Final answer:
Humans and other species like chimpanzees have a high degree of genetic similarity indicating a common ancestor, but allelic variation, which is the differing forms of a gene, contributes to the vast genetic diversity within a species.
Step-by-step explanation:
The genetic similarity between humans and other species like chimpanzees is very high because they share a large portion of their genes, often more than 98%. This high level of genetic similarity indicates a shared ancestry. However, the allelic similarity is much lower due to variations in individual DNA base pairs within alleles. These variations lead to considerable genetic diversity even among closely related individuals within a species.
Each gene can have multiple forms or alleles, and different combinations of these alleles contribute to the unique traits of each individual. This allelic variation is what leads to the differences we observe among individuals of the same species. For example, in humans, genetic variations can influence characteristics such as eye color, height, and susceptibility to certain diseases. While the overall structure of the genome may be similar between two species, the specific alleles present can differ greatly, resulting in distinct physical and behavioral characteristics.
It's important to note that a small genetic change in an organism's DNA can cause significant morphological differences, which is why organisms with a high percentage of shared DNA may still appear very different in terms of anatomy and physiology.