Overview
This lecture explains how new species form through reproductive isolation and genetic divergence, describing mechanisms such as allopatric and sympatric speciation, adaptive radiation, and reproductive barriers.
Defining Species and Reproduction
- A species is a group of organisms that interbreed and produce fertile, viable offspring.
- Species share high levels of DNA alignment, resulting in similar characteristics and successful reproduction.
- Appearances can be misleading; hybrids between species are often infertile, maintaining species boundaries.
- Only heritable genetic changes, passed through gametes, influence evolution within species.
Speciation Mechanisms
- Speciation is the formation of new species from an original population, requiring reproductive isolation.
- Allopatric speciation occurs when populations are geographically separated, allowing independent evolution.
- Sympatric speciation occurs without geographic barriers, often through genetic changes like polyploidy.
Allopatric Speciation
- Geographic separation leads to reduced gene flow and genetic divergence between populations.
- Dispersal (movement to a new area) and vicariance (physical barrier formation) are main mechanisms.
- Greater distance increases the likelihood of speciation due to different environmental pressures.
Adaptive Radiation
- Adaptive radiation is rapid speciation from one ancestor into many species adapted to different niches.
- Island environments, like the Hawaiian honeycreepers, are common sites for adaptive radiation due to isolation.
Sympatric Speciation and Polyploidy
- Sympatric speciation often involves polyploidy, where organisms have extra chromosome sets.
- Autopolyploidy arises from chromosome duplication within one species.
- Allopolyploidy involves chromosome sets from different species.
- Polyploidy is more common in plants than animals.
Reproductive Isolation
- Reproductive barriers prevent gene flow and arise over time due to genetic and phenotypic divergence.
- Prezygotic barriers (before fertilization) include temporal, habitat, behavioral, gametic, and mechanical isolation.
- Postzygotic barriers (after fertilization) include hybrid inviability and hybrid sterility.
Examples and Additional Mechanisms
- Habitat isolation occurs when populations occupy different niches, leading to genetic divergence.
- Sympatric speciation can also result from ecological factors, such as different feeding behaviors.
- Cichlid fish in Lake Victoria and Nicaragua demonstrate rapid sympatric speciation due to ecological differences.
Key Terms & Definitions
- Species — group capable of interbreeding and producing fertile offspring.
- Gene pool — total genetic diversity within a population.
- Speciation — process where new species arise.
- Allopatric speciation — speciation due to geographic separation.
- Sympatric speciation — speciation within the same location.
- Adaptive radiation — rapid diversification from one ancestor into multiple species.
- Polyploidy — condition of having extra chromosome sets.
- Autopolyploidy — polyploidy with chromosomes from one species.
- Allopolyploidy — polyploidy from two different species.
- Reproductive isolation — barriers that prevent species from interbreeding.
- Prezygotic barrier — reproductive barrier before fertilization.
- Postzygotic barrier — reproductive barrier after fertilization.
Action Items / Next Steps
- Watch the recommended video on bird evolution.
- Review diagrams and figures referenced for visual understanding of speciation events.