Chapter 19 — The Evolution of Populations (19.1–19.2)

Population genetics studies…

changes in allele frequencies over time.

Was Darwin’s blending inheritance idea correct or wrong?

Wrong

Modern population genetics was formalized in the

1940s (Modern Synthesis)

Allele

 Different forms of a gene

Allele frequency

How common an allele is in a population

Gene pool

All the alleles in a population

Founder effect

When a few individuals start a new population → limited alleles

Populations in Hardy-Weinberg equilibrium have…

stable allele frequencies

You can use the Hardy-Weinberg equation to…

predict allele distribution

Evolutionary forces are at work if…

observed allele frequencies differ

The Conditions needed for equilibrium

1. No mutation

2. No natural selection

3. No gene flow

4. Random mating

5. Very large population

Random changes in allele frequencies occur…

due to chance

Genetic drift is strongest in..

small populations

Genetic drift can lead to…

loss of alleles

An example of Genetic Drift (READ THE RABBIT ONE)

• Rabbits: brown (B) dominant, white (b) recessive.

• First generation: p = 0.5, q = 0.5.

• Second generation: only half reproduce → p = 0.7, q = 0.3.

• Third generation: by chance, only homozygous dominant reproduce → b allele lost.

Bottleneck Effect

• Population size drastically reduced by chance events or catastrophes.

• Genetic variability decreases.

• Survivors may not represent original population’s alleles.

Examples of the Bottleneck Effect

• natural disasters

• disease

• habitat loss

Gene flow

The movement of alleles between populations

Gene flow occurs when…

individuals migrate to a new location and reproduce

Gene flow can…

increase genetic similarity between populations

Population Genetics

The study of how allele frequencies in a population change over time and how evolutionary forces like mutation, selection, drift, and gene flow affect populations.

Allele frequency

How common an allele is in a population

Gene pool

All alleles in a population.

Founder effect

When a small group starts a new population, reducing genetic diversity.

Hardy-Weinberg Equilibrium

A theoretical state where allele frequencies in a population remain constant over generations, showing no evolution is occurring.

Bottleneck Effect

A sudden reduction in population size caused by a catastrophe or chance event, which decreases genetic variability and may change allele frequencies randomly.

Gene Flow

The movement of alleles between populations when individuals migrate and reproduce, which increases genetic similarity among populations.

Natural Selection

The process by which individuals with advantageous traits survive and reproduce more, causing allele frequencies in a population to change over time.

Key Concept: Populations evolve; individuals do not.

Populations _____, not individuals.

evolve

Individuals with beneficial traits ______________ , changing allele frequencies in the population.

 survive and reproduce more

Types of Natural Selection

1. Stabilizing Selection

2. Directional Selection

3. Diversifying (Disruptive) Selection

Sexual dimorphism

males and females of a species differ in appearance or size.

What is sexual selection / dimorphism often due to?

Mate choice or competition

Evolution has no…

long-term purpose or direction.

What two things are important in shapng traits?

Constraints and tradeoffs

Adaptive evolution occurs under ______ _______ like environment, predators, and mates.

selective pressures

What is a population?

A group of the same species living in the same area.

What is fitness?

Ability to survive and reproduce.

What is an adaptation?

A trait that improves survival or reproduction.

Homologous structures

Similar structures from a common ancestor.

Analogous structures

Same function, different ancestors.

Vestigial structures

Structures with no current function.

Speciation

Formation of new species.

Allopatric speciation

Speciation caused by geographic isolation.

Sympatric speciation

Speciation without geographic separation.

adaptive radiation

One species evolves into many species filling different niches.

Aneuploidy

Too many or too few chromosomes.

Autopolyploidy

Extra chromosome sets from same species (meiosis error).

allopolyploidy

Chromosome sets from two different species

Temporal isolation

Different breeding times

Habitat isolation

Species live in different habitats

behavioral isolation

Different mating behaviors.

mechanical isolation

Incompatible reproductive structures

What is a gametic barrier?

Gametes cannot fuse.

hybrid inviability

Embryo cannot survive development.

hybrid sterility

Hybrid lives but cannot reproduce.

What is a hybrid zone?

Area where two species meet and interbreed.

What is reinforcement?

Hybrids are less fit, so species diverge more.

What is fusion?

Hybrids are fit, species merge back together.

Punctuated equilibrium

Rapid change + long periods of no change.