EEB Class 2/28

Speciation Types

  • Sympatric Speciation: Occurs in the same geographic area without physical barriers.

  • Allopatric Speciation: Involves physical separation (e.g., drying of a pond) leading to the emergence of distinct species.

  • Example in question: Emergence of a blue species distinct from an orange species without isolation.

Mechanisms of Sympatric Speciation

  • Macromutation: A rare large genetic alteration during meiosis can lead to distinct species.

  • Habitat Preferences:

    • Different subpopulations of a species may prefer different depths in a habitat (e.g., shallow vs. deep water).

    • This leads to reduced interactions and, over time, can contribute to speciation.

  • Sexual Selection: Can be a mechanism promoting speciation through mate selection preferences that create barriers.

Evidence of Sympatric Speciation

  • Studies indicate that 25-33% of plant species origins show signs of sympatric speciation, often due to mechanisms like macromutation.

  • Example: Creosote Bush in Northern Mexico

    • Autopolyploidy and allopolyploidy observed due to genetic changes facilitating new species formation.

Allopolyploidy Explained

  • Occurs when hybridization leads to the formation of a new species which is usually sterile due to mismatched chromosome numbers from two distinct species.

  • Concept of prezygotic vs. postzygotic barriers to gene flow explained, with the hybrid being an example of postzygotic barriers.

Case Study: Trigopogon (Flower Species)

  • Native and introduced Trigopogon species in Oregon demonstrate how lack of geographic separation led to hybridization and emergence of new species.

  • Evidence over the last 150 years of speciation occurring due to these overlaps.

Cichlids of Lake Victoria

  • Evolutionary Context: Lake Victoria is geologically young (100,000 years) with over 600 cichlid species.

  • Diversity Mechanisms: Factors like fluctuating lake sizes and sexual selection have contributed to rapid speciation within this lake.

  • Female Preference: Differences in male coloration influence mate selection; changes in light conditions can relax barriers causing hybridization.

Behaviorally Acquired Traits in Speciation

  • Observations from a unique lineage of birds (Geospiza fortis) exhibiting cultural transmission of traits such as song can create barriers to gene flow.

  • The arrival of a new male introduces alleles and behavior, affecting mating patterns and promoting speciation through cultural mechanisms.

Key Definitions

  • Allopatry: Geographic separation leading to speciation.

  • Sympatry: Speciation occurring within the same geographic area without physical separation.

Geographic Barriers Summary

  • Geographic barriers can include lack of water, physical separation like islands or unusual habitat conditions which limit species interaction and gene flow.