What factors contribute to phenotypic variation between organisms?

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Let us first understand what is meant by the term "phenotypic variation". Simply, the phenotype of an organism is its observable characteristics - like how it looks, how it behaves, and so on. Differences between organisms in these observable traits is known as phenotypic variation. 

So for example, a human is very phenotypically different from a snail (and we can say that there is a lot of phenotypic variation between the two). But a human is less phenotypically different from another human (and we can say that there is less phenotypic variation between the two). Co-twins of the same sex would have very little phenotypic variation between them (though there is still some).

Now we understand what we mean by phenotypic variation, we can move on to answering the question. 

Two things contribute to variation in phenotype – variation in genotype, and variation in environment. First let’s deal with variation in genotype.

A genotype is the genetic information of an organism. This genetic information, in the form of DNA, tells cells which protein to encode. Ultimately, the proteins that are encoded decide aspects of what an organism looks like, and how it functions and behaves – in this way, variation in the genotype contributes to variation in the phenotype. The genotype is inherited from an organism’s mother and father (half from the mother, half from the father), so this can be thought of as the inherited aspect of phenotypic variation.

The environment can affect an organism’s phenotype in many different ways. For example, exposure to a high-fat diet would lead to an increase in weight and consequent increase in size, while exposure to a low-fat diet would lead to a decrease in weight and consequent decrease in size. Clearly then, variation in environment contributes to variation in an organism’s phenotype.

Overall, it is the combination of genotypic variation and environmental variation that is responsible for phenotypic variation. Importantly, different phenotypic traits rely on genotype and environment to different degrees. For example, variation in eye colour is entirely down to genotypic variation, but variation in weight is down to both genotypic and environmental variation.

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