Evidence from comparative anatomy

Evolutionary theory predicts that related organisms will share similarities that are derived from common ancestors. Therefore, comparative anatomy is used to determine relationships between different species of organisms which may link them to a common origin.

Homologous structures and divergent evolution

Widely separated groups of organisms would share certain basic features if they have common ancestors. The degree of how closely two organisms are related in evolution can be determined by the extent of resemblance between them.

Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups which have a lot in common.

Fundamentally similar structures are used in deciding how closely related the two organisms are, even though they may serve different functions in the adult stage of the organism’s life cycle. Such structures are described as homologous and suggest a common origin.

These homologous structures are different in the way that each structure is specialized to perform a variety of functions in order to adapt to different environmental conditions unique to the organism. The gradual spreading of the different structures is called divergent evolution.

Example: Pentadactyl limb

All of the animals’ forelimbs as shown in the picture above conform to the basic pentadactyl pattern but are modified for different usages.

The pentadactyl limb is found in all classes of tetrapods (i.e. from amphibians to mammals), and can even be traced back to the fins of certain fossil fishes from which the first amphibians are thought to have evolved. The different forelimbs of all the animals all share the same set of bones - the humerus, the radius, and the ulna. This suggests that even though the bones may be slightly different in shape and size (due to the animals’ different lifestyles), the animals may have all descended from a common ancestor. In the course of evolution, these fundamental structures have been modified to serve different functions and to adapt to different environments and modes of life.

One example is the bat, whereby 4 of the bones in the forelimbs have been elongated and turned into wings for flying, whilst another bone is hooked to allow the bat to hang from trees.

Embryology

Homologous structures are usually present in the organism’s stage of embryonic development, but it is sometimes absent in the adult organism. In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development. Species that look very different from each other in their adult form may have similar embryonic stages.

One such example is the human foetus. The embryo has gill-like structures like those of the fish in one of its developing stages. The embryo also has a tail for a long period of the time during its development, similar to those of our primate relatives. Hence, it can be said that humans and primates or humans and fish share homologous structures.

Source:

http://en.wikipedia.org/wiki/Evidence_of_evolution, 11th April 2007

http://www.sparknotes.com/biology/evolution/evidence/section3.rhtml, 11th April 2007

Ilisa Ishak [Science Investigator]