Speciation and Phylogeny Notes

Speciation and Phylogeny

Speciation

• Species: A group of populations whose members have the potential to interbreed in nature, producing fertile and viable offspring.

Reproductive Isolation

• Reproductive Isolation: Mechanisms that prevent members of different species from interbreeding and producing viable, fertile offspring.
• Prezygotic Barriers: Barriers that occur before the formation of a zygote, blocking fertilization from ever occurring.
  • Temporal Isolation: Species mate at different times of the year.
    • Example: Frogs living in the same pond breeding during different seasons (summer vs. spring).
  • Ecological Isolation: Species occupy different habitats.
    • Example: Lions and tigers potentially interbreeding but usually occupying different habitats.
  • Behavioral Isolation: Species have different courtship behaviors.
    • Example: Certain groups of birds only responding to species-specific mating calls.
  • Mechanical Isolation: Physical differences prevent copulation/pollination.
    • Example: Certain breeds of dog being morphologically incapable of mating due to size.
  • Gametic Isolation: Eggs and sperm are incompatible.
• Postzygotic Barriers: Barriers that occur after the formation of a zygote, resulting in hybrid zygotes that are not viable or fertile.
  • Reduced Hybrid Viability: Hybrids are produced but fail to develop to reproductive maturity.
    • Example: Certain types of frogs forming hybrid tadpoles that die before they can become frogs.
  • Reduced Hybrid Fertility: Hybrids fail to produce functional gametes (sterility).
    • Example: Mules are sterile hybrids resulting from mating between a horse and a donkey.
  • Hybrid Breakdown: F1 hybrids are fertile, but F2 generation fails to develop properly.
    • Example: The offspring of hybrid copepods have less potential for survival or reproduction.
• Speciation: The process by which new species arise.

Modes of Speciation

• Allopatric Speciation ("other homeland"):
  • Geographically isolated populations.
  • Caused by geologic events or processes.
  • Evolves by natural selection and genetic drift.
  • Example: Squirrels on the North/South rims of the Grand Canyon.
  • Geographic Isolation: Can lead to allopatric speciation (Darwin's Finches as an example).
    • Founders arrive.
    • Geographic isolation occurs.
    • Gene pool changes.
• Sympatric Speciation ("together homeland"):
  • Overlapping populations within the same geographic area.
  • Gene flow between subpopulations blocked by:
    • Polyploidy.
    • Habitat differentiation.
    • Sexual selection.
  • Example: Polyploidy in 80% of plants (oats, cotton, potatoes, wheat).
  • Mainly in plants.
  • Non Plants
    • Organisms living in same area, but don’t have opportunity to mate - large range.
• Adaptive Radiation: Rapidly changing populations that originate from one founding species.
  • Due to diverse environments and populations only being able to fill one ecological niche.

Phylogeny

• Phylogeny: The evolutionary history of a species or group of related species.
• Tools to determine evolutionary relationships:
  • Fossils.
  • Morphology (homologous structures).
  • Molecular evidence (DNA, amino acids).
• Animals and fungi are more closely related than either is to plants.
• Legless conditions evolved separately; analogous structures evolved by convergent evolution.
  • Example: Eastern glass lizard and Snakes

Taxonomy

• Taxonomy: Classifying and naming organisms based on similar/different characteristics.
• Ordered division of organisms:
  • Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
  • Mnemonic: Dear King Philip Came Over For Good Spaghetti.
• Each category at any level is called a taxon.
• Binomial Nomenclature: Naming system developed by Carolus Linnaeus (Genus species).
  • Example: Panthera pardus (leopard).
    • Species: Panthera pardus
    • Genus: Panthera
    • Family: Felidae
    • Order: Carnivora
    • Class: Mammalia
    • Phylum: Chordata
    • Kingdom: Animalia
    • Domain: Eukarya

Phylogenetic Tree

• Phylogenetic Tree: Branching diagram that shows the evolutionary history of a group of organisms.
• Branch Point: Where lineages diverge.
• Sister Taxa: Groups of organisms that share an immediate common ancestor.
• Basal Taxon: A lineage that diverges early in the history of a group.
• Polytomy: An unresolved pattern of divergence.
• Branch lengths can represent genetic change or indicate time.

Constructing Phylogenetic Trees

• Divergent vs. Convergent Evolution
• Homology vs. Analogy
• Homologous Structures: Similar structure / Different Function.
  • Example: Human arm, cat foreleg, whale fin, bat wing.
• Analogous Structures: Different structure / Similar Function.
  • Example: Wing of a bird, wing of a butterfly.
• Clade: A group of species that includes an ancestral species + all descendents.
• Cladogram: Diagram that depicts patterns of shared characteristics among groups
• Shared ancestral characteristic (of all vertebrates) = vertebral column
• Shared derived characteristic of mammals = hair
• Monophyletic Group (Clade): A group that includes an ancestral species and all of its descendants.
• Paraphyletic Group: A group that includes an ancestral species and some, but not all, of its descendants.
• Polyphyletic Group: A group that includes distantly related species but does not include their most recent common ancestor.