Lecture 25 - Speciation

Question 1

What is the morphological species concept?

Answer 1

Groups of organisms that appear identical by morphological (anatomical) criteria. It is the oldest and most practical method but can be misleading due to mimics or cryptic species.

Question 2

Give an example of how the morphological species concept can be misleading.

Answer 2

Heliconius butterflies mimic each other, appearing identical to predators but belonging to different species.

Question 3

What is the biological species concept?

Answer 3

Species are groups of interbreeding natural populations that are reproductively isolated from other such groups (E. Mayr 1945).

Question 4

What are the limitations of the biological species concept?

Answer 4

It does not apply to asexual organisms, is difficult to test in practice, cannot classify extinct taxa, and some “good species” still hybridise.

Question 5

What is the null model of speciation?

Answer 5

Populations become separated in space (allopatry), accumulate genetic differences, and develop Bateson-Dobzhansky-Muller incompatibilities (BDMIs), leading to reproductive isolation.

Question 6

What are Bateson-Dobzhansky-Muller incompatibilities (BDMIs)?

Answer 6

Genetic incompatibilities that occur postzygotically, reducing hybrid fitness and contributing to reproductive isolation.

Question 7

What is allopatric speciation?

Answer 7

Speciation that occurs when populations are geographically isolated, as seen in Galápagos finches.

Question 8

Can allopatric speciation occur outside of islands?

Answer 8

Yes, for example, in freshwater lakes or mountain tops where populations are ecologically isolated.

Question 9

What genes are involved in BDMIs?

Answer 9

Example: In Xiphophorus fish, the oncogene Tu and repressor R lead to melanomas in hybrids without the repressor allele.

Question 10

What are prezygotic barriers?

Answer 10

Barriers that act before fertilisation, such as differences in mating behaviour, habitat, or pollinator preference.

Question 11

What is reinforcement in the context of prezygotic barriers?

Answer 11

Selection against unfit hybrids reinforces reproductive isolation by evolving stronger prezygotic barriers.

Question 12

Give an example of reinforcement in prezygotic isolation.

Answer 12

Phlox flowers (P. drummondii) evolve different colours in areas where closely related species overlap to reduce hybridisation.

Question 13

What is the role of ecological selection in speciation?

Answer 13

Strong ecological selection can act as a by-product, favouring traits that reduce hybridisation and enhance reproductive isolation.

Question 14

What is the significance of BDMIs in speciation?

Answer 14

BDMIs represent genetic differences that reduce hybrid viability or fertility, driving the process of speciation.

Question 15

What is postzygotic isolation?

Answer 15

Barriers that reduce fitness after zygote formation, such as hybrid inviability or infertility.

Question 16

What is the difference between prezygotic and postzygotic barriers?

Answer 16

Prezygotic barriers prevent fertilisation (e.g., mate choice, habitat differences), while postzygotic barriers reduce the fitness of hybrids after fertilisation (e.g., hybrid inviability or sterility).

Question 17

Why is allopatric speciation common?

Answer 17

Geographic isolation limits gene flow, allowing populations to diverge genetically through mutation, selection, and drift, often resulting in speciation.

Question 18

How do sky islands contribute to speciation?

Answer 18

Sky islands, such as mountaintops, create isolated microhabitats that limit gene flow between populations, promoting divergence and potential speciation.

Question 19

How does temporal isolation function as a prezygotic barrier?

Answer 19

Temporal isolation occurs when species reproduce at different times (e.g., seasons or times of day), preventing interbreeding.

Question 20

What is an example of a postzygotic barrier in hybrids?

Answer 20

Reduced hybrid viability, as seen in Xiphophorus fish, where certain genetic combinations result in high susceptibility to melanomas and reduced survival.

Question 21

How do ecological differences drive prezygotic barriers?

Answer 21

Differences in habitat use or pollinator preference can prevent interbreeding by favouring distinct ecological niches for each population.

Question 22

What is the role of genetic drift in speciation?

Answer 22

In small, isolated populations, genetic drift can fix random mutations, contributing to divergence and reproductive isolation.

Question 23

Why is reinforcement more common in sympatric populations?

Answer 23

Sympatric populations overlap geographically, so selection acts directly to prevent the wasted effort of producing unfit hybrids, strengthening prezygotic barriers.

Question 24

What is an example of speciation through reinforcement in nature?

Answer 24

Phlox flowers (P. drummondii) evolve distinct colours in overlapping regions to reduce hybridisation, an example of reinforcement.

Question 25

Why are BDMIs considered postzygotic barriers?

Answer 25

They involve genetic incompatibilities that manifest after fertilisation, reducing hybrid fitness through inviability or sterility.

Question 26

What is a hybrid zone?

Answer 26

A geographic area where interbreeding between diverging populations occurs, often producing hybrids with varying fitness.

Question 27

How does pollinator preference contribute to speciation?

Answer 27

When populations adapt to different pollinators, it creates reproductive isolation by limiting cross-pollination, as seen in Mimulus flowers.

Question 28

What is an example of ecological selection leading to speciation?

Answer 28

Divergent habitat use in Rhagoletis fruit flies, where different populations adapted to hawthorn or apple trees, creating strong prezygotic isolation.

Question 29

What is the role of hybrid inviability in speciation?

Answer 29

Hybrid inviability prevents the survival of zygotes or early-stage hybrids, acting as a postzygotic barrier to maintain species boundaries.

Question 30

How do freshwater lakes and ponds promote speciation?

Answer 30

Isolated aquatic environments limit gene flow between fish populations, leading to genetic divergence and potentially speciation.