Study Notes: Evolutionary Genetics (Chapter 26)

Evolutionary Genetics

26.1 Organisms Evolve Through Genetic Change Within Populations

Evolutionary genetics studies how genetic changes drive the evolution of populations and species. Biological evolution is defined as genetic change in a group of organisms over time.

• Biological evolution: Genetic change in a population or group of organisms.

• Two-step process of evolution: 1) Genetic variation arises, 2) Genetic variants increase or decrease in frequency due to evolutionary forces.

• Types of evolution:

  • Anagenesis: Evolutionary change within a single lineage over time.

  • Cladogenesis: Splitting of one lineage into two, resulting in speciation.

• Example: The diagram shows anagenesis as a straight line and cladogenesis as a branching event, representing speciation.

26.2 Genetic Variation in Natural Populations

Genetic variation is essential for evolution to occur. It can be observed at multiple biological levels.

• Molecular variation: Differences in DNA sequences among individuals.

• Protein variation: Differences in protein structure and function due to genetic differences.

• DNA sequence variation: Direct changes in the nucleotide sequences of genes.

• Examples:

  • Frogs with different appearances may belong to the same species.

  • Birds with similar appearances may belong to different species.

Molecular Variation

• Molecular data are genetic and can be used with all organisms.

• Molecular methods allow for the analysis of large amounts of genetic variation.

• All organisms can be compared using molecular data.

• Molecular data are quantifiable and provide insights into evolutionary processes.

• The database of molecular information is extensive and growing.

26.3 New Species Arise Through the Evolution of Reproductive Isolation

Speciation is the process by which new species arise, often through the evolution of reproductive isolation.

• Biological species concept (Ernst Mayr, 1942): A species is a group of organisms capable of interbreeding and producing viable, fertile offspring, but reproductively isolated from other species.

• Reproductive isolation: Prevents gene flow between populations, leading to speciation.

Reproductive Isolating Mechanisms

• Prezygotic isolating mechanisms: Prevent formation of a zygote.

  • Ecological: Differences in habitat.

  • Behavioral: Differences in mating behavior.

  • Temporal: Reproduction occurs at different times.

  • Mechanical: Anatomical differences prevent mating.

  • Gametic: Gametes are incompatible.

• Postzygotic isolating mechanisms: Act after fertilization.

  • Hybrid inviability: Hybrid zygote does not survive.

  • Hybrid sterility: Hybrid is sterile.

  • Hybrid breakdown: F1 hybrids are viable and fertile, but F2 are inviable or sterile.

Type	Characteristics
Prezygotic	Acts before a zygote has formed
Geographical	Differences in habitat; individuals do not meet
Temporal	Reproduction takes place at different times
Mechanical	Anatomical differences prevent copulation
Behavioral	Differences in mating behavior prevent mating
Gametic	Gametes are incompatible or not attracted to each other
Postzygotic	Acts after a zygote has formed
Hybrid inviability	Hybrid zygote does not survive to reproduction
Hybrid sterility	Hybrid is sterile
Hybrid breakdown	F1 hybrids are viable and fertile, but F2 are inviable or sterile

Speciation Mechanisms

• Allopatric speciation: Occurs when a geographic barrier splits a population, preventing gene flow and leading to genetic divergence.

• Sympatric speciation: Occurs without geographic barriers; reproductive isolation evolves within a single population.

• Speciation through polyploidy: Especially common in plants, polyploidy can instantly create reproductive isolation.

• Example: Spartina anglica arose sympatrically through allopolyploidy.

26.4 Reconstructing Evolutionary History: Phylogeny and Phylogenetic Trees

Phylogeny describes the evolutionary relationships among organisms, often visualized as phylogenetic trees.

• Phylogeny: The sequence of evolutionary events in a species or group.

• Phylogenetic tree: Diagram showing evolutionary relationships, based on homology (shared ancestry).

• Homologous structures: Traits inherited from a common ancestor (e.g., bat wing, human arm, cat leg).

• Monophyletic group (clade): All descendants of a common ancestor.

• Nodes/branch points: Indicate divergence events (speciation).

• Tips of branches: Represent modern or extinct species.

• Gene trees: Show evolutionary relationships among genes (e.g., human globin gene family).

Construction of Phylogenetic Trees

• Alignment of homologous sequences: Phylogenetic trees are often constructed from DNA sequence data.

• Methods:

  • Distance approach: Based on genetic distance between sequences.

  • Parsimony approach: Seeks the simplest tree with the fewest evolutionary changes.

How to Read a Phylogenetic Tree

• Vertical axis represents time; branch points indicate speciation events.

• Clades are groups of species sharing a common ancestor.

• Cladogenesis is shown as branching; anagenesis as a straight line.

Gene Duplication and Multigene Families

• Gene duplication events can lead to multigene families, such as the human globin genes.

• Successive duplications allow for functional diversification.

Horizontal Gene Transfer

• Genetic material can be exchanged between different species, especially in prokaryotes.

• This process, called horizontal gene transfer, contributes to genetic diversity and evolution outside of vertical inheritance.

Additional info: These notes expand on the brief points in the slides, providing definitions, examples, and context for key concepts in evolutionary genetics, including speciation, phylogeny, and genetic variation.