Evolution

 

Characters are heritable traits that can be compared across organisms, such as physical characteristics (morphology), genetic sequences, and behavioral traits. A shared character is one that two lineages have in common, and a derived character is one that evolved in the lineage leading up to a clade and that sets members of that clade apart from other individuals. An ancestor in a species had an evolved trait and therefore passed that trait down to its offspring. Shared derived characters can be used to group organisms into clades.

For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have, or historically had, four limbs. If you look at a modern snake you might not see obvious limbs, but fossils show that ancient snakes did have limbs, and some modern snakes actually do retain rudimentary limbs. Four limbs is a shared derived character inherited from a common ancestor that helps set apart this particular clade of vertebrates.

 

Vestigal structures are organs that serve little or no function for an organism. Organisms may posses these structures because they do not affect how well the organism survives so natural selection does not cause the elimination of the structure or organ. Organisms have these to start with because they evolved from structures that the species used to need. One example is the wings of flightless birds. 

 

Homologous structures, or homologies, are structures that have mature forms but develop from the same embryotic tissues. They provide strong evidence that all four limbed vertebrates have descended, with modifications, from common ancestors. The similarities and differeneces between the structures of different organsisms help biologists group animals according to how recently they last shared a common ancestor. 

 

Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms. An example of a modern form of a transitional species is a whale.