Speciation Concepts in Biology

Speciation

• Haldane jokingly said the Creator had an "inordinate fondness for beetles" when asked about natural theology
• Figure 9.1 shows diversity of cichlid fish in Lakes Tanganyika and Malawi. Rate of speciation is very high in these environments.
• The biological species concept (BSC) defines species as groups of interbreeding populations reproductively isolated from others. Gene flow is the key, focusing on reproductive isolation more than phenotypic differences. Cryptic species share little phenotypic difference but are reproductively isolated.
• Problems with BSC: asexuals lack gene flow, extinct groups cannot be assessed, and breeding is impractical for many species.
• Other species concepts include phylogenetic (species distinguished by diagnosable differences in morphology or DNA), evolutionary, recognition, and genealogical species concepts
• Evolution, as a process, has intermediate stages and fuzzy boundaries, meaning one unique classification will not work for all situations.

What are species?

• Biological species concept (BSC): Groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups through gene flow that is not happening.
• Focusing on gene flow (Reproductive Isolation), rather than phenotypic differences.
• Cryptic species isolated but with little phenotypic differentiation.

Problems with the BSC

• Asexuals: no gene flow
• Extinct groups: Cannot conduct breeding experiments
• Gene flow between species: How to observe flow of genes
• Difficulty in breeding experiments

Other species concepts

• Phylogenetic Species Concept: Populations are species if they are "diagnosably" different (morphology, DNA).
• Evolutionary, Recognition/cohesion, and Genealogical Concepts

Issues with the single species concept

• The search for a uniform species concept is unattainable given the diverse nature of evolution.

Example of Post-mating, Pre-zygotic Isolation

• Researchers studied Callosobruchus beetles (maculate vs. subinnotatus) to look for post-mating, pre-zygotic isolation.

Extrinsic Isolation

• Different food resources may result in varying evolutionary tracks, affecting post-zygotic isolation for Darwin's finches, for example.

Geographic Isolation of Speciation

• Allopatric speciation: The most common mode of speciation, which involves geographic separation of populations.
  • Populations become isolated and can evolve independently. If the barrier is removed, the two populations may be unable to interbreed or may have already evolved into distinct species through various evolutionary factors.

Further Points

• Sympatric speciation: Speciation within the same geographic region.
• Ecological isolation: Lack of opportunity to mate due to timing, habit, or space.
• Behavioral/sexual isolation: Individuals prefer to mate with members of their own species.
• Reproductive Isolation: Pre-zygotic/post-zygotic: events occurring before/after fertilization, respectively.

Haldane's Rule

• The heterogametic sex is usually affected more severely by hybrid breakdown if the two sexes are different.

Empirical Evidence and Island Endemics

• Empirical findings support the idea that sexual isolation is significant.
• Studies on islands show that sister species are rarely found in sympatry (co-occurrence in the same geographic area).
• Speciation is less likely in small island environments due to limited gene pools/restricted geographic opportunity.

Dobzhansky-Muller Model

• There's a problem with single-locus incompatibilities.
• Species diverge, then mate incompatibilities evolve.