Class 12 (V2)

Why does chemical combat (e.g., herbicides, insecticides) cause resistance?

Chemicals create strong selective pressure for resistance, conferring a fitness advantage to resistant genotypes that arise from mutations or gene flow.

Annual Loss Due to Weeds

Weeds cause 34% loss of crop yields annually as they compete with crops for light, nutrients, and space.

Sources of Resistance (3 Mechanisms)

Resistance comes from pre-existing genetic variation in the population, new mutations, and gene flow.

Resistance and Population Size

In very large populations, new, simple mutations may be introduced at a high rate.

Gene Flow and Resistance Example

Glyphosate resistance came about by gene flow. In a study of waterhemp, a variant in south Ontario was also found on the border of Kansas Missouri, indicating gene flow.

Selfing vs. Outcrossing Weeds (Resistance Variation)

Outcrossing weeds have more pre-existing resistance variation than selfing weeds.

Strategy to Combat Resistance (Multi-herbicide)

Using multi-herbicide treatment makes new adaptations less likely because it requires more complex adaptation from the pest/weed.

Strategy to Combat Resistance (Rotation)

Rotation of different kinds of herbicides hits weeds with different selection pressures, but this could select for generalized resistance.

Malaria Statistics and Resistance

Malaria causes over 600,000 deaths annually. The strong selective pressure from insecticides (the major prevention strategy) has led to the rapid evolution of resistance in mosquitoes.

"Evolution Proof" Solutions Goal

These solutions aim to minimize selection for mosquito resistance while still reducing malaria transmission, often by targeting reduced infectiousness instead of mortality.

HIV Drug Limitation

Current HIV drugs hinder virus replication but cannot eradicate the virus from latent reservoirs in the body.

HIV Management Strategy (Goal)

The strategy is to use evolution to manage HIV rather than eliminate it.

Multidrug Cocktail Rationale

A multidrug cocktail works because single mutations are unlikely to confer resistance to multiple drugs with different mechanisms of action, and lower viral loads make multiple mutations less likely.

Evolutionary Toggling (Mechanism)

Once resistance evolves, the patient stops taking the drug. The resistance now has an energetic cost, causing non-resistant forms to dominate. The drug is then re-applied to reduce virus levels.

Traditional Cancer Treatment Flaw

Strong, prolonged selection pressures using the same chemotherapy drugs may not be the best solution as it selects for resistance.

Evolutionary-Informed Cancer Alternatives (3)

Cycling drugs, multidrug cocktails, and lower doses of drugs are potential better options.

Challenge in Applying Cancer Theory

Ethical considerations make tests of evolutionary theory for human application challenging.

Evolutionary Rescue (Definition)

When a population suffers a demographic decline due to selection, and then strong selection favors adaptations that help the population grow again.

Human-Caused Environmental Changes (3 Categories)

1. Loss/fragmentation of habitat. 2. Altered abiotic conditions (Temperature, pH, salinity, chemical pollutants, precipitation). 3. Altered biotic conditions (Transport of species, invasive species, diseases).

Chytrid Fungus Epidemic (Effect on Amphibians)

Chytrid fungus infects the skin of amphibians, causing osmotic regulation problems and making it hard to breathe, killing the animal within 120 days.

Chytrid Life Cycle (Mechanism Detail)

Spores swim toward the frogs, burrow into the skin, develop into a thallus, and mature fungus develops into a sporangium, releasing spores after 4–5 days.

Leading Hypothesis for Chytrid Spread

Humans moved the fungus from a region where amphibians were locally adapted to it to the rest of the world where amphibians were strongly susceptible.

Genetic Issues in Conservation Biology (4 Examples)

Loss of genetic diversity & heterozygosity, inbreeding depression, fixation of deleterious alleles, and the inability of populations to adapt.

Probability of Evolutionary Rescue Dependence (3 Factors)

The probability of evolutionary rescue depends on population size, the beneficial mutation rate, and how much fitness was reduced