Saturday, December 5, 2015

Some Thoughts on Plant Evolution in The Urban Jungle

Plants evolve, often more rapidly that animals. We may not notice it since we don't look closely at them. But they do. In a recent paper on plant evolution in The American Journal of Botany the authors remark:

Why might urbanization infl uence plant evolution? Clearly, urban development changes both the biotic and abiotic environment in ways that could alter natural selection and adaptive evolution within plant populations. Urbanization may also infl uence nonadaptive evolution due to altered gene fl ow, genetic drift, or nonrandom mating. For example, urban development causes habitat fragmentation given that buildings and roads are a common feature to every city, where extensive pavement, concrete, and alteration of natural habitats is the rule rather than the exception. Th is fragmentation can limit dispersal and gene fl ow,  leading to greater genetic diff erentiation between populations. It can also infl uence the size of populations and thus the importance of neutral evolution because genetic drift will be greater in  smaller populations. Finally, urban areas can alter mating patterns (e.g., increased selfing) through changes in pollinator communities...

 The authors then make several predictions:
  1. Urban and non-urban populations will diff er in the amount of genetic diversity
  2. Urbanization will alter natural selection on populations
  3. Neutral evolution will be greater in urban areas
  4. Genetic divergence between urban and non-urban populations will be proportional to the size of urban areas
  5. Insect-pollinated plants will evolve greater selfpollination or clonal growth in urban areas
Now I would argue a bit differently. The primary influence on urban diversity will be driven by humans. Humans select plants that they like, like the triploid Hemerocallis. They become invasive via human attraction. They are sterile but aggressive growers pushing out other plants. They are a typical invasive species. Also the introduction of foreign species may very well lead to cross breeding that would have been impossible otherwise. Yet successful growth may be inhibited. Also the use of pesticides and similar plant control biotics will cause a shift in classic evolutionary manners.

Thus the five predictions above I believe are at most a small part of the complexity of urban development. For example I have more than twice the tree species on my small acreage than indigenous plants would provide. Some of the trees are quite hardy and aggressive such as Ginkgo and Metasequoia.

I believe that a more details analysis and monitoring would be quite useful, and that what the authors propose may be quite limited.