What Fodor and Piattelli-Palmarini got wrong – part 3: issues left dangling

In this final post on Fodor and Piatelli-Palmarini's book What Darwin got wrong I will deal with the bits and pieces that didn't really fit into my previous two posts.

Cutting down the tree of life?

The authors discuss the process of horizontal gene transfer, which they call non-Darwinian and non-adaptationist. Horizontal gene transfer refers to the transfer of genetic material from one organism to another in a manner different from the typical genetic inheritance by offspring. Bacteria can be genetically altered through uptake of DNA from their surroundings and lateral gene transfer can occur through cell to cell contact or through a virus. Fodor and Piatelli-Palmarini note that

...horizontal gene transfer is the rule, rather than the exception, in microorganisms, to the point that the very notion of a 'tree of descent' is being questioned. The preferred metaphor in microbiology is that of a bush or a network (Doolittle, 1999).

Is it time to cut down the tree of life? According to the New Scientist magazine (1-24-2009) the answer is yes! Moreover, this implies that Darwin was wrong according to New Scientist. 

There are really two issues here. First, is the tree of life still a good model for evolution? Second, is the tree of life Darwinian? When contemplating the tree of life model we need to realize that horizontal gene transfer as well endosymbiosis (the view that mitochondria and chloroplasts were originally 'free' bacteria that were taken inside a eukaryotic cell) primarily belong to the world of microorganisms. If we wish to hold on to the tree of life model we can take these complications into account. We would have something like this:

However, since these complications primarily occur in microorganisms we can simply ignore them when dealing with animals. For example, the whole evolutionary tree from fish to human remains unaffected. Perhaps that helps to place things into perspective.

Is the tree of life a Darwinian idea? Well, not long after his Beagle voyage he did contemplate the tree of life model:

This is taken from his own notes and the words "I think" show that this is very much a work in progress. When Darwin finally published his ideas two decades later he did not use the tree of life model, but rather he spoke of "descent with modification". David Penny makes this clear in a recent article published in PLoS Biolgy. According to Penny descent with modification encompasses a variety of processes including both horizontal and vertical gene transfer. 

Furthermore, Darwin's main legacy is of course the theory of natural selection. Whatever the mechanisms behind the modifications, natural selection is what largely determines which modifications will spread and which will not.

Adapting to an ecological niche

Fodor and Piatelli-Palmarini complain that evolutionary biologists use the fit between organisms and their ecological niche as evidence for natural selection. Of course organisms fit their ecological niche, because otherwise they wouldn't be with us today:

...a creature's ecology consists of whatever-it-is-about-the-world that makes its phenotype viable. That's to say: it is constituted by those features of the world in virtue of which that kind of creature is able to make a living in the world. In effect, the notions 'ecology' and 'phenotype' (unlike the notions 'environment' and 'phenotype') are interdefined. Since they are, it's hardly surprising that a creature's phenotype reliably turns out to be in good accord with its ecology.

But why are there still animals around? How does evolution deliver phenotypes that fit some ecological niche? Natural selection is Darwin's answer, but Fodor and Piatelli-Palmarini, who reject this solution,  admit that they do not know. Perhaps it's worth while to look at a few ways in which species make a living and ask "if not natural selection, then how did this species get this way?"

A famous example is the adaptive radiation of Darwin's Galapagos finches (and yes, the figure below represents an evolutionary tree).

Natural selection has an elegant and straightforward explanation of the differences between these strongly related bird species (none of which are found anywhere else). A single species made it all the way from South America and then it spread over the different islands and started to adapt to the different ecological niches. Mutations that affected the size and shape of the bird's beak made it better or worse at eating insects, seeds or eggs. We find similar adaptive radiations on other island groups. How to explain this if not be natural selection?

And what about evolutionary arms races?

Why did the cheetah evolve to run so fast and to turn so agile? To catch antelope that are also really fast and agile. Why did species of antelope evolve to run so fast and to turn so agile? Because cheetahs and other big cats are also so fast and agile. Circular reasoning? Yes, put like this it is, but it is of course a gradual process in which any mutation that increases speed or agility in big cats or antelope gives it a survival advantage against members of the same species without the mutation. This keeps on going until we get animals like the cheetah, with a ridiculous speed of 115 km/h, and the wildebeest and Thomson's gazelle with a speed of 80 km/h. Natural selection can make perfect sense of these evolutionary arms races. What is the alternative?

Another great piece of evidence for natural selection comes from mimicking animals. For example, take the butterfly papilio dardanus. Professor Armand Leroi explains the implications of mimicry in this marvelous BBC documentary (go to 25:35).





My personal favorite example of mimicry is the Indonesian mimic octopus:




What can explain the amazing mimicry of this octopus? Natural selection has an answer: it has evolved to act in specific ways in specific situations (see for example this press release from the California Academy of Sciences). When a mutation affects the way the octopus responds to certain stimuli then a response that mimics another species may influence its ability to escape predators, especially if the mimicked species is itself a predator or a venomous animal. The octopus is not aware that it is mimicking another species, it just does what it does because it works. If mimicry is not explained by natural selection then I really would like to hear a plausible alternative. Fodor and Piatelli-Palmarini certainly haven't offered one.

To conclude, the authors have not managed to prove Darwin wrong, at least not with regards to natural selection as the main driving force behind evolution. Sure, there were many things Darwin did not know. He was writing in the mid 19th century which makes it rather obvious that he couldn't be right about everything related to evolution. Yet, the theory of natural selection has stood up amazingly well to all critique. I think "What Darwin didn't know" is a much better way of phrasing the developments in evolutionary biology since Darwin. If you haven't yet had a chance I'd recommend watching the BBC documentary above that takes precisely this approach.