Chapter 15 Mechanisms of Speciation

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Chapter 15Mechanisms of Speciation
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Where are We

• Up to this point we have concentrated on
  evolutionary processes operating within and
  between populations (ie. population genetics)
• Microevolution
• Now we are moving towards more coarse-level
  evolutionary processes operating on larger
  temporal, geographical and phylogenetic scales
• Macroevolution

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Species

• Species form the link between Microevolution and
  Macroevolution
• Processes of Speciation include elements from
  fine scale Microevolution as well as larger
  Macroevolutionary processes

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What is a species?

• Difficult concept to put boundaries on We find
  variation within groups and between groups
• We find phenotypes that are plasticwe find
  genetic differences that do not correspond to any
  visible phenotypic differences

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What is a species?

• The difficulty in defining and identifying
  species points to the gray area that exists
  between species (ie, intermediates, variants,
  races, subspecies exist)
• May be the best evidence that each species was
  not specially created
• Check outwww.pbs.org/wgbh/evolution/library/05/i
  ndex.html

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What is a species?

• Species are a group of interbreeding populations
• All groups of individuals that send genetic
  migrants back and forth belong to a single
  species
• Species have independent evolutionary
  trajectories
• A species evolves without the being influenced by
  gene flow from other groups of individuals
• Species are the fundamental unit of evolutionary
  change

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Typological or Essential Notion of Species

• Individuals were members of a given species if
  they conformed to that type or ideal, in
  certain characters that were essential fixed
  properties
• This forms the idea of a type-specimen or
  type-series for defining a species.
• Essentially all described species in science are
  based on a TYPE

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Typological or Essential Notion of Species

• Basically stems from Linnaeus
• Formally discussed by Mayr
• Relies heavily on fixed (not polymorphic)
  morphological differences unique to a species
• Yet, still required common decent or decent from
  a common ancestor (monophyly!)

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Species concepts

• There are something on the order of 32 recognized
  species conceptsEach seeks to define groups that
  cannot (or do not) talk to each other
  genetically
• Well limit our discussion to 4 basic ideas of
  species delineationBiologicalMorphologicalPhylo
  genetic
• Evolutionary

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Morphological (aka Taxonomic) Species Concept

• Morphological (aka Taxonomic) Species Concept
• Species are defined by differences in characters,
  especially morphologyWorkable for all kinds of
  organisms, extinct or extant

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Morphological (aka Taxonomic) Species Concept
Major Difficulties

• Polymorphism - genetic polymorphisms, sexual
  dimorphism, social castes

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Morphological (aka Taxonomic) Species Concept
Major Difficulties

• Geographic variation - a species may appear
  different in different parts of its range due to
  genes or environment
• Two populations of Thamnophis sirtalis

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Morphological (aka Taxonomic) Species Concept
Major Difficulties

• Cryptic species - genetically distinct units
  might have no outward differences
• Hyla chrysoscelis (4n) and H. versicolor (2n)

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Biological Species Concept

• A group of interbreeding (or potentially
  interbreeding) populations that is reproductively
  isolated from other such groups
• If individuals from two separate populations
  cannot produce fertile offspring they are said to
  be two different species
• A current favorite among biologists, the legal
  definition of species by the USA, CITES, and the
  ESA

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Biological Species ConceptMajor Difficulties

• Fossils and extinct organisms?
• Asexuals?
• Geographically separated groups?
• How much interbreeding is too much?
• What is potentially interbreeding?

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Fem. lion X Male tiger
Male lion X Fem. tiger
zebroid
zonkey
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Evolutionary Species Concept (Simpson, modified
by Wiley)

• "A single lineage of ancestor-descendent
  populations which maintains its identity from
  other such lineages and which has its own
  evolutionary tendencies and historical fate"
• Four logical corollaries
• All organisms, past and present, belong to some
  evolutionary species
• Species must be reproductively isolated from each
  other to the extent that this is required to
  maintain their separate identities, tendencies,
  and fate.
• Evolutionary species may or may not exhibit
  recognizable phenetic differences ( of species
  may be over or under-estimated, usually the
  latter)
• No separate, single evolutionary lineage may be
  subdivided into a series of ancestral and
  descendent species.
• Benefits clear conceptually applies to asexual
  species, through time, and allopatric secies
• Problems application (problem to know the
  future) asexual species (too many independent
  lineages)

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Phylogenetic Species Concept

• A species is the smallest, irreducible cluster of
  organisms that contains all known descendants
  from a common ancestor ? the smallest
  monophyletic group
• Identifiable through phylogenetic reconstruction
  ? any branch is a species defines species as
  evolutionarily meaningful units

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Phylogenetic Species ConceptMajor Difficulty

• Unclear what scale differences between species
  should be separate from variation within a
  species, ie, how much genetic difference is
  enough to consider two groups separate species?

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Restricted Gene Exchange

• Narrow Hybrid Zones genetically distinct pops.
  meet interbreed to a limited extent
  (semispecies, superspeciesWe dont like these
  terms!)
• Sympatric Hybridization some hybridization
  occurs w/in sympatry but only occasionally
• Geographic Variation Genetically distinct pops.
  appear conspecific in certain areas

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Ring Species

• A chain of interbreeding pops. that loop around
  an uninhabited area so that terminal pops. are so
  different when they meet, they cannot interbreed!
• eg. Ensatina salamanders

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Ring Species Ensatina
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Allopatric Populations

• This is one of the main limitations of the
  Biological Species Concept!
• The BSC. uses potentially interbreeding.
• What if populations NEVER come into contact but
  may be able to interbreed in captivity (eg.
  Mountain top species, species that live in patchy
  habitats )

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Example 1 Montane Pitvipers

• Live at high elevation (above 2500M) in Middle
  America
• Most populations are isolated on these mountain
  tops
• Expect restricted gene flow between populations

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Phylogeny of Cerrophidion (Montane Pitvipers)
Cerrophidion godmani
Costa Rica
C. petlacalensis
Veracruz, Mexico
C. tzotzlilorum
Chiapas, Mexico
C. godmani
Oaxaca, Mexico Guatemala
C. godmani
Huehuetenango, Guatemala
C. godmani
N.W. and Interior Guatemala
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C. petlacalensis
C. tzotzliroum
Montane Pitvipers Cerrophidion
C. godmani
C. godmani
C. godmani
C. godmani
HOW MANY SPECIES????
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Side Note Gene VS. Species Trees

• The previous examples were based on one or two
  gene sequences
• We used these to infer the species tree or
  phylogeny
• We made the assumption that the Species Tree
  Gene Tree
• This assumption is based on the assumption that
  the genes we examined evolved with the species
  over time

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Allopatric Populations

• The BSC requires gene flow between populations of
  different species to be prevented
• What are the mechanisms which may prevent gene
  flow and promote speciation
• Well. Allopatry is one, what are the rest?

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Reproductive Barriers 2 Major Categories

• Prezygotic barriers prevents the formation of
  hybrid zygotes
• Postzygotic barriers reduced fitness of hybrid
  zygotes

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Prezygotic Isolation - I

• Temporal Isolation different mating times
• Habitat or Resource Isolation isolation by
  using different habitats, different host plants,
  etc..
• Ethological (Behavioral) Isolation very
  important, courting behavior, seasonal timing of
  mating, species specific recognition, chemical
  recognition of species..

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Prezygotic Isolation - II

• Mechanical (or physical)
  Isolation mainly in plants, yet
  lizards and snakes also show this
  (Hemipenes ? lock and key)
• Gametic Incompatability External
  fertilization Marine Inverts cast sperm eggs
  into the water column (Lysin protein dissolves a
  hole only in egg of same species)

Atractus ventrimaculatus hemipenis
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Hemipenes of snakes (Atractus)
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Postzygotic Isolation

• Hybrid Inviability hybrids fitness ZERO!
• Hybrid Sterility again No fitness conferred
  to hybrids!

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Hybridization

• Occurs when offspring are produced by
  interbreeding between genetically different
  populations.
• Opportunity to study the interactions between
  selection and gene flow in nature

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Hybrid zones

• A hybrid zone is a region where genetically
  distinct populations meet and mate, resulting in
  at least some offspring of mixed ancestry
• Primary
• Secondary

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Hybrid Zones

• Primary Hybrid zone originate in situ as natural
  selection alters allele frequencies in a series
  of more or less continuously distributed
  populations.
• Thus the position of the zone is likely to
  correspond to a sharp change in one or more
  environmental factors

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Hybrid zones

• Secondary hybrid zones are formed when two
  formerly allopatric populations that have become
  genetically differentiated expand so that they
  meet and interbreed (secondary contact).
• Thus, the position of the zone does not
  correspond to sharp changes in env.

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Hybrid Zones

• In primary zones we would expect the zone to
  occupy areas of pronounced ecological change.
• Selection would act only on the differences
  stemming from the environmental gradient.

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Primary Hybrid Zone
Expected pattern of change in the allele freq.
along an environmental gradient
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Hybrid zone

• In contrast, secondary zones will occur wherever
  expanding populations meet, and this does not
  have to be at an ecological border.
• All characters should show a coincident cline.

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Secondary Hybrid Zone
Expected pattern of change in the allele freq.
from secondary contact.
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Hybrid Zones

• The width of the cline will depend on the
  strength of selection, relative to gene flow.

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Hybrid Zones

• Dispersal, selection, and linkage all affect the
  distribution of alleles in a hybrid zone.
• Zone of secondary contact
• If the F1 and backcrosses are partially fertile
  alleles are going to move.
• Rate of gene flow and dispersal distance (s)
  broadens the zone
• Selection narrows the cline

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Hybrid Zones

• The width of a cline of a locus or character in a
  hybrid zone varies, locus to locus, depending on
  the strength of selection on that locus or on
  linked loci.
• Thus even if the fitness of hybrids is quite low,
  alleles at some loci may penetrate far beyond the
  hybrid zone that other loci display
  (introgression)

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Hybrid Zones

• Neutral alleles will show a steeper cline than
  expected if they are linked to a locus that is
  subject to selection

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Hybrid Zones

• Many zones are concordant clines for allozymes
  and other characters.
• Estimate that 37 of the hybrid zones seen in
  North America formed since the last glaciations
• Most zones between pairs of taxa are seen in the
  same place suggesting a common history.

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Hybrid Zones

• In North America, quite often, the hybrid zones
  between pairs of taxa are located in the region
  suggesting a commonality in their evolution.
• This phenomenon is called a suture zone.
• The great Plains region is a major suture zone.
• Also, so is the continental divide (across the
  axis of the Rocky Mts

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EXAMPLE 2Western Diamondback Rattlesnakes

• Lives at lower elevations in hot-dry desert and
  grassland areas across the US and northern Mexico
• No obvious drastic boundaries across its range
  that isolate populations Just a huge range!
• High degree of morphological polymorphism across
  its range

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South-Central Mexico (San Luis Potosi)
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S-Eastern Arizona
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Central Texas
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East-Central Mexico (Veracruz)
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PROPOSED BIOGEOGRAPHIC REGIONS
Colorado Desert Sonoran
Desert Chihuahuan Desert/ Trans
Pecos Region Southern Plains/
Ozarks Coastal Plains
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Is Crotalus atrox monophyletic...??
Crotalus tortugensis
Crotalus atrox
Crotalus atrox
Crotalus ruber
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ML (and Bayesian) Phylogeny for all 50 C. atrox
specimens plus outgoups mt ND4 gene (850bp)
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C. tortugensis
GENE FLOW Between Desert Regions!!!
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Only a single hybrid known Common only along
Platte River in central Nebraska hybrids rare or
unknown elsewhere well studied in only a few
locales
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Hybrid Zones

• Break down in co-adapted gene complexes or
  genomic disruption
• Generally hybrids are less fit, get parasites
  easier etc.
• However in rare cases hybrids have high fitness.

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Fate of hybrid zones

• Hybrid zones may persist indefinitely, with
  selection maintaining steep clines at some loci.
• If zone is a tension zone it may move around
• Section may favor alleles that enhance prezygotic
  isolation, resulting ultimately in full
  reproductive isolation

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Fate of hybrid zones

• Alleles that improve the fitness of hybrids may
  increase in frequency. In the extreme, the
  postzygotic barriers to gene exchange would break
  down, and two populations would become one
  species.
• In at least part of the hybrid zone, the hybrids
  themselves could evolve reproductive isolation
  from the parent forms, and become a 3rd species.

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Speciation
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The History of Life

• We have been considering how the four
  evolutionary forces affect populations
• Mutation, natural selection, genetic drift,
  migration
• Now we will examine how these forces lead to the
  formation of species and higher taxa? How and
  why lineages diversify through time

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The History of Life

• Darwins The Origin of Species
• It is very important to remember that the same
  forces that cause microevolution also cause
  macroevolution
• The mechanisms are identical, only the scale is
  greater

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Most Basic Debated Question

• Is speciation an adaptive process?
• No doubt different sp. occupy different niches
  and have different coadapted genetic traits
• But, this does not necessarily mean that
  speciation is inherently adaptive whereby Natural
  Selection builds isolating mechanisms because
  they achieve these results

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Speciation

• Reproductive isolation evolves as a by-product of
  genetic changes that occur for other reasons, so
  speciation is incidental, non-adaptive
  consequence of divergence between populations.

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How Does Speciation Occur?

• Speciation may be viewed as a three-step process
• Population isolation
• Divergence of phenotypes and genotypes
• Reproductive isolation
• We will examine each step to understand how
  speciation progresses

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Modes of speciation (how species are isolated)

• Two basic groups, along with some special cases
  that dont fit neatly into the paradigm
• Those that rely on geographic isolation
  (Allopatric)
• Those that rely on selection (Sympatric and
  Parapatric)
• Also Speciation through hybridization Speciatio
  n through chromosomal changes (or polyploidy
  events)

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Allopatric Speciation

• Most Common Mode of Speciation
• Thousands of examples known
• Evolution of reproductive barriers between
  populations that are separated by a geographic
  barrier from exchanging genes at more than a
  negligible rate

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Allopatric Speciation

• Physical Isolation
• Migration tends to homogenize genotypes
• When gene flow is stopped, speciation may occur
• Migration-selection balance of water snakes in
  Lake Erie
• Populations did not diverge into banded and plain
  forms because migration kept gene flow active
• If snakes were prevented from immigrating to the
  island, the two forms might diverge into separate
  species by selection, mutation, and drift

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Allopatric Speciation

• Physical Isolation
• Speciation may occur by Vicariance
• A new barrier to gene flow arises
• Mountain range or river
• Or Dispersal (also called Peripatric)
• Individuals migrate to new area and a separated
  from their source population, (OR) individuals on
  the edge of parent species range become isolated

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Allopatry Via Dispersal

• Geographic isolation through dispersal
• 850 species of Hawaiian Drosophila
• Exploit a vast number of niches and have highly
  varied body forms
• Founder Hypothesis of speciation
• Hawaiian Islands have varied ages and they arose
  in a west to east sequence
• Founder flies inhabited western island and
  emigrated eastward as other islands arose

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Allopatry Via Dispersal

• Geographic isolation through dispersal
• Test founder hypothesis with geology of Hawaiian
  Islands and phylogeny of flies
• Predictions of founder hypothesis
• Closely related species will occupy adjacent
  islands
• Branching sequence should partially coincide with
  island emergence sequence
• Used mtDNA to estimate phylogeny
• Founder hypothesis was supported
• (basically, same pattern seen in Galapagos
  tortoises)

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Direction of Colonization
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Allopatry Via Vicariance

• Geographic isolation through vicariance
• Ranges of marine organisms were split when the
  Central American Isthmus closed 3 Mya
• Did this vicariant event cause the speciation
  patterns seen today in snapping shrimp?
• 7 pairs of morphospecies separated by Isthmus

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Allopatry Via Vicariance

• Geographic isolation through vicariance
• A mtDNA phylogeny demonstrates that formation of
  Central America did appear to split species in
  two
• Also revealed Cryptic Species
• Species that cannot be distinguished by
  morphology
• Species were not all the same age
• Isthmus arose disjunctly over a long time period

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Snapping Shrimp
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P Pacific C Carribian
Sister species cut by Panamanian land bridge
formation Pacific and Caribbean sister species
cannot produce viable offspring!
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Other Mechanisms of Isolation

• Changes in Chromosome Number
• Polyploidization can create barriers to gene flow
  that are not geographic
• Very important mechanism of isolation in plants
• 43-70 of extant plant species may have arisen by
  polyploidization
• Much less common in animals but occasionally
  important in barrier creation
• Sometimes smaller scale chromosomal changes can
  cause speciation as well

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What Can Cause Divergence?

• Mutation? - maybe, but we know its incredibly
  slow
• Drift? - SURE, especially when populations are
  small
• Inbreeding? - Yes for genotypes, but only causes
  gene frequency change when coupled with drift or
  selection
• Selection? - Of course, especially if environment
  differs between two populations
• Migration? - NO WAY! Gene flow causes populations
  to converge, not diverge. It will negate other
  differentiating forces (drift and selection).
  Populations must be isolated from one another to
  undergo independent evolution.

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What causes divergence?

• NOTE isolation by itself is not enough to cause
  differentiation, other factors must be acting to
  cause populations to diverge

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Mechanisms of Divergence

• After a barrier has been formed, genetic drift
  and natural selection act on mutations to cause
  divergence in isolated populations
• Drift is most important in small populations
• Most species are formed from few individuals
• Drift may rapidly cause divergence because of
  bottleneck effects

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The Role of Drift

• This is very difficult to determine because it is
  tough to tease apart the magnitude of drifts
  impact
• However, it is definitely a strong force in
  founder effect speciation and other situations
  where Ne is small

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The Role of Selection

• Widely held view that vicariant speciation is
  caused by Natural Selection resulting in pre-
  post zygotic incompatibility
• This occurs while the taxa are allopatric
• Thus, both pre- and post zygotic isolating
  mechanisms are by-products of divergent selection
  that occurs during allopatry

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Categories of Selection

• Ecological Selection side effect of adaptive
  divergence
• Sexual Selection variation in reproductive
  success due to variation in the ability to
  acquire mates

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Ecological Selection

• Almost no evidence but probably is significant
• Requires the genes underlying the adaptive
  differences either have pleiotropic effects
  resulting in reproductive isolation or be closely
  linked to those genes responsible for isolation

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Ecological Selection

• Little known about genes that control pre post
  zygotic isolation
• Sperm lysin genes of marine inverts. prezygotic
  isolation results from constant change in female
  egg proteins which is chased by evolution of
  male lysin proteins
• Ground finch (Geospiza) different beak sizes
  will court different species with wrong head

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Evidence for Ecological Selection ? Parallel
Speciation

• Independent evolution of the same
  reproductively isolated forms in different
  localities suggests that ecological selection
  can result in speciation
• Sticklebacks

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Parallel Speciation Sticklebacks
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Sexual Selection

• Arises from competition among individuals of one
  sex (usually males) for access to the other sex,
  or from the preferences of one sex
• The males secondary sex characteristics
  frequently play a role in both contexts
• Females choose among conspecific males on the
  basis of the state of one or more features in the
  courtship display
• Generally the female prefers the male with more
  elaborate stimuli or traits

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Sexual Selection WHY?

• Females have a fixed maximum fecundity
• Males fecundity essentially based on number of
  mates only
• So, variance in male fitness is always gt female
  fitness

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Sexual Selection

• Why females have preference is controversial
• Runaway selection self reinforcing
• Preference is adaptive Male traits are
  correlated with fitness
• Females prefer traits that will produce more
  competitive sons
• Perceive louder, brighter, etc. against
  background noise
• Females have sensory biases towards certain
  specific stimuli

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Sexual Selection

• The weight of evidence is that sexual selection
  is a major source of pre zygotic isolation
• Sexually selected characters act as a barrier to
  interbreeding (e.g. Physalaemus call preferred
  only intraspecifically)
• High species richness of many taxa in which
  sexual selection appears to be diverse and
  intense

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Sexual Selection

• Many species that are genetically similar and
  have diversified rapidly
• These show females which look very similar yet
  males which look VERY different!!
• Suggests strong female mate choice powering
  speciation (e.g. sea lions!)
• Study by Barroclaugh found the clade with
  sexual dimorphism contains more species

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Sexual Selection

• Hawaiian Drosophila have elaborate courtship
  behaviors and secondary sexual characteristics
• Males meet in leks and fight to attract females
• Males have bizarre head shapes to help them win
  fights

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Sexual Selection

• D. heteroneura have hammerhead and butt heads
• during fights
• D. silvestris have heads just like females and
  grapple during fights
• Evolutionary scenario
• Ancestor of both species had normal heads
• Mutation occurred to cause head-butting behavior
• Mutants won more contests
• Mutation increased to fixation in a
  sub-population
• Additional mutations caused change in head shape
  for better fighting

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Peripatric Speciation

• The process by which drift effects initial
  changes in allele frequencies
• Then selection may act to further diverge colony
  from parent population

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Peripatric Speciation

• How does this differ from Vicariant Allopatry?
• Typically, most allopatric events are thought of
  as a major split leaving two large populations on
  either side, whereas a Peripatric division
  typically results in the nearly intact parent
  pop. and a small founder population (thus, drift
  is very powerful in the founder)

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Peripatric Speciation Remember...

• Genetic change can be very rapid in localized
  populations founded by few individuals cut off
  from gene flow
• Differences in ecological selection can play a
  role because the environment of a small area is
  often more homogenous than the larger area
• So... conflicting pressures that act on a
  widespread species may be less numerous and more
  directed at certain adaptations

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Peripatric Speciation Remember...

• Allele frequencies at certain loci will be
  different from the parent population due to drift
  and founder effects
• Changes in allele freq. could create genotypes
  that have higher fitness that Natural Selection
  may select for or against
• Many species have arisen by peripatric speciation
• eg. Peromysus maniculatus

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Peripatric Example Peromysus maniculatus
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ParapatricSpeciation

• Evolution of reproductive isolation between
  populations that are continuously distributed in
  space, so that there is substantial movement of
  individuals and genes between them

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Parapatric Speciation

• NOTE The observation that two sister species
  have parapatric (or sympatric) ranges does not
  mean that they have speciated parapatrically or
  sympatrically! ? this may instead be allopatry
  followed by secondary contact of ranges once
  distinct

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Parapatric Speciation

• Divergent selection GREATER than gene flow over
  time
• Generally a hybrid zone may be formed
• The degree of selection and gene flow depend on
  the size of the character clines

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Secondary Contact

• If populations come back into contact after steps
  1 and 2, have opportunity to interbreed
• Hybridization is common in plants and birds
• Will hybrids be viable and fertile?
• Will hybrids have characteristics of parent
  species or new characteristics?
• Depends on outcome of speciation event

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Secondary Contact

• Theodosius Dobzhansky formulated the
  reinforcement hypothesis about the third stage of
  speciation
• If populations sufficiently diverged in
  allopatry, hybrids should have reduced fitness
• Should be strong selection for assortive mating
• Selection that reduces fitness of hybrids is
  Reinforcement
• Completes reproductive isolation

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Secondary Contact

• Reinforcement Hypothesis
• Predicts that pre-zygotic isolation will evolve
  in species in secondary contact
• Prezygotic isolating mechanisms prevent
  fertilization
• Mate choice, time of breeding, genetic
  incompatibility
• Postzygotic isolation
• Offspring are sterile
• Reinforcement not necessary

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Secondary Contact

• Study by Coyne and Orr showed that prezygotic
  isolation occurs more often in sympatric than
  allopatric species

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Secondary Contact

• Hybridization
• Hybrids should have reduced fitness by
  reinforcement hypothesis
• Some have normal or increased fitness
• Sorghum is an important crop
• Johnsongrass is closely related weed
• Agriculturalists worried that if they genetically
  engineered herbicide resistance into sorghum it
  might be transferred to johnsongrass

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Secondary Contact

• Hybridization
• Significant gene flow occurred and herbicide
  resistance introgressed into Johnson grass
• If hybrids have increased fitness, will they form
  their own species?
• Biologists attempted to recreate the
  hybridization event that lead to the formation of
  Helianthus anomalous
• Found that certain crosses had higher fitness and
  certain had low
• High fitness hybrids became new species

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Secondary Contact

• Hybridization
• Hybrid zones occur where recently diverged
  populations overlap
• May occur by secondary contact or during
  parapatric (or peripatric) speciation
• Hybrid zones are often present where hybrids have
  equal fitness to parental species
• Size of hybrid zone depends on fitness of hybrids

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Secondary Contact

• Hybridization
• Study on sagebrush in western US
• Basin sagebrush found at low elevations
• Mountain sagebrush found at high elevations
• Species make contact at middle elevations and
  hybridize
• Graham compared several fitness measures of
  hybrids and parentals
• Found that hybrids have superior fitness in
  transitional habitats

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SympatricSpeciation

• Among the most controversial subjects in
  evolutionary biology
• Complete panmictic mating and a reproductive
  isolating mechanism evolves within the population

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Sympatric Speciation Possible Mechanisms

• Polyploidy
• Disruptive selection, whereby certain homozygous
  genotypes have high fitness on one or the other
  of two resources intermediate (heterozygotes)
  has low fitness

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Sympatric Speciation Polyploidism
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Hybridization in Helianthus
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Sympatric Speciation Disruptive Selection

• Selection may favor alleles in homozygous
  condition and you get assortive mating
• But, antagonism likely to arise from alternative
  selection which promotes association between
  alleles for adaptation and alleles for assortive
  mating, and recombination which destroy this
  association
• This is the greatest opponent to the process of
  Sympatric Speciation
• In such a model strong selection and tight
  linkage are required... BUT recombination will
  break this down!

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Sympatric Speciation Disruptive Selection

• Another similar model was proposed by Bush based
  on his work on the apple maggot flies
• Rhagoletis economically important pest which
  are parasites on the fruits of trees in the
  hawthorn clade
• Rhagoletis ID host by sight, tough and smell
• Courtship and mating occur on the fruit
• Females lay eggs in fruit on the tree
• eggs hatch in 2 days and develop after fruits
  fall to ground
• insects burrow into ground, overwinter, and
  emurge next spring

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Mechanisms of Divergence

• Apple and hawthorn maggot flies
• Are they different populations?
• Live in sympatry on adjacent trees
• Recently diverged because apples are not native
  to US
• An allozyme study revealed they do form distinct
  populations
• How have they diverged without initial isolation?
• Did they skip Step 1?

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Mechanisms of Divergence

• Apple and hawthorn maggot flies
• There are other mechanisms for speciation besides
  allopatric
• Through assortive mating there is only 6 gene
  flow among populations
• Separated in time by pupating at different times
  of year
• They are able to maintain distinct populations
  even with gene flow because of strong natural
  selection