Lecture 2: Evolutionary Processes

Genetics Review and Definitions

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA, which is then translated into proteins.

• DNA: Chains of nucleotides that store genetic information.

• RNA: Chains of ribonucleotides, produced by transcription from DNA.

• Proteins: Chains of amino acids, synthesized by translation of RNA.

Process:

• Transcription: DNA → RNA

• Translation: RNA → Protein

Genetics Terminology

• Gene: A sequence of DNA that codes for an individual protein.

• Alleles: Different variants of the same gene.

• Genotype: The actual alleles carried by an organism.

• Phenotype: The visible trait expressed by the organism as a result of its genotype.

• Population: A group of individuals from the same species that live and breed together.

What is Evolution?

Definition and Measurement

Evolution is the change in the genetic makeup of a population over time, specifically the change in allele frequencies. This process can be measured and tested using mathematical models such as the Hardy-Weinberg equilibrium.

• Evolution: Change in allele frequencies in a population over generations.

• Hardy-Weinberg Equilibrium: A model that predicts allele and genotype frequencies in a non-evolving population.

Evolutionary Mechanisms

Main Mechanisms

Four primary mechanisms drive evolutionary change in populations:

• Natural Selection

• Genetic Drift

• Gene Flow

• Mutation

Natural Selection

Mechanism and Effects

Natural selection is the process by which certain alleles increase in frequency due to differential survival and reproduction. Alleles that confer higher fitness become more common in the population.

• Differential survival/reproduction: Individuals with advantageous traits are more likely to survive and reproduce.

• Fitness: The ability of an organism to survive and reproduce in its environment.

Types of Selection

• Directional Selection: Favors one extreme phenotype, shifting the average trait value in one direction.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation and maintaining the average trait value.

• Disruptive Selection: Favors extreme phenotypes at both ends, increasing variation.

• Balancing Selection: Maintains genetic diversity by favoring different alleles under different conditions, frequency-dependent selection, or heterozygote advantage.

• Sexual Selection: A special case of natural selection focused on traits that improve mating success, including female choice and male-male competition.

Examples of Selection

• Directional Selection: Cliff swallow body size shifts after a selection event.

• Stabilizing Selection: Human birth weight, where extremes have higher mortality.

• Disruptive Selection: Black-bellied seed cracker beak length, where only individuals with extreme beak sizes survive.

• Balancing Selection: Sickle cell allele in malaria regions (heterozygote advantage).

Summary Table: Modes of Selection

Mode of Selection	Effect on Phenotype	Effect on Genetic Variation
Directional selection	Favors one extreme phenotype, causing the average phenotype to change in one direction.	Genetic variation is reduced.
Stabilizing selection	Favors phenotypes near the middle of the range, maintaining average phenotype.	Genetic variation is reduced.
Disruptive selection	Favors extreme phenotypes at both ends of the range.	Genetic variation is increased.
Balancing selection	No single phenotype is favored in all populations at all times.	Genetic variation is maintained.

Sexual Selection

Mechanisms

Sexual selection acts on traits that affect an individual's ability to attract mates. It can be directional, stabilizing, or disruptive.

• Male-male competition: Males compete for access to females or resources.

• Female choice: Females select mates based on traits indicating high genetic quality or resource provision.

Fundamental asymmetry of sex: Females typically invest more in offspring than males, leading to choosier females and competitive males.

Examples

• Female Choice: Zebra finches, where females prefer males with brighter coloration.

• Male-Male Competition: Elephant seals, where a few males father most offspring.

• Sexual Dimorphism: Differences in appearance between males and females, such as size, coloration, or ornamentation (e.g., lions, beetles, birds).

Genetic Drift

Mechanism and Effects

Genetic drift is the change in allele frequencies due to random sampling error, especially in small populations. It can lead to loss of genetic variation and fixation of alleles.

• Founder Effect: A new population is established by a small number of individuals, leading to different allele frequencies than the source population.

• Population Bottleneck: A sudden reduction in population size due to high mortality, resulting in random changes in allele frequencies.

Example: Cheetahs have low genetic diversity due to historical bottlenecks.

Gene Flow

Mechanism and Effects

Gene flow is the movement of alleles between populations due to immigration or emigration. It can increase or decrease genetic variation and make populations more genetically similar.

• Effect: Causes allele frequencies in different populations to become more alike.

Mutation

Mechanism and Effects

Mutation is the source of new genetic variation. It occurs due to errors in DNA replication, resulting in gametes (egg or sperm) that carry different alleles from the parent.

• Effect: Increases genetic variation; most mutations are neutral or deleterious, but some can be beneficial.

Population Effects of Evolutionary Mechanisms

Summary Table: Effects on Genetic Variation and Fitness

Process	Effect on Genetic Variation	Effect on Average Fitness
Selection	Maintains, increases, or decreases	Increases
Genetic Drift	Decreases (loss, fixation)	Decreases
Gene Flow	Increases or decreases	Increases or decreases
Mutation	Increases (new alleles)	Random; usually decreases

Key Equations

• Hardy-Weinberg Equation (for allele and genotype frequencies in a non-evolving population):

$p^2 + 2pq + q^2 = 1$

$p + q = 1$

• p: Frequency of one allele

• q: Frequency of the other allele

Additional info: The notes cover core concepts from chapters on Evolution, Genetics, and Population Biology, including definitions, mechanisms, and examples relevant for introductory biology students.