1. Selection and Speciation

2. To Start – Mini Whiteboards
No Notes, write out the key words linked to natural selection
On the other side, write two pieces of evidence that stabilising selection has taken place.

3. Directional Selection
Acts against one of the extremes in a range of phenotypes. As a result one phenotype becomes rare, and an alternative phenotype becomes more common.

4. Directional Selection
The upper graph represents in the frequency of phenotypes in a population. The graph has a normal distribution with a fairly large standard deviation, this is before natural selection has occurred.
The second graph is a frequency distribution of the same population after natural selection has occurred. The standard deviation of this curve is less than the upper curve and its mode has shifted to the right on the x-axis.
Natural selection has caused a genetic change in this population, favouring organisms with a characteristic towards the upper range of the frequency distribution

5. Stabilising Selection
Stabilising Selection acts against both extremes in a range of phenotypes. As a result the variation about the mode is reduced.
After selection, the mode is in the same position: This is the most advantageous phenotype. Stabilising selection has reduced the variation about this modal value.

7. Species Organisms that belong to the same
species are able to breed together to
produce fertile offspring

8. Speciation
Formation of a new species by natural selection is known as SPECIATION
Geographic Isolation:
This can happen when a species are physically separated into two.
It could be due to a mainland population being separated from a population on an island
A creation of a volcano and therefore the two populations are unable to reproduce
This prevents gene flow within the popuation

9. Drosophila
There are many different species of fruit fly. The differences between them are so small, you may not be able to tell one species from another. But the flies can!
Before mating, male and females undergo a courtship ritual

10. The whole sequence, including the response from the female is controlled by genes and is species-specific
If the male does not have the correct courtship dance, the female does not allow him to mate with her

11. If the fruit fly is homozygous recessive for wing length, it will not kill him, however his courtship dance will not attract a female, and he will be unable to pass on his alleles of the gene for wing length.
His mutated gene for wing length will not be passed on. However, suppose these genetic changes had occurred not in an individual but in an entire population that was isolated from another population of the same species – This will be conditions in which speciation can occur

12. Geographic Isolation
Geographic isolation will lead to a population of a species being isolated, and as a result there will be no GENE FLOW to neighbouring populations
Gene mutations occur at a constant and low rate, some are beneficial and result in increasing the organisms reproductive success. This mutation will therefore be passed on.
An accumulation of mutations can occur, which could mean that if the population was reintroduced to the original population, that it would be unable to reproduce. This has resulted in SPECIATION

13. Speciation This can occur if two populations have been separated
The conditions for each population
will now be slightly different
As a result selection may occur in different
directions, some alleles maybe favoured
by one population but selected against in
another.

16. Speciation
An experiment was conducted with fruit flies. One population was split into two and each was fed on different food. After many generations the two populations were placed together and it was observed that they were unable to breed together.
What evidence shows speciation has occurred?(1)
Explain why the experiment resulted in speciation. (3)
Suggest two possible reasons why members of the two populations were not able to breed together (2)

17. Answers The new species were unable to breed with each other (1)
Populations were isolated and fed on different foods (1)
This caused changes to allele frequencies between the populations (1)
Which made them reproductively isolated and eventually resulted in speciation (1)
Seasonal changes (1)
Mechanical changes (1)
Behavioural changes (1)