Lecture 13 - Speciation part 2

Question 1

Why are post-zygotic barriers avoided?

Answer 1

A they are evolutionary wasteful

Question 2

What is the concept of reinforcement?

Answer 2

Hybridization yields to unfit hybrids, so individuals who mate with their own taxon enjoy fitness advantage, natural selection therefore favours the evolution of prezygotic isolation

Question 3

When is prezygotic isolation stronger in: sympatric or allopatric

Answer 3

Sympatric

Question 4

Character displacement

Answer 4

Pattern whereby characters differ more where two taxa are sympatric than where they are allopatric

Question 5

Example of character displacement

Answer 5

When pied flycatcher are allopatric with collard flycatcher, male have similar black and white pattern, when the two species are sympatric a brown form of pied flycatcher is prevalent and preferred by the females

This avoids confusion between other species and reduces the chances of hybridisation

Question 6

Allopatric speciation

Answer 6

Reduction of the initial gene flow (m) is accomplished by a physical barrier extrinsic to the organism

Question 7

m

Answer 7

gene flow

Question 8

What are the two forms of allopatric speciation?

Answer 8

Vicariant speciation - divergence of two large populations

Peripatric Speciation - divergence of a small population from a large ancestral one

Question 9

Parapatric speciation

Answer 9

Divergence of a spatially distinct population between which there is some gene flow (0<m<0.5)

Question 10

Sympatric speciation

Answer 10

The divergence between a single, initially randomly mating population (m=0.5)

Question 11

What is the theory behind allopatric speciation?

Answer 11

Geographical isolation of previously interbreeding populations can occur through different geological or climatic events

Geographic isolation allows populations to diverge without hybridisation events (m=0)

Different habitats may impose different selective pressure on the allele pool of the two populations, or a different set of mutations arises in the two populations, or genetic drift can fix alleles in different isolates

Genetic divergence produces the isolating barrier

Question 12

What is an example of vicariant speciation?

Answer 12

Snapping shrimps: divergence between the pacific and Caribbean populations

The emergence of the Isthmus of Panama in the Pliocene (3mya) divided the populations.

In labs only 1% of the inter-specific mating can be produced between pairs of these species

Question 13

What does the example of the dusky salamanders in the Appalachian mountains demonstrate?

Answer 13

Isolation increases with geographic distance

Question 14

What does the example of the fire bellied toad and the yellow bellied toad demonstrate?

Answer 14

Two species of toad that were reproductively isolated

Then the barrier was lifted and a secoundary hybrid zone formed

This zone had a low fitness

Question 15

How can reproductive isolation arise genetically?

Answer 15

Different mutations arise in the two populations due to:

Natural selection

D-M incompatibilities

Genetic drift

Question 16

What are the two types of natural selection causing different mutations to arise in two different populations?

Answer 16

Ecological selection and speciation

Sexual selection and speciation

Question 17

Example demonstrating ecological speciation

Answer 17

Leaf beetle

3 different populations: Ontario, New york and Georgia

Georgia and New york both feed on maple and Ontario feeds on willow

Ontario and New york are closest geographically however these two populations have the largest sexual isolation as they are ecologically divergenet

Question 18

Example of ecological parallel speciation

Answer 18

three-spined stickleback

There are two ecomorphs: benthic (large, feeds on large food) and limnetic (small, feeds on small food)

Females preferentially mate with the same ecomorph, wheater they are from the same or different lakes

Question 19

Sexual selection

Answer 19

Animals can be sexually isolated by female preference for phenotypic traits of conspecific males

Enhances diversity

Question 20

Genetic Drift

Answer 20

Changes in genotype frequencies due to random fluctuations (random sampling effects in future generations)

Question 21

Where does genetic drift typically occur?

Answer 21

Small populations

Question 22

D-M theory of allele substitution

Answer 22

Barrier separate two populations

New alleles can arise and become fixed in populations

New alleles incompatible in hybrid populations contributing to the isolation between the two populations

Question 23

Peripatric (Founder effect) speciation

Answer 23

In the peripatric speciation one of the two populations is very small

Founder effect speciation often involve the invasion of novel habitats that exert strong selection

Genetic drift became important and can play a role in speciation

Question 24

Example of peripatric speciation

Answer 24

There is variation among the paradise Kingfisher in New Guinea

Although paradise kingfisher varies very little throughout the island, several distinct species differentiated on small islands along the coast

Question 25

What are the two models for parapatric speciation?

Answer 25

Clinal model - single species distributed across a variable environment

Stepping-stone model - discrete populations having restricted gene flow

Question 26

Example of parapatric speciation

Answer 26

A.odoratum is a grass that evolved tolerance to heavy metals near mines

Populations under strong selection for metal tolerance diverged from neighbouring populations growing on uncontaminated soil in terms of tolerance, flowering time and pollinating charecteristic

Question 27

What are the two types of sympatric speciation?

Answer 27

Host/habitat-shift speciation

Polyploid/hybrid speciation

Question 28

Prerequisite for host/habitat-shift speciation?

Answer 28

Powerful disruptive selection imposed by two ecological niches, and association between ecological niche and mate choice

Question 29

Prerequisite for Polyploid/hybrid selection?

Answer 29

Cytological changes

Question 30

Why do people doubt sympatric speciation?

Answer 30

As speciation needs to occur despite high gene flow

how is the frequency of the intermediate genotypes reduced to avoid a conduit gene exchange between incipient species

It is unlikely that complete sexual isolation can evolve by a single mutation because most of the characters are polygenic

Recombination break down the association between loci affecting fitness and loci that govern mating

Question 31

Example of Sympatric speciation in animals?

Answer 31

Rhagoletis pomonella:

There were four biological attributes that causes a reduction of gene flow in sympatry:

Rhagoletis mate only on host fruit

Host preference must have some genetic basis

There must be a genetically based trade-off in fitness associated with colonising and adapting to a new host (alleles suitable for one host may be selected against in the other)

The two plants have different fruiting time, favoring alleles that optimise time for mating, oviposition and eclosion

Question 32

Example of sympatric speciation in plants?

Answer 32

Palms on Lord Howae Island

Pollen could spread across the whole island

Island was established way before the two species

Two plants separated due to soil preference, this then impacted fruiting times which further separated the two

Question 33

Sympatric polyploid/hybrid speciation

Answer 33

Polyploid populations are post-zygotically isolated from their diploid progenitors

Only example known of instantaneous speciation by a single genetic event

Question 34

Autopolyploid

Answer 34

Formed by the fusion of two 2n gametes of the same species

Question 35

Allopolyploid

Answer 35

Formed by the fusion of two 2n gametes of two closely related species

Question 36

Example of hybrid speciation: Tragopogon sp.

Answer 36

The bybrid between tetraploid and diploud ancestor of tragopogon sp. is triploid and therefore is largely sterile, hence the 4n is reproductively isolated from the 2n

Question 37

What is hybrid speciation?

Answer 37

Some recombinant offspring produced by hybrids of two species are fertile but reproductively isolated from parent species

Question 38

Time for speciation (TFS)

Answer 38

Time required for complete RI to evolve since the process started

Question 39

Biological speciation interval (BSI)

Answer 39

Time between the origin of a new species and when that species speciates again

Question 40

How does speciation with RI enable morphological differences to persist in the long term

Answer 40

Local populations may diverge rapidly as a result of selection, but this state may be ephemeral as environmental circumstances changes and the divergence can be lost by a new wave of interbreeding

However if RI is established a sequential series of speciation capturing further differences in the phenotypic traits may results in a long-term trend

Question 41

Rate of allopatric speciation?

Answer 41

Allopatric speciation involving genetic drift in large population can be extremely slow, however in smaller populations involving a combination of drift and selection can have rapid TFS

Nearly all models show that sympatric speciation is fast (rapid TFS), however it happens rarely (long BSI)

Question 42

Example of extremely slow speciation?

Answer 42

Ginkgo biloba

The only extant species of the family Ginkgoacaeae appeared in the early Jurassic. This was a widespread group showing a failure to diversify

Question 43

Examples of very fast speciation?

Answer 43

Polyploid speciation

Hybrid speciation

Adaptive radiation - rapid speciation and phenotypic adaptation to exploit unoccupied niches