# starlarvae's thread hijack (from, "Does evolution follow the scientific method? If so, how?")

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Such tools will tell you if the observed change in allele frequency statistically deviates from a null model. If observed changes in allele frequency deviate from a null model (barring some caveats) the population is, by definition, under selection at the genetic loci which are being tested.

Yes, the population is "by definition" under selection. The definition is derived statistically. It is a contrivance. If allele frequency deviates from null model, you can define the difference as due to "selection" or anything else you care to invent, but all you really know is that the frequency deviates. Who knows what's causing it? Statistics can't tell you.

That's not a confusion of causality and correlation, that's measurement. As an analogy, if readings on a thermometer are going up over time, it's getting hotter.

But wait. The deviation from the null model is a measurement, yes. BUT the attribution of the deviation to natural selection is precisely a confusion of causality and correlation. If thermometer reading goes up, it's getting hotter. That's all the thermometer can tell you. It says nothing about the source of the heat. Measuring deviation from the null model tells you how far the allele frequencies deviate. But that's it. It says nothing about the source/cause of the deviation.

If what you're saying is the detection of selection doesn't immediately allow one to determine the source of selection, that's quite right - and I don;t think anyone has suggested that.

But you're not detecting selection. You're detecting only the deviation in allele frequency, then defining that as a measure of selection pressure. That's called Begging the Question: You assume that natural selection is responsible for the deviation, then you take the deviation to be evidence of selection.

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Yes, the population is "by definition" under selection. The definition is derived statistically. It is a contrivance. If allele frequency deviates from null model, you can define the difference as due to "selection" or anything else you care to invent, but all you really know is that the frequency deviates. Who knows what's causing it? Statistics can't tell you.

But wait. The deviation from the null model is a measurement, yes. BUT the attribution of the deviation to natural selection is precisely a confusion of causality and correlation. If thermometer reading goes up, it's getting hotter. That's all the thermometer can tell you. It says nothing about the source of the heat. Measuring deviation from the null model tells you how far the allele frequencies deviate. But that's it. It says nothing about the source/cause of the deviation.

But you're not detecting selection. You're detecting only the deviation in allele frequency, then defining that as a measure of selection pressure. That's called Begging the Question: You assume that natural selection is responsible for the deviation, then you take the deviation to be evidence of selection.

Deviation from the null is EXACTLY what selection is.

Its population genetics 101. Hardy-Weinberg Equilibrium. Given a large enough population size and absence of various factors LIKE selection, allele frequencies do not change. This is has been mathematically worked out and demonstrated. Changes in allele frequency are always do to one of four forces....genetic drift, natural selection, gene flow, and mutation. Genetic drift and mutation are inherent in the null model which selection is tested against. gene flow can be identified by multiple signatures. That leaves only selection to explain the deviations observed.

In this, the tests are actually superior because they are not biased in anyway by any presumption of what the selective force is or what adaptation should look like.

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Deviation from the null is EXACTLY what selection is.

Deviation from the null is EXACTLY what differential reproductive success is. What causes differences in reproductive success? who knows?

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What causes differences in reproductive success? who knows?

People who have studied evolution.

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A change in temperature on a thermometer is a change in the volume of the mercury inside it... what causes the change in volume? Who knows? [/sarcasm]

The use of HWE deviation as a measure of the strength of selection is observationally validated e.g.

Much like the use of a thermometer to measure temperature. The argument that we don't actually know what causes deviations from HWE is about as relevant as arguments that we don't know for sure that Zeroth's Law explains variations in the volume of a fluid, etc.

All good points. We're honing in on philosophy of science. Namely, when science articulates theories and mechanisms to explain observations -- to get underneath descriptions of what is observed and propose causes as to why the given observations occur and not others -- then is science making an ontological (or epistemological) claim, or is science making a pragmatic claim?

Does science claim to tell us about reality and the way things actually are? Or does science claim only to figure out what observations will obtain in certain situations? Does science explain observations or just interpret them using vocabularies that scientists have agreed to use for such interpretations?

Specific to the topic of evolution:

Are we using "natural selection" with the claim that we're explaining a process that shapes allele distributions? Or do we just agree that "natural selection" is our shorthand term for allele distributions that conform to certain statistical parameters?

Edited by starlarvae