Population genetics explores the distribution and change of allele frequencies in populations.
Cladogram Basics
Cladogram: A diagram that shows the evolutionary relationships between different species.
Plant Development Timeline
Timeline Components:
- Cherry Tree
- Pine Tree
- Fern
- Moss
Key Features of Various Plants:
Woody Stems
- Pine Tree: Lives in wet areas, produces seeds on short shoots.
- Cherry Tree: Has a woody stem and photosynthesis occurs primarily through the stem.
Vascular Tissue
- Present in woody plants (e.g., Cherry and Pine Trees).
- Absent in non-woody plants (e.g., Moss).
Darwin's contributions to evolutionary theory were limited by his lack of knowledge about genetics and DNA.
Key challenges faced by Darwin:
1. Source of variation within species.
2. Process of inheritance of traits across generations.
Populations and Gene Pools
Population: Group of individuals of the same species in a defined area.
Gene Pool: The complete set of genetic information within a given population, encompassing all alleles for each gene
Allele Frequency
Definition: The proportion of a specific allele among all alleles for that gene in a population.
Sources of Genetic Variation
Mutations
Definition: A heritable change in genetic information.
- Can lead to the formation of new alleles.
Natural Selection: Can act on the resulting phenotypic variations.
Mutation Rates: Generally lower in plants and animals, and higher in prokaryotes due to faster reproduction.
Impact of Mutations:
- Can be harmful, neutral, or beneficial; most are neutral to harmful.
Recombination Processes
Genetic Recombination:
1. Independent Assortment: Random segregation of homologous chromosome pairs during gamete formation.
2. Crossing-Over: Exchange of DNA segments between homologous chromosomes during meiosis, increasing genetic diversity.
Natural Selection
Overview of Natural Selection
Focus on how natural selection impacts allele frequencies and population phenotypes.
Reproductive Success is quantified through relative fitness, which considers survival and reproduction capabilities relative to others.
Measuring Effects of Natural Selection
Changes in phenotypic averages can provide insights into selection effects.
Modes of Natural Selection
Directional Selection: Favors one extreme phenotype, moving the mean in that direction.
Disruptive Selection: Favors extremes of phenotypes, potentially leading to speciation.
Examples of Natural Selection Effects
Directional Selection: Longer necks in giraffes.
Stabilizing Selection: Human birth weights.
Disruptive Selection: Color variation in rock pocket mice.
Genetic Drift and Population Changes
Genetic Drift
Definition: Random fluctuation of allele frequencies in small populations due to chance events.
Consequences: Can reduce genetic diversity, not driven by selection mechanisms.
Bottleneck Effect
Occurs when a significant reduction in population size leads to altered allele frequencies and loss of genetic diversity.
Examples of causes: Natural disasters, habitat destruction, and overhunting.
Cheetah Population: Example of severe bottleneck, resulting in low genetic diversity due to historical events.
Founder Effect
Happens when a new population is established by a small subgroup of the original population, potentially resulting in different allele frequencies.
Gene Flow
Definition: Movement of alleles between populations through immigration/emigration of individuals.
Example: Pollen dispersal from one flower population to another, allowing allele mixing.
Scenarios Review
Scenario analysis to determine if they exemplify bottleneck effect, founder effect, or gene flow:
1. Black robin population decline due to habitat loss - Bottleneck effect.
2. Amish population polydactyly - Founder effect.
3. Genetic diversity loss in marble trout - Bottleneck effect.
4. Pollen dispersal leading to trait mixing - Gene flow.
Speciation
Definition and Introduction
Species: A group capable of interbreeding to produce viable, fertile offspring.
Speciation: The evolutionary process by which new biological species arise, leading to life's diversity.
Reproductive Isolation
Occurs when populations are prevented from interbreeding through mechanisms that maintain gene flow barriers.
Types of barriers:
1. Prezygotic: Prevent mating or hinder fertilization.
2. Postzygotic: Prevent development of viable, fertile hybrids.
Prezygotic Isolation Mechanisms
Geographic Isolation: Separation of species due to physical barriers (e.g., mountains).
- Example: Western vs Eastern bluebirds.
Temporal Isolation: Species breed at different times, preventing mating.
- Example: Mating times of western and eastern spotted skunks.