Observed evolution (antibiotic resistance, pesticide resistance) Knowledge-Based Study

Study notes

1. Evolution in Real Time

   Evolution is often viewed as a slow process taking millions of years, but evolutionary change can occur rapidly enough to be directly observed within human lifespans. When a population with high reproductive rates experiences strong environmental pressures, natural selection causes measurable shifts in allele frequencies over relatively few generations.

   These real-time observations provide concrete, empirical evidence of microevolution in action. Two classic examples of directly observable natural selection are antibiotic resistance in bacteria and pesticide resistance in agricultural pests.

   How Bacterial Populations Evolve Resistance

   Antibiotics are chemical compounds used to inhibit bacterial growth or kill bacterial cells. However, widespread and repeated antibiotic use creates intense selective pressure on bacterial populations.

   • Pre-existing Genetic Variation: Random mutations occasionally produce alleles that enable individual bacteria to neutralize an antibiotic, pump it out of the cell, or alter the target binding site.
   • Selective Advantage: When exposed to an antibiotic, susceptible bacteria die off rapidly, while individuals carrying resistance alleles survive.
   • Differential Reproduction: Surviving bacteria reproduce by binary fission, passing the resistance genes to subsequent generations. Resistance can also spread horizontally between bacteria via conjugation, transformation, or transduction.
   • Population Shift: Over time, the frequency of resistant bacteria increases significantly, causing the antibiotic to lose therapeutic effectiveness.

   Chemical Control and Insect Adaptation

   Similar to antibiotic resistance, the emergence of pesticide resistance among agricultural pests and disease vectors illustrates rapid natural selection driven by chemical agents.

   When an insecticide (such as DDT) is applied to a crop field or urban habitat:

   1. The vast majority of insects are killed because they carry common, non-resistant alleles.
   2. A tiny fraction of insects already possess rare genetic mutations that allow them to detoxify or tolerate the chemical compound.
   3. These resistant survivors produce offspring that inherit the protective alleles.
   4. With repeated applications of the same pesticide, the proportion of resistant individuals increases until the standard application rate no longer controls the population.

   Crucial Concepts to Remember

   Understanding observed evolution requires distinguishing between natural selection and acquired traits:

   • Populations evolve, not individuals: An individual bacterium or insect does not "develop" resistance during its lifetime. It is either born with a genetic variant that confers resistance or it is not.
   • Selection acts on existing variation: Antibiotics and pesticides do not induce or create beneficial mutations on demand. Mutations arise randomly prior to chemical exposure; the chemical acts merely as the selective filter.
   • Evolution is context-dependent: A resistance allele may carry a metabolic fitness cost in an environment free of antibiotics or pesticides, but it confers a decisive survival advantage in their presence.