Human-Induced Selective Pressures

Same mechanism, new source of pressure

Humans change the environment so often and so strongly that our activities have become a major source of selective pressure, but the underlying mechanism is exactly the one from Lesson 1.

A human-induced selective pressure is any change caused by human activity, pesticides, antibiotics, harvesting, pollution or habitat alteration, that makes some heritable variants survive and reproduce more than others, thereby driving evolution in the population.

The key point is that humans do not change how evolution works. We simply supply a new and often intense selective pressure. Variation must still already exist; the trait must still be heritable; differential reproduction still does the work. What is different is the speed: because human pressures (like blanket antibiotic use) can be extremely strong, the population can shift in just a few generations.

Pause, copy the highlighted definition into your book before moving on.

How a chemical selects for resistance step by step

We just said human pressures use the ordinary mechanism. That raises a question: what does the clearest case, chemical resistance, actually look like? This card answers it → the step-by-step rise of resistant bacteria.

A chemical such as an antibiotic does not target the susceptible cells' ability to mutate, it simply removes them, leaving the already-resistant minority to multiply.

The same logic applies to insecticides (DDT-resistant mosquitoes) and herbicides (resistant weeds). In each case the chemical is the human-induced selective pressure, and the resistant variant, which arose earlier by chance mutation, is favoured.

Chemicals (antibiotics, pesticides, herbicides) act as human-induced selective pressures: they kill susceptible individuals, leaving rare pre-existing resistant variants to survive and reproduce. The chemical selects for resistance; it does not create the resistance allele.

Pause, copy the three-step resistance sequence and the highlighted summary into your book.

Artificial selection and unintended pressures

We just saw chemicals select accidentally. That raises a question: is human selection always accidental? This card answers it → it ranges from fully deliberate breeding to side-effects nobody intended.

Human-induced pressures fall on a spectrum from deliberate selective breeding to entirely unintended consequences of other activities.

Artificial selection (deliberate): humans choose which individuals breed, wild cabbage was bred into broccoli, cauliflower and kale; wolves were bred into hundreds of dog varieties. Here the human is the selective agent, choosing the trait on purpose.

Unintended pressures (accidental): antibiotic resistance, pesticide resistance, and size-selective fishing are not goals, they are by-products. Humans changed the environment for another reason and the population evolved as a side-effect.

An Australian worked example, and it runs both ways. Cane toads were deliberately released in Queensland in 1935 to control cane beetles, a human decision that became a powerful selective pressure on two fronts. In the toads themselves, individuals at the leading edge of the invasion have longer legs and travel further per night than toads from long-settled populations, and the invasion front has accelerated from roughly 10 km per year in the 1940s to more than 50 km per year (Phillips, Brown, Webb and Shine, Nature, 2006). In native predators, snakes from toad-exposed regions have relatively smaller heads than snakes from toad-free regions, because snakes able to swallow a large toad were the ones killed by it (Phillips and Shine, PNAS, 2004), and red-bellied black snakes from long-exposed populations are also physiologically more resistant to the toad toxin itself (Phillips and Shine, Proceedings of the Royal Society B, 2006). Humans supplied the pressure; natural selection did the rest.

Both are human-induced and both work by natural selection's mechanism. The difference is only in the intent behind the pressure. (Note: in artificial selection, humans replace the natural environment as the agent doing the choosing, but heritable variation and differential reproduction are still required.)

Human-induced pressures range from deliberate artificial selection (selective breeding, e.g. wild cabbage → broccoli/kale; wolf → dog breeds) to accidental side-effects (antibiotic/pesticide resistance, size-selective fishing). Both work by the natural-selection mechanism; they differ only in human intent.

Add the deliberate-vs-accidental distinction, and the cane toad example, to your notes before the check below.