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

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.

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?

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”.

Ernst Mayr

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Mayr circa 2000 Mayr 1928

1904 – 2005

The importance of

reproductive isolation

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.











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!

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

Morphological Species Definition



Heliconius/Ithomiine

mimicry complex

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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”

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)

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.

Speciation: the “Patrics”



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.

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

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.



Speciation in Nicaraguan

Cichlids

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

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

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

Accelerating Speciation

Sexual Selection: Fisher’s

Runaway ProcessQuickTime™ and a

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Founder EventsQuickTime™ and a

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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

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?)

The effects of Reinforcment

Pairs of Drosophila Taxa

Galapagos and Cocos Islands

Galapagos Finches

Another tree

Finch Ecology

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?

Hybridization and Speciation

Hybridization can be a source of new species

Loren Rieseberg

Hybrid Zones

Hybrid Zones cont.

Forces on hybrid zone

Fate of the Hybrid?

Hybrid has high fitness

throughout regionHybrid has high

fitness in hybrid

zone

Hybrid has

low fitness

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.


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)


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.

Documents

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