The plan Describe hypotheses for the origin of major body plan changes. Taxonomy Describe the catalogue of diversity. An adaptation which was honed by natural selection in one environment becomes useful in a new environment. The required heavy hind limbs and large balancing tail was originally an adaptation for living in a swamp. The failure of the fold to take place may have lead to the formation of digits in tetrapods.
Aquatic salamanders retain gills as adults instead of developing lungs.
The human skull frontal bones grow at a faster rate than in the chimp allowing for a deeper brain pan. In multicellular organisms, different sets of genes may be expressed in each cell allowing for dedicated cell function. Example is the gamete producing cell in many colonial algae. Determines which specialization each segment will develop. Example is the legs growing from the heads of Drosophila when the inappropriate homeotic loci are turned on.