WEIT Chap3

What are vestigial organs ?

cf. p.56

Vestigial organs are so called not because they have no function but because "it no longer performs the function for which it evolved". These organs are present because our organim has inherited them from the anatomy of their ancestors. They are traces of an organism's evolutionary history. We know this thanks to the fossils and from the pattern of ancestry.

Examples: Flightless birds have wings and they have exactly the same bones as in wings of species that can fly. In some birds, wings don't seem to have any function, they're just remnants. In others, wings have new fuctions. Ostriches can't fly, but they still use their wings when they run, for balance or to protect their babies from the sun. In penguins, the ancestral wings are now flippers. Another vestigial feature that proves evolution is the appendix. This organ, also known as the vermiform, is a thin cylinder that forms the end of the pouch that is situated at the junction of our small and large intestines. It was used before by the herbivorous animals and our leaf-eating ancestors but has no value for humans any more. This is why it's a vestigial organ. Then we can ask a question. If our appendix is useless, then why do we still have one? The answer is that we don't know.

== How come the vestigial organs appear? ==

What are atavisms ?

p.56 ss.

Sporadically expressed remnants of ancestral features. Difference with vestigial traits : occur only occasionally. Has to recapitulate an ancestral trait. Come from reexpression of genes that were functional in ancestors but were silenced by natural selection when no longer needed.

These dormant genes can be reawakened when something goes awry in the development but the information is degraded during the time it remains unused in the genome.

Examples :

" coccygeal projection " known as the human tail : we still carry a developmental program for making tails but those genes are deactivated in human fetuses.

Dead genes

Genes that make a trait that is no longer used don't disappear from the genome but are inactivated during evolution. This has been confirmed by the new technologies that allowed to "read" the DNA. These genes were thus present in our ancestors and then silenced in some of the descendants.

Why do all flightless birds have wings ?

cf. p.57

Are vestigial traits useless?

cf. p.58

Why haven't vestigial organs completely disappeared?

The vestigial organs, in some cases, have almost disappeared. However, if an organ such as wings begin to assume new uses as with the ostrich who can't fly but can maintain balance, mate and threaten enemies thanks to them,then the wings will be maintain by natural selection even if it will be in a form that doesn't allow the birds to fly. In other species, wings are beginning to disappear but we can only see the middle of the process.
Another example is the appendix. It was useful for our leaf-eating ancestors but is not anymore for us. Then why does it still exist? It is hard to answer. Maybe it's in its way out or maybe selection can't shrink the appendix without hurting us. Indeed, a smaller appendix has a higher risk to be blocked. cf. p.59