Community Change: Persistence
Last week I began to answer the question “How, and why, do we see such a large variety of different species on the planet today?” by talking about how evolution can cause splits in a lineage and result in different species over time.
Today, I am going to talk about Persistence – what it is and how it affects a community over time.
Now, the dictionary definition for the word persistence is as follows:
Persistence, NOUN; The fact of continuing in an opinion or course of action in spite of difficulty or opposition.
However, I shall be using a slightly modified definition when discussing the fossil record. The definition I will be using is “persistence is the apparent lack of change in a fossil community over time.” This is where, if you were to track a community over a geological period, you would see no or very little change to those organisms.
This is seen in the fossil record quite often, a study done by the palaeontologist DiMichele in 2005 showed that plant fossils in the Carboniferous (about 360-300 Ma) showed no change for long periods of time, with sudden changes over short periods of time. This is called Punctuated Equilibrium – a theory devised by Niles Eldredge and Stephen Jay Gould in the 70s – and is a theory to account for these sudden bursts of change in the fossil record.
The theory of punctuated equilibrium is that change in the fossil record is rare and rapid, taking place in events called cladogenesis in which new species are formed (note, that it does not say that new orders and classes are being created out of the blue). In between these events is a period of stasis, where little evolution takes place. However, is punctuated equilibrium a reasonable theory for explaining this persistence?
The problem with the theory is that it does not allow for any change from one generation to the next – which there is, of course – it is observable. Last week, when I talked about rabbits (who have a very quick generation time relative to the larger mammals) who can show this (in the words of Kim Sterelny) “wobble about the phenotypic mean” (which just means, variation in a species over generations) very quickly. Even humans show this “wobble” – think of how subtly different you look to your parents, then compare that to your grandparents and so on. There will be similarities, but also quite a few differences from one generation to the next.
When I defined the word “persistence”, I deliberately added the word “apparent” in to the definition. Remember that the fossil record is extremely bias in what it shows us. If you were to walk outside, fall into a ditch and die – what would happen to you, assuming that no human beings were around to find you?
Chances are, you would either be eaten by wild animals or just decompose – only leaving your bones remaining. Now, the chances of them being buried and then surviving compaction and fossilisation are very low – and even still, we only have your bones. We cannot see what your face looked like, what your organs looked like or what you ate for breakfast that morning. In a similar way, we have no idea what the soft bodied parts of an organism looked like (but we can make a very good guess at it!) or how they were used. When I say the “apparent lack of change”, we cannot account for changes in ecology (where and how the organism lived) or body plan (what the organism looked like compared to another fossil in terms of soft tissue). The phyetic gradualism theory could, therefore, work perfectly – it’s just that we cannot see it working.Eventually, smaller changes could lead to a visible change that we see in the fossil record.
Smaller scale instances of persistence can also be explained by evolved ecological relationships. The relationship, for example, between “Darwin’s Moth” and “Darwin’s Orchid” is a perfect example of this. The moth has evolved a particularly long proboscis in order to feed off the plant, which has developed a particularly long spur. The moth is perfectly evolved to feed off this plant, the the plant has evolved in such a way to ensure that it is getting pollinated. As long as one member of the evolutionary relationship survives (does not go extinct for any reason) then the other will also flourish and persist.
DiMichele also mentioned in his 2005 study about the “Theory of Large Numbers” – a theory that literally says that the more prevalent organisms will remain the most common due to their higher offspring count. This has links to another theory devised by the biologist Hubbell in 2001 – the Unified Theory of Biodiversity (UTB). The UTB says that any given community will have a constant number of individuals and new individuals can only join the community when an old one has died. There are three different ways of replacement of the old member;
- A new member of the same species is born – a reproduction event
- A member of the metacommunity joins the community – an incorporation event
- A new birth results in a new species – a speciation event
The theory takes the probability of any of the events happening, as well as the probability of an individual dying, to be equal. Species will persist due to the chance of a new species surviving being very low, allowing the numerically superior species (through the Theory of Large Numbers) to survive.
Persistence can explain why we see stasis in fossil communities over time. The evolution of life on Earth is not as simple as what we can see from the fossil record, and we need to consider these periods of “stasis” for all the possibilities that they might be.
Next week, in the final week of this mini-series on Community Change, I will be discussing what happens when everything goes wrong – Mass Extinctions and Interchange.
Until then, have a wonderful week.
Persistence is not a topic which is commonly written about in literature, however some of the ideas can be read up upon. If you want to know more about punctuated equilibrium and the minds behind it, picking up any of Goulds books wouldn’t be a bad idea. Wonderful Life, although not directly about the theory, is a good read and (although controversial) can help with ideas of evolution on an Earth were life was just taking a hold. Other than this, any books on evolution might mention some of the ideas in this post.
If you are looking into more academic papers, then the one I have heavily used in this article is the one by DiMichele et al. titled LONG-TERM STASIS IN ECOLOGICAL ASSEMBLAGES: Evidence from the Fossil Record – which is one I would highly recommend. Another good read is a paper by Abrams, P written in 2000 titled THE EVOLUTION OF PREDATOR-PREY INTERACTIONS: Theory and Evidence.