Northwestern Garter Snake
Transcript of Northwestern Garter Snake
I. What is a species?I. What is a species?
II. Isolating MechanismsII. Isolating Mechanisms
III. Models of SpeciationIII. Models of Speciation
SpeciationSpeciation
Species-a group that shares specific properties
Genus-a group that shares generic properties
Aristotle coined the term species.Aristotle coined the term species.
AristotleAristotle(384(384--322 BC)322 BC)
Linnaean SystemLinnaean Systemof Classificationof Classification
Carl von LinnCarl von LinnJJa.k.a. Linnaeusa.k.a. Linnaeus
(1707(1707--1778)1778)
Definitions of SpeciesDefinitions of Species
•• Morphological speciesMorphological species
•• Biological speciesBiological species
•• Evolutionary speciesEvolutionary species
A species is a group of individuals that are structurally similar.
Morphological SpeciesMorphological Species
Two species Two species or one?or one?
A species is a group of individuals that are structurally similar.
Morphological SpeciesMorphological Species
Two species Two species or one?or one?
♂ ♀
Cryptic species of parasitic fliesCryptic species of parasitic flies((ApocephalusApocephalus paraponeraeparaponerae))
Some flies feed and lay eggs in an injured worker ant. Larvae develop inside the ant and consume their host.
Species of flies are separated by the ability to use specific species of ants.
Distantly related flies have converged on the same species of ants. The two species of flies are genetically distinct, but phenotypically indistinguishable.
A species is a group of potentially interbreeding individuals.
Note: Not all individuals have to interbreed, but they must have the potential to do so.
[Source: [Source: MayrMayr 1942, 1942, Systematics and the Origin of SpeciesSystematics and the Origin of Species, Columbia University Press], Columbia University Press]
Biological SpeciesBiological Species
Ernst Ernst MayrMayr19271927
A species is a group of potentially interbreeding individuals.
But what about asexual organisms?
Biological Species ConceptBiological Species Concept
Oat aphid (Rhopalosiphum padi)
A species is a lineage that maintains its integrity A species is a lineage that maintains its integrity through space and timethrough space and time..
[Source: [Source: WiensWiens 2004, 2004, The American NaturalistThe American Naturalist 163: 914163: 914--923]923]
Evolutionary Species ConceptEvolutionary Species Concept
A BA B
AncestorAncestor
Sympatry-two groups do occur in the same place
Allopatry-two groups don’t occur in the same place
Speciation depends on ecological distributions.Speciation depends on ecological distributions.
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Evolutionary and Biological species concepts
Allopatric speciation Sympatric speciation
evolution of morphological
differences
evolution of reproductive
isolation
Morphological speciesBiological species
Evolutionary species
geographic isolation
Morphological speciesBiological species
Evolutionary species
Allopatric speciation Sympatric speciation
evolution of morphological
differences
evolution of reproductive
isolation
geographic isolation
evolution of reproductive
isolation
Evolutionary & Biological species
Isolating MechanismsIsolating Mechanisms
•• SpatialSpatial•• TemporalTemporal•• BehavioralBehavioral•• MechanicalMechanical•• GameticGametic
Spider Spider wortwort((TradescantiaTradescantia spp.)spp.)
TradescantiaTradescantia canaliculatacanaliculata--full sun on edge of cliffsfull sun on edge of cliffs--flowers in springflowers in spring
TradescantiaTradescantia subaspersasubaspersa--full shade of deep forestfull shade of deep forest--flowers in summerflowers in summer
Both spatial and Both spatial and temporal isolation temporal isolation prevent interbreeding.prevent interbreeding.
Spatial & Temporal Isolating MechanismsSpatial & Temporal Isolating Mechanisms
Fruit flies courtingFruit flies courting
In In DrosophilaDrosophila, a female , a female attracts a male with attracts a male with chemicals on the surface of chemicals on the surface of her body. her body.
The chemicals on females The chemicals on females of of Drosophila melanogaster Drosophila melanogaster are unattractive to males of are unattractive to males of Drosophila Drosophila simulanssimulans (and (and vice versa).vice versa).
Behavioral Isolating MechanismsBehavioral Isolating Mechanisms
Mechanical Isolating MechanismsMechanical Isolating Mechanisms
Salvia mellifera
Salvia apiana
A) Only large pollinators can pick up pollen from the anthers of S. apiana.
B) Only small pollinators can land on the flower of S. mellifera.
AB
Pollination of Sage
GameticGametic Isolating MechanismsIsolating Mechanisms
Fruit flies matingFruit flies mating
Sperm or egg is destroyed, Sperm or egg is destroyed, or sperm cannot fertilize or sperm cannot fertilize eggs. eggs.
For example, when a male For example, when a male fly inseminates a female of fly inseminates a female of another species, her vagina another species, her vagina swells and prevents swells and prevents fertilization. fertilization.
Isolating MechanismsIsolating Mechanisms
•• SpatialSpatial•• TemporalTemporal•• BehavioralBehavioral•• MechanicalMechanical•• GameticGametic
PrePre--zygoticzygotic
•• SpatialSpatial•• TemporalTemporal•• BehavioralBehavioral•• MechanicalMechanical•• GameticGametic
•• Hybrid Hybrid InviabilityInviability•• Hybrid SterilityHybrid Sterility
Isolating MechanismsIsolating Mechanisms
PrePre--zygoticzygotic
PostPost--zygoticzygotic
Hybrid Hybrid InviabilityInviability
Green frogGreen frog
Many hybrids die either in Many hybrids die either in the embryonic or juvenile the embryonic or juvenile stage. stage.
For example, frogs of the For example, frogs of the genus genus RanaRana hybridize but hybridize but the resulting offspring die the resulting offspring die as embryos. as embryos.
Hybrid SterilityHybrid Sterility
A mule killing a mountain lionA mule killing a mountain lion
Some hybrids survive just Some hybrids survive just fine, but cannot reproduce. fine, but cannot reproduce.
For example, mules are For example, mules are the sterile hybrids of the sterile hybrids of horses and donkeys. Yet, horses and donkeys. Yet, they are quite viable as they are quite viable as hybrids (see left).hybrids (see left).
Fruit fliesFruit flies
Drosophila Drosophila pseudoobscurapseudoobscura--low elevation (warm)low elevation (warm)--mates in eveningmates in evening
Drosophila Drosophila persimilispersimilis--high elevation (cool)high elevation (cool)--mates in morningmates in morning
Even if interbreeding Even if interbreeding occurs, hybrids have very occurs, hybrids have very low fertility.low fertility.
PrePre-- & Post& Post--zygotic Isolating Mechanismszygotic Isolating Mechanisms
How do new biological species form?How do new biological species form?
OROR
How do isolating mechanisms evolve?How do isolating mechanisms evolve?
MayrMayr’’ss Model of Model of AllopatricAllopatric SpeciationSpeciationA single species over a broad range.
A geographic barrier arises that prevents gene flow. Populations evolve independently.
The barrier disappears but the populations are no longer capable of interbreeding.
Seven Pairs of Species(P =Pacific; C= Caribbean)
snapping shrimp(Alpheus spp.)
[Source: Knowlton et al. 1993, Science 260:1629-1632]
ReinforcementReinforcement
When contact reoccurs between populations, When contact reoccurs between populations, selection can produce isolating mechanisms selection can produce isolating mechanisms that finalize speciation.that finalize speciation.
Usually, preUsually, pre--zygotic isolating mechanisms zygotic isolating mechanisms evolve to reinforce postevolve to reinforce post--zygotic isolating zygotic isolating mechanisms. mechanisms.
Why do you think reinforcement occurs?Why do you think reinforcement occurs?
Sympatric SpeciationSympatric Speciation
•• Speciation between populations living in the Speciation between populations living in the same areasame area
•• Proposed in the 1930Proposed in the 1930’’s but not appreciated s but not appreciated until the mid 1980until the mid 1980’’ss
•• Usually requires strong disruptive selection Usually requires strong disruptive selection
[Source: Via 2001, [Source: Via 2001, Trends in Ecology & EvolutionTrends in Ecology & Evolution 16:38116:381--390]390]
Disruptive Selection & Disruptive Selection & AssortativeAssortative MatingMating
more fit least fit more fit
Assortativemating
Assortativemating
On Isla Santa Cruz, beaks sizes of Darwin’s finches vary greatly.
Source: Hendry et al. 2006, Proc. R. Soc. Lond.
two birds caught in the same net
Factors Promoting Sympatric SpeciationFactors Promoting Sympatric Speciation
•• Habitat choice and mate choice are each Habitat choice and mate choice are each determined by one genedetermined by one gene
•• Genes for habitat choice and mating Genes for habitat choice and mating choice are physically linked (i.e., on the choice are physically linked (i.e., on the same chromosome).same chromosome).
•• Even better, one gene could affect both Even better, one gene could affect both habitat choice and mate choice. habitat choice and mate choice.
[Source: Via 2001, [Source: Via 2001, Trends in Ecology & EvolutionTrends in Ecology & Evolution 16:38116:381--390]390]
Sympatric Speciation by PolyploidySympatric Speciation by Polyploidy(duplication of chromosomes within a species)(duplication of chromosomes within a species)
Tetraploids (4 N) are formed when chromosomes do not segregate during meiosis.
Most sexually reproducing organisms are diploid (two copies of each gene = 2N). These copies segregate during meisosis.
Tetraploidcell
Diploid cell Hybrids
are sterile