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SPECIES AND SPECIATION

SPECIES AND SPECIATION. Two major categories of evolutionary change: Evolutionary change within lineages over time (anagenesis). Splitting of lineages over time (cladogenesis). MAJOR ISSUES IN SPECIATION THEORY. What is a species? The role of geographic isolation in divergence.

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SPECIES AND SPECIATION

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  1. SPECIES AND SPECIATION • Two major categories of evolutionary change: • Evolutionary change within lineages over time (anagenesis). • Splitting of lineages over time (cladogenesis)

  2. MAJOR ISSUES IN SPECIATION THEORY • What is a species? • The role ofgeographic isolation in divergence. • Do population bottlenecks facilitate changes in genetic architecture? • The role of natural selection in speciation. • Prezygotic vs. postzygotic reproductive isolation. • The genetic mechanisms responsible for reproductive isolation.

  3. SPECIATION • The process by which one genetically-cohesive population splits into two or more reproductively-isolated populations. • Requires the disruption of gene flow and the evolution of reproductive isolating mechanisms (RIMs).

  4. WHAT IS A SPECIES???

  5. DECIDING WHAT CONSTITUTES A SPECIES CAN BE COMPLEX From Doyen and Slobodchikoff, 1974, Systematic Zoology 23:239-247

  6. Certainly no clear line of demarcation has yet been drawn between species and sub-species – that is, the forms which…come very near to, but do not quite arrive at, the rank of species. …A well-marked variety may therefore be called an incipient species. …From these remarks it will be seen that I look at the term species as one arbitrarily given. Darwin, TheOrigin of Species

  7. THE BIOLOGICAL SPECIES CONCEPT “Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups.” E. Mayr (1942)

  8. PROBLEMS WITH THE BIOLOGICAL SPECIES CONCEPT • Difficult to apply to fossils • Asexual organisms don’t fit the criteria

  9. THE DIFFICULTY OF RECOGNIZING SPECIES • For sympatric species, it usually is clear -- If they do not interbreed, then they are good species. • For allopatric populations, it is less clear -- It may be difficult assess whether they are “potentially interbreeding”. • It may also be difficult to decide what constitutes reproductive isolation. • - If individuals from different populations produce sterile hybrids, then they are certainly reproductively isolated. • - What if they produce partially sterile hybrids?

  10. OTHER SPECIES CONCEPTS • PHYLOGENETIC SPECIES CONCEPT: An irreducible cluster of organisms that is diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent. • EVOLUTIONARY SPECIES CONCEPT: a species is a single lineage of populations or organisms that maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate.

  11. STRENGTHS AND WEAKNESSES: EVOLUTIONARY SPECIES CONCEPT • STRENGTHS • It describes the essence of what we mean when we talk about species. • In principle it could be used for almost any organismal group. • WEAKNESS • In practice, it is nebulous and impossible to apply.

  12. STRENGTHS AND WEAKNESSES: PHYLOGENETIC SPECIES CONCEPT • STRENGTHS • It contains a historical component. • It provides specific criteria that can be diagnosed in natural populations. • WEAKNESS • It is somewhat vague. What exactly is meant by “irreducible cluster” and “diagnosably different”? • Taken literally and to the extreme, it is absurd.

  13. STRENGTHS AND WEAKNESSES: PHYLOGENETIC SPECIES CONCEPT RECENT DEBATE: Pro: Species Concepts and Phylogenetic Theory by Wheeler and Meier (2000) Con: “Cladists in wonderland” by Avise in Evolution 54:1828-1832 (2000). • Bottom line: • The Biological Species concept is the major species concept used by modern biologists in practice. • However, an increasingly large group (of cladists) is applying the phylogenetic species concept (or derivations of it), particularly in regard to conservation genetics.

  14. KEY FEATURES OF OPERATIONAL SPECIES CONCEPTS • Reproductive cohesion within species. • Reproductive isolation from other such groups. • Recognition that species are dynamic evolutionary lineages, not static “types”.

  15. U. S. ENDANGERED SPECIES ACT • Legal Definition: Species includes: “any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature.” • Operational Definition: Evolutionary Significant Units (ESU): “a population (or group of populations) that • 1) is reproductively isolated from other conspecific population units, and • 2) represents an important component in the evolutionary legacy of the species.”

  16. OPERATIONAL SPECIES CONCEPTS PROVIDE A MEANS TO PROTECT: • Individual populations • Threatened portions of species ranges • Ecologically distinct populations Example: Separate runs of salmon

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