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Lecture 14

Biology 1001A Lecture 14.pdf

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Biology 1001A
Beth Mac Dougall- Shackleton

Biology 1001A | 2012 LECTURE NOTES Lecture 14 Selection & Other Evolutionary Mechanisms –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– - look at how the factors we will discuss will affect allele frequency and how this will affect genetic diversity - there are various forms of variance on a locus Directional Selection - why hasn’t directional selection already removed all genetic variation for traits related to fitness? - most natural populations contain a huge amount of genetic variation, especially at quantitative traits related to fitness - if the environment changes, then an allele which was previously associated with negative characteristics can now confer an advantage in the new environment - a phenotype with selective advantage in one area may not necessarily have a selective advantage in another area - an example of this is the British peppered moth - the pale moths would hide on the pale lichen on trees, but the soot from the industrial evolution changed the selection pressure in the environment - now the pale moths were less able to hide, whereas the darker moths were now better able to hide from predators - because of this, the - adaptations (traits that increase bearer’s relative fitness) are environment specific Disruptive Selection - more common - involves an advantage to individuals at the extreme ends of the spectrum - it is possible for the intermediate individuals to disappear entirely Frequency-dependent Selection - negative frequency-dependence - ex. predators form search images of prey - whichever form in the population is rare has higher fitness than that which is common - predators will preferentially target organisms which are present in greater quantity - once the population of of the common form has been sufficiently reduced by predation, another form will become more common and the predators will preferentially target that form - because of the periodic change in the preferences of the predators, the allele frequencies will be maintained at similar levels indefinitely and it is highly unlikely that any one form will be removed over time - advantage to rare forms is also referred to as balancing selection - positive frequency-dependent - ex. warning colouration Biology 1001A | 2012 - different colours within a population; the more prevalent the colour, the safer the organism will be because the predator is more likely to have past experience with the unpalatability of the species - predators learn Selection –> adaptation, not perfection - limited by available genetic variation - limited by dominance relationships - plays catch-up after environmental changes - trade-offs (compromise between competing demands) Selection is the only force driving adaptive evolution - other forces –> nonadaptive evolution - recall Hardy-Weinberg assumptions - very large population size - no mutation - no gene flow - random mating - no selection - genetic drift: whenever population size less than infinite - random, unpredictable changes in allele frequencies due to sampling error - drift strongest in small populations, and populations suddenly becoming small (bottlenecks, founder events) - some species such as cheetahs have almost no genetic diversity due to a bottleneck in the past when the majority of the population died off and population was rebuilt from limited variation - drift often opposes evolution - outcome depends on strength of selection, and population size - the more favourable the allele, the better the chances that it spreads throughout the population - the mostly occurs in small populations - how does genetic drift affect variation within a population - genetic drift tends to decrease the genetic variation with an population - how does drift affect variation (differences) among populations - genetic drift tends to increase differences between populations - chance will cause things to go in opposite directions - mutation creates new alleles - mutation is not “random” but not directed toward the needs of the organism
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