BIOL 467 Lecture Notes - Hybrid Zone, Mate Choice, Assortative Mating
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Chapter 16- Mechanisms of Speciation
16.1- Species Concepts
•Species is the smallest evolutionarily independent unit
•Evolutionary independence occurs when mutation, selection, gene flow, and drift operate on
populations separately
•Evolution consists of changes in allele frequencies, and species form a boundary for the spread
of alleles
oAs a result, different species follow different evolutionary trajectories
oThe essence of speciation is lack of gene flow
•Species consist of interbreeding populations that evolve independently of other populations
•The three most important “species concepts”:
oMorphospecies Concept
oBiological Species Concept
oPhylogenetic Species Concept
Each of the three agrees that species are evolutionarily independent units that are
isolated by lack of gene flow, but each employs a different criterion for the
determining that independence is actually in effect
The Morphospecies Concept
•In traditional cultures, people name species based on morphological similarities and differences
•In biology, careful analyses of phenotypic differences are the basis of identifying morphospecies
•Morphospecies can be identified in species that are extinct or living, and in species that
reproduce sexually or asexually
•Fossil species that differed in color or the anatomy of soft tissues cannot be distinguished
•Neither can populations that are similar in morphology but were strongly divergent in traits like
songs, temperature, or drought tolerance, habitat use, or courtship displays
oSpecies like these are called cryptic species
Species that are indistinguishable morphologically, but divergent in songs, calls,
odor, or other traits
The Biological Species Concept
•Under this concept, criterion for identifying evolutionary independence is reproductive isolation
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•If populations of organisms do not hybridize regularly in nature, or if they fail to produce fertile
offspring when they do, then they are reproductively isolated and considered good species
•Great strength of this concept is that reproductive isolation is a meaningful criterion for
identifying species because it confirms lack of gene flow
•Lack of gene flow is the litmus test of evolutionary independence in organisms that reproduce
sexually
The Phylogenetic Species Concept
•Systematists are biologists who are responsible for classifying the diversity of life
•This concept is also known as the genealogical species concept
oThis approach focuses on a criterion for identifying species called monophyly
•Under this concept, species are identified by estimating the phylogeny of closely related
populations and finding the smallest monophyletic groups
oOn a tree like this, species form the tips
•Rationale behind the phylogenetic species concept is that traits can only distinguish populations
on a phylogeny, if the populations have been isolated in terms of gene flow and have diverged
genetically, and possibly morphologically as well
•To be called separate phylogenetic species, populations must have been evolutionarily
independent long enough for the diagnostic traits to have evolved
•Populations within species have shared, derived traits that distinguish them from populations of
other species
•Species are named on the basis of statistically significant differences in the traits used to
estimate the phylogeny
•Recent analyses have found that the phylogenetic species concept often distinguishes a series
of cryptic species in populations that were formerly considered a single species.
Applying Species Concepts: Two Case Histories
•Species can be identified by distinctive morphological traits, reproductive isolation, and/or
phylogenetic independence. Each species concept has advantages and disadvantages.
•Employing more than one species concept can help biologists recognize diversity and organize
research on its consequences
•Read example on Marine Copepods and Elephants on Pages 609-610
16.2- Mechanisms of Genetic Isolation
•Speciation has been hypothesized to be a three-stage process:
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