Sunday, March 16, 2008

Paucity of useful books on mammals

A quote from a 1993 book called Mammal Phylogeny:

"Prior to the advent of phylogenetic systematics... the influence of environment over natural selection was virtually the only mechanism invoked to describe morphological patterns discovered in the history. It was argued that similar environmental demands... led to the convergent evolution of 'mammalian' characters in many different lineages. Implicit in many discussions is the thought that convergence was so prevalent that the true genealogy could never be known with any precision.Phylogenetic systematics has turned our attention from many of these issues. The discovery of monophyletic taxa replaced definitions of grades as the central issue in understanding early mammalian history." (pp130-131; my emphasis).

It goes on to say there is no doubt mammalia is monophyletic. Further: "Previous views saw taxa as classes defined by characters, while contemporary phylogenetics views taxa as individuals defined by common ancestry".

This gives some context to the difficulty I had in finding useful books on mammalian evolution in the university library (UNSW). Most of them were not recent enough to encompass the revolution in evolutionary analysis. One of the more promising titles had an unpromising date from the 1960s, last updated 1972.

The quote above suggests the cladistic approach to analysis has cleared the decks, so to speak. On the one hand, the attempt to group all species into equivalent hierarchical sets was obviously thoroughly doomed. This, even apart from the quite vexed issue of delineating evolutionary lineages into species in the first place.

On the other hand, systematic phylogeny has immeasurably clarified evolutionary relationships, which to my mind should be rather paramount.

On the other, other hand, I can greatly empathise with adherents of Linnean taxonomy, whose tidy world is so greatly threatened. But they should acknowledge the appropriateness of the threat: phylum, order, class, family, and species were always arbitrary, and give unhelpful illusions of equivalence that just do not exist.



Another difficulty for me with libraries is the sort of books I'm looking for: non-therians, that is mammals that are not placental (which also excludes marsupials). Most evolutionary biologists happen to be very much focused on extant species and lineages. Thus most books on mammalian history treat eutherians or metatherians - or monotremes at a pinch.


Reference
Szalay, F, Novacek, M and McKenna, M (eds.) (1993): Mammal Phylogeny. Springer, New York.

2 comments:

Anne Gilbert said...

Of course any taxonomic system is at least somewhat arbitrary. The "Linnean" system is very neat and orderly --- and arbitrary. Cladistics is messier, and probably "truer" in reflecting evolutionary change through time. But, like the process of evolution itself, it too is messy. And both are arbitrary, to some extent, in deicidng where a species, genus, order, etc., begins or ends.
Anne G

Stephen Simmonds said...

Anne,

Yes, spot on. It's about the human effort to create an orderly framework where one doesn't exist. Ideally, we'd figure some tool or mechanism to describe this messy map. I'd say we're in an interregnum right now. I reckon there must be a more useful analytical method than simply cladograms.


(And if you ever come across useful resources for specifically non-therian mammals, please let me know...)