If you were to put an animal with a certain type of seemingly advantageous "trait" into an environment where the trait was not necessarily needed, ex: Putting an animal with a high tolerance for cold into a mild climate, what kind of natural selection occur? Balancing?

  • $\begingroup$ It becomes redundant and has no positive selection pressure, so it becomes vestigial, become intermediary performance, changes in quantity to something useful but less adapted to the extreme, and may even disappear, although highly useful genes tend to stay hidden in the chromosomes for many millions of years rather than just disappear. $\endgroup$ Feb 14, 2019 at 5:18

1 Answer 1


Recipe for selection

There is selection on a specific phenotypic trait if and only if

  1. There is variance for this trait in the population
  2. This variance is, in part at least, explained by genetic variance
  3. The trait covaries with fitness

The first two points can be reduced to a single point by stating "The trait has some non-zero heritability in the population". For more information about heritability, please have a look at Why is a heritability coefficient not an index of how “genetic” something is?.

This recipe is often called Lewontin recipe.

Applying the recipe to your case

So, in the model you consider, some individuals are cold tolerant and some aren't, so the first point is checked!

We will assume that the difference in cold tolerance is caused by genetic differences. Second point is checked.

Does the trait covary with fitness? As you defined that, in the environment considered, being cold tolerant does not have any impact on fitness, then no, there is no covariance between the trait and fitness. The third point is not checked and therefore, there is no selection happening.

Are you sure there is no cost to being cold tolerant?

Note, btw, that it is not rare to observe (or at least to assume) that some adaptation such as cold tolerance would come at a comparable cost when the environment is warm enough.

For example, increased cold tolerance might be mediated via anti-freeze proteins. The production of such proteins might be costly for the individual. In such case, we would have a covariance between fitness and the trait (cold tolerant individuals would have lower fitness that individuals that are not cold tolerant) and hence selection should act as to reduce the frequency of individuals that are cold tolerant in the population (always assuming an environment that is warm).

What else could cause the frequency of cold tolerance to evolve?

In absence of selection, the frequency of cold tolerance individuals might still evolve in the population. Things like genetic drift, selection at linked site or selection at correlated trait or migration can typically affect this frequency.

  • $\begingroup$ What if I were to phrase the question, "after putting them into the mild climate, would you expect positive, purifying, or balancing selection to occur?" Thus making you choose one? Or is there no grounds for any of the three? $\endgroup$ Feb 14, 2019 at 7:38
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    $\begingroup$ Real world example - Primates' ability to synthesize vitamin C (clearly a good thing for most species) held no benefit to species that ate a very fruit-rich diet, and the vitamin C synthesis pathway gradually accumulated mutations and become non-functional. $\endgroup$
    – iayork
    Feb 14, 2019 at 12:43
  • $\begingroup$ @iayork - and would that be a kind of "selection" on the gene? $\endgroup$ Feb 14, 2019 at 19:33
  • $\begingroup$ @biostudent4451 it would be the absence of selection, allowing random drift leading to mutational inactivation (possibly also some negative selection reducing metabolic costs but I don't know if that's been demonstrated) $\endgroup$
    – iayork
    Feb 14, 2019 at 19:36
  • $\begingroup$ @BioStudent4451 would you expect positive, purifying, or balancing selection to occur? I would expect, no selection. Hence none of the three you mention (assuming mild climate and not fitness trade-off of any kind). $\endgroup$
    – Remi.b
    Feb 14, 2019 at 19:46

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