Voir la source de 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 organism 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. <br>

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. <br>Another remnant of primate ancestry is our vestigial tail: the coccyx.
== 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.<br>

 
Examples : <br>

 
" 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
   
By looking at some animals like the ostrich, the kiwi or even at the penguin, we notice that all flightless birds have wings. These birds no longer use their wings to take off. These organs are sometimes very small and hidden under the feathers, keeping us from seeing them properly but that doesn’t mean they are useless. <br>
The wings can simply be vestigial traits which means that they were an evolution in an ancestor but they are now useless (as in the ostrich) or have acquired new traits (as in the penguin) : the ostrich’s wings don’t allow it to fly but it uses them for balance (« by extending them to the sides »), while it is running. The penguin’s wings, allow it to swim extremely fast as they evolved into flippers. The birds can also use them in mating displays or even to threat their enemies.


Despite their new uses, the flightless birds’ wings nevertheless display the same bones as the species that can fly, showing that these animals are effectively the result of evolution.

=Are vestigial traits useless?=

No a vestigial traits isn't useless. Thanks to the vestigial traits, new uses of those traits have been developed and it is very important because it SHOWS evolution! Opponents of evolution often use the argument when we talk about vestigial traits as evidence of evolution that "the features aren't useless". They say that vestigial traits are useful for something and that we haven't discovered yet what they're for. What they don't understand is that evolutionary theory doesn't say that vestigial traits are useless, it says that it's not functional because "it no longer performs the function for which it evolves". So even if it's in different ways, both agree that vestigial traits aren't useless. Darwin used to say that "an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose". This is all about it: Organs may be no more useful for one thing but evolve in order to developed another caracterisct. That's why vestigial traits aren't 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.<br/> 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

=Palimpsests in Embryos=

Palimpsets can be compared to recycled manuscripts that were used in medieval Europe. It's a kind of process that consists in re-using some informations as clues to prove evolution. Indeed "within the bodies of animals and plants lie clues to their ancestry, clues that are testimony to evolution." Today we have proved that these palimpsests exist thanks to the DNA sequences. 
All vertebrates (fishes, reptiles, birds, amphibians and mammals) begin development in the same way, looking like an embryonic fish. During our embryonical development, our fishlike circulatory system turns into one similar to that of embryonic amphibians. Then it continues to change into a circulatory system resembling that of embryonic reptiles. Finally it changes still further and turns into a true mammalian circulatory system. In the development of an organism, we also see its own development. Each vertebrate undergoes development in a series of stages, and the sequence of those stages follows the evolutionary sequence of its ancestors. <br>
As fish and shark embryos develop, the first arch becomes the jaw and the rest become gill structure. But in other vertebrates that don't have gills as adults, these arches turn into very different structures that make up the head.