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Continuing
Evolution

Explore how natural selection, genetic drift, and other mechanisms continue to shape populations today: evolution is not a thing of the past.

AP Biology

What Is Continuing Evolution?

Evolution is not a historical event that stopped millions of years ago. Populations are still evolving right now. Any time allele frequencies in a population change over generations, evolution is occurring. This includes changes driven by natural selection, mutation, gene flow, genetic drift, and sexual selection.

Evidence of Ongoing Evolution

Antibiotic Resistance
Bacteria evolve resistance to antibiotics within years, demonstrating natural selection in real time.
Pesticide Resistance
Insect populations develop resistance to pesticides through selection for resistant alleles.
Darwin's Finches
Beak sizes in Galapagos finches shift measurably across drought and wet years within single decades.

A bacterial colony before and after selective pressure

Two agar microcolony states show intact teal rods and collapsed translucent cells; the companion model tracks susceptible and resistant counts through selection.In a closed population with pre-existing resistance, differential survival and reproduction raise the resistant fraction from 20% before exposure to about 67% among survivors and 80% after regrowth.

Selection changes population frequency

Selection changes population frequency. In a closed population with pre-existing resistance, differential survival and reproduction raise the resistant fraction from 20% before exposure to about 67% among survivors and 80% after regrowth.In a closed population with pre-existing resistance, differential survival and reproduction raise the resistant fraction from 20% before exposure to about 67% among survivors and 80% after regrowth.

Mechanisms Driving Evolution Today

Allele Frequency Change Natural Selection Mutation Gene Flow Genetic Drift Sexual Selection All five mechanisms act on populations today

The Hardy-Weinberg Null Model

The Hardy-Weinberg principle describes a population that is not evolving. It serves as a null hypothesis: if observed allele frequencies deviate from predictions, at least one evolutionary mechanism is at work.

Visualizing Allele Frequency Shifts

Frequency of allele A Generations 0 0.5 1.0 0 20 40 60 80 Natural Selection Genetic Drift

Selection vs Drift: Key Distinction

Natural Selection Genetic Drift
Mechanism Differential survival and reproduction Random sampling of alleles
Direction Predictable: favors adaptive alleles Random: no consistent direction
Effect of pop. size Acts in populations of any size Strongest in small populations
Leads to adaptation? Yes: increases fitness No: can fix harmful alleles
Example Antibiotic-resistant bacteria Bottleneck in cheetahs

Why Continuing Evolution Matters

1
Medicine: Antibiotic and antiviral resistance makes treating infections harder every year: understanding evolution guides treatment strategies.
2
Agriculture: Pesticide resistance in insects and herbicide resistance in weeds threaten crop yields worldwide.
3
Conservation: Small populations face genetic drift and inbreeding, reducing genetic diversity and adaptive potential.
4
Public Health: Flu vaccines must be updated annually because influenza viruses evolve rapidly through mutation and selection.

How to Detect Ongoing Evolution

1
Establish baseline allele frequencies using Hardy-Weinberg: calculate expected \( p^2 \), \( 2pq \), and \( q^2 \).
2
Collect observed genotype frequencies from the population across multiple generations.
3
Compare observed vs. expected. Use a chi-square (\( \chi^2 \)) test to determine if the deviation is statistically significant.
4
Identify the mechanism: Is the deviation directional (selection)? Random (drift)? Influx of new alleles (gene flow)?
5
Conclude: If allele frequencies changed significantly, the population is evolving.

Explain why preserved ancestors and replicated lineages strengthen inference.

A revived frozen ancestor and derived population are compared in a matched competition assay after acclimation. No fitness result is assumed.

A revived frozen ancestor and derived population are compared in a matched competition assay after acclimation. No fitness result is assumed. The contextual image is illustrative.
A revived frozen ancestor and derived population are compared in a matched competition assay after acclimation. No fitness result is assumed.

Frozen ancestors and twelve parallel futures

A researcher retrieving a vial from a frozen archive, beside parallel culture flasks and a comparison workspace.Frozen archives allow ancestral and descendant samples to be revived and compared in a common environment. Twelve independently maintained lineages provide replication while retaining the possibility of different evolutionary outcomes.

Frozen archives support replicated comparisons

Frozen archives support replicated comparisons. Frozen archives allow ancestral and descendant samples to be revived and compared in a common environment. Twelve independently maintained lineages provide replication while retaining the possibility of different evolutionary outcomes.Frozen archives allow ancestral and descendant samples to be revived and compared in a common environment. Twelve independently maintained lineages provide replication while retaining the possibility of different evolutionary outcomes.

Example: Antibiotic Resistance

A hospital tracks Staphylococcus aureus infections. In 2010, 5% of isolates were methicillin-resistant (MRSA). By 2020, 40% were resistant. Is this evolution?

Distinguish horizontal acquisition from differential reproduction.

A plasmid transfer strand can pass through contact, followed by complementary strand synthesis. The donor chromosome stays in the donor; retention in descendants is a selected possibility.

Exact schematic
Exact schematic
Selected retained-plasmid lineage
Selected retained-plasmid lineage
A plasmid transfer strand can pass through contact, followed by complementary strand synthesis. The donor chromosome stays in the donor; retention in descendants is a selected possibility. The contextual image is illustrative.
A plasmid transfer strand can pass through contact, followed by complementary strand synthesis. The donor chromosome stays in the donor; retention in descendants is a selected possibility.

Example: Bottleneck Effect

Before Bottleneck Catastrophic event Survivors After Recovery 4 alleles represented Only 2 alleles remain Genetic diversity lost

Example: Hardy-Weinberg Test

In a population of 500 wildflowers, flower color is controlled by one gene with two alleles. 320 are red (\(AA\) or \(Aa\)) and 180 are white (\(aa\)). Is the population in Hardy-Weinberg equilibrium?

1
Find \( q^2 \): frequency of aa = \( \frac{180}{500} = 0.36 \)
2
Find \( q \): \( q = \sqrt{0.36} = 0.60 \)
3
Find \( p \): \( p = 1 - q = 1 - 0.60 = 0.40 \)
4
Expected genotypes: \( p^2 = 0.16 \) (\(AA\)), \( 2pq = 0.48 \) (\(Aa\)), \( q^2 = 0.36 \) (\(aa\))
5
Compare with observed: If observed genotype counts match expected, the population is in equilibrium. If they deviate significantly (\( \chi^2 \) test), evolution is occurring.

Types of Natural Selection

Directional Trait value Stabilizing Trait value Disruptive Trait value Original distribution After selection

Gene Flow and the Founder Effect

Gene Flow
Movement of alleles between populations via migration. Homogenizes allele frequencies across populations and introduces new alleles.
Founder Effect
A small group colonizes a new area. The new population's allele frequencies differ by chance from the source: a form of genetic drift.

Common Mistakes

✕ "Organisms evolve on purpose"
Evolution has no goal or direction. Organisms do not choose to evolve: selection acts on existing variation.
✕ "Individuals evolve"
Populations evolve, not individuals. An individual's genotype does not change during its lifetime.
✕ "Mutations arise because they're needed"
Mutations are random. They are not generated in response to environmental pressures. Selection then favors beneficial ones.

Summary of Evolutionary Mechanisms

Mechanism Effect on Allele Freq. Random or Directed?
Natural Selection Shifts toward adaptive alleles Directed
Genetic Drift Random fluctuation; fixation or loss Random
Mutation Introduces new alleles Random
Gene Flow Homogenizes populations Directed (by migration)
Sexual Selection Shifts toward preferred traits Directed

Question 1 of 5

A population of beetles shows a change in the frequency of a shell-color allele over 10 generations. Which statement best describes this observation?

Question 2 of 5

Which of the following is NOT a condition required for Hardy-Weinberg equilibrium?

Question 3 of 5

In a population, \( q^2 = 0.09 \). Assuming Hardy-Weinberg equilibrium, what is the frequency of heterozygous individuals (\( 2pq \))?

Question 4 of 5

A hurricane drastically reduces a large lizard population to just 12 individuals. The surviving group has different allele frequencies than the original population. This is an example of:

Question 5 of 5

Bacteria exposed to an antibiotic develop resistance over several generations. Which statement correctly explains this?