Fair question, but remember that the rational sciences - including commonly accepted mechanisms of evolution or the differentiation of species - were not part of Tolkien's mythos, which is most explicitly outlined in the early chapters of the Silmarillion.
The Elves and Dwarves were explicitly the designed outcome of 'sub-creation' (secondary or consequent developments) by the Valar, and contained to some degree universal traits representative of their creators - for example, the Dwarves prediliction for the depths, and for the use of trees in the making. The Ents were also 'made' in this way, and so on.
Whilst these 'subcreative' events took place over a relatively long period of time, and were affected by what one might call 'chance', Eru's hand is apparent. In addition, Tolkien revisited the First and Second Age repeatedly in notes, incomplete revisions or contextual additions, and the cosmology should in fairness be viewed as unfinished and not definitive, though full of interesting details (on a side note, evolution theory today is neither flawless nor comprehensively definitive).
Rather than looking for exact causal models or a replication of empirical science, perhaps it's rewarding to look at why Tolkien focused on particular kinds of creature, what their significance was, what identifications we can find collectively or individually - and, perhaps most of all, his attentiveness to details of personality or community (which, among other things, certainly distinguishes his work and gives it conviction and substance). I wonder if there are any works (ancient or modern) with the same range of large, thriving, conscious (in a human sense), moral communities that interact with each other in so many different ways. This facet of his work may only be a part of whatever has made LotR such a success, but it represents a devotion to, and expansive vision of, a single narrative framework across the lifetime of the author which may well be unique.
Peace [img]smilies/smile.gif[/img]
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