bioe 109 summer 2009 lecture 11-part ii speciation

BIOE 109Summer 2009

Lecture 11-Part IISpeciation

What is speciation?

What is speciation?

• in Darwin’s words, speciation is the “multiplication of species”.

What is speciation?


• according to the BSC, speciation occurs when populations evolve reproductive isolating mechanisms.

What is speciation?



• the barriers may act to prevent fertilization – this is pre-zygotic isolation.

What is speciation?




• may involve changes in location or timing of breeding, or courtship.

What is speciation?




• may involve changes in location or timing of breeding, or courtship.

• barriers also occur if hybrids are inviable or sterile – this is post-zygotic isolation.

Modes of Speciation

Modes of Speciation

1. Allopatric speciation

Modes of Speciation


• reproductive isolation occurs in complete geographic isolation.

Modes of Speciation


• reproductive isolation occurs in complete geographic isolation (no gene flow).

Geographic isolation can rise from dispersal or vicariance

Modes of Speciation


• reproductive isolation occurs in complete geographic isolation (no gene flow).

Example: Hawaiian Drosophila

HawaiianDrosophila

D. suzukii

D. microthrix

D. nigribasis

Speciation by island-hopping

Modes of Speciation

2. Parapatric speciation

Modes of Speciation


• reproductive isolation occurs without complete geographic isolation (some gene flow).

Modes of Speciation


• reproductive isolation occurs without complete geographic isolation (some gene flow). Example: ring species of salamanders (Ensatina) in CA

Ensatina salamanders

Ring species – evidence for parapatric speciation

Modes of Speciation

3. Sympatric speciation

Modes of Speciation


• reproductive isolation evolves with complete geographic overlap.

Modes of Speciation


• reproductive isolation evolves with complete geographic overlap.

Example: the apple maggot fly, Rhagoletis pomonella?

Apple maggot fly Hawthorn fly

Speciation due to host specialization in this case

Original population

Initial step of speciation

Evolution of reproductiveisolation

Allopatric peripatric parapatric sympatric

Modes of speciation: summary

Barrier formation

Newniche

Newniche

Geneticpolymorphism

In isolation In isolation In adjacentniche

Within thepopulation

What evolutionary processes are involved in speciation?


1. Natural selection


1. Natural selection • driven by different abiotic conditions (e.g., temperature, altitude) and biotic conditions (e.g., competitors, parasites).



2. Sexual selection



2. Sexual selection

• both female choice and male-male competition can promote rapid divergence (e.g., Hawaiian Drosophila).



2. Sexual selection

• both female choice and male-male competition can promote rapid divergence (e.g., Hawaiian Drosophila).

• sexual antagonistic selection too!

Male-male competition in Hawaiian Drosophila

Establish territoryOn a lek by head butting

Fight over displayTerritory by grappling


3. Random genetic drift



• may involve founder effects and genetic bottlenecks.



• may involve founder effects and genetic bottlenecks.

• alleles that are neutral in one environment may not be neutral in another!

Some generalities

1. The magnitude of pre-zygotic and post-zygotic isolation both increase with the time.

Some generalities

1. The magnitude of prezygotic and postzygotic isolation both increase with the time. • in Drosophila, it takes about 1.5 to 3 million years for complete isolation to evolve.

Some generalities


• in marine bivalves, it may take 4 to 6 million years!

Some generalities


• in marine bivalves, it may take 4 to 6 million years!

2. Among recently separated groups, pre-zygotic isolation is generally stronger than post-zygotic isolation.

Some generalities

3. In the early stages of speciation, hybrid sterility or inviability is almost always seen in the heterogametic sex.

Some generalities


• for example, D. simulans and D. mauritiana female hybrids are completely viable yet male hybrids are completely sterile!

Some generalities


• for example, D. simulans and D. mauritiana female hybrids are completely viable yet male hybrids are completely sterile!

• this is called Haldane’s rule.

J.B.S. Haldane (1892-1964)

What causes post-zygotic isolation?

What causes postzygotic isolation?

• the underlying mechanism is called Dobzhansky-Muller incompatibility:

Dobzhansky and Muller were incompatible!

“Balanced” school

“Classical” school



Ancestral Pop: A1A1B1B1




Derived Pops: A2A2B1B1 A1A1B2B2




Derived Pops: A2A2B1B1 A1A1B2B2

Hybrids: A1A2B1B2 fitness

Differences between plant and animal speciation


• in plants, polyploidization is a major mode of speciation.



• polyploidization refers to the retention of extra sets of chromosomes (i.e., tetraploids, octoploids, etc.)



• polyploidization refers to the retention of extra sets of chromosomes (i.e., tetraploids, octoploids, etc.)

• there are two types of polyploids: autopolyploids and allopolyploids.


• autopolyploids add chromosomal sets from the same species:



Species 1 x Species 1 Species 2 (2N = 4) (2N = 4) (4N = 8)




• allopolyploids combine chromosomal sets from different species:

Secondary contact and reinforcement


• secondary contact occurs when two formerly allopatric populations meet.



Three outcomes are possible:




1. No interbreeding occurs





• isolating mechanisms in place – speciation completed.






2. Introgression






2. Introgression

• no isolating mechanisms in place – populations merge completely.


3. Partial interbreeding occurs



• some isolating mechanisms in place – a hybrid zone forms (but hybrids are less fit).



• some isolating mechanisms in place – a hybrid zone forms (but hybrids are less fit).

• reinforcement should occur to “complete” the process by the evolution of additional pre-zygotic barriers.

Evidence for reinforcement in Drosophila


• Coyne & Orr (1997) compared sister species of Drosophila that were either allopatric or sympatric.



For each species pair they estimated:




1. The degree of pre-mating isolation from mate choice experiments.




1. The degree of premating isolation from mate choice experiments.

2. The degree of genetic divergence using allozymes.

Ecological speciation in sticklebacks

1. Colonization by marine stickleback ~10,000 years ago



2. Adaptation to freshwater environment



2. Adaptation to freshwater environment

3. Secondary invasion by marine stickleback



4. Evolution of limnetic and benthic sticklebacks


Evidence for secondary invasion hypothesis


1. Only low elevation lakes possess limnetic and benthic species pairs.



2. Cores from lakes with limnetic and benthic species pairs show evidence of salt water influx (e.g, clams, etc.).



2. Cores from lakes with limnetic and benthic species pairs show evidence of salt water influx (e.g, clams etc.).

3. Higher elevation lakes have neither limnetic and benthic species pairs nor evidence of salt water influx.

What types of genes are involved in speciation?

Example: desat-2 in D. melanogaster



• D. melanogaster has radiated out of Africa with humans and lives all over the world (in our garbage cans).




• female flies from Africa (A) possess a different cuticular hydrocarbon than cosmopolitan females (C).




• female flies from Africa (A) possess a different cuticular hydrocarbon than cosmopolitan females (C).

• difference due to a different position of a single double bond.

2. desat-2 in D. melanogaster

• the desat-2 mutation also affects mate choice.



• when A females are placed with A and C males, they only mate with the former.




• this modified hydrocarbon affects female smell – in effect they wear a different “perfume”.




• this modified hydrocarbon affects female smell – in effect they wear a different “perfume”.

• the A females are not courted very intensely by C males.

bioe 109 summer 2009 lecture 11-part ii speciation

Documents