Speciation in Evolutionary Biology

Speciation

• Speciation is the process by which one species splits into two or more species
• Biological diversity largely comes from speciation
• Understanding how new species arise is a key study in evolutionary biology

Learning Objectives

• Define speciation, reproductive barriers, and reproductive isolation
• Outline various species concepts, advantages, and disadvantages
  • Discuss why species concepts may not always agree when considering speciation as a process
• Describe pre- and postzygotic barriers and classify examples
• Define allopatric and sympatric speciation with respect to population geography
• Explain the mechanisms and geographic context of speciation
• Summarize experimental evolution techniques, and how lab experiments provide insight into speciation
• Outline how ecologically-dependent postzygotic isolation provides evidence for ecological speciation and reciprocal transplant experiments to test this

Outline

• Speciation & species concepts
• Reproductive barriers
• Geography of speciation
• Mechanisms of speciation
• Studying speciation

Species Concepts

• Species is Latin for "kind" or "appearance"
• Speciation is a process, and taxa vary in how far they are along
• Different definitions for species may not agree
• More than 20 different definitions have been proposed (biological, phylogenetic, cohesion, recognition, ecological, internodal, morphological)
• Understanding how new species arise is the goal

Biological Species Concept (BSC)

• Biological species are groups that actually or potentially interbreed and produce fertile offspring in nature
• Populations are part of the same species if reproductively compatible; different species if reproductively isolated
• The ability to form fertile offspring means gene flow is possible
• Reproductive isolation is the opposite of compatibility and is the result of reproductive barriers
  • Biological features that reduce or prevent gene exchange with other groups

Dis/Advantages of the BSC

• Biologically relevant; lack of gene flow means separate gene pools
• Provides a straightforward approach to studying speciation by understanding reproductive barriers
• Can't be applied to asexual taxa or fossils
• Difficult to test reproductive barriers

Other Species Concepts

• Morphological - based on morphological similarity
• Can be applied to fossils, but is subjective, potentially missing cryptic species
• Ecological - based on ecological niche (physical and environmental conditions; interactions with other species)
• Can be applied to asexual taxa, but not fossils, and is subjective

Case Studies

• Freshwater threespine sticklebacks: pelvic spines for protection
• Ring species: a chain of geographically connected populations in which the populations at the ends of the chain are reproductively isolated

Reproductive Barriers

• Can be prezygotic (before fertilization) or postzygotic (after fertilization)
  • Prezygotic barriers block fertilization
    • Habitat, temporal, behavioral/pollinator, mechanical, gametic isolation
  • Postzygotic barriers reduce the survival/reproductive success of hybrids

Geographic Context of Speciation

• Allopatric: Gene flow is interrupted by an extrinsic barrier. Populations evolve independently, accumulating reproductive barriers. Secondary contact can result in reproductive isolation.
• Sympatric: Speciation occurs in the absence of any extrinsic barrier; populations live in the same region. It's difficult to overcome initial unrestricted gene flow. Can occur via polyploidization or strong disruptive selection with positive assortative mating. Restricted conditions and biogeographic evidence suggest sympatric speciation is relatively rare

Mechanisms of Speciation

• Ecological: Reproductive isolation arises from adaptation to different environments/niches; Natural selection in different populations; phenotypic differences cause reproductive isolation.
• Polyploidization: Increase in chromosome sets (2n,3n, 4n, etc.). Postzygotic isolation; Instantaneous due to unbalanced gametes. Common in ferns and plants
• Reinforcement: Strengthening of prezygotic barriers via natural selection in response to reduced hybrid fitness. Expected outcome: stronger prezygotic isolation in sympatry than allopatry, requiring secondary contact

Studying Speciation

• Laboratory studies: Test the feasibility of various speciation mechanisms and explore conditions under which they are more/less likely to occur
• Speciation in nature: Test hypotheses about the causes of existing reproductive isolation between species. Examples are testing for ecologically-dependent RI

Ecological Speciation (Examining Speciation In Nature)

• Ecologically dependent postzygotic isolation occurs when hybrid fitness is reduced due to a mismatch between hybrid phenotype and environment (reduced hybrid fitness has an ecological basis)
• In the absence of an intermediate environment, divergent selection between environments is the predicted outcome. A unique prediction of ecological speciation
• Can be tested through reciprocal transplant of parental species and hybrids across different habitats

Studying Speciation: Laboratory Experiments

• Experimental evolution is a technique used to study speciation
• Multiple, replicate populations are derived from a common ancestor
• Their evolution is tracked under controlled conditions
• Can be used to test ecological speciation
  • Ecological speciation predicts stronger behavioral isolation between populations adapted to different environments compared to populations independently adapted to the same environment.