The Origin of Species

The Origin of Species Study Notes

Student’s Learning Objectives

  • Explain the various models of speciation.
  • Give examples that support the various models of speciation.
  • Explain what is meant by "Cryptic Species."

Models of Speciation

Models of speciation explain how reproductive isolation arises:

Allopatric Speciation
  • Definition: The evolution of new species after populations have been separated geographically, preventing gene flow between them.
  • Example Scenario: A river slices across the range of a species of terrestrial beetle.
    • The two populations are physically isolated yet evolve through genetic drift and natural selection.
  • Effects on Allele Frequencies:
    • Existing alleles may become more common in one population while becoming less common or absent in the other.
    • Mutations arising in one population are less likely to be present in the other due to the physical separation.
  • Reproductive Barriers Created:
    • Differences in breeding timing.
    • Alleles that make males attractive to females in one population may render them unattractive to females in the other population.
  • Reintegration Post-Separation:
    • A scenario where the river dividing the two populations dries up may allow them to rejoin.
    • If reproductive barriers are weak, the rejoined populations will interbreed easily.
    • If reproductive barriers are strong, populations may not interbreed, indicating complete speciation.
  • Evidence of Allopatric Speciation Model:
    • Until about three million years ago, the Caribbean Sea and Pacific Ocean were joined by the Atrato Seaway.
    • Some shrimp species inhabited ranges stretching from the Pacific Ocean to the Caribbean Sea.
    • Three million years ago, the two water bodies became entirely cut off from one another, leading to split populations now isolated by a terrestrial barrier.
    • Genetic studies demonstrate findings where the closest relatives of Caribbean shrimp are Pacific species, reinforcing allopatric speciation evidence.
Sympatric Speciation
  • Definition: The evolution of new species within a single, physically connected geographic area that still has extensive gene flow.
    • This model starts with a population of interbreeding individuals occupying the same range.
    • Individuals begin to mate nonrandomly, often with those genetically or phenotypically similar.
    • This leads to the original population diverging into multiple populations divided by reproductive barriers.
  • Sympatric Speciation via Disruptive Selection:
    • Example: Two sympatric populations of finch birds evolve.
    • One group feeds on small seeds while the other on large seeds.
    • Without nonrandom mating, the two populations would not diverge genetically, preventing speciation.
    • Hybrid fitness issue: Produced offspring (hybrids) between different specialists may be better suited for a novel medium seed niche, but if no such seeds exist, natural selection will act against these hybrids.
  • Evidence of the Sympatric Speciation Model:
    • Lord Howe Island serves as a case for once-contiguous populations of palm trees that evolved separately.
    • Two soil types led to the segregation of species based on their flowering times; Howea belmoreana (neutral and low pH soils) and Howea forsteriana (high pH soils).
    • Flowering time differences (6 weeks apart) reduced likelihood of pollination and hybridization.
    • This evolution emphasizes how ecological conditions lead to sympatric speciation, citing Savolainen et al. (2006).
Comparison of Allopatric and Sympatric Speciation
  • Allopatric Speciation:
    • Characterized by geographic separation involving vicariance or dispersal, leading to reproductive isolation over time.
    • Ancestral populations face geographical barriers resulting in large, isolated populations.
  • Sympatric Speciation:
    • Occurs within the same geographic locale, where disruptive selection fosters genetic divergence within the original population, eventually leading to distinct species.
Ecological Speciation
  • Definition: The evolution of reproductive barriers between populations through adaptation to varying ecological niches or environments.
  • Populations adapt to differing ecological conditions (e.g., food sources, habitats) causing reduced contacts and mating opportunities.
  • Hybrid Fitness Issues:
    • Hybrids resulting from populations adapted to different environments may display lower reproductive success and fitness, struggling to compete in the respective habitats.
Example of Ecological Speciation
  • Polar Bears and Brown Bears:
    • Polar bears have evolved from brown bears by adapting to the Arctic marine environment.
    • The ecological niche of polar bears is linked to specific selection pressures, such as the trade of the APOB gene associated with high cholesterol due to seal-based diets.
    • Despite their divergence, occurrences of hybridization between polar bears and brown bears have been documented.

Cryptic Species

  • Definition: Lineages historically treated as one species due to morphological similarities but are genetically distinct.
  • Cryptic species can diverge from ancestral populations without developing distinguishable morphological characteristics.
Importance of Identifying Cryptic Species
  • Conservation policies necessitate identification of species to implement effective preservation strategies.
  • Cryptic species often possess diverse habitat requirements and varying threat levels; overlooking them could result in unnoticed extinctions.
  • They significantly enhance the understanding of biological diversity on our planet.
Final Clarifying Question
  • Definition within context: A cryptic species is a species that appears morphologically similar to another but is genetically distinct.