Question

What is a transition feature? What is vestigial feature?

How doe these support the idea of

common descent?

Answer 1

Transition feature is feature acting as a bridge between an organism and a distinct organism that evolved from it. In other words, transition feature is also used to define an intermediate organism between two different organisms, the latter evolving from the transition one.

An example of transitional feature is Archeopteryx lithographica. It is a bird transitioning from reptilians to birds. It is considered as a transitional fossil and shares characters of dinosaurs and modern birds. The fossils of Archeopteryx have indicated they lived about 150 million years ago during the Jurassic period. The species had characters of birds such as wings for flight and could fly to some extent, but major resemblance to reptiles was found like three toed claws (two in birds), toothed beak, free hand bones and metatarsals (unlike fused in birds), nasal opening in the forward portion of skull, and long tail with separate vertebrae instead of fused vertebrae and short tail in birds.

So it can be said that a transition feature is the one which contains features of both the organisms, the organism from which it is transitioning and the other, to which it is transitioning during evolution.

Vestigial feature is defined as a character or feature in the body of an organism that functioned earlier but with evolution, it was no longer needed but does exist in the organism without any role to play.

Example:

Presence of tail bone (coccyx) in humans is a vestigial part of what happened to be an external tail in human ancestors. The coccyx is present in modern humans as a support to the muscles but does not do its intended function any more.

Wisdom teeth are no longer required in modern humans, as the diet of humans have changed considerably from that of our ancestors, they had large jaws and excessively chewed the raw plant and animal food leading to erosion of their teeth and giving space for the third molar set to arise. But with change in food habits and eating cooked food prevented excess use of the teeth for chewing, thus maintaining them in good shape and decreasing the need of the wisdom teeth with time making it vestigial.

Reduced pelvic girdles and the hind limbs in snakes are considered vestigial features.

Transitional features suggest common descent because it shares features common to two groups suggesting one as the ancestor and the one is the descendant of the former. This implies that the organisms have evolved from one to other suggesting a common beginning for all the organisms (common descent).

Vestigial feature suggest common descent because theses features were functional in ancestral species but due to natural selection, they were no longer required. These features in an organism can be considered homologous to those functional features in a different organism, suggesting divergence during evolution and a common ancestory.

Example Appendix in humans and rodents such as rabbits: In humans, appendix is vestigial as it is no longer required to secrete cellulase enzyme for cellulose degradation as human switched to cooked food. But herbivores take in raw plants abundant in cellulose, hence require a functional appendix fro cellulase secretion. This shows that both the organisms have a common decent but due to change in environmental factors one diverged and evolved differently from the other.