descendants, the labeled descendants a1 and m1 differ the most from (A), while the intermediate offspring (not labeled) are more similar to (A). The vertical line above (A) designates offspring that are more or less identical to (A). Similarly, for example, the vertical line above (F) indicates that (F) does not change at all over time.
Now compare carefully the variations at each time step that survive for some time (perhaps even to the top of the diagram) to those that go extinct more or less quickly. You will find that it is for the most part the more extreme variations which survive, while the less extreme variations disappear.
What Darwin is trying to illustrate is his “principle of divergence”. This is an aspect of Darwin’s work that did not get accepted in modern evolutionary theory, and so it is always in danger of being in our blind spot when we read the historical sources.
For some time in the 1840s and 1850s, Darwin was worried that natural selection explained adaptation but not divergence. Why would there be many different species living today and not just a small number of highly adapted ones? What explains the abundant divergence of lineages in addition to their transformation toward better adapted forms?
Darwin’s answer was that there was an advantage to ecological differentiation:
We can clearly see this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural powers of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting waters, and some perhaps becoming less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal became, the more places they would be enabled to occupy. (p. 113)
Darwin next discusses a botanical example. He reports an experiment showing that if a plot of land is sown with one species of grass and another plot with several species, then the second plot will support a greater overall biological mass. In line with the quote above, he interprets this as showing that several divergent species can make better use of a plot of land than a single species can:
I cannot doubt that in the course of many thousands of generations, the most distinct varieties of any one species of grass would always have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of species. (p. 114, my emphasis)
Thus, the diagram is supposed to illustrate the origin of species by the principle of divergence:
Only those variations which are in some way profitable will be preserved or naturally selected. And here the importance of the principle of benefit being derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer dotted lines) being preserved and accumulated by natural selection. (p. 117)
What Darwin is describing is a form of sympatric speciation, or of a species splitting into two species in the absence of geographical barriers. For much of the 20th century, the modern synthesis of evolution mainly recognized allopatric speciation, where a population is divided by a geographical barrier (such as a mountain, a river, and so on) into two populations, which then experience different selection pressures until they are sufficiently different to stop interbreeding and thus to count as different species. Sympatric speciation such as Darwin considered was thought not to occur in nature. In recent decades, however, evolutionists have started to take the possibility of sympatric speciation much more seriously again. Perhaps this will make it easier for the next generation of scholars to understand Darwin’s diagram in its intended context.