Hormones are crucial for the development of organisms. In the tropical butterfly, Bicyclus anynana, ecdysone affects eyespot size on the ventral wings and developmental time, both important fitness components. This is reflected in a strong genetic correlation between the traits, and there is evidence that ecdysone is the mechanism for this connection: high levels of ecdysone result in large eyespots and fast development, while low lead to small eyespots and slow development. They selected simultaneously on these two traits and succeeded not only in making more extreme phenotypes, but also in breaking the connection and obtaining phenotypes with large eyespots and slow development, or with small eyespots with fast development. They examined ecdysone physiology to show that both the sensitivity to ecdsyone and the titers of the hormone are only affected by selection on developmental time. This result is important because it shows that in the field the adaptation in life history to changing environments is not hampered by a hormonal mechanism that controls several traits. It further shows that the developmental system is flexible enough to allow evolution in directions opposing the correlation: phenotypes can change by genetic variation in other, independent, pathways.
For more information about this article and other articles in the May issue, see The American Naturalist online: www.journals.uchicago.edu/AN