I previously talked about an earlier paper where Eldredge and Gould put forward their concept of punctuated equilibria. This was a rather controversial concept at it's time. I think it's gone through the infamous 'stages of truth' series (which weirdly are tied up with Darwin's bulldog Huxley and famed embryologist von Baer as detailed here); first ridicule, then violent opposition, and then the claim that it's trivially and obviously true.
Or, as the claim was initially stated by Schopenhaur:
Reading these papers you see a lot of work that seems trivially obvious today: allopatric speciation, local sections don't necessarily represent the wide ranging species, speciation is a process of branching, etc. But some other aspects of these papers haven't had their brief victory celebration yet (species selection and speciation through regulatory gene revolutions).
"To truth only a brief celebration of victory is allowed between the two long periods during which it is condemned as paradoxical, or disparaged as trivial."
Gould and Eldredge's major gripe is that gradualism prevents examination of the tempo and mode of evolution, and that in particular gradualism can't be refuted by fossil evidence because the evidence has been heavily interpreted under the gradualistic framework. Punctuated Equilibria, they argue, allows for fair assessment of evolutionary tempos, and once we understand that we can make some inferences about the mode that evolution operates through. They state that if the tempo is punctuational, then the mode is 'speciation' or branching.
By "Tempo", G&E borrow from George Gaylord Simpson (a leader in the Modern Synthesis) and mean variation in rates of evolution between lineages, while "Mode" is the mechanism that produces the variation in rates.
Tempo, once the blinkering effect of gradualism can be removed, can be observed empirically, and punctuated equilibria is a mode that can be inferred from it. G&E feel that punctuated equilibria will be 'orders of magnitude' more important than phyletic gradualism, and that phyletic gradualism occurs 'hardly ever'. Importantly in this work, the fossil records sometimes failure to display radical morphological change, rather than being a 'failure to record' information is actual information, stasis, they reiterate is data. This is a necessary implication of applying the neontologist's allopatric speciation to the paleontologist's fossil record.
G&E also make it clear that they want two major changes to occur in the way paleontologists do their work (or rather did, since the initial paper on this topic was from more than 30 years ago); 1) evolutionary trends are the result of (higher level) species selection; and 2) workers need to quantitatively study the evolution of entire ecosystems and their members. They particularly want for workers to quantify geographic variation within a population and compare that to stratigraphic variation along and between lineages before anyone can really talk about stasis or punctuation (although they seem to relax that standard sometimes when people make claims of finding punctuation).
They then go through a number of studies that have supported or contra-indicated punctuated equilibria and tests for it, amoung them Stanley 1975 (pdf); Hecht 1974 (a longstanding and well-respected Chair of Biology at CUNY Queens College); Hayami & Ozawa 1975; Makurath & Anderson 1973; Gingerich 1974, 1976, 1977 (pdf); and Klapper & Johnson 1973 (whom they seem to paint as what I will call 'naive gradualists'). They also take to task workers who publish on evolutionary trends when all they really have are three data points, an original population, and then say one where the individuals are bigger on average, and then a third population where they're a little bigger again, on average. Trends really have to be based on many points, not just a few, G&E go through some basic statistics on why this is so.
G&E also look at a few cases where other authors have calculated the rates of gradualistic change, finding rates along the lines of 10% per million years. This means a fantastically small amount of change per year or per generation, which should just be wiped out by genetic drift. It also implies uninterupted multi-million year long selection pressures, which is terribly odd, and also seems to beg the question of why not select for more change over a shorter span of time?
Speciation Theory
All of this is well and good and most workers today try to follow these recommendations, good papers carry them out fully, examining entire faunas, gathering solid data for statistical analysis, paying attention to stratigraphy, etc, and really great papers make explicit statements about null & multiple working hypotheses and put their theoretical assumptions up front.Beyond this, G&E start trending into more controversial territory. They enter this territory by way of an opening analogy:
"speciation is the raw material of macroevolution, and genetic substitution within populations cannot be simply extrapolated [into macroevolution] [...] We therfore challenged [that] change in gene frequency within populations is hte building block of major evolutionary events"Species selection/species sorting is selection at a heirachical level higher than the cannonical individual, it is something that is still strongly debated today. As they presented it here and in their 1972 paper, marginal species randomly enter peripheral environments. These sub-populations respond especially well to their surroundings in some types of peripheral environments. From this, an overall effect arises, a trend. Lets say a population is exposed to cold conditions at multiple points (and even at many times) along the edge of it's range, and that this species always tends to strongly react to 'cold' by forming thicker fur, bulkier body-types, etc. This means that, of the varieties of this species that are out there, a bunch of them are going to be cold-adapted, and over geologic time-scales, you're often going to have cold-adapted sub-types pop up. There's a good chance that the cold-adapted subtype, simply by the numbers, and through the actions following allopatric speciation, will tend to be successful and replace it's parent variety/sub-species/species. And this process repeats. The "net effect" is a trend in body-type, fur, etc.
This is a messy idea, and a big problem is, what are G&E saying is actually going on? To continue with the above example, does the trend towards 'cold-types' happen because of differential reproductive success of some individuals within a population, or is it happening because of the success of the species as a whole? How do we distinguish selection for cold-type individuals against selection for cold-type species?
Also, in the above, and maybe this is just my misunderstanding, the overall environment doesn't need to get cold, the whole lineage can show a trend towards the 'cold-type' while the temperature across the overall range remains the same, the cold-types win out because there's always a bunch of them around and they can expand out of their limited allopatric range. Perhaps that'd be a good test for species selection, a non-adaptive trend that starts out as an adaptation to a local environment (this is the opposite of how Gould often talks about adaptations, with most current 'adaptations' starting off in a different functional context, or iow as 'exaptations', in his coinage).
G&E talk about their ideas about trends and species selection being a necessary/logical consequence of two things, 1) the occurence of punctuated equilibria (itself a logical consequence of allopatric speciation applied to the fossil record) and 2) that the morphology associated with a speciation event is random with respect to the direction of evolutionary trends within the group (their so-called "Wright's Rule").
G&E also wander a bit too closely to the line of '"explains everything" when it comes to punctuated equilibria also. They consider anagenesis to be simply the result of species selection over many, many splitting events (making it something like a trend). But anagenesis is most people's word for "phyletic gradualism". So they appear to be saying that punctuated equilibria can actually explain the very process that it's set up antithetically to, perhaps this dialectic 'negation of negation' is the Marxist-Hegelian spirit at work in Gould. Regardless, a theory that explains everything in the universe is a useless theory (consider the "godidit" idea, even contradicting evidence can be explained as "godidit"). Perhaps, similarly, a theory that can explain everything under it's ambit is a little too good (and by implication we're probably deceiving ourselves and the theory is ultimately wrong or more limited). Along these lines, G&E even claim that punctuated equilibria can now explain events below the species level, especially within asexual species; we already have a theory that explains those phenomena, it's the standard modern synthesis, there's no need to tack on punctuations.
G&E also step into what I think is unfamiliar territory for them, and they really step on it. Perhaps in the 70s it was debateble if they were right or wrong, but their idea that speciation occurs with a 'genetic revolution' (admittedly this is attributed back to Mayr), and that this revolution involves drastic re-structuring of the regulatory parts of the genome, is terribly wrong. When you start writing things like (and this is actually Carson 1975 that G&E are quoting):
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| No, not that Carson |
"Speciation is considered to be initiated when an unusual forced reorganization of the epistatic supergenes of the closed variability system occurs"
you know you're in trouble: time to take a step back and re-evaluate (see here for a more sympathetic and fuller discussion).
G&E are particularly wedded to the idea that evolution occurs through variation in the tempo and mode of development, which is why Gould is so interested in heterochrony and paedomorphy. Perhaps Carson's statements were just too 'in line' and tempting with their thinking to prevent them from stepping into this topic. I just want to be clear, this issue of the genetics of speciation was a lively topic for a long period of time, there's nothing 'invalid' about it obviously, it just seems to me the G&E overstepped, widely, by taking a side on this issue in this paper.
G&E, despite getting some things very wrong, got a lot right. In particular, Figure 1 shows how they see punctuated equilibirum, with it's branching pattern of speciation (they call it a 'v' pattern and contrast it with what others call a 'y' patter). Figure 1 (Figure 8 in their paper) is basically what any phylogenetic tree looks like in modern papers today.
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| Figure 1 |
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| Figure 2 |
This is a minor point in some ways perhaps, but it illustrates that G&E were definitely on the right side with respect to branching and clades.
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