"I look at the term 'species' as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other." (Origin of Species).
Does Darwinian theory commit you to nominalism? Logan Paul Gage, in his essay "Can a Thomist be a Darwinist," gives this as a reason why Thomists shouldn't embrace Darwinism.
From God and Evolution (Seattle: Discovery Institute Press, 2010).