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Speciation

Overview of Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It typically involves three main steps: population isolation, phenotypic divergence, and the evolution of reproductive isolation.

• Population Isolation: Populations become separated by physical, ecological, or behavioral barriers, preventing gene flow.

• Phenotypic Divergence: Isolated populations experience genetic drift and natural selection, leading to differences in traits.

• Reproductive Isolation: Mechanisms evolve that prevent interbreeding between diverging populations, solidifying speciation.

Genetic Drift and Natural Selection in Speciation

• Genetic Drift: Random changes in allele frequencies can lead to genetic differentiation between isolated populations.

• Natural Selection: Different environments impose distinct selective pressures, driving adaptive divergence.

Ecological Speciation

Ecological speciation occurs when barriers to gene flow evolve as a result of divergent natural selection between environments. This process links ecological divergence directly to reproductive isolation.

• Requirements for Ecological Speciation:

  1. A source of divergent selection (e.g., different habitats or resources)

  2. A form of reproductive isolation (prezygotic or postzygotic)

  3. A mechanism linking divergent selection to reproductive isolation

Mechanisms of Reproductive Isolation

Premating and Postzygotic Isolation

• Premating Isolation: Mechanisms that prevent mating between populations, such as behavioral differences or habitat preferences.

• Postzygotic Isolation: Mechanisms that reduce the fitness of hybrids, including intrinsic (developmental defects) and extrinsic (environmentally dependent) factors.

Darwin’s Dilemma and Hybrid Fitness

Natural selection typically favors traits that increase fitness, but the evolution of reproductive isolation can sometimes result in the production of unfit hybrids.

• Intrinsic Isolation: Hybrid inviability or sterility due to genetic incompatibilities, independent of environment.

• Extrinsic Isolation: Hybrids are less fit because they occupy intermediate niches that are not optimal in either parental environment.

Bateson-Dobzhansky-Muller (BDM) Model

The BDM model explains how genetic incompatibilities can accumulate between isolated populations, leading to extrinsic inviability when hybrids are formed.

• As populations diverge, substitutions accumulate independently. Some genes from one population may perform poorly in the genetic background of the other due to local adaptation.

Reinforcement

Reinforcement is the process by which natural selection increases reproductive isolation between populations in sympatry, favoring individuals that avoid producing unfit hybrids.

• Females and males that preferentially mate with their own species have higher fitness.

• Selection favors alleles that enhance mating discrimination.

Case Study: Sticklebacks and Adaptive Divergence

Adaptive Change After Colonization

The three-spined stickleback (*Gasterosteus aculeatus*) is a model organism for studying ecological speciation. After colonizing freshwater lakes, sticklebacks diverged into benthic (bottom-dwelling) and limnetic (open-water) forms.

• Growth Rate: Growth rate is a key fitness trait, with each form exhibiting highest growth in its native niche.

• Sympatric vs. Allopatric Populations: In lakes with both forms (sympatry), clear phenotypic differentiation is observed; in lakes with only one form (allopatry), differentiation is absent.

Evolution of Reproductive Isolation in Sticklebacks

• Premating Isolation: Male sticklebacks show preference for mates based on size and form, reinforcing reproductive barriers.

• Postzygotic Isolation: Hybrids between benthic and limnetic forms have reduced fitness, especially in natural environments.

Genomic Perspectives on Speciation

Genic View of Speciation

The genic view of speciation emphasizes that adaptation and reproductive isolation can be driven by specific genes or gene sets, rather than the entire genome.

• Polygenic Adaptation: Multiple genes, often in linkage disequilibrium, contribute to adaptive divergence.

• Revision of the Biological Species Concept (BSC): The BSC focuses on genome-wide reproductive isolation, but the genic view highlights the importance of differential adaptation at the gene level.

Genomic Evidence from Sticklebacks

• Genomic studies reveal that adaptive divergence often involves regulatory and coding changes at specific loci.

• Key genes, such as EDA, are associated with morphological and ecological differences between marine and freshwater sticklebacks.

Table: Examples of Speciation by Natural Selection

The following table summarizes recent studies on populations diverging due to differences in habitat or resource use:

Species	Type of Divergence	Citation
Three-spine sticklebacks (freshwater fish)	Benthic (bottom-dwelling) versus limnetic (open-water-dwelling) forms	Rundle, H.D. et al. 2000. Science 287:306–308.
Lake Nicaragua cichlids	Benthic versus limnetic forms	Barluenga et al. 2006. Nature 439:719–723.
Sockeye salmon	Sea-run versus lake-dwelling populations	Wood, C.C. and C.J. Foote. 1996. Evolution 50:1265–1279.
Sockeye salmon	Populations that breed in beach versus river habitats	Hendry, A.P. et al. 2000. Science 290:516–518.
Blue tits (birds)	Populations that breed in deciduous versus evergreen habitats	Blondel, J. et al. 1999. Science 285:1399–1402.
Warbler finches (birds)	High versus low elevation	Tonnis et al. 2003. Proceedings of the Royal Society of London B 272:819–826.
Crossbills (birds)	Food source (type of conifer)	Benkman, 2003. Evolution 57:1176–1181.
Indigobirds	Different hosts of nest parasites	Payne et al. 2004. Nature 928–931.
Mycorrhizal fungi	Different host species (orchids)	Otero et al. 2004. Proceedings of the Royal Society of London B 271:1799–1806.
Heliconius butterflies	Different habitats and types of warning coloration	Jiggins, C.D. et al. 2001. Nature 411:302–305.

Summary

Speciation is a multifaceted process driven by genetic drift, natural selection, and ecological factors. The evolution of reproductive isolation, both prezygotic and postzygotic, is central to the formation of new species. Genomic studies, particularly in model systems like sticklebacks, reveal that speciation often involves changes at specific loci, supporting the genic view of speciation. Ecological speciation highlights the importance of divergent selection in different environments as a powerful force in generating biodiversity.