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Evidence for Evolution

Biology (Year 12)

Comparative Anatomy

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Ben Whitten

Comparative anatomy is a tool used to establish evolutionary relationships between species based on structural similarities and differences between organisms. The main areas of comparative anatomy include;

  • Embryology

  • Homologous structures

  • Analogous structures

  • Vestigial structures

Embryology Embryology is the comparative study of vertebrate embryos which share similarities across a large range of species, even with relatively distant ancestors. The structural similarities found in embryos of fish, humans and many other organisms suggest that they all share a common ancestor from the past. In the phylum Chordata, there are similarities in Chordate embryos, particularly between dorsal nerve cords, which suggests that there is a common ancestor from which all Chordata have evolved.

Homologous Structures

Homologous structures are features of organisms that were present in common ancestors, but have evolved to perform different functions in different descendant species. For example, this is seen in the human hand. Organisms like a whale, a bat and a human all have pentadactyl limbs (containing a radius, ulna, carpal, humerus), but for different functions. The bat wing is adapted for flight, the whale flipper is adapted for aquatic environments and the human hand is adapted for different functions. When adaptive radiation occurs (a rapid form of evolution), the species retain the same basic structures as they have the same genetic history. For example, all lizards have scales, yet they vary in colour, hardness, shape, function and temperature.

Vestigial Structures

Vestigial structures are those that have come from a common ancestor, but they no longer have a function in descendant species. This means that in the past they were useful, but have since become useless. They are usually rudimentary (undeveloped) or atrophied (loss of muscle tissue). A key example is a human appendix. It's suggested that we had the appendix to help digest plant matter, as we pertained a herbivorous diet which has since become redundant. Other examples include the coccyx (tailbone).

Analogous Structures Analogous structures are structures in different organisms that serve the same purpose, but don't have the same structure. These types of structures arise by convergent evolution. A key example is the wing of insects and birds used for flying. Both serve the same purpose, but do not have the same structure.

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