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By Sam Lucas
Comparative anatomy is the study of similarities and differences within the structures of organisms. Physical features may be considered homologous or analogous, but what does this mean?
Homologous structures are similar physical features in organisms that share a common ancestor, but the features serve completely different functions. An example of homologous structures are the limbs of humans, cats, whales, and bats. Regardless of whether it is an arm, leg, flipper or wing, these structures are built upon the same bone structure.
Diagram from: //www.ck12.org/book/cbse_biology_book_class_xii/section/9.9/
Homologies are the result of divergent evolution. Divergent evolution is the process in which organisms from the same common ancestor evolve and accumulate differences, often resulting in a new species. This may occur due to pressures such as changes in abiotic or biotic factors within the environment.
On the opposite side of the spectrum, analogous structures are similar physical features in organisms that do not share a common ancestor. Instead, these structures are related to one another because they perform the same function. An example of this are the wings of a bat and the wings of a bird. They have completely different bone structure, but their wings share the same function, allowing the animal to take flight.
Diagram from: //evolution.berkeley.edu/evolibrary/article/_0_0/evo_09
Analogies are a result of convergent evolution. Convergent evolution is the process in which two organisms that do not share a common ancestor evolve and develop similarities independently of one another. These similarities form because the animals either live in comparable environments or they experience the same environmental pressures, resulting in the evolution of these features.
Next time you are comparing two organisms try to think about how they are related (or not) to one another. The answers may surprise you!
Comment
Since a phylogenetic tree is a hypothesis about evolutionary relationships, we want to use characters that are reliable indicators of common ancestry to build that tree. We use homologous characters — characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. An example of homologous characters is the four limbs of tetrapods. Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature — so the presence of four limbs is a homology.
Not all characters are homologies. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences.
Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings. This idea is illustrated by the phylogeny below, which is based on a large number of other characters.
Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they have both experienced natural selection that shaped them to play a key role in flight. Analogies are the result of convergent evolution.
Interestingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs.
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