1. Species and Speciation

2. How can genetic variation be increased or decreased in a population?

3. Increased:
Mutations
Gene flow (movement of genes from one population to another), immigration

4. Decreased Emigration Non-random mating Natural selection
Genetic drift (catastrophe leading to reduced numbers in a small population)

5. The formation of new species = speciation
Definition of species:
A species consists of groups of similar individuals who can interbreed with each other to produce fertile offspring but do not naturally interbreed with members of other species.

6. Demes
A species usually exists as distinct populations may be separated geographically. These local interbreeding populations are called demes.
Organisms mostly interbreed within the deme rather than with members of other populations, therefore, demes often develop slightly different allele frequencies, giving each different characteristics.

7. ‘Species’ is tricky to define
Boundaries of a species gene pool can be unclear .
eg: closely related species of the dog family can interbreed
Also, species can show a gradual change in phenotype over a geographical area. This gradual change is called a cline. This often occurs over the length of a country or continent.

8. Clines

9. Ring species – a special type of cline
If a cline forms a ring, (eg. across a continent) demes A and E may be unable to breed when they meet, although, the intermediate forms can still interbreed. Are A and E still the same species or two separate species? C D B E A

10. Reproductive Isolating mechanisms
Reproductive isolating mechanisms prevent successful breeding between different species. They are barriers to gene flow.
Most species have more than one isolating mechanism operating to maintain a distinct gene pool.

11. Geographical barriers
Physical barriers such as mountains, rivers, oceans and deserts prevent gene flow between two different species.

12. Prezygotic (before fertilisation) isolating mechanisms
These operate prior to mating
Include:1) habitat preference
Timing of mating
Behavioural incompatibility
Structural incompatibility
Gamete mortality

13. 1) Habitat preference

14. 2) Timing of mating
Closely related species may have quite different breeding seasons

15. 3) Behavioural incompatibility
These behaviours ensure mating within species.

16. 4) Structural incompatibility

17. Gamete mortality
Even if mating occurs, the sperm of one species may not be able to survive in the reproductive tract of another species.

18. Postzygotic Isolating mechanisms
These act after fertilisation to prevent successful reproduction.
Include: 1) hybrid sterility
2) hybrid inviability
3) hybrid breakdown

19. 1) Hybrid sterility
Even if two species mate and produce offspring, the offspring may be sterile. This is common in the horse family.
One cause of this sterility is the failure of meiosis to produce normal gametes. This can occur if parents are different in chromosome number.

20. Zebronky or zedonk

21. 2) Hybrid inviability
A zygote is formed but fails to develop properly. Sometimes fail to divide because of unmatched chromosome numbers from each gamete.

22. 3) Hybrid breakdown
The first generation offspring (F1) are fertile but the second generation (F2) are infertile or inviable.

23. Types of speciation Allopatric speciation
This occurs when species become geographically separated, each being subjected to different natural selection pressures, and finally establishing reproductive isolating mechanisms.

24. Sympatric speciation
Occurs when a population forms a new species within the same area as the parent species.
There is no geographical separation.
More rare than allopatric speciation.

25. Two situations where sympatric speciation thought to occur:
Speciation through niche differentiation – there may be a change in host preference, food preference or habitat preference;
Instant speciation as a result of polyploidy (particularly in plants).

26. Stages in the development of a new species: