Final answer:
To determine the relatedness of two species, genetic similarity is key, although body shape and behavior can also provide clues. Most definitive evidence comes from genetic overlaps, indicating common ancestry.
Step-by-step explanation:
To determine how closely two species are related, scientists primarily consider genetic similarity. However, other factors such as body shape (morphologic evidence) and behavior can also provide insights into their relationship. Genetic similarity is the most definitive of these measures due to the direct inheritance of DNA from common ancestors. Organisms that have a high degree of genetic overlap are considered to be more closely related. Characteristics that are both morphologically and genetically derived from a common ancestor are known as homologous structures. These can include shared physical features, like the arrangement of bones in the limbs of various animals. Conversely, analogous structures may perform similar functions but do not indicate close evolutionary relationships as they arise from convergent evolution.