The two kinds of variation are phenotypic (changes in outward physical traits) and genotypic (changes in the organism's underlying genetic makeup — basically, the DNA sequence of its genes).
Individuals with the same phenotype can have different genotypes, and individuals with the same genotype can have different phenotypes. Height, for example, has both a genetic component and an environmental component. You'd think that two people with identical genetic makeup (such as identical twins) would be the same height, but if only one received a healthy diet, the result could be two adults of different heights. In this example, two people with the same genotype have different phenotypes. Conversely, two people with different genetic make-ups can be the same height. In this case, the two individuals have different genotypes but the same phenotypes.
¿jtJABEft Keeping this distinction in mind is important when you think about how natural selection can cause evolutionary change. Because natural selection doesn't "see" the different genotypes, it acts only on the phenotypes — the physical features of the organism that can influence its survival and reproduction. But evolution can occur only if phenotypic differences are correlated with genotypic differences.
Imagine that being taller carries a selective advantage — that is, taller folks are better at surviving and reproducing. In the case of the identical twins with different heights, the taller of the two identical twins would be more likely to contribute genes to the next generation, but because the height difference was due to environmental factors and not genetic makeup, the genetic composition of the next generation doesn't change. In the case of the two unrelated people of the same height, the two individuals have different genotypes, but because natural selection acts only on phenotypes, both individuals are the same height, so they have an equal chance of contributing genes to the next generation.
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