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You learned about homologous traits, now begin to explore analogous traits. They are similar in a way. However, analogous traits do not come from a common ancestor.
You learned about homologous traits, now begin to explore analogous traits. They are similar in a way. However, analogous traits do not come from a common ancestor.
See the examples below and complete the note page as you read.
See the examples below and complete the note page as you read.
Not all Similarities are Homologous
Not all Similarities are Homologous
In the game you played at the beginning of this module, you decided that some legs were "not like the others." The matching limbs in the game have bones, and you eliminated the limbs that don't have bones.
Since the octopus, sea star and grasshopper limbs don't have bones, you concluded that they are probably not homologous to tetrapod limbs.
Homologies are inherited from common ancestors. The octopus limb could only be homologous to the lizard limb if they both inherited the limb from a common ancestor. The phylogenetic tree below shows how the octopus is related to tetrapods, and the points in their evolutionary histories when their limbs evolved.
Tetrapod and octopus limbs evolved independently after their point of common ancestry, so they were not inherited from a common ancestor. Therefore, they are not homologous. The same is true of the grasshopper leg and the sea star arm. Simple observation tells us that these limbs are probably not homologous to the tetrapod limb, because they have such different structure.
Similar structures that evolved independently are called analogous structures, or analogies.
How do Analogies Evolve?
How do Analogies Evolve?
Often, two species face a similar problem or challenge. Evolution may then shape both of them in similar ways — resulting in analogous structures. Analogous traits in biology are structures that share a resemblance, but have different origins. For example, the wings of a fly, a moth, a bat, and a bird are analogous because they developed independently as adaptations to a common function—flying. The presence of wings do not hint at a common ancestor.
In many cases analogous structures, tend to become similar in appearance by a process termed convergence. Similar environmental pressure may have driven the analogous traits. The fly and a moth, who developed the ability to fly may have each lived in an environment where it was beneficial to fly, collect more food, thus being able to reproduce and pass down their traits. Scientists understand that if environmental pressure drove one species to develop a particular trait, it is not unlikely that another species living in close proximity may also develop that trait.
When finished, proceed to the next section and watch how Speciation can be represented on a Phylogenetic Tree.
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