For selection to change the next generation, the differences among individuals must:

1. Importantly, these differences must have some genetic basis, otherwise the selection will not lead to change in the next generation. 2. This is critical because variation among individuals can be caused by non-genetic reasons. 3. An individual might be taller because of better nutrition rather than different genes. 4. A difference with no genetic basis cannot be passed on, so selecting on it changes nothing. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 18 'Evolution and the Origin of Species', sections 18.1-18.3_