I believe that lots of biologists think like this. Let's discuss a quotation from The Blind Watchmaker (1986, p.178-179) by Richard Dawkins. How many readers agree with him?
After many generations of cumulative selection in a particular place, the local animals and plants become well fitted to the conditions, for instance weather conditions, in that place. If it is cold the animals come to have thick coats of hair, or feathers. If it is dry they evolve leathery or waxy waterproof skins to conserve what little water there is. The adaptations to local conditions affect every part of the body, its shape and color, its internal organs, its behavior, and the chemistry of its cells.
If the conditions in which a lineage of animals lives remain constant; say it is dry and hot and has been so without a break for 100 generations, evolution in that lineage is likely to come to a halt, at least as far as adaptations to temperatures and humidity are concerned. The animals will become as well fitted as they can be to local conditions. This doesn't mean that they couldn't be completely redesigned to be even better. It does mean that they can't improve themselves by any small (and therefore likely) evolutionary step: none of their immediate neighbors in the local equivalents of 'biomorph space' would do any better.
Evolution will come to a standstill until something in the conditions changes: the onset of an ice age, a change in the average rainfall of the area, a shift in the prevailing wind. Such changes do happen when we are dealing with a timescale as long as the evolutionary one. As a consequence, evolution normally does not come to a halt, but constantly 'tracks' changing environment. If there is a steady downward drift in the average temperatures in the area, a drift that persists over centuries, successive generations of animals will be propelled by a steady selection 'pressure' in the direction, say, of growing longer coats of hair. If, after a few thousand years of reduced temperature the trend reverses and average temperatures creep up again, the animals will come under the influence of the new selection pressure, and will be pushed towards growing shorter coats again.
Prothero, D.R., Syverson, V.J., Raymond, K.R., Madan, M., Molina, M., Fragomeni, A., DeSantis, S., Sutyagina, A., and Gage, G.L. (2012) Size and shape stasis in late Pleistocene mammals and birds from Rancho La Brea during the Last Glacial–Interglacial cycle. Quaternary Science Reviews 56:1–10. [doi: 10.1016/j.quascirev.2012.08.015]
I'm not yet sure what to think. It seems to me like the authors of that paper are over-interpreting their results (and over-interpreting the predictions of the evolutionary "rules" they cite). Clearly the animals at the earlier few thousand years of the ice age were already adapted to the colder temperatures. If they weren't, then they would have died. These may represent a small fraction of their original populations, carrying a few rare alleles, which immediately launched toward fixation at the start of the ice age. Then you wouldn't necessarily expect to see morphological change later on.
ReplyDeleteIf morphological changes would have resulted in better survival, then clearly those changes would have happened. They didn't, so I have to conclude that they wouldn't have helped. This doesn't rule out other scenarios where morphological change CAN help.
My thinking is not very advanced on this topic. I'll have to ponder it a bit more.
I think there's a couple of problems with this whole issue. First of all, Dawkins is writing a popular science book, not actually fleshing out the neo-Darwinian synthesis and it's predictions. However problematic this will become when actual evolutionary biologists have to explain how evolution really works, the general public will not be interested in buying a book chuck full of statistical calculations, graphs and formulae.
ReplyDeleteOn the other hand we already know Dawkins is an excellent writer and has something of a gift explaining complex subjects with metaphors, so why he chose to make almost the same level of overgeneralization in this instance as was expressed in the paper, I can't say. His example is missing several key factors, such as population size and the strength of the selective pressure, for him to be able to conclude anything from his simple scenario.
When you ask whether we agree with him, my own answer is that I don't. It's hard for me to imagine much change achieving fixation in a population in a mere 100 generations, especially if I don't know the population size or the strength of the selective pressure. But that's also why I disagree with the statement in the paleontology paper, because it seems to operate on the same overgeneralization Dawkins is committing to here, and this is not the actual neo-Darwinian synthesis, which is a mathematical statistical theory where numbers like selection coefficient and population size matter. So both the authors of the paper in their statement about what neo-Darwinism predicts, and Richard Dawkins is wrong.
Ironically, Dawkins is using an example about fur coat lenght, which is the kind of adaptation to climate change that you won't detect in a series of fossils. And those species could then be interpreted as "evolutionary static" and "not sensitive to environment" ;-)
ReplyDeleteAmazing isn't it? There are over one hundred species in the tar pits and God arranged to evolve all of them over a period of 35,000 years in a way that's undetectable by peleontologists. :-)
DeleteMy sister just bought a furless rat. Poor creature is ugly as fuck, but to compensate for this lack of fur it has a much higher metabolism to keep warm. You wouldn't detect this in the fossil record.
DeleteGranted, if all of those >100 species really show no change at all over 35000 years of climate change, it's unlikely they all just evolved in ways other than skeletal morphology. But that's not the same as saying none of them did.
God? Sorry, Laurence, I'm not understanding the meaning of your reply :-/
DeleteDoes it say on their degrees if they are serious or not serious scientists? Just kidding!
ReplyDeleteCreatures do adapt to the area and right quick. i say its from innate triggers. Not slow selection on reproduction results.!
Envirorment change demands quick acting change to survive. !!
there's no time for lingering. if it wasn't important cghange there would be no need to change.
Looking at the life around me, I conclude that the primary adaptation to local environmental change was to move to a different habitat. Given the variability in conditions mostly adaptability, rather than adaptations, seem to have been selected. In the organisms that have specialized in a very narrow niche, extinction seems to be the usual outcome. In either case, though, I thought that genetic change was inevitable, with natural selection acting usually to conserve the morphology, particularly for breeding purposes. All flesh is grass, and only natural selection keeps it from withering away.
ReplyDeleteI've even speculated that sexual isolating mechanisms serving as compatibility markers for partners could be a mechanism for speciation, instead of being markers for adaptive fitness to the environment.
I think color is something capable of changing very quickly, and multiple experiments with fish and other animals have confirmed that in the cases studied at least, color is prone to change in very few generations.
ReplyDeleteThe only study I know on changes in shape is that with the lizards on Pod Mrcaru, where there was a change in head size and cecal valves occurred in the digestive system during only a 36 year stretch.
So it does seem like evolution can happen fast, but all these examples have been where humans came in, and directly modified the environment is an IMMENSELY drastic way.
I am pretty much totally uninformed on any studies trying to follow a less sudden shift in habitat, or a shift that was caused by natural disasters rather than direct and intended manipulation.
In sum I currently think fast, "sensitive" change does happen, but only in specific scenarios and probably with specific species, not broadly. But I'm also not yet qualified by any means to have any of my view to hold any real weight.
And holy cow did I fuck up the grammar in that last paragraph to hell and back.
DeleteI wouldn't be surprised if Dawkins apologized for twenty pages that the description would be, of course, an oversimplification (I have not read this book, but if the selfish gene and the extended phenotype are good guides to his general writing ...)
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteI made a quick check of some Bergmann Rule research papers and yes, the phenomenon is rampant in birds and large mammals. There is even good evidence that some British birds have become smaller since the 1970s, when temperatures were about 1o C. lower than today! With all of that, I see a disconnect. Modern animals measured by mastozoologists and ornithologists are able to evolve and maintain latitudinal size gradients against gene flow and even change size in 30 years (say 60 generations) in Britain. Today, there is less evidence of stasis than for adaptation. Also, in a similar vein, I remember an old paper (in the 1970s, perhaps American Naturalist) showing that English sparrows recently colonizing US cities varied quite markedly from one place to another. If stasis does not hold in real time, why the hell did it hold 35,000 years ago? I think some one should actually read the damn paper.
ReplyDeleteUsing paleotemperatures and known Bergman size gradients, how much should the Los Angeles species have changed in size? Could their samples sizes have detected the expected magnitude of change? Are the samples biased by age classes, or are they extra variable because of the necessity of grouping animals that died over periods of decades? Los Angeles is by the sea; did the oceanic climate buffer local climatic variation? Did mammals migrate further south during the winter? In other words, are we dealing with stasis, or with artifacts and faulty premises?
My feeling is, while respecting the hard work put into the study, that palenotological data are not ecologically dense enough to draw strong conclusions.
@Benson:
Delete"I remember an old paper (in the 1970s, perhaps American Naturalist) showing that English sparrows recently colonizing US cities varied quite markedly from one place to another. If stasis does not hold in real time, why the hell did it hold 35,000 years ago?"
Well, you said "recently". The idea with punctuated equilibrium and stasis is that you have a small population separated from the parent population and it will go through relatively fast evolutionary change (including genetic drift) until it stops due to population size, adaptation, etc. So it's not surprising that in the particular example you're using (without me reading the paper) those recent "invaders" are now changing fast. When their population size grows beyond a certain threshold and genetic drift becomes less effective, and as adaptation (if any) stops its major effects, stasis will kick in.
Of course, this is a very simpliflied description. I also agree with other people have said. Morphological change evaluated from the fossil record can only show, well, morphological variation, and even here it won't show you everything in the vast majority of cases (fur, etc). Physiological changes are far harder to see. Molecular are next to impossible. If we define evolution as a change in allele frequencies in a population over time, it is evident that evolution never stops, not even during "stasis". The vast majority of mutations are neutral. New alleles are produced all the time, neutral or quasi-neutral, spreading to varying degress through the population due to genetic drift or other factors. Some get fixed, others don't, depending on population sizes and adaptational value, if any. How con we measure all this from fossils? We can't.
Stasis is a nice concept that I think elucidates a lot about the processes of evolution, but it is difficult to draw to many generalized conclusion for fossil data. We are still stuck, including most paleontologists, and because most people that "built" the theory of evolution were zoologists (with a particular interest in birds, I'd say), on the idea that evolution of big fat animals that leave fossils is representative of everything that is going on. It's not surprising that Gould wrote his massive magnus opus on evolution and, from a cursory look at it, there's very little molecular data. Microorganisms are almost left unmentioned.
@ Pedro:
DeleteI found the paper on the rapid evolution of house sparrows. It is older than I remembered:
Richard F. Johnson + Robert K. Selander. 1964. House sparrows: Rapid evolution of races in North America. Science, 144 (3618): 548-550.
It is more relevant than I thought too. Here is the abstract:
“Conspicuous adaptive differentiation in color and size has occurred in the house sparrow (Passer domesticus) in North America and the Hawaiian Islands since its introduction in the middle of the 19th century. Patterns of geographic variation in North America parallel those shown by native polytypic species, in conformity with Gloger's and Bergmann's ecogeographic rules. Racial differentiation of house sparrow populations may require no more than 50 years.”
Wow!! In short, several dozen house sparrows were introduced into the U.S., in New York, in 1851. They later spread across much of North America. Today the sparrows descended from that original introduction have evolved to be relatively smaller at warmer (southern) sites (Bergmann’s rule) than cooler ones, and plumage evolved to be darker in humid than in dry regions (Golger’s Rule). In approximately 100 years the birds had evolved to conform to the expectations of adaptive evolution. Since rain and temperature regimes tend to be rather similar over contiguous areas, different populations (cities) of sparrows within geographic regions (aided by gene flow) often became quite similar, giving them the properties of geographic races. Within regions, in spite of gene flow from around the edges, natural selection has caused sparrows maintain their characteristic appearance.
Stasis is an effect, not a cause. It means ‘little or no phenotypic change over many generations.’ The 20,000 years covered by Prothero’s study is ‘many generations’; and the lack of change in the face of climate fluctuation is perplexing and begs for an explanation. However, ecological studies on modern bird and mammal species show that body size commonly changes with temperature and that the direction of change nearly always agrees with Bergmann’s rule. After becoming adapted, the animals enter stasis because there is no selection to continue change. Presumable the phenotype becomes optimized with respect to competing ecological requirements.
Analogous situations occur in insects. European fruit flies have bigger wings at the northern edges of their ranges (Scandinavia). One species, introduced from Europe into North and South America, spread out over each continent, and flies re-evolved large wings in more polar regions of each. It seems that flies cannot beat their wings as fast when it is cooler, requiring larger wings to stay aloft. After adapting, the good times arrive, and there is stasis.
In grasshoppers the relation is inverted: body size becomes smaller the colder it gets. Where this has been studied, it is because colder climates have shorter growing seasons, and the insects have less time to get big before they must reproduce. Insects that show this pattern, in contrast to the fruit fly, have only one generation a year. The evidence shows that natural selection adapts animals to climate in different ways, and when success can be increased through small simple changes in body mass, limb size (including wings), body pigmentation and the like, given sufficient selection pressure and genetic variability, evolution will likely occur rapidly.
W. Benson, I tend to think as you do, that "If stasis does not hold in real time, why the hell did it hold 35,000 years ago?"
ReplyDeleteand my skepticism stems from the huge variety of plants one finds occupying and coexisting on plots of land in forests, perhaps especially rainforests. Including the existence of older, less evolved types of plants coexisting alongside more modern species.
If stasis is normative, then it seems to me (with the recognition that there may be a glaring error in my thinking), that the explanation for the variety would necessarily be that some species of plants migrated to different areas, evolved in their newer regions, and then made their way back again when the climate restored itself to the climate they had been accustomed to before migrating (or was now of a type that the new species could thrive in). It seems further like this would have needed to be the case for a large percentage of these plants, in order to account for the huge numbers of different species that we now see coexisting. Meaning the same plot of land, over millions of years, would have seen a huge amount of waxing and waning, accommodating itself to the loss of numerous types of plants (and the nutrients lost by their absence), re-accommodated itself when the new species re-migrated back to the plot, and so forth. The plants that remained would not have been able to dominate the necessary elements for their survival - sunlight, water, soil nutrients, root space, etc - while different members of their species tracked to other environments, evolved, and returned, so as to act as a barrier to encroachment by newer rivals, although it seems to me as if they should have been able to have done so.
Is stasis the correct assumption when considering the vast array of plant life?
just by way of reference, from about.com
Delete" Experts say that just a four-square-mile area of rainforest may contain as many as 1,500 different types of flowering plants and 750 species of trees"
I don't believe those "experts." Even if they were to say "varieties" instead of "species" i'd still say it's an exaggeration. Using their definition of "species" they would have to say that there are several hundred "species" of humans since the ones living in Ireland are obviously different from the ones living in Japan.
Deletebut anyway it's still a lot. And they all have more or less the same basic needs. So it would seem like the reason there are so many varieties is that they scratched out little niches for themselves within the larger environment. Is that right?
DeleteNot that they kept going, coming back in modified forms, going again, coming back again, etc. etc.
But a lot of those niches would have arisen in response to each other, I would think. Little differences in nutrients based on the other types of plants that were near them, among other things.
But where did they all COME from if stasis is the most natural way of existing? Until I read these recent articles I just assumed they mostly evolved onsite. To my inexpert way of thinking this stasis-as-norm model looks like a horse chasing its tail around.
The problem here is one of jumping into hasty generalizations out of single examples. It is worse if we were to think that evolutionary processes produced visible anatomical changes at a precise rhythm. Anatomical stasis has been demonstrated. That does not mean that every lineage has had, will have, the very same periods of stasis, or that their stases last for the very same period of time. Thus, some lineages will appear to be changing continuously, others will show the stasis periods much more clearly. Evolution is not planned. SIngle lineages will appear to be changing constantly for some periods of time, not so much for other periods ... Stasis is real. It is not rhythmical though.
Deleteandyboerger asks,
DeleteSo it would seem like the reason there are so many varieties is that they scratched out little niches for themselves within the larger environment. Is that right?
No, I don't think that's right. According to my view of evolution, species are subdivided into many localized demes that are somewhat genetically isolated from one another. (This could be temporary.) Under such circumstances it's quite likely that random alleles will become accidentally fixed in the deme by random genetic drift. Thus, when you compare a bunch of these populations it's likely that you will find small differences that are localized to a particular area where there's substantial inbreeding.
This has nothing to do with adaptation or environmental niches.
If you're an expert at recognizing these tiny differences, and you're a splitter as opposed to a lumper, you will declare a new species.
This same phenomenon is observed in our species. There's a huge variety of different visible phenotypes and many of the variants are localized to different demes within our population. We don't call them species because we know that the individuals can interbreed without any loss of viability.
Stasis, does not mean lack of variation. It's an entirely different timescale. It means that there is no overall direction in evolution even though there might be a lot of variation within the population.
Larry, understood. And clear. Thank you.
DeleteLarry said:
Delete"Stasis, does not mean lack of variation. It's an entirely different timescale. It means that there is no overall direction in evolution even though there might be a lot of variation within the population."
The differing definitions/interpretations of words like stasis, variation, evolution, speciation, etc., are one of the things that causes disagreement and debate between scientists (and then god pushers come along and try to use that to their advantage).
In the paper about short-term stasis the authors said:
"Yet since 1863 it has been well known that Pleistocene animals and plants do not show much morphological change or speciation in response to the glacial–interglacial climate cycles. (my bold)
And they also said:
"Yet statistical analysis (ANOVA for parametric samples; Kruskal–Wallis test for non-parametric samples) showed that none of the Pleistocene pit samples is statistically distinct from the rest, indicating complete stasis from 35 ka to 9 ka. (my bold)
And:
"Thus, the data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change that occurred at 20 ka..." (my bold)
I realize that there is data provided in the paper that explains the conclusions in more detail but I also think that the wording used by the authors can be construed as somewhat contradictory or misleading, or at least confusing and inconsistent.
For example, what is "complete" stasis, as compared to just stasis, and exactly how well does "complete stasis" align with "do not show much morphological change or speciation"? And then there's the "unresponsive" claim. If they were "unresponsive" and there was "complete stasis" then how could there have been ANY morphological change or speciation. Saying that they "do not show much morphological change or speciation" doesn't mean that they showed NO morphological change or speciation.
Now, before anybody jumps on me, I know that learned people can pretty much figure out from the paper what the authors actually discovered but I think that the authors of a scientific paper should be very careful in their wording (including in the abstract) and that they should always provide their definition/interpretation of key words and phrases. Otherwise, disagreement, debate, and confusion are likely, even among scientists, and the less learned the people who read the papers are, the more disagreement, debate, and confusion there is going to be.
@ Larry
DeleteI just opened my copy of Flora da Reserva Ducke, a biological reserve in 'monotonous' lowland forest next to Manaus, Brazil. Ghillean T. Prance (then Director of the Kew Botanic Garden) comments in the Forward: "This small area of 100 km2 harbours 2200 species of Amazonian Vascular plants." The tree list (trunks > 4 inches) gives 1180 species (I counted them!) Prance found that one hectare plots in the same region contain ~250 good tree species. The Flora book I cite does not just list species, it describes them and gives photographic keys for separating them. Areas at the base of the Andes and in the East Indies are said to be considerably more diverse.
@andyboerger
Hi! Yes, it does boggle the mind how many trees can grow in the same small patch of tropical forest. Ecologists are divided on the reason. I think the explanation with the most evidence to back it up is that tropical trees, like much life in the tropics, are controlled by disease and parasites (including insect pests), especially when they are seedlings and become stressed in the forest shade. Since most parasites are highly host specific, pest pressure on young plants is stronger near adult trees of the same species (adults are carriers of the specialized diseases and insect pests). This produces a zone around adult plants where only plants of other species grow well. Many studies by dedicated ecologists have shown that seedlings in tropical forests grow and survive better when they are far from adult plants of the same species. This effect is much stronger in tropical forests, and tropical forests have more tree species, because (we think) more constant warm, humid conditions favor diseases and pests.
As to the rest, trees would speciate in the standard ways (isolation by rivers, mountains, or dry strips; hybridization, specialization to extreme soils, etc. etc.) and after speciation disperse (seeds carried by a migrating bird, for example) back again. As you say, survival after getting back depends on lucking into a ‘niche’, in this case, being defended against native pests.
There was a flurry of speculation in the 1980s that the tropics were subject to climate fluctuations that accelerated evolution and promoted speciation and, ultimately, increased species diversity. New evidence has not supported this idea. The tropics, at least South America, although not immune to the great climatic cycles, seem to have not changed much in many millions of years. Little change, little extinction, high diversity.
That said, Larry is right to stress population isolation and the role of drift in producing genetic individuality. However, tropical nature is biologically challenging in a way that a biochemist may not appreciate, and I suspect that much of the difference arising in isolated populations, in the tropics and elsewhere, is from selection. (In the same breath, I admit that biochemistry is complex in ways that a naturalist could hardly appreciate.) The drift/selection dichotomy is of course an empirical question that can be solved by evidence.
I am impressed with you interest in the topic. However, ecology and evolution may be conceptually more complex than you might imagine. Although it is somewhat dated, Eric Pianka’s 1999 (6th edition) book 'Evolutionary Ecology', for about $10 used, provides a good start on what real ecologists’ think is going on. Robert Ricklefs’s 'Economy of Nature' (2008) is another good read.
W. Benson, thank you for the above. Although the explanation of, largely, host-specific parasites does serve well to explain the advantage of plants having distinct characteristics (the information about saplings thriving AWAY from their elders is fascinating) at the same time it boggles the mind even FURTHER in that it increases, rather than decreases the amount of evolution that has to be taking place on small tracts of forest because it throws in hundreds, if not thousands, of highly specialized predators to an already overcrowded mix!
DeleteAs you write, ecology and evolution is very probably 'conceptually more complex' than I can imagine. Thanks for the book recommendations; I shall try to un-boggle my mind a bit more. :)
I don't believe lots of biologists "views organisms as infinitely sensitive and responsive to their environments". Evolvability is a new and popular concept which deals directly with the problem how easy populations can respond to selective pressure. And we know from theory and many empirical studies that populations are not "infinitely sensitive" - they can get stuck on local peaks, in the parlance of fitness landscapes. "Infinitely sensitive" is by now a total straw man.
ReplyDeleteHere's a link to a new article that (to me) seems topical to this thread:
ReplyDeletehttp://www.sciencedaily.com/releases/2012/10/121025130922.htm