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Natural Selection
Continued

Explore the mechanisms, types, and evidence for natural selection: from directional and stabilizing selection to the Hardy-Weinberg principle and real-world applications.

AP Biology

What is Natural Selection?

Natural selection is the process by which organisms with heritable traits that are better suited to their environment tend to survive and reproduce at higher rates than organisms without those traits. Over generations, this shifts the allele frequencies in a population.

Four Conditions for Natural Selection

1
Variation: Individuals in a population differ in their traits (phenotypic variation).
2
Heritability: Some of this variation is passed from parents to offspring through genes.
3
Overproduction: More offspring are produced than can survive given limited resources.
4
Differential survival and reproduction: Individuals with advantageous traits leave more offspring (higher fitness).

Fitness and Adaptation

Biological Fitness
The relative ability of an organism to survive and reproduce in a particular environment. Measured by number of viable offspring.
Adaptation
A heritable trait that increases fitness in a given environment. Adaptations accumulate over many generations via selection.

Three Types of Selection

Directional Trait Value shift Stabilizing Trait Value narrow narrow Disruptive Trait Value split

Directional Selection in Detail

Directional selection occurs when individuals at one extreme of a phenotypic range have higher fitness. Over time, the population's trait distribution shifts toward that extreme.

Example: Peppered Moths
During the Industrial Revolution, soot darkened tree bark. Dark-colored moths had higher survival; the population shifted from mostly light to mostly dark.
Example: Antibiotic Resistance
When exposed to antibiotics, bacteria with resistance alleles survive and reproduce. The population shifts toward resistance.

Stabilizing vs Disruptive

Stabilizing Selection
Favors intermediate phenotypes. Extremes are selected against. Example: human birth weight: very small or very large babies have lower survival rates.
Disruptive Selection
Favors both extremes over intermediate phenotypes. Can lead to speciation. Example: African seed-cracker finches with either large or small beaks, but not medium.

Natural Selection vs Genetic Drift

Natural Selection Genetic Drift
Mechanism Differential survival and reproduction Random sampling of alleles
Direction Non-random: favors adaptive traits Random: no direction
Effect on fitness Increases population fitness May increase or decrease fitness
Population size Effective in any size Strongest in small populations
Outcome Adaptation Loss or fixation of alleles

The Hardy-Weinberg Principle

The Hardy-Weinberg equations describe a population in genetic equilibrium: no evolution is occurring. When natural selection (or other forces) acts, allele frequencies deviate from these predictions.

Sexual Selection

Sexual selection is a special case of natural selection where traits are favored because they increase mating success, even if they reduce survival. It operates through two mechanisms:

Intersexual Selection
Mate choice: One sex (often female) chooses mates based on traits like bright plumage or elaborate courtship. Example: peacock tail feathers.
Intrasexual Selection
Competition: Members of the same sex compete for access to mates. Example: male elk battling with antlers for dominance.

How to Identify Selection Type

1
Look at the trait distribution before and after selection. Has the mean shifted, narrowed, or split?
2
Identify which phenotypes have higher fitness. Is it one extreme, the middle, or both extremes?
3
Mean shifts one direction: Directional. Variance decreases: Stabilizing. Bimodal distribution: Disruptive.
4
Check for randomness. If the change seems random with no fitness advantage, consider genetic drift instead.

Example: Sickle Cell and Selection

In regions with malaria, the sickle-cell allele (\( H^S \)) is maintained at a higher frequency than expected. Let us analyze this using Hardy-Weinberg.

Example: Galapagos Finches

Beak Depth (mm) Frequency Before drought After drought Shift

Calculating Relative Fitness

Relative fitness (\( w \)) compares the reproductive success of a genotype to the most fit genotype. The most fit genotype is assigned \( w = 1.0 \).

Evidence for Natural Selection

Fossil Record
Transitional fossils show gradual changes in traits over time, consistent with selection pressures.
Homologous Structures
Similar bone structures in different species suggest common ancestry modified by different selective pressures.
Molecular Biology
DNA sequence similarities and shared pseudogenes confirm evolutionary relationships shaped by selection.

Field models test camouflage-dependent attack rates

Four separate enclosures show pale/dark substrates, paired pale/dark clay models, two closed mesh tops and two open tops. No measured survival result depicted. Separate exact panel: Four treatment cells cross light and dark substrate with predator access and exclusion, each beginning with equal light and dark model prey.Compare light and dark models on both substrates, with predator-access and exclusion treatments. Randomized independent enclosures test camouflage-dependent attack rates; reproductive success requires additional evidence.

Test camouflage-dependent attack rates

Test camouflage-dependent attack rates. Compare light and dark models on both substrates, with predator-access and exclusion treatments. Randomized independent enclosures test camouflage-dependent attack rates; reproductive success requires additional evidence.Compare light and dark models on both substrates, with predator-access and exclusion treatments. Randomized independent enclosures test camouflage-dependent attack rates; reproductive success requires additional evidence.

Common Mistakes

✕ "Organisms evolve on purpose"
Natural selection has no goal or direction. Organisms do not "try" to evolve. Traits that happen to increase fitness are simply passed on at higher rates.
✕ "Individuals evolve"
An individual's genotype is fixed at birth. Only populations can evolve: allele frequencies change across generations, not within a single organism's lifetime.
✕ "Survival of the fittest = strongest"
Fitness is about reproductive success, not physical strength. The "fittest" organism is the one that leaves the most viable offspring in a given environment.

Selection Types: Summary

Directional Stabilizing Disruptive
Favored phenotype One extreme Intermediate Both extremes
Effect on mean Shifts in one direction No change May not change
Effect on variance Decreases (one tail) Decreases (both tails) Increases
Example Peppered moths Human birth weight Finch beak sizes
Frequency in nature Common Most common Rare

Question 1 of 5

A population of rabbits shows a shift in fur color from mostly light brown to mostly dark brown after a forest fire darkens the soil. This is an example of:

Question 2 of 5

In a population where \( p = 0.6 \) and \( q = 0.4 \), what is the expected frequency of heterozygous individuals under Hardy-Weinberg equilibrium?

Question 3 of 5

Human birth weight is an example of stabilizing selection because:

Question 4 of 5

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

Question 5 of 5

The sickle-cell allele persists at relatively high frequencies in malaria-endemic regions because: