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Lecture 20

Lecture 20.docx

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Biological Sciences
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Maydianne Andrade

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Lecture 20- Modes of Selection and Speciation 1. Modes of Selection - Directional, Stabilizing, Disruptive 2. Species and Speciation -Isolation: Allopatry and Sympatry -Divergence: Drift, Natural and Sexual Selection -Reproductive Isolation: Hybrid fitness How do Taxa Diversify? - Phylogenies –branching from a common ancestor 1. Directional Selection -selection on one tail of the distribution - fitness increases or decrease with changes in trait value - measured using the selection differentials we looked at -eg: Alpine skypilote flower in the tundra and finch beak size during draught - selection within ONE generation (not over several generation) 2. Stabilizing selection -selection of center of distribution - the trait values in the middle have the highest fitness -The mean trait value has no change because it (the average) is what is being selected for -the tails however are being cut shorter -Eg: Birth weight in humans – mortality is lowest (so fitness is highest) when looking at the center of the distribution (average) 3. Disruptive Selection - selection on both tails of the distribution -fitness is lowest at the average for the population - so after selection, the average (mean) stays the same but the population distribution is widened increase in variance - or can lead to bimodal distribution (two different distributions) -Eg: bill size in black – bellied seedcracker -Exam question!!! Which of these patterns of selection would be most common in a stable environment? stabilizing selection because in a stable environment, there is not need for much variation for the individual to do well. -Disruptive selection is mostly seen in a novel environment What is a Species? -When recognizing a species it has to follow two rules: 1. Species are evolutionarily independent –mechanisms of evolution operate separately in different species 2. Species consist of interbreeding populations (if sexual)-there is gene flow or migration among the populations of a species this will homogenize frequencies Three Steps to How Species Form: 1. Isolation - species remain similar across populations because of gene flow. So in order to form species we need the isolation of species to stop the gene flow so that the mechanisms of evolution can work separately and the population can begin to look and be different from each other 2. Divergence - mutation, drift and selection alter traits of isolated populations 3. Reproductive Isolation -once the population reaches reproductive isolation and then they somehow meet with a population of the original species, either A) the traits have changed so much that they can no longer interbreed, or B)the hybrids do so poorly that there is strong selection for the mechanisms that maintain the genetic isolation. 1. Isolation Can be done in two ways: a) Allopatric speciation – populations become GEOGRAPHICALLY isolated I)Dispersal and colonization of a new habitat -Eg: a small population of fish diverge to a new habitat and then evolve to new environment Question: are the allele frequencies in the new population going to be equal to the allele frequencies in the original population and colony? No because of the founders affect -Eg: Hawaiian Drosophila – Founders hypothesis prediction: The most ancient species on the genus would be found on the oldest island and the newest species would be found on the newest island consistent with speciation. II)Vicariance: a geographic event that splits the species distribution forming subsets of the population - could be something very fast or very slow -then divergence in the newly separated groups. -Eg: the land between North and South America – Central America before there was nothing connecting NA and SA but 3 million years ago, land emerged as water decreased. So then there was a separation between the species of the Pacific Ocean and the species in the Caribbean Sea (a vicariance event) creating a land bridge - we would predict that the species on one side of the bridge would be more similar to the species on the other side of the bridge than the species on the same side of the bridge. (with geographic speciation, we would predict that the species on the same side would be most similar each other , but that is not the case with a vicariance event) - this can be tested  that is what you find (the species on the opposite side of the bridge are more similar) -pairs of species on opposite side of bridge are sister species. b) Sympatric speciation – populations are still in the same geographic area but speciation can still occur If there is some genetic incompatibility that occurs or through disruptive selection and divergent mate choice which decreases gene flow. I) By Genetic Isolation: more likely to occur in plants- occurs more when large-scale chromosomal changes occur  the mutation causes the offspring to no longer be compatible to the parent species.  so they form a new species on their own. -Eg: Polyploidization normal breeding results in N but mut
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