Population Genetics, Speciation, Phylogeny, and Fungi: General Biology Study Notes

Microevolution and Population Genetics

Microevolution

Microevolution refers to changes in allele frequencies within a population across generations. These changes are the foundation of evolutionary biology and are driven by several mechanisms.

• Definition: A change in allele frequencies in a population over generations.

• Mechanisms: Natural selection, genetic drift, gene flow.

Gene Pool & Population

The gene pool is the collection of all alleles in a population. Understanding the gene pool is essential for studying genetic variation and evolution.

• Population: A group of individuals of the same species living in the same area and interbreeding.

• Gene pool: All alleles present in a population.

• Each genotype and allele has a frequency in the population that can be calculated.

Genotype and Allele Frequency Calculations

Calculating genotype and allele frequencies is fundamental in population genetics. These calculations help track genetic changes over time.

• Genotype frequency: Number of individuals with a genotype divided by total individuals.

• Allele frequency: Number of copies of an allele divided by total alleles.

Example Calculation:

• Genotypes: 120 CRCR, 160 CRCW, 20 CWCW

• Total individuals: 300

• Frequency of CRCR: 120/300 = 0.4

• Frequency of CRCW: 160/300 = 0.53

• Frequency of CWCW: 20/300 = 0.07

• Total alleles: 600 (2 per individual)

• Frequency of CR allele: (2x120 + 160)/600 = 0.67

• Frequency of CW allele: (2x20 + 160)/600 = 0.33

Equation:

• $p + q = 1$ (where p and q are frequencies of two alleles)

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model for allele and genotype frequencies in a non-evolving population.

• Equation: $p^2 + 2pq + q^2 = 1$

• p: Frequency of one allele

• q: Frequency of the other allele

• p^2: Homozygous dominant

• 2pq: Heterozygous

• q^2: Homozygous recessive

Conditions for Hardy-Weinberg Equilibrium

For a population to remain in Hardy-Weinberg equilibrium, five conditions must be met:

• No mutations

• Random mating

• No natural selection

• Large population size

• No migration (gene flow)

Mechanisms of Evolution

Evolutionary mechanisms disrupt Hardy-Weinberg equilibrium and lead to changes in allele frequencies.

• Mutation: Changes in DNA introduce new alleles.

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

• Gene flow: Movement of alleles between populations.

• Natural selection: Differential survival and reproduction favor certain alleles.

Types of Natural Selection

Directional, Disruptive, and Stabilizing Selection

Natural selection can act in different ways on the phenotypic range of a population.

• Directional selection: Favors individuals at one extreme of the phenotypic range.

• Disruptive selection: Favors individuals at both extremes of the phenotypic range.

• Stabilizing selection: Favors intermediate variants and acts against extreme phenotypes.

Example: Human birth weight is an example of stabilizing selection.

Speciation and Reproductive Barriers

Reproductive Barriers

Reproductive barriers prevent species from interbreeding and maintain species boundaries.

• Prezygotic barriers: Prevent fertilization (e.g., habitat, temporal, behavioral isolation).

• Postzygotic barriers: Occur after fertilization (e.g., reduced hybrid viability or fertility).

Mechanisms of Speciation

Speciation can occur via geographic or non-geographic separation.

• Allopatric speciation: Geographic separation leads to speciation.

• Sympatric speciation: Speciation without geographic separation.

Phylogeny and Systematics

Phylogeny

Phylogeny is the evolutionary history of a species or group, often represented as a branching diagram called a phylogenetic tree.

• Phylogenetic tree: Diagram showing evolutionary relationships.

Key Concepts in Phylogeny

• Rooted tree: Shows divergence from a common ancestor.

• Branch point: Represents divergence of two evolutionary lineages.

• Sister taxa: Groups that share an immediate common ancestor.

• Basal taxon: Lineage that diverges early in the history of a group.

• Analogy: Similarity due to convergent evolution.

• Homology: Similarity due to shared ancestry.

Cladistics and Character Tables

Cladistics is a method of classifying species based on shared derived characteristics.

• Character table: Used to construct phylogenetic trees by showing presence/absence of traits.

• Outgroup: Species or group closely related but not part of the group being studied.

• Ingroup: Species or group being studied.

Term	Definition
Outgroup	Species closely related but not part of the group being studied
Ingroup	Species being studied
Character table	Shows presence/absence of traits for each species

Maximum Parsimony

Maximum parsimony is a principle stating that the simplest explanation (fewest evolutionary events) is preferred in constructing phylogenetic trees.

• Application: Used to infer evolutionary relationships.

Fungi: Structure, Life Cycle, and Importance

What are Fungi?

Fungi are eukaryotic heterotrophs that absorb nutrients from their environment. They play essential roles in ecosystems as decomposers, mutualists, and pathogens.

• Definition: Eukaryotic heterotrophs that absorb nutrients.

• Roles: Decomposers, parasites, mutualists.

Key Characteristics of Fungi

• Use hydrolytic enzymes to digest compounds.

• Can be unicellular (yeasts) or multicellular (filamentous forms).

• Cell walls contain chitin.

• Hyphae form networks called mycelia.

Mycorrhizae

Mycorrhizae are mutualistic associations between fungi and plant roots, enhancing nutrient exchange.

• Plants: Supply organic nutrients to fungi.

• Fungi: Supply water and minerals to plants.

Types of Fungi

• Endomycorrhizal fungi: Grow inside root cells.

• Arbuscular mycorrhizal fungi: Penetrate plant cell walls but not the cell membrane.

Fungal Reproduction

• Most fungi reproduce both sexually and asexually.

• Sexual cycle includes plasmogamy (fusion of cytoplasm) and karyogamy (fusion of nuclei).

• Haploid spores are produced and dispersed.

Major Fungal Groups

• Ascomycetes: Produce spores in sac-like structures called asci.

• Basidiomycetes: Produce spores in club-shaped basidia.

Ecological Roles of Fungi

• Decompose organic material, recycling carbon, nitrogen, and other elements.

• Form mutualistic relationships (e.g., lichens, mycorrhizae).

• Can infect plants, animals, and humans (e.g., ringworm, athlete's foot, Candida albicans).

Lichens

Lichens are symbiotic associations between fungi and photosynthetic microorganisms (algae or cyanobacteria).

• Fungi provide shelter, water, and minerals.

• Algae or cyanobacteria provide sugars via photosynthesis.

Fungi in Human Life

• Pathogens: Cause diseases (e.g., ringworm, athlete's foot, Candida infections).

• Practical uses: Mushrooms, truffles, yeast (bread, beer), antibiotics (penicillin).

Genetic Research

• Yeast Saccharomyces cerevisiae has been genetically modified to produce human glycoproteins, including insulin-like growth factor.

Additional info: These notes cover topics from chapters on population genetics, speciation, phylogeny, and fungi, relevant to a General Biology college course.