Species and Speciation

Species and Speciation Notes

Species Definition

• Key point of the Darwinian revolution: species are not fixed.
• Evolution leads to the question: How should we define a species?
• Biological Species Concept (BSC):
  • Species are groups of populations that are actually or potentially interbreeding.
  • Species are reproductively isolated from other such groups.
  • They have a closed gene pool - mutations spread within but not across species.
• The BSC is more useful in theory than in practice due to:
  • Hybridization at species boundaries.
  • Difficulties in identifying species clearly.

Leakiness of Species Boundaries

• Two closely related species may hybridize, especially near species boundaries.
• Hybridization is more common in plants than animals, and DNA evidence suggests species boundaries can be flexible.
• Example: Hybridization in towee species (Fig 21.4).

Other Species Concepts Broadening the BSC

• Morphospecies Concept: Classifies species based on morphological similarity.
• Ecological Species Concept: Defines species via their niche or ecological role.
• Phylogenetic Species Concept: Based on evolutionary relationships or phylogeny.

Morphospecies Concept

• Idea: Individuals of the same species share similar appearances.
• Application challenges:
  • Convergent evolution may lead to similar appearances between different species.
  • Polymorphisms: Individuals may look different even though they belong to the same species.
  • Cryptic species: Different species that appear identical but require genetic analysis to identify differences.

Ecological Species Concept

• Concept of a one-to-one correspondence between species and their ecological niche.
• Example: Ladybugs (Henosepilachna species) are reproductively isolated due to different host plants.

Phylogenetic Species Concept

• Centers on shared common ancestry among species.
• Questions the applicability of species classification on a broad scale (e.g., are all mammals one species?).

Reproductive Isolation Mechanisms

• Extrinsic Isolation: Geographic barriers leading to isolation.
• Intrinsic Reproductive Isolating Mechanisms include:
  • Ecological isolation: Two species occupy different habitats.
  • Temporal isolation: Different breeding times prevent interbreeding.
  • Behavioral isolation: Unique courtship rituals prevent mating.
  • Mechanical isolation: Physical differences prevent successful mating.
  • Gametic isolation: Sperm from one species fails to fertilize eggs of another.
  • Reduced hybrid viability: Hybrids are less viable.
  • Reduced hybrid fertility: Hybrids survive but are infertile.
  • Hybrid breakdown: Offspring of hybrids may be fertile, but those of their offspring are not.

Speciation Overview

• Speciation is the development of reproductive isolation and occurs through genetic divergence of populations, typically following geographic separation (allopatric speciation).
• Types of geographic separation:
  • Dispersal: Migration to new geographical areas.
  • Vicariance: Emergence of geographical barriers.

Allopatric Speciation

• Examples: Allopatric and peripatric speciation.
• Vicariance creates barriers within a population, leading to divergence.
• Dispersion results in new populations forming, potentially leading to faster speciation due to stronger selection pressures in different environments.

Examples in Kingfishers

• Large populations on New Guinea are separated by mountain ranges with more diversity in smaller island populations.

Adaptive Radiation

• Rapid diversification (burst of speciation) often occurs when ecological opportunities rise (open niches).
• Example: Darwin’s Finches illustrate how natural selection accelerates speciation and adaptation.

Selection Mode Changes

• Mechanisms of selection may change upon environmental shifts, influencing evolutionary pathways.

Sympatric Speciation

• Speciation can occur without geographic separation via strong natural selection counteracting gene flow.
• Example: Host-race formation in specialized insects, such as the apple maggot fly, Rhagoletis pomonella.

Disruptive Selection as a Mechanism

• Can lead to sympatric speciation by favoring extremes over intermediates.

Summary of Speciation Types

• Allopatric Speciation: Occurs through geographic separation (vicariance or dispersal).
• Sympatric Speciation: Occurs within the same geographic location, driven by disruptive selection or host specialization.

Instantaneous Speciation via Polyploidy

• Common in plants through:
  • Autopolyploidy: Backcrossing produces non-viable hybrids.
  • Allopolyploidy: Hybridization between species leads to new chromosomal combinations.

Case Studies on Instantaneous Speciation

• Example: Hybridization observed in finches on Daphne major leading to new species with distinct features.

Visual Synthesis of Speciation

• Highlights the roles of genetic drift and natural selection in divergence between populations leading to speciation.
• Speciation occurs when genetic divergence renders populations unable to produce viable, fertile offspring.