1. Natural Selection
What is natural selection? Natural selection is the way in which nature favours the reproductive success of some individuals within a population over others.

2. Initial Population

3. Natural Selection
Fast Cheetah
Slow Cheetah

4. Natural Selection

5. Population after Natural Selection

6. Genetic Drift evolution without selection

7. Catastrophe!!

9. After the catastrophe, the population isn’t the same as the initial population.

10. What we know so far… Populations can change over time
Natural Selection
Genetic Drift
These are examples of microevolution

11. Microevolution
Microevolution: changes in allele frequencies and physical traits within a population and species
So we know that alleles that allow cheetahs to run faster will be favoured
But how did the cheetah arise as a new species?

12. Speciation

13. What is a Species? All members of a population with a common gene pool
These members are able to interbreed under normal conditions and produce fertile offspring

14. How Species Arise
In the formation of a new species, one constant is always seen:
Distinct features arise that isolate the new species reproductively (and therefore genetically) from other species

15. How Species Arise
For a variety of reasons, members of the new species are not able to produce fertile young with other species
This is known as Reproductive Isolation

16. Mechanisms of Reproductive Isolation
Any behavioural, structural, or biochemical trait that prevents individuals from reproducing successfully
Two types
Prezygotic Mechanisms
Postzygotic Mechanisms

17. Prezygotic Mechanism
Prezygotic Mechanism: A reproductive isolating mechanism that prevents fertilization and zygote formation

18. Prezygotic Mechanism
These are traits between species that prevent them from being able to create a zygote (fertilized egg)
These species may live in the same region, but these factors prevent breeding from occurring
There are five types of prezygotic mechanisms

19. 1. Behavioural Isolation
Different species use unique courtship rituals to find a mate
Example: Birds of Paradise

20. 2. Temporal Isolation Different species breed at different times
Example: Field Crickets

21. 3. Ecological Isolation
Similar species may inhabit different habitats within a region
Example: Eastern and Mountain Bluebirds

22. 4. Mechanical Isolation
Differences in physical features that make two species unable to mate
Example: Damselflies

23. 5. Gamete Isolation
Male gametes are unable to recognize and fertilize egg cells of another species
Example: Marine Animals

24. Postzygotic Mechanism
Postzygotic Mechanism: An isolating mechanism that prevents maturation and reproduction in the offspring of interspecies reproduction

25. Postzygotic Mechanism
In this case, species are able to mate and create a zygote
The zygote is either unable to make it to term, or the offspring is sterile

26. Postzygotic Mechanisms
There are three types of postzygotic mechanisms:

27. 1. Zygotic Mortality Mating and fertilization are possible
Genetic differences result in a zygote that is unable to develop
Example: Sheep and Goats

28. 2. Hybrid Inviability
A hybrid individual develops, but cannot survive to maturity
May die before birth
May die before reaching maturity
Example: Tigers and Leopards

29. 3. Hybrid Infertility
Hybrid offspring remain healthy and viable but are sterile
Unable to reproduce and have their own offspring
Example: Mules

30. Allopatric Speciation
Allopatric Speciation: the formation of a new species as the result of evolutionary changes after a period of geographic isolation
Physical isolation means that these species are no longer able to exchange genetic info
Mutations that arise in one population will not be shared with the other

31. Allopatric Speciation
After some time has passed in geographic isolation, a reproductive isolating mechanism will develop

32. Mechanisms of Allopatric Speciation
How does this occur?
Isolated on remote islands
Mountain ranges form between populations
Continental drift

34. Sympatric Speciation
Sympatric Speciation: The formation of new species within the same geographic area
This occurs as populations become genetically isolated from the larger population

35. Sympatric Speciation This can occur through human action
The Hawthorne Fly:
Apples were introduced to North America from Europe between 1800 and 1850
Some flies started laying eggs on apples
Today there are two species:
one whose maggots feed mostly on apples
One whose maggots feed mostly on hawthorn

36. This is an example of disruptive selection!

37. Sympatric Speciation Instant Sympatric Speciation
Occurs in species that can reproduce asexually
A random mutation in an individual may eventually lead to the formation of a new species
This has probably happened many times in plants – many are able to sexually and asexually reproduce

38. Polyploidy Remember meiosis?
cells should have 2n chromosomes: one from each parent
Problems during meiosis can lead to cells with more than 2n chromosomes:
A cell may actually end up with 3 or 4 copies of a chromosome

39. Polyploidy
Polyploids can successfully breed with other polyploids but not with the original 2n ancestor

40. Polyploidy
Incredibly important in the evolution of plants

41. Polyploidy Eastern Treefrog has four sets of chromosomes
nearly identical to the Cope’s Gray Treefrog, which has two sets of chromosomes
Suggests that the Eastern Treefrog has evolved from the Cope’s Treefrog

42. Homework Q’s
PG. 340 #2, 5, 6
PG. 372 #20, 37