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Chapter 16

CHM101H1 Chapter Notes - Chapter 16: Assortative Mating, Zygosity, Hybrid Zone

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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
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
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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