A homologous trait is one that is seen in different species that come from a common ancestor. This can be seen between the human and monkeys. Humans have a tailbone that resembles the tail of monkeys. It is referred to as the vestigial and is the remains of where a tail would be. In humans, the tailbone is basically useless, while in monkeys, their tail is used for many different things. They use it for balance or as a fifth limb for climbing and swinging. These traits are different between the two species because one is useful while the other is just a remnant of evolution. Since humans and monkeys are both considered primates, a primate would be the common ancestor.
An analogous trait is one that is similar between two species but not due to genetics. This can be seen in the fins of dolphins and penguins. A penguin is a bird and a dolphin is a mammal but they both have fins that perform the same function, navigation through water. Since both belong to a different class of vertebrates they're evolutionary line does not cross paths at all.
It is often tempting to assume that vestigial organs have no use to the organism, but often there are functions we are not aware of until that structure is gone. For instance, the appendix (early in life) may actually play a role in our immune system, and the same has been argued for our tonsils, which may be why doctors aren't so quick to have those removed anymore. The human tail bone actually serves as a very crucial muscle attachment point for the muscles in our perineum, particularly those involved in supporting the pelvic floor. You would notice if you were missing these!
ReplyDeleteThat aside, there are distinctly different functions to the tail of the monkey and what is left of the tail in humans. Recognize that not all monkeys possess the ability to use the tail for climbing. That is only the case in New World monkeys. Old World monkeys (like baboons) don't have this function.
I agree that both organisms are primates, but how does this confirm that these traits are homologous? What do we know about early primates (and mammals in general) that confirms homologous status of these structures?
Good choice and explanation of your analogous traits.
With regard to this statement: "Since both belong to a different class of vertebrates they're evolutionary line does not cross paths at all"
Birds and mammals share a common ancestor. Both arose from early reptiles and therefore the potential for common ancestry. The key to your trait pairs, however, is that these traits are in completely different locations on the body, which says a lot about their lack of common descent. The penguin "fin" is a wing, which is a derived trait that evolved from an early reptilian fore-limb. The dorsal fin on the dolphin didn't arise from a fore-limb at all, but developed independently along the back for the same reason fish have these structures, to help them navigate through the water.
Now if you had chosen to compare the flipper of the penguin with the lateral fins of the dolphin (the pair below the dorsal fin on either side of the body), would those have been analogs or homologs?
Good images.
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