Podos states that his findings were consistent with the proposed vocal constraint hypothesis, which essentially states that as beak size changes, various levels of beak mobility will in turn change song quality and sound. His data suggests that the change in sound was correlated to both beak size as well as mass, however he later presented contrary evidence for the mass’s effect on this. He then proposes that several factors, including resource changes leading to beak size adaptation, as well as cultural evolution and various acoustic environment. Finally, he suggests that the importance of prezygotic isolating mechanisms (mating songs) are indirectly influenced by adaptive diversity of beaks. This particular experimental group is powerful as there is a high number of species within a relatively small area and opportunity for hybridization. Despite this, there has been rapid speciation and he believes that his songs help ensure “correct” mating occurs.
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The initial question presented at the beginning of the article pertains to the evolution of flight in regards to evolutionary advantage during the intermediate stages. There are two rival theories; the”tree down” hypothesis as well as the “ground up” hypothesis that support conflicting ancestral behavior that eventually transitioned to modern flight. Dial’s interpretation of his data, specifically the correlation between the WAIR facilitating movement on coarse surfaces, such as tree bark, as well as the change in the birds’ center of gravity while performing WAIR, both support the ground-up hypothesis as these qualities seem to be an escape mechanism for terrestrial birds. In addition, Dial goes on to suggest that proto-birds, such as archaeopteryx, had bone structure to support antero-posterioral movement, which is ideal for the movements necessary for WAIR utilization. Furthermore, he proposes that wing-angle adjustment used for varying levels of incline could have also been used as a tool for controlled descent and body manipulation while in mid air, which could have also contributed to the eventual evolution of flight.
Stable isotope analysis is a process where one analyzing varying levels of common isotopes in tissue levels. This information is useful as concentrations of various isotopes are predictable and tissue changes are accumulated via diet. However, each isotope has a turnover rate which can range from days to months, which may skew any data being used to determine locations of breeding or molting grounds. Feathers are an exception to this turnover process as they rapidly develop and then become metabolically inert, essentially freezing their isotope concentrations in place. Then, using, a mass spectrometer, one can identify what was consumed during feather development and then use this information to determine where molting or breeding grounds are. While this is still an inaccurate process and unrefined process, there is opportunity for more improvement with multiple isotope analysis as well as more accurate geographical isotope predictions.
Correlated evolution of morphology and vocal signal structure in Darwin's Finches
In this article, Podos challenges the classical theory that mating signal (i.e song) evolution is an accidental byproduct of speciation operating on other environmental factors. Specifically, he refers to beak morphology and questions how the adaptive diversification of beaks has influenced the evolution of song quality by either enhancing or constraining vocal abilities. Considering he begins the article by noting that songs are often used for mating, I infer that he suspects that evolutionary diversification of break size and shape (secondary to ecological changes) may indirectly lead to speciation by changing mating calls.
Figure 1. Oxalis Triangularis Development. Comparison of the change in pigmententation during development. Immature speciemens (right) produce green foliage. Mature (left) speciments produce purple leaves. Creative commons licensed image by https://flic.kr/p/frB4rf.