When studying animals' relationships to one another, scientists often make use of features that are similar among different species. Some features, such as the wings of a bird and the wings of a butterfly, are the result of a phenomenon known as "convergent evolution" (when the same selective pressure causes similar features to develop in genetically distinct organisms) and do not indicate a close genetic relationship. Other features known as homologous traits, do show a close relationship.
A homologous trait is a feature that stems from a common ancestor. The species share the trait because they are linked by a common ancestor that passed the trait to further generations. This is in opposition to an analogous trait. Animals that share an analogous trait developed the feature independent of one another, not through a shared ancestor. To help understand the difference, think of four people with red hair. Cousins Jane and Bob have naturally red hair because their grandmother had red hair. Jane and Bob exhibit a homologous trait. Two strangers Sally and Roger dyed their hair red because they thought it would look interesting. Their red hair is analogous. Note that in the wild, animals do not choose analogous traits.
Acquiring the Traits
Homologous traits stem from a common ancestor. This ancestor can be relatively recent, such as the first primate ancestor that had no tail and passed on this trait to humans, gorillas, baboons and other apes. Homologous features may have also come from a common ancestor far in the past such as the skeletal system shared by all vertebrates or the feature of hair shared by all mammals.
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Observing the Traits
A classic example of a homologous trait is the arm and hand or paw of many animals. Thanks to a shared ancestor, creatures such as bats, birds, whales, reptiles and people all have the same basic arm structure with a single upper arm bone, two forearm bones and an array of bones at the end called the phalanges, carpals and metacarpals. Bats and humans show another homologous trait: five digits on the "hand."
Homologies include more than traits that can be seen by observing a species' outer features or skeleton. Geneticists now use common traits found in DNA to trace the evolutionary relationships between species.