Origin of Species

Origin of Species

How does variation persist?

• All of the ways that variation is created
• Mutation
• Recombination
• Independent Assortment of Chromosomes
• Fertilization
• Disruptive Selection
• Gene Flow
• Negative Frequency-Dependent Selection: a trait is only advantageous when it is rare
• Heterozygote Advantage: having two different alleles for the same gene produces the greatest fitness Example: Sickle-Cell Anemia

Types of Evolution

• Microevolution: evolution within populations or species (smaller changes)
• Can accumulate to become macroevolution
• Macroevolution: evolution above the species level
• Novel traits ("game changers")
• Can result from mass-extinction which open up the environment for rapid evolution

Biological Species Concept

• Biological Species Concept: a "species" is a group of interbreeding organisms that can produce viable offspring in nature
• Issues:
• doesn't work with asexual organisms
• some species can reproduce with other species: hybrids
• Reproductive Isolation: a "species" should not be able to breed with other "species"
• Prezygotic Isolation: barriers to reproduction before fertilization
• Ecological
• Temporal
• Behavioral
• Mechanical
• Gamete
• Postzygotic Isolation: species can mate, but the resulting offspring fail
• zygote death
• infertile F1 offspring
• low hybrid fitness Example: "Hinny" is a sterile cross between Horse and Donkey

Speciation

• Allopatric Speciation: speciation occurs due to a geographic barrier
• most common
• after two groups become isolated they differentiate

• Sympatric Speciation: speciation without a geographic barrier
• rare
• assortative mating
• microhabitat specialization
• disruptive selection
• After reproductive isolation
• no gene flow between populations
• evolution occurs separately for the two populations
• eventually the two populations become different enough to prevent mating