Modes of Selection and Speciation lectures -

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University of Toronto Scarborough
Biological Sciences
Maydianne Andrade

BIOB51Fall2011 Modes of Selection and Speciation LectureDirectional Selectiono Fitness increases or decreases with increased trait valueo Mean trait value changes it goes up or it goes down o Example the bill size in finches during the droughtthe mean changes to greater beak size there are more birds with greater bigger size than the average before the droughtthere is a decrease in variation of beak size after the drought Stabilizing Selectiono Intermediate trait values have the highest fitness o So the mean trait value does not change o Phenotypic variationstrong decrease in variationo Example birthweight in humans most around the same intermediate value because that is the weight at which most children survive aka optimumbirth weight is the mean birthweight Disruptive Selection o Extreme trait valueshighest fitness o There is no change in the mean trait value post selectiono But the phenotypic variation increases o May lead to a bimodal distribution of trait valueso Example bill size in blackbellied seedcrackerbig beak eats big seeds and small beak eats small seeds the intermediate beak on the other hand though is not good at eating big nor small seeds so there is not many intermediate sized beak birds out there the majority of birds are at the extreme sizes How do Species For o Isolationno gene flow between populationso Divergence mutation drift selection after traits of isolated populationso Reproductive Isolation divergent traits prevent interbreeding even if groups have secondary contactif hybrids do poorly strong selection for mechanisms that maintain genetic isolationCoevolution of Plants and Pollinatorso Plant depends on animal for pollinationo Animal gets nectar and pollen from the planto Classic example of coevolution o Evolution of angiosperms linked to evolution of pollinatorsfossil record and phylogenetic analyses showo Structure colour and scent of flowers match sensory abilities and feeding structures of major pollinators o Geographic variation in flower species can lead to divergence in pollinator populationsStar Orchid and MothsCoevolution Exampleo Moth pollinated plants often have spurs or tubes the exact length of a certain moths tongueo The star orchid of Madagascar has a nectar tube of 11 inches long o Darwin predicted the existence of a Madagascan moth pollinator with a tongue at least 11 inches longhe was widely ridiculed for this prediction o 40 years later the Madagascan hawk moth was actually discoveredit has a 12 inch long tongue1
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