Recently, I published a review paper in Molecular Ecology with my co-authors, Ruth Rivkin, Marc TJ Johnson, Jason Munshi-South, and Brian Verrelli. In this paper, we discussed how urbanization influences gene flow and genetic drift. We looked at 167 primary literature papers to see what researchers have found.
Competing Models of Urban Effects
You might not know this, but there are two competing models that seek to explain how urbanization influences gene flow and genetic drift.
The first one is the “Urban Fragmentation” Model. This model states that as urbanization grows, it fragments and eliminates natural habitat. This results in populations becoming isolated, reducing gene flow between populations and increasing genetic drift within populations.
The competitor is the “Urban Facilitation” Model. This model states that urbanization brings organisms together, usually via human-mediated movement. This results in increased gene flow between populations and reduced genetic drift within populations.
Box 1. (a) The initial prediction of the urban fragmentation model is that genetic diversity (blue solid line/text) will decrease within and differentiation (green dashed line/text) will increase between urban habitats. However, these predictions may change when factoring in other variables.
(b) Environmental variation can contribute to different responses to urbanization, where responses may be constrained in urban habitats regardless of the variation in nonurban habitats. We advocate for explicit urban/nonurban comparisons to help delineate these responses.
(c) Variation in life history across taxa may result in alternate predictions being made (e.g., urban facilitation model). For example, highly mobile organisms may benefit from urbanization and exhibit greater genetic diversity in, and less differentiation among, urban habitats.
(d) Such variation can also be parsed out of the data by using spatially explicit models, such as memgene (Galpern, Peres‐Neto, Polfus, & Manseau, 2014). Circles of similar size and colour (light vs. dark) represent individuals with similar scores, which indicate that they are associated with similar genetic and spatial factors (i.e., in the same genetic neighbourhood). Population structure due to habitat fragmentation can be more accurately identified by considering landscape features that may contribute to fragmentation.
(e) The choice of molecular marker can influence the predictive ability of the data. Highly variable microsatellites (open circles) may obscure patterns in the data due to homoplasy. However, the sheer abundance of SNPs (solid circles) will reduce the influence of extreme data points, increasing the likelihood of higher confidence in resolving patterns in the data.
Measures of Gene Flow and Genetic Drift
It is nearly impossible to directly measure gene flow and genetic drift in natural populations. Because of this, we tend to measure other variables as a proxy for gene flow and genetic drift. Typically, for genetic drift we measure genetic diversity. Genetic diversity within a population is expected to go down as genetic drift increases. There are lots of ways to measure genetic diversity, we found that the most common types in the literature were Allelic Richness (Ar), Expected (HE) and Observed (HO) Heterozygosity, and Inbreeding (FIS). Similarly, there are lots of ways to measure gene flow, but the most common is genetic differentiation (FST).
Urban Fragmentation vs Urban Facilitation
After looking through all the different papers and collecting all the different measures of gene flow and genetic diversity, we built two models to test our predictions: one that just looked at urban vs non-urban and the second that looked at several explanatory variables.
We found that under our simple model, urbanization reduced gene flow and increased genetic drift. But this was only significant for Ar and trending for the other measures (Figure 2).
With our model that had all the explanatory variables, there was no significant effect of urbanization on either gene flow or genetic drift.
But Why?
We did find that when we combed through our data, urbanization still did have an effect on gene flow and genetic drift. In fact 90% of the studies show an effect. So why doesn’t our model come out as significant? Well, that is because ~50% of the studies support Urban Fragmentation and ~40% of the studies support Urban Facilitation!
Looking through these results, we couldn’t find a trend as to why one organism would experience facilitation while another would experience fragmentation. In fact, we looked really closely at organisms in studies and even similar kinds of organisms experienced urbanization in different ways!
This field of urban evolution is still super new. In the coming years, when we collect more data, maybe we will find a trend as to why some organisms experience urban facilitation and others experience urban fragmentation.
I am interested in urban ecology and evolution. Urbanization can have drastic impacts on local ecosystems, both negative and positive, and creates a new environment for organisms to experience. To understand the impacts that urbanization plays on organisms, I look through the lense of population genetics and molecular evolution.
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