Evolutionary biologists don't tend to like Plato very much. Consider the following quote from Richard Dawkins:
" If, like Aristotle, you treat all flesh-and-blood rabbits as imperfect approximations to an ideal Platonic rabbit, it won’t occur to you that rabbits might have evolved from a non-rabbit ancestor, and might evolve into a non-rabbit descendant."
Dawkins explicitly draws on the renowned evolutionary biologist Ernst Mayr, who argued that the "dark hand of Plato" was the reason why evolutionary theory took so long to be (almost) fully accepted in the West. I will briefly address the above criticism and some others in this entry.
The thought here is that if one believes that members of a species all reflect the same unchangeable form, one is unlikely to accept that the ancestors of the members of this species belonged to a different species. Dawkins and Mayr may be right that Plato's influence has contributed to the great resistance to evolutionary theory, and even that platonic realism itself makes evolutionary ideas seem unintuitive. Nevertheless, it is important to emphasize that nothing in the theory of platonic realism contradicts the notion that a given species can evolve from another species. This is because the claim that all members of a species participate in the same form is perfectly compatible with the claim that said members have a disposition to bring about (however indirectly) members of a different species, who participate in a different form.
A more worrying objection is voiced by Prof Volker Sommer, a primatologist at UCL. Sommer argues that the categories that species constitute have "blurred edges". The evolutionary account of biological diversity entails that, rather than being discrete, sharply distinguished 'boxes', species are continuous with one another, in much the same way that colors are. Thus, just as there is no sharp boundary between orange and red, there may be no sharp boundaries separating two species, which means that there will be 'borderline' cases where there is no fact of the matter as to whether a given organism belongs to one species or another.
Sommer suggests that, because of this, there is a sense in which, far from merely defining preexisting "borders of the world", zoologists "create worlds". That is, the species to which an organism may or may not belong is a convenient artificial grouping, rather than a real or natural one. We create such artificial groupings all the time: for example, I might conceptually group some human beings under the name 'friends', and others under the name 'acquaintances'. Groupings like this are practically useful, because they help me navigate the complex world of human relationships (for example, to decide whose email to answer first, and so on). But they aren't real, in the way that we'd expect species to be real.
Interestingly, when I took his module back at UCL, I once heard Sommer say in a lecture that it was ironic for Darwin to have named his book The Origin of Species, since his theory implied that there really are no such things as species.
But if species aren't real /natural groupings, they can't be said to reflect forms -- as we saw previously, part of the point of positing a realm of forms is to explain why such real groupings exist.
This problem is well-known to philosophers of biology, and there is no way a single blog entry could do it justice. Hence I will content myself with a few points:
1. The objection, at most, shows that members of the same biological species don't participate in the same form. This is much more modest than saying that no grouping is made up of things participating in the same form. Realists don't need to believe that all the names and categories we use are real and not artificial, and hence linked to their respective forms. Few, for instance, would say that there is a form of the 'democrat' or of 'being an acquaintance'.
2. The objection relies on an inference from 'this grouping has vague boundaries' to 'this grouping is artificial, rather than natural'. But it really isn't obvious that a grouping doesn't have anything to do with the real structure of the world if it has borderline cases. Take two kinds, 'bird' and 'zird'. Something is a member of 'zird' if it is either a bird or a mobile phone. 'Zird' is an artificial grouping if there ever was one. The evolutionary story tells us that members of the kind 'bird' are descendants of the kind 'non-avian dinosaur'. This means that there almost certainly have existed organisms for which it is vague whether they belonged to 'bird' or 'non-avian dinosaur'. So 'bird' has vague boundaries. But it just seems wrong to say that 'bird' is on par with 'zird', both being equally artificial groupings. For one thing, that an animal exemplifies 'bird' explains much more than that it exemplifies 'zird'. The fact that Pete the parakeet is a member of 'bird' explains quite a lot about Pete, e.g. the fact that he has feathers and a beak. Indeed, these traits are what we would expect of a member of 'bird'. In contrast, the fact that he is also a member of 'zird' explains very little about him. It may perhaps explain the fact that he is a material thing, since all members of 'zird' are material things. But this explanation in terms of being a zird is totally unnecessary, as the fact that he is a bird can explain this just as well (since all birds are material things).
Of course, those suspicious of allegedly 'natural' groupings could retort that this just makes 'bird' more useful than 'zird', not any more real (see Nanay 2011 for a view along these lines), since it helps us make more predictions about the features of members of 'bird'. I would tentatively submit that high explanatory and predictive power is just part of what it means for a grouping to be 'natural'. In fact, I can't really see what more one could ask of a given grouping for it to genuinely count as 'natural', i.e. somehow reflective of the structure of nature. In that case, saying, 'I see that the grouping 'bird' helps you explain and predict many features of the members of 'bird', but is it a natural grouping?' would be conceptually confused, like saying 'I can see that you are single and male, but are you a bachelor?'.
If I'm right, the fact that species (like Homo sapiens) have vague boundaries does not imply that they are purely artificial groupings, and hence that they cannot have a corresponding platonic form, which their members participate in.
These responses to Sommer's objection may not be decisive (at least in their current form), but they should at least give evolutionary biologists an opportunity to reconsider their natural aversion to platonism.