4. Adaptation sometimes occurs in the following fashion: an environment changes, then a new mutation that produces some fitness-promoting effect arises, and then it sweeps to fixation. But most often adaptation starts from "standing genetic variation" - that is, from asllelic variants that are already present in populations at somewhat low frequency. These standing variants are just drifting about neutrally in frequency before the environment changes, and then, once the environment changes, selection favors of one of the formerly neutral variants. Given that standing genetic variation may be important for adaptation to a changing environment, why might we worry that urbanization has reduced populations' ability to adapt? If the genetic variation is already low before the environment changes, then there may be little standing genetic variation available to respond to the new selection pressures, leaving the population with few options for adaptation. 5. Mutation rates in urban settings are often higher because of pollution. These elevated mutation rates probably haven't shaped the evolutionary change of many urban populations just yet - cities simply haven't been around long enough for that to take effect-but if we let ourselves suppose that cities (and therefore scientists!) will persist over the next few hundred thousand or million years, we can then consider the long-term consequences of elevated mutation rates for urban organisms. If we suppose that a pair of urban and rural bird populations, say, come to be good species and then remain at their current population size over evolutionary time, do you expect equal rates of neutral evolution in the two species? Elevated mutation rates can increase the rate of neutral evolution because they introduce more genetic variation into a population, increasing the chances that new neutral mutations will arise and become fixed. Population size is decreased in urbanization so genetic drift has a higher effect fixing neutral mutations that could otherwise be advantageous in the occurrence of a change in the environment.
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4- Adaptation sometimes occurs in the following fashion- an environmen.docx
1. 4. Adaptation sometimes occurs in the following fashion: an environment changes, then a new
mutation that produces some fitness-promoting effect arises, and then it sweeps to fixation. But
most often adaptation starts from "standing genetic variation" - that is, from asllelic variants that
are already present in populations at somewhat low frequency. These standing variants are just
drifting about neutrally in frequency before the environment changes, and then, once the
environment changes, selection favors of one of the formerly neutral variants. Given that
standing genetic variation may be important for adaptation to a changing environment, why
might we worry that urbanization has reduced populations' ability to adapt? If the genetic
variation is already low before the environment changes, then there may be little standing genetic
variation available to respond to the new selection pressures, leaving the population with few
options for adaptation. 5. Mutation rates in urban settings are often higher because of pollution.
These elevated mutation rates probably haven't shaped the evolutionary change of many urban
populations just yet - cities simply haven't been around long enough for that to take effect-but if
we let ourselves suppose that cities (and therefore scientists!) will persist over the next few
hundred thousand or million years, we can then consider the long-term consequences of elevated
mutation rates for urban organisms. If we suppose that a pair of urban and rural bird populations,
say, come to be good species and then remain at their current population size over evolutionary
time, do you expect equal rates of neutral evolution in the two species? Elevated mutation rates
can increase the rate of neutral evolution because they introduce more genetic variation into a
population, increasing the chances that new neutral mutations will arise and become fixed.
Population size is decreased in urbanization so genetic drift has a higher effect fixing neutral
mutations that could otherwise be advantageous in the occurrence of a change in the
environment.
