I believe that the phenotype is what dictates the direction of selection. Well, one may ask, what it is that determines the phenotype? The answer to that is obviously the genotype. But it should understood that the genotype is expressed in the phenotype and it is the phenotype that is in direct contact with the environment and not the genotype.
Am going to animate the genotype for clarity. The genotype needs to propagate itself from generation to generation and in such it decides to create a vehicle to propagate it, this is the phenotype. In a related but slightly different example is in sexual reproduction. A typical human has 46 chromosomes but when the need to reproduce arises, only 23 chromosomes are used in reproduction. The body acts as a vehicle that carries the genotype.
In the above example, the environment, which is the source of selection pressure, acts on the phenotype and in this case the general body (I am aware of the miss coming of this example). But now on a practical examples let me look into one of the possible reasons for speciation and that is population bottle necks and founder effect.
Assuming that a population undergoes a bottle neck let’s say a disease, and a fraction of the population is resistant to the disease. It must be understood that diseases affect metabolic pathways by the production of toxins most of which are protein in nature. Therefore, a resistant individual will probably have an alternative pathway or produce antitoxins in response to the toxins. It is clear that these are phenotypic traits. Now consider the same population but this time the individuals are not resistant but through human intervention we are able to exogenously apply the antitoxins. It goes without saying that the individuals will survive irrespective of the fact that their genotype do not code for the ‘external phenotype’. A clearer testimony to my theory lies on the observation by most scholars that in most developed nations survival no longer depends on the genotype.
Modern medicine has done away with the need to have advantageous genotypes. Through the laws of probability, I tend to believe that individuals with the same genotype will not necessarily have the same phenotype.
Consider identical twins. They have the same nucleotide sequence in all the 46 chromosomes that constitute the human genotype. However, their fingerprints are never identical. Let us theoretically assume that one type of fingerprint is selected sexually in a population. It follows that the twin with the selected fingerprint will have a higher reproductive success. By this we clearly observe that it is the phenotype that is selected for.
Nothing makes it more beautiful than through the study of phenotypic variance. One of the causes of phenotypic variance lies in the genetic-environmental interaction variance. This is a situation where the expression of a phenotype depends on the environment in which a genotype is found in. Assume an example like phenylketonuria (PKU), a human genetic condition caused by a mutation that makes the liver unable to produce an enzyme that convert phenylalanine into its next ketone-form through deamination. High levels of phenylalanine in the blood leads to mental retardation in infants. However, in the absence of a diet rich in phenylalanine the mental retardation does not occur. We witness that the phenotypes, mental retardation and normal mental health, are expressed due to environmental conditions and not the genotype and in humans mental retardation is selected against sexually (I have no statistical data to support this claim). i would love to hear your reactions to the above post.