Mechanisms of Evolution and Patterns of Natural Selection

Mechanisms of Evolution and Patterns of Natural Selection

Darwin's Four Postulates of Natural Selection

Natural selection is a fundamental mechanism of evolution, first described by Charles Darwin. It explains how populations change over time due to differences in survival and reproduction among individuals. Darwin's theory is based on four key postulates:

Darwin’s Postulates	Salmon Example
1. Variation exists among individuals in a population.	Some salmon are slow, some are fast. Some salmon have better eyesight. Some salmon are twitchier than others.
2. Some of this variation is heritable.	These traits (speed, eyesight, twitchiness) must be genetically controlled and passed from parents to offspring.
3. More offspring are born than can survive.	Many salmon will not survive because they will get eaten by bears or die from other causes.
4. Individuals with certain traits are more likely to survive and reproduce, passing those traits to their offspring.	Salmon that are faster, have better eyesight, or are twitchier are more likely to avoid being eaten and will survive to reproduce, passing these advantageous traits to their offspring.

Genetic Variation and Hardy-Weinberg Principle

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations. It predicts how gene frequencies will be inherited from one generation to the next in the absence of evolutionary forces.

• Hardy-Weinberg Equilibrium: A population is in Hardy-Weinberg equilibrium if allele and genotype frequencies remain constant from generation to generation, provided that no mutation, migration, selection, or genetic drift occurs, and mating is random.

• Mutation: Mutations are the only source of new genetic variation (new alleles) in a population.

• Effect on Fitness: If a mutation does not affect an organism's ability to survive and reproduce (its fitness), it is considered neutral.

Example: In a population of 10,000 fruit flies with no selection pressure, a white fruit fly appears after several generations due to mutation. This population is not in Hardy-Weinberg equilibrium because mutation introduces new alleles.

Mechanisms of Evolution: Mutation, Natural Selection, and Gene Flow

Evolution in populations can occur through several mechanisms, including mutation, natural selection, and gene flow.

• Mutation: Introduces new genetic variation into a population.

• Natural Selection: Favors individuals with advantageous traits, increasing their frequency in the population.

• Gene Flow: Movement of alleles between populations through migration, which can increase or decrease genetic variation depending on the context.

Example Scenario: If white fruit flies are given a 10% fitness advantage and black fruit flies immigrate into the population, both directional natural selection (favoring white flies) and gene flow (introducing black alleles) are at work. Directional selection decreases genetic variation and increases the frequency of the advantageous trait, while gene flow can increase or decrease variation depending on the direction of migration.

Patterns of Natural Selection

Natural selection can act on traits in three main patterns, each affecting the distribution of traits in a population differently:

Pattern	Description	Effect on Population	Example
Stabilizing Selection	Favors intermediate phenotypes and selects against extremes.	Reduces variation; average trait value remains the same.	Human birth weight: very small and very large babies have lower survival rates.
Directional Selection	Favors one extreme phenotype over others.	Shifts the average trait value in one direction.	Antibiotic resistance in bacteria: resistant bacteria survive and reproduce.
Disruptive Selection	Favors both extreme phenotypes over intermediate ones.	Increases variation; can lead to two distinct groups.	Darwin's finches: birds with either very large or very small beaks survive better than those with intermediate beaks.

Visual Representation: (See diagrams in original notes for bell curve shifts.)

Key Terms and Definitions

• Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Adaptation: A heritable trait that increases an organism's fitness in a particular environment.

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

Summary Table: Mechanisms of Evolution

Mechanism	Effect on Genetic Variation	Effect on Fitness
Mutation	Increases	Usually neutral or deleterious; rarely beneficial
Natural Selection	Can increase or decrease	Increases average fitness
Gene Flow	Can increase or decrease	Can increase or decrease fitness depending on context
Genetic Drift	Decreases (especially in small populations)	Random; can decrease fitness

Additional info:

• Hardy-Weinberg equations for allele and genotype frequencies:

  • Allele frequencies: $p + q = 1$

  • Genotype frequencies: $p^2 + 2pq + q^2 = 1$

• Stabilizing, directional, and disruptive selection are often illustrated with bell curves showing changes in trait distribution over time.