Fallacy?

Unwarranted extrapolation

A process seen working on a small scale is assumed to work on a far larger one, without evidence that it can.

Also called: Hasty generalisation · Micro-to-macro leap

The test

Has the process been seen doing the kind of thing claimed, or only a small amount of something else?

All science extrapolates. We have not watched every falling object, yet we trust the law of gravity. Extrapolation becomes a fallacy when there is a reason to think the process might not scale, and the reason is not addressed. Changing the size of a beak and building a new organ may be the same kind of thing, done more times. Or they may be different kinds of thing. That is precisely the question.

This is the most common reason claims on this site receive a grade of D, which the grading scale names Extrapolation.

Seen on both sides

In mainstream arguments
It would be an unwarranted extrapolation to take changes in finch beak size over a few drought seasons as showing how new body plans arise.
In critical arguments
It would be an unwarranted extrapolation to take a set of loss-of-function adaptations in laboratory bacteria as showing that mutations can never add a new function anywhere.
Not a fallacy when

There is independent evidence that nothing changes between the scales, or the mechanism is understood well enough to show that it scales.

Diagnosed on this site

  • Pillar 4 · Mutation & New InformationClear case
    In the pillar as a whole

    From what mutation and selection have been seen to do, over decades, to what they are said to have done over the history of life.

  • Claim № 4.01 · Mutation & new informationClear case
    In the claim as taught

    “Over vast spans of time … accounts for the diversity of life” extends observed tuning and breaking of genes to the origin of every biological novelty, without showing that the process scales.

  • Claim № 4.01 · Mutation & new informationPossible
    In the alternative reading

    Behe's “first rule” generalises from laboratory and short-term studies, which favour quick gains by losing function. Whether the rule holds over longer times and larger populations is an open question, and should be tested rather than assumed.

  • Claim № 4.02 · Duplication & divergenceClear case
    In the claim as taught

    The experiments show a duplicated gene specialising in an activity the ancestor already had. The claim as taught covers the origin of new functions in general.

  • Claim № 4.02 · Duplication & divergencePossible
    In the alternative reading

    Gauger and Axe studied one pair of related enzymes. Treating their estimate as a general limit on enzyme evolution generalises from a single case, as their critics point out.

  • Claim № 4.03 · How rare are working proteins?Clear case
    In the claim as taught

    From proteins that tolerate substitutions, and binders improved by selection, to the origin of every protein fold. The claim as taught presents the conclusion as a description of what happened.

  • Claim № 4.03 · How rare are working proteins?Possible
    In the alternative reading

    Axe's figure of about 1 in 1077 was measured for one enzyme domain. Applied to all folds, it generalises from a single case. The card says this openly; the argument still leans on the number.

Further reading

  1. Hume D. An Enquiry Concerning Human Understanding, §IV (1748), on the limits of inference from observed cases.