Friday, May 13, 2005

Split Up

DarkSyd at UTI has a great series on hominid evolution.

This got me to thinking (again) about how taxonomists overly split the hominid lineage. If we were talking about any other mammal, everything within the genus Homo would be one species (possibly broken into subspecies), and all apes would be classified as a single genus (or maybe just Homo, Pan and Gorilla). Of course, mammalian taxa are overly split relative to other vertebrate taxa, vertebrate taxa are overly split relative to other animal taxa, and (I'm assuming) animal taxa are overly split relative to plants, fungi, and prokaryotes.

Taxonomy is a combination of observation and classification (a fairly objective process) and hierarchical naming (more subjective). Some of the oversplitting of certain taxa could be due to the amount of knowledge we have regarding those taxa. For example, we know much more about primates because we are interested in our origins so paleontologists search for those fossils. This contrasts with, say, what we know about insects -- there has been much less study on these critters relative to the amount of species diversity within this group. This is the subjectivity involved in observation and classification.

The other reason for oversplitting of hominids, mammals, animals, etc is that we tend to oversplit things that are similar to us. We put chimpanzees in a separate genus (we even put other hominids in separate genera!) despite the fact that we diverged from them within the last 10 million years. Contrast this with another well characterized group: Drosophila. According to the most recent molecular dating, the last common ancestor of all Drosophila species existed approximately 60 million year ago, yet they are considered a single genus. Granted, this genus is split into subgenera, groups, subgroups, species complexes, species, and (sometimes) subspecies, but they are all still considered to be in the same genus.

I think I'm done beating this drum to a dead horse -- to mix my metaphors.


At 8:11 PM, Blogger John Wilkins said...

The problem arises because there are fixed ranks in the Linnaean classificatory scheme, and so when a richly divergent group is found, the need is to shoehorn that diversity at some arbitrary or subjective level (e.g., family, genera or species).

Phylogenetic schemes lack this absolute ranking, and so they have less problem here, but then it becomes hard to communicate and name groups of interest. So long as it is realised that Linnaean ranks are purely arbitrary and hence are incommensurable, there is no problem using them.

At 2:10 PM, Blogger GrrlScientist said...

Another problem lies in using morphology (and behavior, in some cases) to classify animals and plants. Basically, we don't know the "value" that morphological changes represent to these taxa being studied, and in fact, we don't even know how to rank the importance of genetic changes in taxa being analyzed/ranked using DNA data. In the case of primates, we rank small differences as having huge implications/importance, but generally do not assign the same importance to similarly big changes in non-primate taxa.

So, in the absence of this information, we cannot really design a truly objective ranking/classification system for living creatures. However, each ranking/classification system used is interesting (in my humble opinion) because it represents how another evolutionary biologist thinks about these issues and reveals the importance that s/he places on these differences.


At 9:32 AM, Blogger RPM said...

Given the amazing periodicity of the molecular clock, I don't see why we can't say a certain amount of DNA divergence puts a taxon at a certain hierarchical level. Or, if you prefer real time, we could say that divergence at some mya equals a particular taxonomical classification. For those taxa were the fossil record is sparse, we could use molecular tools to approximate divergence times.

This could be applied to everything above the species level, as species level classifications are an entirely different discussion.

Granted, these new standards would lead to entirely new classifications for many taxa and require a complete overhaul of the current system -- not very practical, I guess.


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