Posts Tagged ‘punctuated equilibrium’
stuff on punctuated equilibrium and organic complexity
isolation, strong selection, rapid change – an example of peripatric speciation
I’ve read a few books by Steven Jay Gould, and have found them sometimes a bit heavy-handed and clever-clever, though I’m sure I’ve learned a lot from them. I’ve also strongly disagreed with his notion of NOMA or non-overlapping magisteria, and have written about it in the long ago. The notion didn’t last long, it seems. So I’m wondering about how the concept of punctuated equilibrium, developed by Gould and others back in the day, is faring.
The idea was first proposed by Gould and Niles Eldredge in 1972. The PBS evolution library puts it this way:
… species are generally stable, changing little for millions of years. This leisurely pace is “punctuated” by a rapid burst of change that results in a new species and that leaves few fossils behind. According to this idea, the changes leading to a new species don’t usually occur in the mainstream population of an organism, where changes wouldn’t endure because of so much interbreeding among like creatures. Rather, speciation is more likely at the edge of a population, where a small group can easily become separated geographically from the main body and undergo changes that can create a survival advantage and thus produce a new, non-interbreeding species.
This is strange and interesting. I had thought that the ‘rapid burst of change’ was considered to be due to a sudden change in external conditions, brought about by, say, a burst of volcanic activity, or a massive meteorite, or whatever caused the Cambrian explosion (of new life forms) over 500 million years ago. I might return to that shortly. The type of speciation mentioned above, in which there is separation and isolation, creating unique circumstances for diversification in small populations, is called peripatric speciation. It’s one of the four modes of speciation generally recognised, the others being allopatric, parapatric and sympatric. Peripatric speciation is described as ‘a special version of the allopatric speciation mode’. A good example of this allopatric speciation mode, it seems to me, is the separation of the ancestors of bonobos and chimps into these two species about two million years ago. To back me up here’s a quote from UC Berkeley’s Understanding Evolution website:
Allopatric speciation is just a fancy name for speciation by geographic isolation… In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Isolation might occur because of great distance or a physical barrier, such as a desert or river…
So to round this out, I must inform myself about parapatric and sympatric speciation, which means trying to put other people’s explanations into my own words. Parapatric speciation occurs when there’s no outside barrier to gene flow (the movement of genetic material, often in the form of individual carriers of genetic material, from one population to another). Think of human migration, where the mixing of genes might not be random, due to culture barriers, but of course the chances of mixing would be much greater than if there was no migration. Sympatric speciation involves a reduction of gene flow without distancing or physical barriers. In humans it could happen when a particular set of humans breeds only with others of that set. This would take a very long time, and for humans it would involve a very artificial situation, but it has been known to occur, though rarely, in some species of insects and fish.
So, back to punctuated equilibrium. That this concept has been explored, with some evidence found but also disputed, is perhaps an indication of how novel and complex the whole topic of evolution still is. We look for traces of evolution in the fossil record, but that record is piecemeal and easily subject to ‘contamination’ over time. To be honest, much of what I’ve read from the proponents of punctuated equilibria goes over my head, requiring, it seems, an understanding of macroevolution, cladogenesis, phyletic gradualism and much much more. So I’m at a crossroads, sort of. Should I just give up, or plough on, with my not very efficient plough? How important is the topic? I really don’t know.
Here’s an example of the problem, as I see it, from ScienceDirect.com:
Cladogenesis is a phenomenon of evolution that occurs by the divergence of taxa due to positive selection for the adaptation of sister populations from a common ancestor to different environments due to their anatomical, morphological, geographic, temporal, ecological, and/or ethological (behavioural) isolation.
Ah, so that’s what cladogenesis is! But really, what’s a clado (a clade?) and what are taxa? Who’s this being written for? Obviously not dilettantes like me. I’ve turned instead to videos, and found one by someone I know well – sort of. Robert Sapolsky, whose big books Behave and Determined I’ve read with great interest in recent years. Sapolsky gives the story of gradualism versus punctuated equilibrium a humorous twist, with the proponents of gradualism calling the punctuators ‘jerks’, while they in turn were called ‘creeps’. Maybe, but is this really a big issue? Isn’t it quite reasonable to assume that just as the Earth’s biosphere itself has changed gradually but with the odd convulsion along the way, both forms of speciation, or many forms, have occurred?
So I’ve finally found a more enlightening (for me) video, obviously made some time ago, because it featured both Gould, who died in 2002, and Eldridge talking about their work, in good old layperson terms.
Anyway, for me the controversy or non-controversy over punctuated equilibrium is nowhere near as interesting as the one highlighted by Gould in his 1996 book Life’s Grandeur:
The basic theory of natural selection offers no statement about general progress, and supplies no mechanism whereby overall advance might be expected. Yet both Western culture and the undeniable facts of a fossil record that started with bacteria alone, and has now produced exalted us, cry out in unison for a rationale that will place progress into the centre of evolutionary theory.
This is particularly fascinating/amusing for me, as it plays into the ‘directed evolution’ concept which seems to me to be the god-botherers’ last hope for their world-view, though it seems to rule out ‘young Earth creationism’, though you never know with these people. I recall the late Cardinal George Pell, Australia’s most controversial Catholic, trying to argue for God-directed evolution shortly before sex-based controversies caught up with him and he disappeared from public view. Unfortunately, you need to make scientific sense of what this God-thing is, given that our planet is only one of probably billions out there, each of which has its story to tell, as part of a 13.8 billion-year universe.
Gould tries to deal with this conundrum – the undirected nature of evolution, from the point of view of ‘progress’, and how this process has produced ‘exalted us’ – in the last 60 pages or so of Life’s Grandeur, but I find that his tendency to use verbose language really gets in the way of comprehension (the key seems to be a fixed boundary condition) – so much so that I prefer to look to other sources. A research article from 2000, ‘Evolution of biological complexity’, points out, fairly obviously, that the key is to effectively define and perhaps quantify complexity – a difficult task, it seems. After all, complexity is complex.
Definitions may vary and not everyone is likely to agree, but what this article focuses on is genomic complexity, and information – ‘the amount of information a sequence stores about its environment’. I’m not sure what’s meant by a sequence here – perhaps a sequence in an evolutionary chain. The article is long and, for me, forbiddingly mathematical, but as it concludes in the abstract..
We show that, because natural selection forces genomes to behave as a natural “Maxwell Demon,” within a fixed environment, genomic complexity is forced [my emphasis] to increase.
So what is a ‘Maxwell Demon’ (I do know that it comes from James Clerk Maxwell, and that it has to do with the second law of thermodynamics), and how has this ‘proof’ fared in the 14 years since it was published?
Does the emergence of complexity of life on Earth need to be explained? Some, of course, want to explain it in terms of ‘purpose’, opening the door to agency and all that stuff, but I’m not going there. Others might worry about it in terms of that pesky second law (entropy always increases), but then there are ‘closed systems’, such as the Earth’s biosphere, which temporarily act against the ‘growth’ of entropy. Don’t worry, life will become extinct here eventually, and the whole planet will fall apart – the centre doesn’t hold.
Darwin himself was a bit contradictory about this issue. Often he enjoyed, it seems, to point out the directionlessness (to coin a word?) of natural selection, as it seemed to stick in the craws of some, but more often he fell in with his own culture and class and wrote of savages and the less advanced, in at least pseudo-evolutionary terms.
Anyway, complexity has arisen, or evolved, on this planet, so I’ll try to think about it further, with the help of reading and listening to others, in my next post.
References
https://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
https://evolution.berkeley.edu/modes-of-speciation/peripatric-speciation/
http://www.scholarpedia.org/article/Punctuated_equilibria
Stephen Jay Gould, Life’s Grandeur: the spread of excellence from Plato to Darwin, 1996