Fodor’s intensional criticism of evolution is that the process of evolution is unable to make the necessary distinctions in selecting traits. This is to say that evolution itself cannot select for specific traits. If evolution can’t select for traits, then we will definitely not be able to figure out what’s happening based on evolution. Hence evolution is not a good theory.
Does evolution need a mechanism to select for certain traits?
No, there is no need for a mechanism which decides that it wants a certain trait and then systematically selects for that trait.
Instead evolution is more like a Plinko / Pachinko machine with moving pins and prize locations. Organisms – the balls – live and die by bouncing off whatever exists in their environment – pins and prizes -. Whoever happens to land in a good location gets to have their genes replicated. In this setup there is no need to appeal to some evolutionary mechanism to select traits because with the environment and organism described, the evolutionary traits that will be selected are probabilistically determined.
Our evolutionary explanations, then, describe the environment – the position of the Plinko pins and prizes – and the biology of the organism – the shape and location of the Plinko ball – to show why that organism ended up in a position to replicate. If we want to describe how we evolved to have hands, for example, we show how organisms that more consistently landed in the right locations had the traits that led to us having hands, and not because there was a mechanism to pick out ‘having hand traits’ at the start.
Therefore Fodor’s argument from intensionality is a straw man: Evolution does not need to be able to make the distinctions that Fodor says it needs to make. Hence there is no problem within evolution.
[EDIT: I’ve put up a new analysis (24 March 2010) of Fodor’s argument here: Hypotheses Natura Non Fingo]