1. Unit 1: 1.7 Evolution - Speciation
Higher Biology
Unit 1:
1.7 Evolution - Speciation

2. Species
A species is defined as a group of organisms with similar characteristics that are able to interbreed to produce fertile offspring
They are genetically isolated from other organisms
The numbers and kinds of species are always changing
Some will be in a stable relationship with the environment
Some are moving towards extinction
Some are undergoing speciation

3. Speciation Speciation is the formation of a new species
It is brought about by evolution as a result of isolation, mutation, and selection
There are 2 types of speciation
Allopatric
Sympatric

4. Allopatric speciation
Allopatric speciation occurs when interbreeding is restricted by a geographical barrier such as river, sea, mountain or desert

5. Large population occupies same habitat. Interbreed freely.
Population becomes split into 2 by geographical barrier
Mutations occur at random. New variation occurs within each group
Natural selection affects each group in different ways favouring alleles that promote survival in each habitat
Natural selection continues to occur over many generation until the 2 groups are genetically distinct
If the barrier is removed the 2 population cannot interbreed. They are separate species

6. Sympatric speciation
Sympatric speciation occurs when two or more populations live in close proximity but become genetically isolated
Reproduction between them is prevented by behaviours, ecological barriers or changes in sets of chromosomes in plants (polyploidy)
Sympatric speciation is promoted by disruptive selection

7. Large population occupies same habitat. Interbreed freely
Alternative ecological niche becomes available
Some members of the population exploit new niche
2 populations are formed each exploiting different niche and no longer interbreed
Mutations occur to allow better exploitation of different niches
Natural selection occurs. 2 distinct populations are formed with different genetic sequences and different niches

9. Polyploidy
If plants undergo a mutation that occurs in polyploidy they will no longer be able to interbreed with the plants that produced it due to differences in chromosome numbers
This plant is genetically isolated and distinct from other plants
A new species has formed “overnight”

11. Darwin’s finches are believed to have undergone both types of speciation. Allopatric speciation occurred when populations of birds were carried to different islands by weather conditions. The sea acted as the barrier in this case.
Sympatric speciation occurred when a population of birds occupied the same island but started to exploit different ecological niches, for example the types of food eaten.

12. Hybrid Zones
A region may be occupied by a number of populations that vary in their ability to interbreed
The areas in which interbreeding between populations may occur are known as hybrid zones

13. Genes can transfer from population A to E through B, C and D
Genes can transfer from population A to E through B, C and D. If population B, C or D were to disappear this gene flow would stop. C
Hybrid zones B D A E