Lecture 20: Speciation I

• What is a species?

• How to recognize a species?

– 3 concepts of species

• Speciation processes

– Isolation: Dispersal, Vicariance

– Divergence

– Reinforcement

Phylogenetics

• Nodes:

–branch points(speciation)

–Splitting of lineagesfrom a commonancestor

Cichlid fish diversity

1000’s of species “Species flocks”

Monophyletic

group

?

Speciation mechanisms?

Tim

e

?

Species Concepts

“No one definition has

as yet satisfied all

naturalists; yet every

naturalist knows

vaguely what he means

when he speaks of a

species.”

(Darwin, 1859)

Species Concepts

• Groups of similarindividuals

• Evolutionarilyindependent

• Isolated: lack of geneflow with otherindependent units

• How to identify species:3 concepts

1. Morphospecies

• Morphological

similarities and

differences

• What about

differences other

than morphology?

Eurytemora affinis

Rhagoletis pomonella

• Copepod

• Eurytemora

affinis

2. Biological Species Concept

• A species is a

group of– potentially

interbreeding

populations

– reproductively

isolated from other

such groups.

• Copepod

• Eurytemora

affinis

3. Phylogenetic species concept

• Populations thatform the smallestmonophyletic groupwith diagnosticsynapomorphics

• Use phylogeneticsto identifytaxonomic groups(including species) Fig 15.2

Eurytemora affinis

• Phylogeny

identifies groups

of populations,

suggesting

cryptic species

• Interpopulation

crosses show

reproductive

isolation Fig 15.3: COI, 16s DNA sequences

Hawaiian

crickets

Laupala

• Monophyletic

populations correspond

to morphospecies

Mendelson & Shaw 2004, Nature

Allopatric Speciation

IsolationColonization

Vicariance

DivergenceDrift

Natural selection

Sexual selection

Secondary contactHybrid offspring

unfit (postzygotic

isolation)

Selection against

hybridization(prezygotic

isolation)

Reinforcement

(?)

Allopatric Speciation

1. initial isolation

Isolation

1. initial isolation Can happen by

Dispersal or by

Vicariance

Dispersal and

isolation?

• Hawaiian crickets: Closely related species

should be found on adjacent islands

Dispersal and

isolation?

• Closely related

species ARE found

on adjacent islands

Mendelson & Shaw 2004, Nature

Vicariance and Isolation

Vicariance and isolation

• Snapping shrimp

• Panama Isthmus:3-4 millionyears old

• 7 Caribbean,Pacific sistermorphospecies

Figure 12.8

Isolation by vicariance

• Confirmed: Sister

morphospecies are

closest relatives

• Evolution of DNA

sequenceFigure 12.8

Speciation

• Isolation alone

does not lead to

new species