Understand that variation within a species can be genetic, environmental or a combination of both.
1. variation = difference we can see in the phenotype (how things appear)
it is possible to count and measure these differences in graphic form.
An individual has a phenotype --> genotype --> which is modified by the environment.
variation pop = variation genotype = variation environment
Differences in the appearance of individuals in a species population is because those individual have different genotypes and are living+surviving in different environments.
Graph "1" - suggests variation in the pop (species) is due to the variation in genotypes. (environment is normal here)
Discontinuous variation --->
Graph "2" suggests variation in the pop and variation in species.
The variation here is caused by genetic variation in the genotype
eg: Height in humans (affected by your diet)
Combo of genes + the environment.
these groups are modified by the environment which results in a smooth curve.
3rd case: Variation in a population (phenotypic variation) is due to environmental variation. The genes have no role to play in differences in pop.
eg: the home language you speak.