Overview
This lecture explains how new species form, the genetic and reproductive barriers involved, and distinguishes between allopatric and sympatric speciation, including adaptive radiation.
Defining and Identifying Species
- A species is a group of organisms that interbreed and produce fertile, viable offspring.
- Members of the same species have highly similar DNA and share reproductive behaviors.
- Appearance alone is not a reliable indicator of species; genetic compatibility determines speciation.
- A gene pool is the collection of all gene variants present in a species’ population.
Speciation Overview
- Speciation is the formation of two species from one original species.
- Allopatric speciation involves geographic separation; sympatric speciation occurs without physical barriers.
- A population must become reproductively isolated for speciation to occur.
- Adaptive radiation is when one species rapidly diversifies into many new forms, often in isolated environments.
Allopatric Speciation and Adaptive Radiation
- Geographic barriers (rivers, valleys, islands) can isolate populations and reduce gene flow.
- Isolated populations adapt to new environments, leading to genetic divergence.
- Dispersal (movement to new areas) and vicariance (physical barriers) are two ways isolation occurs.
- The further populations are separated, the more likely speciation is due to different selective pressures.
- Adaptive radiation results in multiple new species from a single common ancestor due to varied niches.
Sympatric Speciation
- Sympatric speciation happens within a single geographic area, without a physical barrier.
- Polyploidy, especially in plants, results in reproductive isolation due to chromosome set changes.
- Autopolyploidy involves chromosome duplication within one species; allopolyploidy involves hybridization between species.
- New feeding strategies or behaviors in the same habitat can also lead to speciation.
Reproductive Isolation Mechanisms
- Prezygotic barriers prevent mating or fertilization (temporal, habitat, behavioral, gametic, and mechanical isolation).
- Postzygotic barriers occur after fertilization, leading to nonviable or sterile hybrids.
- Examples include different breeding seasons, habitat preferences, mating behaviors, and incompatible reproductive organs.
Key Terms & Definitions
- Species — group of organisms that can interbreed and produce fertile offspring.
- Gene pool — all genetic variants in a population.
- Speciation — process by which new species arise.
- Allopatric speciation — speciation due to geographic separation.
- Sympatric speciation — speciation within the same geographic area.
- Adaptive radiation — rapid evolution of many species from one ancestor.
- Polyploidy — condition of having extra chromosome sets.
- Autopolyploidy — chromosome duplication within one species.
- Allopolyploidy — chromosome combination from different species.
- Prezygotic barrier — reproductive barrier before fertilization.
- Postzygotic barrier — reproductive barrier after fertilization.
- Reproductive isolation — inability of different populations to interbreed.
Action Items / Next Steps
- Watch the suggested video on bird evolution to reinforce concepts of speciation.
- Review figures referenced for examples of speciation and reproductive barriers.