Northwestern Garter Snake

Northwestern Garter SnakeNorthwestern Garter Snake((ThamnophisThamnophis ordinoidesordinoides))

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

The RingThe Ring

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.

??

Evolutionary and Biological species concepts

Allopatric speciation Sympatric speciation

evolution of morphological

differences

evolution of reproductive

isolation

Morphological speciesBiological species

Evolutionary species

geographic isolation


Evolutionary species



differences


isolation



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.



PrePre--zygoticzygotic

HybridizationHybridization


•• Hybrid Hybrid InviabilityInviability•• Hybrid SterilityHybrid Sterility


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

Beak size constrains the song of a bird.

Small-beaked male

Large-beaked maleFr

eque

ncy

(kH

z)Fr

eque

ncy

(kH

z)

Time (s)

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


Evolutionary speciesEvolutionary & Biological species



differences


isolation



isolation

Northwestern Garter Snake

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