I understand the difference between the genotype of an organism and the phenotype of an organism, (genotype being the genetic code, phenotype being the physical expression), however I am unsure about what this figure mean when it says "Selection that takes place in the environment is based on the specific phenotype of an organism-its traits, not the underlying genotype".

(misconceptions about evolution)


I thought the genotype controlled the phenotype, so what does the figure mean exactly when its saying that natural selection isn't based on the genotype?

Thank you!

  • $\begingroup$ See my comment on Remi.B’s answer below. To say that the genotype controls the phenotype is true to a first approximation. It would be more accurate to say the genotype at one locus influences the organism’s phenotype, because the cumulative affect of the genetic background also influences that phenotype. Hence, there are people walking around carrying one or perhaps even two alleles of mutations linked to specific diseases, yet these people have no disease symptoms (and may never develop them). Of course there are other mutant alleles with 100 % penetrance and expressivity. $\endgroup$
    – mdperry
    Sep 15, 2018 at 19:47

1 Answer 1


The difficulty is in the semantic. In the sentence

Selection acts on the genotype because that is what is inherited

it is unclear what is meant by "acts on". The point that the author(s) of this misconception were probably trying to make is that what define the reproductive success of an individual is the phenotype of an individual. In other words, it is the expression of those genes that affect the probability of the genes to be passed on but it is not the actual genetic sequence itself (or at least less directly so).

Let's assume a haploid case where at a given gene, allele A means 'tall' and allele B means 'short' and let's assume that size matters in defining fitness. It is ultimately not whether an individual carry the allele A or B that matters but whether the individual is tall or short. If by some fantastically random event there is an individual with allele B that is tall then this individual will rather have the same fitness as the tall individuals and not the same fitness as the B carrier individuals.

  • $\begingroup$ Alleles can have variable penetrance, and variable expressivity. In a diploid species consider the heterozygous individuals all carrying the same dominant allele, or m/+. If all of those hets display the mutant phenotype, then it is 100 per cent penetrant. However, if only some of the hets display the mutant phenotype then the allele has variable penetrance (that you can likely measure). Similarly, among all of the hets displaying the mutant phenotype, there may be different degrees of severity, so the allele could have variable expressivity. Same can be said for recessive homozygotes. $\endgroup$
    – mdperry
    Sep 15, 2018 at 19:40
  • $\begingroup$ Of course. Never did I meant that there is a one-to-one function between genotype and phenotype. I would agree that my answer is a bit poorly phrased. I've had a hard time to write a pleasant to read answer here for some reason! $\endgroup$
    – Remi.b
    Sep 15, 2018 at 20:44
  • $\begingroup$ I almost started my comment with “You probably know this . . . “ $\endgroup$
    – mdperry
    Sep 15, 2018 at 23:39

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