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Lecture

BIOL 467 Lecture Notes - Hybrid Zone, Mate Choice, Assortative Mating

7 Pages
133 Views
Winter 2013

Department
Biology (Sci)
Course Code
BIOL 467
Professor
Imad Mansour

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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
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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Description
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 o As a result, different species follow different evolutionary trajectories o The 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”: o Morphospecies Concept o Biological Species Concept o Phylogenetic 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 o Species 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 • 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 o This 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 o On 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: o Initial step that isolates populations o Second step that results in divergence in traits such as mating system or habitat use o Final step that produces reproductive isolation • Isolation and divergence steps were thought to take place over time and to occur while populations were located in different geographic areas • Final phase was hypothesized to occur when these diverged populations came back into physical contact- an event known as secondary contact o Secondary contact- when two populations that have diverged in isolation from a common ancestor are reunited geographically • It is now clear that, isolation and divergence steps that initiate speciation frequently take place at the same time and in the same place o Also it appears likely that in a significant number of speciation events or even a majority, the third phase never occurs • Isolation/divergence/secondary contact hypothesis provides a useful framework for analyzing how speciation takes place • First step in speciation is genetic isolation • Physical separation or changes in chromosome complements can reduce gene flow between populations • Once gene flow is dramatically reduced or ceases, evolutionary independence begins and speciation is underway • Therefore, speciation process begins when gene flow is disrupted and populations become genetically isolated Physical Isolation as a Barrier to Gene Flow • Geographic isolation produces reproductive isolation, and thus genetic isolation • Allopatric Model- the hypothesis that speciation occurs when populations become geographically isolated and diverge because selection and drift act on them independently • Essence of allopatric speciation is that physical isolation creates an effective barrier to gene flow • Geographic isolation has been an important trigger for the second stage in the speciation process: genetic & ecological divergence • Geographic isolation can come about through dispersal and colonization of new habitats or through vicariance events, where an existing range is split by a new physical barrier • Dispersal is when a population in one area splits into two and one moves to another isolated area like from mainland to an island • Vicariance is when a large area is populated and some natural cause occurs and divides the area into two separate lands, dividing one from the other by nature • Look at page 612 for better understanding. • The dispersal-and-colonization hypothesis makes two predictions based on these facts: o Closely related species should almost always be found on adjacent islands o At least some sequences of branching events should correspond to the sequence in which islands were formed • Dispersal to novel environments has proven to be a general mechanism for initiating speciation • Populations can become geographically isolated when individuals colonize a new habitat • Vicariance events split a species’ distribution into two or more i
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