Industrial melanism illustrates evolutionary pressure/selection pressure. But in punctuated equilibria and large evolution, selection pressure won’t work.
Industrial melanism: during the Industrial Revolution, coal soot turned plants and buildings dark grey, and evolutionary pressure caused the normal scalloped ‘hazel’ moth, at left, to largely disappear, while it caused a a darker version to appear, at right. After the soot and coal pollution were reduced, selection pressure then forced the hazel coloration to re-emerge and dominate.
I am pre-publishing this sequence of essays here and in social media to elicit comments and other feedback. They will form the framework for my next book, Darwin, Dada, Dalí, Duke, & Devadevàya. Please feel free to comment below, or contact me.
Yellow Grasshoppers, Green Grasshoppers, & Selection Pressure
Even after my children were reading avidly on their own, I continued to read to them at night. A couple of years back, I bought a copy of The Evolution of Calpurnia Tate to read with my daughter. It was fun, but I picked it because it discusses things I want to share with her: biology, obviously; but even more, women in science (or in this case, girls in science). I strongly recommend it for any young reader. Or even older ones.
In the book, Calpurnia lives in rural east Texas, and the story begins during a hot, dry summer. Calpurnia’s grandfather is an amateur biologist who spends his days in his cottage laboratory, which the children have learned to avoid. One day, however, Calpurnia screws up her courage, goes in to his laboratory, and asks the old man a question: Why is she finding yellow grasshoppers everywhere, but no green ones?
He tells her to figure it out.
So she thinks about it. She finally reports back to him: In the dead, yellow grass, the green grasshoppers get eaten. The yellow ones are harder to spot, and survive. Impressed, her grandfather brings her on as an apprentice, and they become research collaborators, and good friends.
Industrial Melanism
In this problem of grasshopper coloration, the biologist will immediately recognize the parallel with industrial melanism. In the English industrial revolution of the late 19th century, the scalloped hazel moth went from, well, hazel, to very nearly black. Biologists realized that the lighter-colored moths were no longer camouflaged against the soot-covered trees, while the darker variations were better hidden, and had a better chance of survival. Once the factories reduced the burning of coal, however, the trees returned to their earlier colors, and the moths shifted back to hazel.
The biologist J.B.S. Haldane[1]Scifi fans will be interested to learn that Haldane was the model for genius/polymath Hari Seldon in Isaac Asimov’s Foundation trilogy. used industrial melanism as one of the earliest illustrations of Darwinian gradualism.
Haldane was also the first to notice the geographic correlation between certain blood diseases and the prevalence of malaria. Indeed, those diseases—sickle cell, the thalassemias, and others—despite the suffering and even death they create for about a fourth of children in those areas, confer resistance to malaria to roughly half of the entire population. It was the first demonstration of human evolution.
The Gaussian Distribution & Evolutionary Pressure
These shifts in color and blood diseases can be explained by evolutionary pressure: a higher survival rate for one gene over another leads to an increase of the first, and a decrease of the second.
(Click for larger image.) Evolutionary Pressure/Selection Pressure: the frequency of a trait, measured on the vertical axis, moves left and right on the horizontal axis in response to selection pressure.
Evolutionary pressure is typically illustrated with a Gaussian distribution, commonly referred to as a ‘bell curve.’ The bell curve describes almost any variable trait, in any population: height, weight, speed, intelligence, etc. It is one of the most important concepts in statistics, and essential for understanding any large population or group, from people to astronomy to scientific experiments to mass manufacturing.
For the moths and grasshoppers we are considering here, it describes color variations. If we imagine that moths vary from near-white to near black, and grasshoppers vary from light yellow to dark green, then we can see how their colors, and the bell curve with them, will shift left or right whenever the background colors change. (The hemoglobin diseases are slightly different in that there is no smooth range of variations, but only three distinct possibilities: both genes with the trait, both genes without the trait, or one & one. But their frequencies nevertheless respond to selection pressure, although the process is slower and more complicated.)
This brings up a question we previously asked: How do we explain a completely new trait, something that isn’t even on the bell curve, nor on any previously existing bell curve? How does the bell curve and selection pressure help us to understand a completely new allele, a neollele? That’s the problem: If it isn’t on an existing bell curve, our current models can’t explain it. We need an entirely new bell curve.
We need a bell curve that did not previously exist.
That is the difference we discussed between small, probable, or emphatic evolution; and large, improbable, or innovative evolution. Large evolution requires completely new genes, traits, and bell curves, things that previously did not even exist.
The Limits of Evolutionary Pressure
Which leads to a critical insight: evolutionary pressure, acting as it does through small evolution and Darwinian gradualism, can’t produce large evolution. Again, small evolution is emphatic evolution, because it emphasizes genes that are already in the pool. Evolutionary pressure can only work to select among existing genes, alleles, and individuals. But if the necessary genes, alleles, and individuals don’t currently exist, increased selection pressure will not produce them.
In this case, evolutionary pressure accomplishes nothing other than decreasing the population, even driving it to extinction. Which is what we see, new selection pressures often drive populations to extinction.
A biologist’s perspective of history: the horrors of Malthus, and why progress is largely a recent phenomenon.
Click image for more information.
Stress & Mutation
This cannot be overstated: evolutionary pressure does not produce large evolution. With, however, one small caveat: a fair amount of research shows that some populations under stress increase their rate of mutation, i.e., they throw out more random attempts, increasing their chances of finding a solution.
I have reservations about the applicability of that work. First of all, in a future post I will suggest that an ‘increased level of mutation’ may largely be an artifact. Second, is it that there are really more mutations generated? Or is it that for populations under stress, more mutations are tolerated in reproduction?
For that second point, let me offer an example. As women get older, it is well known that they tend to have more babies with Down Syndrome, which is caused by the genetic variation of three chromosomes rather than the typical two, resulting in Trisomy 21. Some 30 years ago I saw a paper on the genetics of spontaneous abortions in humans. The researcher found that the rate of of this trisomy actually appeared to be consistent in all ages, but with advancing reproductive age, women spontaneously aborted them less often.
The biologist (and probably the economist) will immediately see the utility in this approach, as described by the strategies of ‘risk prone’ and ‘risk averse’. As women approach menopause and their remaining chances for reproduction decrease, they become risk prone, i.e., more willing to take a gamble on higher-risk strategies. This means they are more willing to take a risk on different offspring that are less likely to survive, but that just might generate a completely new solution… which, of course, would give us a completely new bell curve.[2]Unfortunately, I have subsequently been unable to locate the paper, despite repeated attempts. For our considerations here, it is still a valid illustration of risk prone strategies, even if I am not … Continue reading So perhaps stress causes a population to generate more mutations. But it is also likely that stress causes a population to tolerate more mutations.
Errors in Selection Pressure
The advances of the New Synthesis, statistics, and approaches such as the bell curve have greatly advanced biology and our understanding of evolution. Punctuated Equilibria give us yet another example of how these concepts might be further extended to provide us with more tools. Selection pressure is one area where this is particularly true. There are many articles and papers which propose that evolutionary pressure might explain various aspects of large evolution. One article recently appeared speculating that deep-sea ‘oases’ such as whale falls and thermal seeps might drive the evolution of deep-sea life.
On an even more important topic, some authors suggest that selection pressure might explain the emergence of humanity, specifically, the evolution of our brains and our language. Considering these from the perspective of the improbability inherent to punctuated equilibria, human intelligence and language would appear to be particularly large evolutionary events: so far as we know, despite the innumerable rich and amazing innovations of nature, these two have appeared exactly once in 3 billion years of life on Earth. Nevertheless, there are published speculations that our brains were produced by a highly variable environment; or that our brain size is predicted by social pressure or fruit-eating; or that increased mental activity forced brain evolution, rather than vice versa. We have noted how our mental ability and our language are tightly interwoven; but that is not the same as the speculations on the evolutionary pressures that might have caused the evolution of language, and along with it, how tool-making may have caused our brains to expand, instead of the other way around.
There are other theories, such as how the opening of the savannas caused our ancestors to walk upright. Some suggest that the extinction of large mammals ‘forced’ humanity to create civilization. And then there is the proposal that psychedelic drugs generated human intelligence.
These are a quick sample, and largely taken from the lay literature. There are many others in the biological and social science journals. We need to step back from these, and consider evolutionary processes in light of the improbability of punctuated equilibria. If an evolutionary advance truly represents a new and highly innovative change, i.e., a change which requires radically different alleles that did not previously exist in the gene pool, then we need to recognize that selection pressure, so important in Darwinian gradualism, is insufficient to explain these things. We need to consider other approaches.
As noted, punctuated equilibria gives us an opportunity to add to Darwinian gradualism, and to expand the toolbox of the New Synthesis. This consideration of evolutionary pressure, and the limits to which it can cause evolution, can expand our approaches, and allow us to explore more advanced evolutionary processes.
For an ecological explanation of sexual dimorphism, click here.
Pictures of moths before and after industrial melanism courtesy of Janet Graham and Donald Hobern under the terms of CC 2.0. These photographs were modified by trimming, shrinking, and merging the two photos.
Footnotes
| ↑1 | Scifi fans will be interested to learn that Haldane was the model for genius/polymath Hari Seldon in Isaac Asimov’s Foundation trilogy. |
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| ↑2 | Unfortunately, I have subsequently been unable to locate the paper, despite repeated attempts. For our considerations here, it is still a valid illustration of risk prone strategies, even if I am not quoting the research accurately. |