There appears to be low addictive genetic variation

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variation, they all have very similar bill length and depth when compared to the natives. There appears to be low addictive genetic variation, which means that there is little deviation from the mean, contributing to the similarity of all the hybrids. Despite all this, the hybrid appears to be very successful. This could be due to their bill and body size. The body size of the new species is between G. Fortis and G. conirostris, but closer to Fortis. This could be due to more material coming from the Fortis. However, the bill size is much closer to that of the G. conirostris. This shift in allometry appears to be due to natural selection. This was further confirmed in the lab by experiments which show increased allometry due to artificial selection, iii showing that selection could well be the driving force. The increased bill size could also be caused by transgressive segregation, when complementing alleles from the parents causes the offspring to become particularly expressive for a trait iv . In regards to the effect of the larger bills, a look at mortality in 2009 showed that the survivors had a much larger average bill size when compared to those who died, suggesting a correlation between survival and bill size. Bill depth appears to be correlated to the body mass of the birds, which means that, due to the larger size of the hybrids, the depth of the bills can also increase, which gives them the selective advantage compared to the natives. S. Lamichhaney et. al. also investigated the development of reproductive isolation. This occurred due to differences in song and morphology. The hybrids prefer to mate with specimens with similar songs and, since all the hybrids have a different song compared to the natives of the island, (imperfectly learned from the original immigrant) this creates a premating (prezygotic) barrier to reproduction. This effect was studied through a surrogate experiment using models of finches and tape recorded songs. v Furthermore, the hybrids appear to prefer mates with similar bill size to body size proportions.
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All of these factors contribute to the isolation of the hybrid as a new species. This, coupled with the
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