Evolution

A favorite evolutionary discussion is about homologous and analogous structures or body parts.

During the discussion of evolution, we include investigations into homologous and analogous structures in an animal’s body.
 * Homologous vs. Analogous**

According to Websters Dictionary, it means: • “derived from or developed in response to organisms of the same species” • “having the same relative position, value, or structure”
 * What are homologous structures?**

Homologous structures are body parts that are similar due to shared ancestry. There is a similarity of the structure, physiology, or development of these structures in different species of organisms based upon their descent from a common evolutionary ancestor.

Mammals with appendages have similar bone structure, although it may not be exactly the same due to changes in function. For example a human’s arm has a humerus and an ulna and radius leading to the wrist area. This leads to the metacarpals (hand) and phalanges (fingers). These same bones can be found in bats, whales, birds, etc. [|Drawing of homologous structures]

Often these structures are what we refer to as developmental; they arose from the same embryonic structures but then became modified based on the species. [|Homologous structures in mammalian embryos]


 * What are Analogous structures?**

According to Websters dictionary: • resemblance in some particulars between things otherwise unlike • correspondence in function between anatomical parts of different structure and origin

Analogous structures are similar in function, not necessarily structure. Compare the wing of a bird with the wing of an insect. Structurally they are not similar but the function is similar; allows organism to move through the air. These structures are not based on common ancestry and do not reflect evolutionary closeness among the organisms.

However, the wings of a bird and a bat are homologous and analogous because they share a common structure and a common function. Because of the homologous structures, there is an evolutionary closeness – they are both mammals.

[|Another comparison of homologous and analogous structures]


 * //The existence of homologous and analogous structures leads to a discussion of Divergent and Convergent evolution.//**


 * Divergent evolution**

In the case of [|divergent evolution], similarity is due to the common origin. The structures are homologous and the adaptations, over time, have not yet completely obscured the underlying similarity. Again, compare the human arm, the fin of the whale and the wings of a bird. Although the structures are similar in composition, they have been adapted for the environment; i.e. flying, swimming and walking.

A good example of divergent evolution is the [|Darwin's finches], which has now over 80 varieties which all diverged from one original species of finch. This is also referred to as adaptive radiation.


 * Convergent Evolution**

[|Convergent evolution] arises when there are some sort of environmental factors that require a specific structure for survival in that environment. For example, the need for fins for propelling though the water. The structures do not have to have a similar skeletal structure to enable this ability. Again, compare the fin of a trout with the fin of a whale or seal. Or compare the wing of a bird with the wing of an insect. These structures come from different points of origin but through adaptation became similar in function.

Watch **“[|How Do We Know Evolution Happens]**?” from PBS.

==**I would like to hear from the class about other analogous structures. It might be a bit tricky to state a few more homologous structures but if you dig deep enough into your textbook or the Internet, you will find some. Please share your results.**==