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Recently, research into embryonic development has given us an even better insight into how major structural changes might occur in a given population of organisms. We now understand that there are two major types of genes: developmental and “house-keeping” genes. Developmental genes are those that are expressed during embryonic development, and their proteins control the symmetry, skeletal development, organ placement, and overall form of the developing animal. In contrast , “house-keeping” genes are expressed during the animal’s daily life to generate proteins which keep the cells, tissues, and organs in the body functioning properly. As you might suspect, mutations in developmental genes can have radical consequences for body form and function, whereas mutations in “house-keeping” genes tend to affect the health and reproductive success of the post-embryonic animal.