Final answer:
Differences in traits among siblings are called variations, which result from the combination of genes inherited from their parents. These variations, which are genetic, create diversity within a species and can be influenced by gene expression and environmental factors. Adaptations are specific variations that have evolved to aid survival.
Step-by-step explanation:
Differences in traits among siblings are called variations, which explain why siblings do not look exactly alike. These variations are the result of the specific combination of genes inherited from both parents. Even a slightly altered sequence of nucleotides within a gene can lead to different traits such as hair or eye color. Moreover, it's essential to understand that traits must have a genetic basis for natural selection to have an effect in subsequent generations. This genetic basis is crucial because it's heritable and hence can contribute to evolutionary changes over time.
While genetic differences are often responsible for the phenotypic differences we observe, gene expression also plays a key role. An individual might be taller not solely because of genetic differences but also because of better nutrition, an environmental factor. However, in terms of heredity, it is the genetic variations that are passed down and contribute to the diversity within a species.
Adaptations are another aspect of variation, but they specifically refer to heritable traits that have developed to aid the survival and reproduction of an organism within its environment. These adaptations are the result of genetic variations that over time come to better match the population to its environment, such as the varying beak sizes of finches that evolved to handle different types of food sources.