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Lecture 11

Lecture 11: "Speciation II"

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Western University
Biology 3466B
Yolanda Morbey

Evolution Lecture No. 11: Speciation II rd Wednesday October 3 , 2012 Divergence By Natural Selection: -In the case study between the two races of maggot flies, the apple and the hawthorn variety, the allele frequency of theAcon-2 95 gene (an important one involved in the speciation process) was examined. As they have both adapted to the phenology of their respective trees, neither one’s would survive well under the selective pressures of the other’s host tree. This is a key study to examine because it shows how studying incipient species (racial sub-species) is often more revealing of the process of speciation than is studying fully distinct species. Divergence In Drosophila: -Drosophila, the fruit fly genus studied in nearly every single scientific publication, has quite an interesting history of divergence spanning across the islands of Hawaii. Because the land masses of this archipelago are estimated to be formed recently (within the last million years), it is quite amazing that Drosophila has diverged through both allopatry (by the geographic isolation of each island to the other)and sympatry (by the process of sexual selection). Let it also be known that the more ancestral species come from islands towards the west, as those are the land masses that supposedly formed first. Divergence By Sexual Selection: -Sexual selection is a form of selection that is due to the differences among individuals in their ability to obtain mates. Opposed to natural selection in shaping the allele frequencies of a population, sexual selection is perhaps better described as a form of reproductive isolation due to behaviour among individuals. In the case of Drosophila heteroneura, males with wider heads are more favourable mates as they establish dominance in head-butting mating rituals. Drosophila silvestris on the other hand, attracts mates through mating rituals more akin to wrestling. So it should be clear, that the different mating behaviours of species (sexual selection) further act as a form of reproductive isolation between species. -In examining the data of studies of Drosophila heteroneura, it becomes clear that sexual selection is an important mechanism for evolutionary changes within a given population. The first graph shows that male head width is proportional to the amount of copulation. In the second graph, we can also deduce that the male winners of mating rituals tend to have wider heads. Darwin’s Finches & Divergence By Sexual Selection: -The finches that Darwin examined on his voyage to the Galapagos all evolved from a single ancestor, which apparently had migrated to the islands from mainland Ecuador. Though Darwin probably didn’t know it at the time, the very distinct morphologies of the birds due to their distinct foraging styles was a prime example of just how quickly sexual selection reproductively isolates entire species. The Galapagos & Darwin’s Finches: -Looking at the number of finch species that are present on each island of the Galapagos, it still remains unclear if speciation had occurred in the finches either from allopatry or sympatry because there is obvious geographic isolation, but the mating songs of the finch species diverged in correspondence to their profound changes in bill size. These diverging populations are isolated after secondary contact mostly due to this shift in birdsong melodies. This as we will see, is closely connected to the phenomenon known as ecological speciation. Ecological Speciation: -Ecological speciation is the evolution of reproductive barriers between populations by adaptation to different environments or ecological niches. It is known for occurring in either sympatry (like where the two maggot fly races were evolving to the different tree phenologies) or allopatry (like where Darwin’s finches were evolving to different niches on different islands). Reproductive barriers are also a significant by product of ecological selection. Various Kinds Of Ecological Speciation: -There are three main f
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