Speciation (Siva-Jothy)

Question 1

Speciation

Answer 1

The splitting of one species into two. It can result in generations of new; species, genera, orders, classes or phyla.

There are two factors that influence speciation:

• Disruptive selection
• Gene flow

Question 2

Gene Flow

Answer 2

Exchange of genes between populations as a result of movement and interbreeding of individuals, often when two populations are geographically close to each other.

Question 3

Relationship between speciation and gene flow?

Answer 3

For speciation to occur, there must be a high level of disruptive selection and a low level of gene flow.

(Direction/stabilising selection + gene flow, is what hold a species together and results in no speciation).

Question 4

Changes during speciation

Answer 4

• Divergence: Species adapt to different environments.

- Reproductive Isolation: Species lose the ability to interbreed.

Question 5

Large scale reproductive isolation

Answer 5

-Mountain range, sea, land bridge, river, desert, ice flows

Question 6

Small scale reproductive isolation

Answer 6

Different rainfall, backgrounds, temperatures, etc.

Question 7

Characteristics that produce reproductive isolation

Answer 7

Live in different habitats, they look different, they behave different (courtship behaviour), the breed at different times during the year, their sex cells are incompatible, their offspring do not survive or are sterile.

Question 8

Island Endemics

Answer 8

Species only found in one place, common on isolated islands

• Chaffinches, found on canary islands, different to those found in the UK.
• Australia is full of endemics. As it was separated from the mainland a long time ago and has since got further away due to continental drift.

Question 9

Types of speciation

Answer 9

There are two theories, both that put a different amount of importance to gene flow. These theories aren’t mutually exclusive.

• Allopatric speciation
• Sympatric speciation

Question 10

Allopatric speciation

Answer 10

Reproductive isolation –> Divergence

Allopatric speciation results from being in a different location. First there is reproductive isolation which results in divergence.
-The reproductive isolation prevents gene flow.

Firstly, the plant is distributed across the mountain and valley, showing little clinal variation. Sea levels rise and one side of the mountain become wetter, whereas one stays dry. The plants that grew in the valley die because of flooding.

• The plants on each side of the mountain range are unable to reproduce with each other due to the large body of water and adapt to their new ecological conditions.
• Once the sea levels go down again, the reproductive barrier is removed, and the plants are able to recolonise old ground. But they are unable to reproduce, so a new species is formed. Alternatively, they might be able to hybridise, which means it is not full speciation, but eventually will be

Question 11

Sympatric Speciation

Answer 11

Divergence –> Reproductive isolation

Divergence of form occurs, in which disruptive selection causes one species to diverge into two.
This results in reproductive isolation.
A x A = fit offspring
A x B = unfit offspring
B x B = fit offspring.
-A x B = hybrid offspring, which is adapted to neither environment. These offspring are removed by post-zygotic selection.
Gametes fuse to form zygote, and the selection process happens the zygote has formed, as there is no mechanism already in place to stop parents from mating.
-As the offspring is not well-adapted to the environment and/or sterile. It is a waste of time and energy for the parents to mate again, so they learn not to mate with each other.
-Selection favours pre-zygotic selection, as no energy is wasted producing non-viable offspring.

Question 12

Is disruptive selection ever enough to overcome gene flow?

Answer 12

YES.
Agrostis tenuis:
A species of wind pollinated grass that grows on and around slag heaps. There are two variations showing ongoing sympatric speciation.
AA = growing off slag heap
RR = growing on slag heap. Enzyme composition to reduce toxicity of heavy metals on slag heap.
The hybrid AR doesn’t grow as quickly, either on or off the slay heap, so is outcompeted and selected against.
-On the slag heap, it doesn’t grow as quickly because although it has some of the enzymes required, it doesn’t have them all and cannot produce them quick or efficiently as RR so is outcompeted.
-Off the slag heap, it also doesn’t grow as quickly because energy is wasted on making unnecessary detoxifying enzymes when they’re not required off the slag heap, so are again outcompeted by AA.