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Speciation Chapter 16
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Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

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Page 1: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation

Chapter 16

Page 2: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

The big problem in Speciation

Define Species!!!!!!!!!!!!!!

Biological species concept

Phylogenetic species concept

Morphological species concept

Cohesion species concept

Evolutionary species concept

Lots of other species concepts.

Page 3: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Why is this a problem

Without a clear species concept we cannot be clear

about what we are talking about!

How can we talk about mechanisms of speciation if we

don’t know what a species is?

More fundamentally:

Mayr thought that species were a “natural” unit.

Is this true?

Page 4: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

The Biological Species

Concept

BSC: A species is a group of actually or potentially

interbreeding individuals that are reproductively isolated

by other such groups. (Ernst Mayr)

Species is a group of organisms sharing a gene pool

Species boundaries are defined by the limits of gene flow.

Ultimately this definition is about reproductive isolation

It is the most widely accepted and conceptually clearest

definition, and underlies our common ideas of “species”.

Page 5: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Ernst Mayr

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Mayr circa 2000 Mayr 1928

1904 – 2005

Page 6: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

The importance of

reproductive isolation

Page 7: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Phylogenetic Species concept

A monophyletic group of organisms that is

phylogenetically distinct from other such groups.

This is consistent with the BSC.

How “far down” the tree you need should go is an issue.

Page 8: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Phylogenetic Species concept

A monophyletic group of organisms that is

phylogenetically distinct from other such groups.

This is consistent with the BSC.

How “far down” the tree you need should go is an issue.

Page 9: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Phylogenetic Species concept

A monophyletic group of organisms that is

phylogenetically distinct from other such groups.

This is consistent with the BSC.

How “far down” the tree you need should go is an issue.

Page 10: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Morphological Species

concept

A group of organisms that are sufficiently similar in

appearance behavior etc. that they can be logically

grouped.

Don’t kid yourselves. This is the one we use!

Page 11: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Biological Species DefinitionProblem: What is potentially interbreeding?

The liger is the world's largest big cat. An average male liger weighs over 900 pounds and

standing almost 12 feet tall.The reason that they are called a liger is because the father

was a lion and the mother was a tiger. If the situation was reversed and the mother was a

lion and the father was a tiger, he would be called a tigon, and would be a dwarf instead

of a giant. A fully grown tigon is usually less than 350lbs. Ligers are not sterile, and they

can reproduce.

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“Artists” renditionIt's pretty much my favorite

animal. It's like a lion and a

tiger mixed... bred for its

skills in magic.

Napoleon Dynamite

Page 12: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Morphological Species Definition

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Heliconius/Ithomiine

mimicry complex

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Page 13: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Isolating Mechanisms

The BSC is really about isolating mechanisms

Two general classes of isolating mechanisms

Premating isolating mechanisms

Postmating isolating mechanisms

Generally Postmating isolating mechanisms are considered

to be artifacts of phylogenetic distance

Premating isolating mechanisms are thought to evolve to

avoid “wasting gametes”

Page 14: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Premating Isolating

Mechanisms

Potential Mates do not meet (seasonal or habitat isolation.).

Lions and Tigers live in different habitats. For

this reason there are no wild Ligers or Tigons

Potential Mates meet but do not mate (behavioral Isolation)

Many species have elaborate mating rituals

that (among other things) keep species

separate

Some examples:

Copulation is attempted, but no transfer of sperm takes

place (mechanical isolation)

Page 15: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Post Mating Isolating

Mechanisms•Sperm Transfer takes place, but egg is not fertilized

(gametic mortality).

•Egg is fertilized, but zygote dies (zygote mortality)

•Zygote produces an F1 hybrid, but zygote dies (zygote

mortality)

•F1 hybrid is fully viable, but partially or completely

sterile (hybrid sterility)

•F1 hybrid is viable, but ecologically unable to survive

(hybrid inviability)

Thought to be the side product of evolutionary divergence.

Page 16: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation: the “Patrics”

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Speciation

Common elements to all models of speciation:

•Reproductive isolation

•Time for independent evolution to occur

Mayr introduced the allopatric speciation model. It is the

only model accepted as definitely true.

Sympatric models have been controversial, but evidence

is accumulating that sympatric speciation does occur.

Page 18: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Dobzhansky-Muller Model

The Dobzhansky-Muller model argues that at least two

interacting genetic changes must occur for there to be

speciation

mutationmutation mutation

mutation

Page 19: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Sympatric Speciation

Sympatric speciation is the most controversial

Postulates NO barrier to gene flow at the

beginning

Speciation arises due to disruptive selection

Theory says that even a small amount of gene flow

should stop this from happening.

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Page 20: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Nicaraguan

Cichlids

Nature 439, 719-723 (9 February 2006)Marta Barluenga et al

Page 21: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Oceanic Palms

Lord Howe Island

Howia forsteriana is shown in black (n = 1,677)

and H. belmoreana in grey (n = 4,542)

Nature 441, 210-213(11 May 2006)Vincent Savolainen et al

Page 22: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Causes of Divergence

Genetic Drift -- SLOW

Mutation -- SLOW

Divergent Selection -- Probably the most common cause

Different Environments -- see table 16.1

Different genetic complexes

Sexual selection

Genetic drift and selection alone do not cause

reproductive isolation unless (1) there is mutation, or (2)

they diverge into sufficiently different habitats that the

hybrid has low fitness

Page 23: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model
Page 24: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model
Page 25: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Accelerating Speciation

Sexual Selection: Fisher’s

Runaway ProcessQuickTime™ and a

Photo - JPEG decompressorare needed to see this picture.

Founder EventsQuickTime™ and a

Photo - JPEG decompressorare needed to see this picture.

Page 26: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Gene Interaction and Speciation:

Dominance by Additive Epistasis

B1B1 B1B2 B2B2

Freq (A1) = 1 1 0Freq (A1) = 0.5

Freq (A1) = 0

0 001 0 —1

—1

In a Metapopulation fixed for the B2

allele the introduction of a B1 allele (by

mutation or migration) can lead to

speciation due to underdominance at the

A locus

Page 27: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Reinforcement

After the initial stages of reproductive isolation have

occurred selection may favor further isolation

1) Initial divergence

2) Species come back into contact

3) Hybrids have low fitness

4) Selection favors females that mate with their own

species

Reinforcement favors the evolution of premating isolating

characters. (why?)

Page 28: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

The effects of Reinforcment

Pairs of Drosophila Taxa

Page 29: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Galapagos and Cocos Islands

Page 30: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Galapagos Finches

Page 31: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Another tree

Page 32: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Finch Ecology

Page 33: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Galapagos

Finches

13 species on Galapagos, 1 on Cocos

Galapagos has more diverse habitats than Cocos

Galapagos has more area than Cocos

Galapagos has more islands than Cocos (1 island)

Which is the cause of speciation on Galapagos?

Page 34: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Hybridization and Speciation

Hybridization can be a source of new species

Loren Rieseberg

Page 35: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Hybrid Zones

Page 36: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Hybrid Zones cont.

Page 37: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Forces on hybrid zone

Page 38: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Fate of the Hybrid?

Hybrid has high fitness

throughout regionHybrid has high

fitness in hybrid

zone

Hybrid has

low fitness

Page 39: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Continuous

Populations

1 .5 1

1.51

.5.5 0

AA

Aa

aa

BB Bb bb

a)

1 .5 1

1.51

.5.5 0

AA

Aa

aa

BB Bb bb

1 .5 1

1.51

.5.5 0

AA

Aa

aa

BB Bb bb

a)

1 .5 .92

1.5.92

.5.5 0

AA

Aa

aa

BB Bb bb

b)

1 .5 .92

1.5.92

.5.5 0

AA

Aa

aa

BB Bb bb

.5 .92

1.5

.50

.5 .92

1.5

.50

AA

Aa

aa

BB Bb bb

b)

In theory speciation can take

place in a continuous population.

Consider a population with:

•multiple loci

•Underdominance

•Multiply underdominant

genotypes have zero fitness.

Page 40: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Continuous

Populations

t = 50 t = 150 t = 250

= 0

= 0.1

t = 1

b) c) d)

e) f) g)

a)

t = 50 t = 150 t = 250

= 0

= 0.1

t = 1

b) c) d)

e) f) g)

a)

b) c) d)

e) f) g)

a)

Page 41: Speciation - The University of Vermontbiology/Classes/271/species.pdf · is accumulating that sympatric speciation does occur. Dobzhansky-Muller Model The Dobzhansky-Muller model

Speciation in Continuous

Populations

• Generations to Speciation.

When mating is spatially localized,

mild underdominance persists for

extended duration (blue and red

lines).

• Decreasing the epistatic coefficient

by an order of magnitude increases

the time to speciation by an order

of magnitude.

• Generations to fixation of a

single genotype. Speciation never

occurs with panmixia.