Final answer:
Heritability is a statistic signifying the proportion of phenotypic variation attributable to genetic factors in a population, but it is not fixed and varies with environment and population. Relative fitness determines evolutionary outcomes based on an individual's reproductive success compared to others in the population.
Step-by-step explanation:
When we say "heritability is not an absolute measure," we mean that heritability is a statistic that represents the proportion of observed variation in a particular trait among individuals in a population that can be attributed to inherited genetic factors.
However, this figure does not convey a fixed value for a trait's expression, because heritability can vary depending on the environment and the population studied. It is also important to recognize that traits are often controlled by a combination of many genes and potentially by epigenetic factors as well, thereby making the concept of heritability complex and relative.
Relative fitness also plays a crucial role in determining which individuals contribute more offspring to the next generation, thereby shaping the population's evolution. This is based on the idea that it is not an individual's absolute fitness that matters, but how its fitness measures up against others within the same population.
The theory of evolution acknowledges that natural selection cannot create perfect organisms because it can only operate on existing genetic variations and mutations that occur, limiting the potential for change even in the presence of evolutionary forces.