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Evolutionary Mechanisms
and Natural Selection

Master the core forces that drive evolution: natural selection, genetic drift, gene flow, and mutation: the foundations of population genetics for the MCAT.

MCAT Biological Foundations

What Is Evolution?

Evolution is the change in allele frequencies within a population over successive generations. It is not about individual organisms changing: it operates at the population level.

The Four Evolutionary Forces

Natural Selection
Differential survival and reproduction based on phenotype: the only force that is adaptive.
Genetic Drift
Random fluctuations in allele frequencies, most powerful in small populations.
Gene Flow
Migration of alleles between populations, which tends to reduce differences between them.
Mutation
Random changes in DNA sequence: the ultimate source of all new genetic variation.

Why Natural Selection Matters

Natural selection is the only evolutionary mechanism that leads to adaptation: a better fit between organisms and their environment. Three conditions must be met:

1
Variation: Individuals in a population differ in their traits.
2
Heritability: At least some of that variation is genetically inherited.
3
Differential Fitness: Some variants survive and reproduce more successfully than others.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg model predicts allele and genotype frequencies in a non-evolving population. It serves as a null hypothesis: deviations indicate evolution is occurring.

Five Conditions for Equilibrium 1 No mutation 2 No gene flow (migration) 3 No natural selection 4 No genetic drift (infinite population) 5 Random mating Allele Frequencies Stay Constant

Types of Natural Selection

Directional Favors one extreme Stabilizing Favors the average Disruptive Favors both extremes Trait Value (dashed = original, solid = after selection)

Genetic Drift: Random Change

Genetic drift is the random change in allele frequencies due to chance sampling events. Its effects are strongest in small populations.

Bottleneck Effect
A drastic reduction in population size (disaster, disease) randomly eliminates alleles, reducing genetic diversity.
Founder Effect
A small group colonizes a new area. The new population's allele frequencies differ from the source by chance.

Gene Flow and Mutation

Pop A p = 0.7 Migration Migration Pop B p = 0.3 Gene flow homogenizes allele frequencies between populations
Gene Flow
Movement of alleles between populations via migration. Reduces divergence and increases genetic similarity.
Mutation
Random DNA changes that create new alleles. Very slow on its own but provides the raw material for all other forces.

Adaptive vs Non-Adaptive Mechanisms

Adaptive Non-Adaptive
Mechanism Natural Selection Drift, Gene Flow, Mutation
Direction Toward better fit Random or homogenizing
Population Size Effect Strong in all sizes Drift strongest in small
Creates Variation? No (acts on existing) Mutation: yes; others: no
Leads to Adaptation? Yes No

Fitness and the Selection Coefficient

Fitness (\( w \)) measures the relative reproductive success of a genotype. The selection coefficient (\( s \)) quantifies how much selection acts against a genotype:

\( s = 0 \)
No selective disadvantage: the genotype is equally fit.
\( 0 < s < 1 \)
Partial disadvantage: selection slowly reduces the allele's frequency.
\( s = 1 \)
Lethal genotype: completely removed each generation (e.g., many homozygous recessive diseases).

How to Solve Hardy-Weinberg Problems

1
Identify the given information: Usually the frequency of the homozygous recessive phenotype (\( q^2 \)).
2
Solve for \( q \): Take the square root: \( q = \sqrt{q^2} \).
3
Find \( p \): Use \( p = 1 - q \).
4
Calculate genotype frequencies: Homozygous dominant \( = p^2 \), heterozygous \( = 2pq \), homozygous recessive \( = q^2 \).
5
Verify: Confirm that \( p^2 + 2pq + q^2 = 1 \).

Example 1: Cystic Fibrosis Frequency

Cystic fibrosis is autosomal recessive. In a population of 10,000, 100 individuals are affected. What fraction are carriers?

1
Find \( q^2 \): Affected = homozygous recessive: \( q^2 = \frac{100}{10{,}000} = 0.01 \)
2
Solve for \( q \): \( q = \sqrt{0.01} = 0.1 \)
3
Find \( p \): \( p = 1 - 0.1 = 0.9 \)
4
Carrier frequency: \( 2pq = 2(0.9)(0.1) = 0.18 \) = 18%

Example 2: Detecting Evolution

In a population, the expected HWE genotype frequencies for a locus are: AA = 0.49, Aa = 0.42, aa = 0.09. Observed: AA = 0.60, Aa = 0.20, aa = 0.20. Is this population evolving?

Expected (HWE) Observed 0.49 AA 0.42 Aa 0.09 aa 0.60 AA 0.20 Aa 0.20 aa Observed ≠ Expected Population is NOT in HWE: evolution is occurring

Example 3: Heterozygote Advantage

Sickle-cell trait: heterozygotes (HbA/HbS) resist malaria, while HbS/HbS homozygotes develop sickle-cell disease. In malaria-endemic regions, which genotype has the highest fitness?

HbA/HbA
Normal hemoglobin. Susceptible to malaria. Fitness: \( w \approx 0.9 \)
HbA/HbS
Carrier. Resists malaria. Highest fitness: \( w = 1.0 \)
HbS/HbS
Sickle-cell disease. Lowest fitness: \( w \approx 0.2 \)

Common Mistakes

✕ Confusing Fitness with Strength
Biological fitness = reproductive success, not physical ability. An organism that survives but doesn't reproduce has fitness of zero.
✕ Drift Is Directional
Genetic drift is random, not directional. It does not push alleles toward any particular frequency: it can fix or eliminate alleles by chance.
✕ Mutation Drives Evolution
Mutation alone changes allele frequencies very slowly. It creates variation, but natural selection and drift are the main forces that change frequencies.

Summary of Evolutionary Mechanisms

Mechanism Effect Adaptive? Population Size
Natural Selection Changes allele freq. toward fitness Yes All sizes
Genetic Drift Random allele freq. change No Strongest in small
Gene Flow Homogenizes populations No Any with migration
Mutation Creates new alleles No Very slow alone

Question 1 of 5

In a population of 1,000 individuals, 40 are homozygous recessive (\( aa \)) for a trait. Assuming Hardy-Weinberg equilibrium, what is the expected frequency of heterozygous carriers?

Question 2 of 5

Which evolutionary mechanism is the only one that consistently leads to adaptation?

Question 3 of 5

A hurricane reduces an island bird population from 5,000 to 50 individuals. The surviving birds happen to have a higher frequency of blue plumage than the original population. This is an example of:

Question 4 of 5

Sickle-cell trait (HbA/HbS heterozygotes) provides resistance to malaria. In malaria-endemic regions, both the HbA and HbS alleles are maintained at relatively stable frequencies. This pattern is best explained by:

Question 5 of 5

Which of the following would violate Hardy-Weinberg equilibrium assumptions?