Final answer:
Phenotypic variation within a population is the observable traits differing among individuals, rooted in genetic diversity which allows populations to adapt to changing environments. Heritability measures the portion of this variation that is due to genetic differences, important for natural selection and evolution.
Step-by-step explanation:
Phenotypic variation in a population indicates the different physical traits that are observable among individuals, such as size, shape, or color. These variations arise from the genetic diversity within a population, which is critical for the population's adaptation to changing environments. Genetic variation provides selective advantages in a population, which is necessary for evolutionary pressures and natural selection to act upon.
An example of population variation can be seen in the different fur colors and patterns found in domestic dogs. This phenotypic diversity is a result of various factors, including differences in genes and how they are expressed, as well as the environment influencing growth and development.
Factors such as gene expression and environmental conditions can also affect phenotypic variation. For instance, identical twins can display differences, not due to different genes but because of differences in expression during development or in response to environmental factors.
In conclusion, while phenotypic variation can have a genetic basis, heritability is the fraction of this variation that can be attributed to genetic differences within a population. The greater the heritability, the more a population's phenotypic variation can be shaped by natural selection and thus evolve over time.