Ten assertions about evolution20 Aug 2006
I thought for a long time about how to respond to Razib's challenge:
My post asking to define evolution in less than 10 words elicited a lot of response (some of it outside the parameters I set in regards to length). So I figured I'd give this sort of thing another shot, again, with parameters which all are welcome to violate, but which I set for myself to prune my tendency toward qualifying verbosity. Below the fold are "10 assertions of 10 words or less" which I believe that the public should know about evolutionary science. I did this in 3.5 minutes, typing out what came to mind and checking word count in M$ Word. Obviously the assertions reflect my biases, though if I can get others to bite perhaps we can establish a "core" which we can all agree upon as necessary to a genuine understanding of evolutionary science.
The problem with compressing things into 10 words is that it always leaves the terms undefined. And since these are supposed to represent what I think the public should know, I certainly can't assume that the terms will be understood already. Which really means that what I think the public should know is what evolution is, and the like -- in other words, definitions of terms.
And definitions of terms are totally boring. Nobody who's reading this wants to read definitions. So they're out, at least mostly.
Of course, the fun of this is writing some assertions that will gin up some controversy. Like Razib's "The difference between micro & macroevolution is semantic." Or Afarensis's "Most people do not understand punctuated equilibria." But the thing is, if I only have 10 things to get the public to remember, do I choose controversial things? The first is plausibly false (insofar as a lot of evolutionary biologists think there are mechanisms driving speciation and the evolution of higher-rank taxa that are never manifested microevolutionarily), the second introduces a controversy without the possibility of explanation (I'm not sure a 10-word explanation is even possible).
And yet, if I avoid all controversial things, it really seems to suck the life out of the science. I mean, imagine 10 assertions about physics today without mentioning string theory. It's controversial, but it's all a whole lot of people care about!
The other question is, how representative should the list be? The other lists are pretty much centralized on population genetics (the first two weeks of my intro class), but have little to say about branches of evolutionary biology like paleontology, evolutionary ecology, agricultural genetics, evolutionary medicine, and (dare-I-say) anthropology. With one exception -- several include "humans are still evolving", which really seems to me like little more than a truism, since it follows from other principles. More interesting would be to convey some notion of how humans are evolving, or in which direction -- although that certainly won't satisfy my "no controversy" condition. More important, stuff we want the public to know about evolution surely includes things like why extinction happens, where all those delicious plants came from, why those antibiotics don't work anymore and why you shouldn't marry your sister.
So I thought three things are desirable. First, the list should be broad, so that it connects to things people are likely to already know, or hear on the news. That makes it relevant. Second, it should not introduce terms needlessly. Natural selection is one of the most important achievements of science, and everyone needs to know about it. Genetic drift is interesting and all, but it is not all that important to ordinary people. To the extent that it is relevant (in conservation, for example), it can be introduced with a concept people already have a vague concept of -- inbreeding.
Third, many of these assertions are reducible to equations. But most people don't relate to equations, and won't remember numbers, so I've tried to be clear about the direction of relationships without quantifying them.
And I admit it, I couldn't get them all to 10 words. I tried very hard, but in the end I thought better to be clear than to make people consult their dictionaries. I added a little cheat by giving each assertion a "category"; that helps to clarify what they refer to.
OK, so here's the list:
- Genetics: Hidden "recessive" genes can cause phenotypic traits to "skip generations."
- Population genetics: Inbreeding increases the risk that offspring will express harmful recessive genes.
- Demography: A slight intrinsic growth rate over many generations causes a population to grow exponentially in numbers.
- Natural selection: A gene that increases the intrinsic growth rate of its carriers thereby increases its representation in future generations.
- Ecology: Selection shapes interactions with other organisms, including mutualism, predation, competition, and parasitism.
- Kin selection: Helping relatives will help an individual's own genes, proportionally with relatedness.
- Species: Selection and interbreeding can make populations more similar; selection and inbreeding can make them more different.
- Paleontology: A species' future is contingent on its history; new adaptations must build on or delete the old.
- Extinction: Rare, inbred, or specialized species have more trouble surviving ecological disturbances and may become extinct.
- Agriculture: Humans changed the ecology of domesticated plants and animals, exerting selection on them.
- Medicine: Pests, parasites, and diseases evolve rapidly, often overcoming human interventions.
Compiling this list really brought home to me how difficult some of the concepts are. So I jotted down some questions as I was doing it, with my thought process about how to answer them:
Boy, why did you wimp out on species? My main problem was that the species concepts familiar to most people (a) don't mention natural selection in any way, and so don't really give an evolutionary account of speciation, and (b) are highly animal-centric. Sure, interbreeding is necessary for species cohesion in sexual organisms, over millions of years at least. But what about asexuals? And what about good "species" that interbreed readily given opportunities? And ring species? I know that my readers know all these problems and many more, and the list of problems never ends. I thought for a long time, but I think I'm just too close to the topic to come up with a one-sentence species assertion that I would foist on anybody. And hey, Darwin wimped out on species, so why shouldn't I?
Why define selection in terms of intrinsic growth rate? For one thing, most short definitions of natural selection are really unsatisfactory. Darwin defined it by analogy. The usual population genetics definition ("correlation between a genotype and reproductive success" or some such) is entirely silent about how this correlation comes about. Fisher's treatment of selection in terms of intrinsic growth rate has the benefit of being very clear, and it can be related to something people are likely to understand. Additionally, it inspired much later mathematical work, including Hamilton's treatment of senescence.
Why waste a whole assertion on defining intrinsic growth rate? Hey, if I thought people actually knew this, I wouldn't. Compound interest generally comes as a surprise to my undergraduates, so I think it's worth explicitly connecting Darwinism to Malthus. But I felt guilty burning an assertion on mathematics instead of biology, so I expanded the list to eleven.
Why is "recessive" important enough to belong on the list? Genes are the chief unobservable entities of biology. Mendel could posit their existence because of the proportion of offspring that exhibited traits that were concealed in their F1 hybrid parents. People are already familiar with the idea of traits "skipping generations", but they generally don't realize that this phenomenon requires the gene theory (that is, it is inconsistent with blending inheritance).
Why "ecological disturbances" instead of "environmental changes"? First, "disturbances" brings to mind human-induced habitat loss, without limiting it to that. Second, "ecology" carries a meaning of intricate relationships, while "environment" is associated by most people with the weather. Not that the weather isn't important, but for the most part its importance is channeled through ecological relationships.
Why "genes" and not "alleles"? I wavered on this, too, but went in favor of genes throughout because it is consistent with the use of many genetics texts and also is a term people are likely to have heard.
Why no adaptation? I kept an assertion about adaptation on the list for a long time. It is a logical intermediate between the definition of selection and the mention of ecological interactions. I guess my attitude came to be very similar to my attitude about "fitness" (which I also ended up not using): it confuses people more than it clarifies. Both "adaptation" and "fitness" are readily misinterpreted as purposive, and ordinary people cannot help but understand them that way. (To my mind, that confusion explains much of the appeal of Dawkins' "Selfish Gene": not that it clarifies adaptation or fitness, which it doesn't, but that it reframes their "purposiveness" in terms orthogonal or reversed from Western moral sensibilities.) For a long time, I had the word "fitness" in the natural selection assertion, but ended up taking it out for simplicity.
"Humans changed the ecology..?" The distinction between natural and artificial selection simply confuses people. Humans certainly did not create their environments from scratch (we sure wouldn't have created brucellosis!), and they remain under non-human-mediated selection for most of their biology. I don't propose doing away with the artificial-natural distinction, but reframing domestication in terms of ecology does allow more interesting thoughts. For instance, humans are mutualists with many domesticated species. Humans work hard to eliminate natural predation on domesticated species, and they still compete with weed species and suffer from diseases.
Well, anyway, that's the list. I think I'll try to come up with 10 paragraphs, which might actually be a great help for my classes. If I were going to add more assertions, I would first add one about senescence and death, then probably something about brain evolution, and possibly mass extinctions. I can think of many more concrete events that people should know before I would add a lot more about genetic mechanisms.
Meanwhile, I've been working on a list that will be a bit more fun...