Some background Human civilization is a recent turn of events in the nearly four-billion-year history of life. As such, cities entail novel pressures that can shape organismal evolution. Urbanization touches on just about every evolutionary force you can imagine: - Drift and gene flow: urbanization fragments habitats and gives rise to smaller somewhat isolated subpopulations - Selection: urbanization creates novel selection pressures (e.g., different soundscapes for birdsongs, different heat sensitivities owing to the urban heat island effect for fungal growth, different densities of hosts for parasites, to name just a few) - Mutation: urbanization exposes organisms to higher concentrations of human-made mutagen in their environment On this example we'll examine how urbanization might be shaping both micro- and macroevolutionary phenomena in a variety of systems. 1. In New York City, there isn't extensive gene flow between the uptown and downtown subpopulations of rats. Let's suppose we sampled 50 uptown rats and 50 downtown rats and genotyped them at 60,000 SNPs across the entire genome. In these 100 rats would we expect to see Hardy-Weinberg proportions at most of the 60,000 loci, only at some of them, or virtually none of them? Justify your prediction. 2. In contrast to the rat story, in a study of bumblebees, scientists detected extensive gene flow between rural and urban bees. However, despite this gene flow, rural and urban bees are locally adapted to their environments. Specifically, in cities, bees have shorter tongues, which permits them to feed on a wider variety of flowers, which are much less abundant in cities than in rural settings. If we were hoping to locate the gene or genes responsible for this tongue adaptation, would we find all of the gene flow helpful or harmful in our efforts? Explain your choice by detailing how gene flow might help or hinder our methods for detecting selection. 3. Crepis sancta is a weed that can be found in urban and rural settings. Each plant is capable of producing two types of seed: dispersing and non-dispersing. In urban habitats plants produce relatively more of the non-dispersing type. It would be easy to declare, from my armchair, that this difference is a local adaptation for city living - namely, because there is so much pavement in cities, dispersing seeds might land where there is no soil, and that would waste resources, lowering fitness. But we should strive to be better than mere armchair story-tellers. What additional work should I do to make a strong case that this difference is indeed an adaptation produced by natural selection? Briefly describe two steps I can take to provide some of the necessary evidence for adaptation. 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 .

1. Some background Human civilization is a recent turn of events in the nearly four-billion-year
history of life. As such, cities entail novel pressures that can shape organismal evolution.
Urbanization touches on just about every evolutionary force you can imagine: - Drift and gene
flow: urbanization fragments habitats and gives rise to smaller somewhat isolated subpopulations
- Selection: urbanization creates novel selection pressures (e.g., different soundscapes for
birdsongs, different heat sensitivities owing to the urban heat island effect for fungal growth,
different densities of hosts for parasites, to name just a few) - Mutation: urbanization exposes
organisms to higher concentrations of human-made mutagen in their environment On this
example we'll examine how urbanization might be shaping both micro- and macroevolutionary
phenomena in a variety of systems. 1. In New York City, there isn't extensive gene flow between
the uptown and downtown subpopulations of rats. Let's suppose we sampled 50 uptown rats and
50 downtown rats and genotyped them at 60,000 SNPs across the entire genome. In these 100
rats would we expect to see Hardy-Weinberg proportions at most of the 60,000 loci, only at some
of them, or virtually none of them? Justify your prediction. 2. In contrast to the rat story, in a
study of bumblebees, scientists detected extensive gene flow between rural and urban bees.
However, despite this gene flow, rural and urban bees are locally adapted to their environments.
Specifically, in cities, bees have shorter tongues, which permits them to feed on a wider variety
of flowers, which are much less abundant in cities than in rural settings. If we were hoping to
locate the gene or genes responsible for this tongue adaptation, would we find all of the gene
flow helpful or harmful in our efforts? Explain your choice by detailing how gene flow might
help or hinder our methods for detecting selection. 3. Crepis sancta is a weed that can be found in
urban and rural settings. Each plant is capable of producing two types of seed: dispersing and
non-dispersing. In urban habitats plants produce relatively more of the non-dispersing type. It
would be easy to declare, from my armchair, that this difference is a local adaptation for city
living - namely, because there is so much pavement in cities, dispersing seeds might land where
there is no soil, and that would waste resources, lowering fitness. But we should strive to be
better than mere armchair story-tellers. What additional work should I do to make a strong case
that this difference is indeed an adaptation produced by natural selection? Briefly describe two
steps I can take to provide some of the necessary evidence for adaptation. 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? 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 yetcities 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?