5. PESTICIDES. In wild grasslands up to one third of the living mass of plants is eaten by herbivores. In our crop fields just 10 percent is eaten. The difference is in part the result of the more than 2.3 billion kilograms of pesticides we use annually to control pests. Though in holding back the pests, we also kill many beneficial species and favor varieties resistant to our pesticides. Resistance to pesticides has evolved in hundreds of species of insects. In addition to pesticides for insects, farmers also use fungicides to kill fungi. Nearly all fungicides have led to the evolution of new resistant strains of plant pathogens (Gould).
6. HERBICIDES. Any patch of land, left alone, will tend to sprout with plants bent on outcompeting each other, rising higher and higher into the sky to win access to the sun. Once, we prevented such competition by weeding our fields and sorting crop seeds from weed seeds, one by one. This selection depended on visual acuity and caused multiple lineages of weeds to evolve seeds resembling those of our crops. Now we exclude weeds using herbicides, whether in our lawns or our fields, before they bear their seeds. The weeds evolve resistance to herbicides, becoming invisible to our chemicals rather than our eyes. More than a hundred species of weeds have evolved resistance to one or another herbicide. We clear the ground, till the soil and spray the fertilizer and herbicide, and when we do, row by row the resistant weeds grow.
7. ENVIRONMENTAL TOXINS. The environmental toxins we produce are everywhere. Often they influence the health and well-being of species around us; sometimes they also influence their evolution. PCBs (aka polychlorinated biphenyls) were once used in industrial coolants. Whereas PCBs are good coolants, they are toxic. PCBs kill fish and other animals, in part by blocking one of the receptors in their bodies, AHR2. The fish with ordinary receptors simply died where PCBs were plentiful, leaving behind food and habitat. Those fish with slightly different receptors, to which the PCBs bound less well, survived and eventually thrived. PCBs were never meant to be used to control other species. Nevertheless, they had the effect of killing some (but not all) of the species and individuals they came into contact with, strongly favoring the individuals with resistance of one form or another. Nor are PCBs unique. Many of our pollutants—be they heavy metals, cadmium, oil and others—appear to lead to rapid evolution of tolerant and, at least sometimes, toxic creatures.
8. OF MICE (AND RATS) AND MEN. Mice and rats have been following humans since at least the origins of agriculture more than 10,000 years ago. It is easy to imagine we have probably been trying to kill them for nearly as long. More recently, however, we've been poisoning these pests, offering them tempting treats laced with deadly chemicals. Rats living in forests and other wild places are attracted to new foods in particular and so feed readily from such baits. Rats living with humans are not, at least not anymore. Present them with a new food and they will wait. Several authors have suggested that this "neophobia" in urban rats has evolved in response to the threat posed to rats and mice by our new "foods." For now, the little we know about the evolution of neophobia fits with this idea. The clearest evolutionary change in rats and mice as a result of our interference has been the evolution of resistance to the rat poison warfarin. We then created superwarfarin to target these resistant populations, but resistance to this poison has recently evolved (Mayumi et al., 2008). Once again our garden of neglect is seemingly growing out of our control.
9. URBAN JUNGLE. Plant species living in urban environments tend to be surrounded by patches of habitat less suitable than the ones in which they are situated. Seeds that disperse far from their mothers are more likely to end up in those less suitable surroundings (think: concrete or pavement; Cheptou et al., 2008). As a consequence some city plants have evolved to produce fewer, larger seeds that fall near them rather than smaller ones that can disperse farther away. Although this type of quick evolution lends a short-term survival advantage, it may mean that these plants are less robust to adapt to a changing environment in the future. Meanwhile, thousands of other city species are acquiring new survival mechanisms despite the ways we build our cities, whether that means evolving the ability to eat concrete, call more loudly to their mates or simply find a place among our towers of glass and steel to hide.