In a real live class, I let the students’ responses determine the order and depth for examining selection after I introduce the concept. It’s a fair bet that my “no purpose, no point” argument doesn’t sink in right away; it’s counter to everything they’ve seen or heard before, or at least to the constructs they’ve formed based on what they’ve seen or heard. Even the ones who proclaim themselves knowledgeable, and that they already know there’s no goal to selection, balk when I say species longevity is, if anything, made briefer by strong selection, or that polar bears did not become white in order to hunt more effectively. People are happy to say “no purpose, no point” as long as the lovely Nature Haiku is still maintained: something new is made, something has been achieved, something is better, something has triumphed, something is beautiful. They’re not going to give it up easily. It’s important for the teaching that I see where this group of students is with the idea and proceed with the right exercises or examples for them.
As a blogging person, though, I don’t have much to go on once having posted Adapt this. It’s a different social context and there’s no way to see where people are at, or what combination of the thousands of neat points and examples will be my best shot at improving the collective understanding in the next round. I’ll open with an abstract point:
Let’s see how it goes in saying next that selection isn’t an ongoing, reliable, there-for-sure phenomenon. Given the following three conditions (or better, circumstances), it does occur, but none of the three is guaranteed to be present at all times for all things.
Selection occurs a lot, but always with a huge “if and when” qualifier in place. Each condition’s internal limitations and nuances deserve a close look too.
- Variation – You might be surprised at how much something varies when it all looks the same to us. Also, quite a bit of variation “hides” and only occurs infrequently, so that you might look across a population and see nothing despite several, or dozens, or even hundreds of variations out there, each with its own statistical presence.
- Inheritance – there’s a lot more than genes to developing a trait. Most discussion of inheritance unwittingly holds constant a remarkable number of contingent events and conditions, which properly ought to be included in this component.
- Reproductive success – I said nothing about mortality/survivorship. Nothing, because these are relevant only if they affect reproductive success, and I assert with some force that that is a serious if, and at most a qualified subroutine rather than a defining component.
Oh yeah! I keep thinking it’s obvious, but that’s not fair – consider that selection, when present, occurs on a spectrum from strong to weak, meaning, roughly, how fast … and which is profoundly depending on how varying, how inheritable, and how drastic the impact/effect of variation is. It’s easy to miss the fact that a very drastic impact/effect is a weak selective phenomenon unless inheritance is very high too.
You can put those points together to critique the familiar phrasing, “selection favoring or for” something. “For” is a retrospective phrasing, rather than a process-based one, and is easily mis-read as purposeful or design-oriented, or at the very least, that the process finds a specific outcome at which it’s “done.”
Better to state it clearly, that selection favors nothing – the circumstances of life blowtorch everything, and if the three conditions obtain, then changes in the population occur, are selected, because something is consistently getting blowtorched the least. In this, selection is very much like Groucho Marx’s song from Horse Features, or the first half of the song anyway:
The concept I called “adversity” in the video includes but is not limited to selection pressure. I’m talking about the ongoing tendency for the exigencies of the living experience to kill, ruin, prevent, and diminish it, such that whatever reproduction happens is like an island in a sea of it not happening. (technical point: when I say reproduction, I mean only regarding offspring who themselves will do so or contribute to it)
Selection doesn’t have to result in fixation (roughly, uniformity) or really any conceivable end-state, and furthermore, is readily susceptible to shifts in “direction” (itself not really a helpful word, “direction” implies somewhere to go) as long as the relevant inheritable variation is available. That’s what happens in Darwin’s finches all the time. One of the most interesting causes of such “directional” shifts – really merely a change-over in what traits happen to be least blowtorched this time – concerns the commonality of the traits, demographically.
There are really two insights from the orchids: that the frequency-dependence, results in flux, or cycling of the colors’ relative abundance; and that this is an excellent example of reproductive impact (differences in fitness) without any change in life-details or the individual experiences of the organisms.
Ah, Astanyax mexicanus, you awesome fish, on a par with armored vs. unarmored sticklebacks. Clarification: although I cited repro/energy as the limiting factor of the selection’s extent, it could just as well have been inheritance or variation. If selection took the population to the point of minimal genetic variation regarding the trait, despite a fair level of variation remaing, then it would stop there; similarly, if selection hit a point at which there simply wasn’t any variation – the fish didn’t have any “less eye” to select for – then it would stop there. I don’t know which of the three is really it, hence my phrasing about “my understanding of the situation.”
I’m sure you spot the final point of this post, concerning “vestigial” as a term. It’s often cited as one of Darwin’s points of evidence, but I think it’s mis-read – the point is not whether the organ’s presumed lack of function means something, but rather that it’s a homologue in a creature who’s definitely not using it the way we’re familiar with in some other creature. As with most of his points in this part of The Origin of Species, the real insight lies in appreciating the wide range of what a given organ can do in all its manifestations across the species who have it. I think the whole “function” content can safely be left out of that point entirely, as it’s wrapped up in utility-thinking, in that the creature does or doesn’t need it. Without such presumed purpose in there, “function” is impossible to define because you can always just back it up to “being part of a living body without making it drop dead.”
I led this post with the caudal vertebrae in humans, more accurately in all apes, because it’s a great example of these points. First, yes, the human tail “functions” insofar as it doesn’t kill us all prior to birth, and it also maintains muscle attachment sites if you’re looking for a reassuring “thing to do,” unlike the cave fishes’ eyes. But it’s also perfectly fair to say that relative to the longer, unfused, externally evident tails in, say, monkeys, it’s not doing much. If not “nothing,” then certainly not much that could be grounds for popping the champagne cork for selection. It’s also interesting to consider whether selection regarding any sort of tail (including the one we have) even happens in us – after all, we didn’t evolve it the way it is, that happened back in the origin of apes and we merely have what we-as-apes have in this regard.
Really, I’m doing this all backwards. I shouldn’t be starting with selection at all. The biggest picture is macroevolutionary, including the origins of new species and the extinctions of existing ones, and considering the parameters of developmental genetics within broad groups. Then diving into a single species, it’s easier to understand the default neutrality of everything, and the various ways in which creatures can change without selection at all. Then it makes sense to see when and how selection happens.
In the 1990s, Evo Devo biologists took to describing selection as the frosting on the cake made of the history and larger context I wrote of above. It’s a really good analogy actually. The big obstacle here is thinking of selection as the “reason for it all,” whether as something to be driven toward (cue Nature Special voice, “adapted to its surroundings”) or as something that makes everything happen. That fallacy forces extraneous concepts like function and purpose into the discussion, and thus seeks a narrative. And I know for sure that I’m not even going to talk about the technical term fitness until that obstacle’s out of the way.
[a lot of this owes much to Richard Lewontin and to Stephen Jay Gould]
Links: Cave fish not blind to attractions of surface-dwelling cousins, Negative frequency-dependent selection maintains a dramatic flower color polymorphism in the rewardless orchid Dactylorhyiza sambucina (PDF), The three necessary and sufficient conditions of natural selection (to see how someone else talks about it)
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