Next time you feel ignored at the dinner party table throw out the comment: "I see the morphologists and the molecular biologists are having a right old battle for supremacy in biological systematics. My money is on the molecules."
If you are lucky, the company will include the avuncular Dr Des Higgins of the biochemistry department, UCC, who will give an entertaining account of the general subject. If Des isn't available you may have to ship some heavy weather - but you will have grabbed attention.
Biological systematics is the study of the taxonomic classification of biological organisms, what is related to what, and what is descended from what. Traditionally, classifications are based on structural evidence, morphology. Pathways of evolutionary descent from now extinct ancestors have relied on examination and comparisons of the structures of fossil skeletons and the bones and soft tissues of living species.
An alternative way to study systematics is to analyse the sequence of information in the genetic material (DNA) or the proteins (another expression of the genetic information of organisms). The more closely related two organisms are, the more similar their DNA and proteins will be.
Proteins are made of chains of chemicals called amino acids. There are 20 amino acids. The protein cytochrome C has a chain of about 100 amino acids.
When you compare the amino acid sequence of yeast and horse cytochorome C, you find 48 differences. These are two widely separated species. Only two amino acids differ between the cytochrome C molecules of ducks and chickens, two closely related species.
Taxonomic classification of biological organisms and taxonomic trees tracing evolutionary descent, based on traditional morphological evidence, are well established. When these classifications and trees are drawn up based on the new molecular technique the broad pattern agrees very well with the traditional method. However, as more comparative molecular data become available, differences in detail of increasing importance are coming to light between the two methods.
A few weeks ago I was browsing an article in the science journal Nature which discussed the current state of the methods of drawing evolutionary trees, specifically comparing the traditional morphological with the newer molecular technique. My eye was arrested by the name Desmond Higgins, the co-author, with Dan Graur of Tel Aviv University, of a seminal paper in this field.
I was naturally interested because Des Higgins occupies the office next door to me at work. Nature is probably the most prestigious science journal in the world, so when it picks out your work for special mention you know you are making an impact.
Graur and Higgins in a 1994 paper in the journal, Molecular Biology and Evolution, cast doubt on the accuracy, in some instances, of the traditional method of morphology. The specific case in question, the origin of whales, is one of the most enduring evolutionary mysteries. How did whales make the transition from a terrestrial to a fully aquatic existence?
THE traditional system classifies cetaceans (whales, dolphins and porpoises) as a sister group to the Artiodactyla. Artiodactyls are eventoed hoofed mammals, divided into four sub-groups: camels and llamas; cattle and deer; pigs and peccaries; and hippopotamuses.
Modern cetaceans don't have toes, but early fossil whales did have even numbered appendages. The teeth of ancient cetaceans also look identical to the teeth of an extinct group called the mesonychids, with whom they are now classified as a sister group to the Artiodactyla.
Graur and Higgins compared the whale DNA that is present in little organelles in the cell called mitochondria, with mitochondrial DNA in various other species. Their results placed whales right in the middle of the Artiodactyla. Other workers extended these studies and showed that whales are closer to hippos than to any other artiodactyl.
The modern hippopotamus is native to Africa and is one of the largest four-footed animals. It reaches a length of 14 feet and a weight of 3.6 metric tons. It is semi-aquatic, spending most of the day with only its eyes, nose and ears above water. It can spend up to 30 minutes underwater.
During the day it feeds on aquatic vegetation and emerges from the water at night to feed on land plants. Hippos often sink to the river bottom and take a walk browsing on the vegetation.
Hippos travel in herds of about 40 animals. Cows bear one calf at a time and are ferociously protective of it. Given all of this it is not difficult to visualise how the modern hippopotamus and the modern whale had a recent common ancestor.
The origin of whales is not the only area in which the traditional approach has produced interpretations at variance with the picture revealed by the molecular techniques. Rival camps have developed, each proclaiming superiority for its own technique. Arguments between rivals can reach that white heat of intensity capable of being generated only in the ruffled feathers of academics.
A compromise position has recently emerged called "total evidence" in which both molecular and morphological evidence is combined in the same analysis. However, there is disagreement over how to analyse the data in order to prevent one type of evidence dominating. Clearly there is still some way to go before things quieten down in this field.
William Reville is a senior lecturer in bio-chemistry and director of microscopy at UCC.