What is meant by 'variability in fitness' within a phenotype?

'Variability in fitness' within a phenotype refers to the average ability of that phenotype to produce mature and fertile offspring compared to other phenotypes. This concept is crucial in evolutionary biology, as it highlights how different traits can influence reproductive success within a population. Fitness is often measured by the number of offspring produced that survive to reproductive age, meaning that a phenotype's fitness is not just about survival, but also about successful reproduction.

This understanding encompasses various factors such as survival, mating success, and the environment's role in shaping which traits are favorable. When we talk about variability, we imply that not all individuals with the same phenotype will produce the same number of offspring due to factors like genetic differences, environmental conditions, and interactions with other organisms. Thus, the correct answer encapsulates the essence of fitness variability within a phenotype, tying it directly to reproductive success in the context of evolutionary pressures.

In contrast, the other options touch on related concepts but do not capture the specific meaning of 'variability in fitness.' For instance, variations in survival rates or adaptation to environmental changes are important topics in ecology and evolutionary theory but do not directly address the reproductive aspect that defines fitness variability among phenotypes.