In the last few days a piece of research has been published which could potentially change our approach to studying evolutionary patterns in the fossil record. So far I have only managed to read the press release and the abstract, so much of what I say could be a complete misunderstanding pulled from my backside. Since the 1970s there has been a debate in evolutionary palaeobiology with regards to the ways in which a lineage evolves. At times this debate has seemed like it has reached its peak, but then something changes the game. On my old blog I did a poorly formatted post explaining punctuated equilibrium, which may be some decent background reading, but I intend to do an update sometime soon anyway. Here is an attempt at a brief explanation of what happened in the debate:
- Evolutionary biologists tended to assume that a species evolved with most change taking place within the lineage (often labelled phyletic gradualism).
- In the 1970s, Eldredge and Gould came along and pointed out that the fossil record shows that a species appears geologically abruptly and changes little during their duration. Most morphological evolution takes place at speciation. This is called punctuated equilibria.
- The introduction of punctuated equilibria caused rich debate, resulting in a lot of study into the patterns of evolution in the fossil record (and some additional concepts which will not be discussed here).
- Examples of phyletic gradualism were found in the fossil record, as was another pattern, punctuated anagenesis, however, punctuated equilibria appeared to be the most common pattern.
One of the issues has always been the techniques used to study these patterns, not to mention, there is a degree of subjectivity when trying to determine which best fits the data (it is often not as unambiguous as the conclusions would have you think). The earliest studies, including those by Eldredge and Gould, tend not to be discussed, as techniques have been refined and many old examples are thrown out or surpassed. The new study claims to have used a technique which was previously unavailable and has some results which may have us scrambling back to the rocks for more samples, or reassessing old studies to see if our conclusions were wrong.
Old studies on evolutionary mode appear to have largely focussed on a single morphological trait, whereas this study looks at a suite of traits of a single lineage, and does so using several different species. They found widespread evidence for mosaic evolution, where characters change within a species at different rates. With this taken into account, analysing a single trait cannot give us enough information as to what is going on with the species; one particular trait might show a pattern which looks punctuated, another may show gradualism, yet neither show what is going on in the bigger picture. And, as is mentioned in the abstract, a single trait may show one particular mode of evolution, whilst the rest of the characters may show another, so the overall change does not fit with the first character.
I’d need to read the paper before I could say much more, but if what it claims is accurate, then this debate may be reignited. This could get interesting…