1. Evolutionary Change without Selection

2. Natural Selection The way nature selects for some traits over others
Selection is NOT random
BUT evolutionary change doesn’t always happen this way
Picture of a bird

3. Change without Selection
Changes within a gene pool can occur without the influence of successful traits
These changes tend to reduce genetic diversity within a population
Spix’s Macaw

4. Change without Selection
Change without selection occurs through genetic drift, the bottleneck effect, and the founder effect.
Allele: a variation of a gene

5. Genetic Drift
Genetic drift: The random shifting of the genetic make-up between generations
This shifting is entirely due to chance
So some generations may see more of a particular feature, and others may see less of that feature

6. Genetic Drift and Population
Population size has a major effect on genetic drift
Large populations tend to produce a stabilizing effect on alleles
Other ½ of pg 331 chart

7. Genetic Drift and Large Populations
So in large populations, we expect to see a lot of genetic variability
One trait may be more common in a generation, and less common in another generation
Overall though, genetic variability remains fairly constant

8. Genetic Drift and Population
A particular trait is more likely to become very common or to disappear in a small population
Chart pg 331

9. Genetic Drift and Small Populations
So small populations tend to have less genetic variation
Certain features become more common within that population, and certain features are lost

10. Variety and Large Populations
So in order to maintain a high level of genetic variability, a large population is best
Features remain in the population at a fairly constant level

11. Genetic Bottlenecks
Bottleneck: A dramatic reduction in population size, usually resulting in significant genetic drift, due to catastrophe
A population of that suddenly drops to 50 will experience a significant genetic bottleneck

12. How Does it Happen?
A population reduction can occur in any number of ways:
Overhunting
Habitat loss
Natural disaster

13. How Does it Happen?
The surviving population is likely to have a smaller gene pool than the initial population
These particular traits will be passed along
In this way, certain traits become favoured, while certain alleles disappear

14. As the Population Recovers
If the population is allowed to recover, that population will be limited to those few traits left as a result of the bottleneck

15. Results of a Bottleneck
Overall, a bottleneck reduces the size of the gene pool, and genetic variety
This is generally harmful to the remaining population
Increased vulnerability to disease
Low reproductive success
Increased infant mortality

16. Cheetahs
The cheetah species suffered a severe bottleneck about 10,000 years ago
As few as 7 cheetahs survived, and all cheetahs today are descendents of these

17. Elephant Seals Population reduced to as few as 20 in the 1890s
Today over 125,000 exist, but all are genetically very similar

18. The Founder Effect
The Founder Effect: occurs when a small number of individuals separate from their original population and establish a new population

19. The Founder Effect
The founding population would have a unique mix of alleles different than the larger original population
By chance, some alleles that are less common in the larger population will be more common in the founding population

20. Frequency Differences
Perhaps a rare variant of a gene occurs once in every people in the large population
But in the founding population, that trait occurs in 1 of the 20 founders
Original Population = 1/10000 (0.1%)
Founding Population = 1/20 (5%)
This represents an 50x increase in the frequency for that variant

21. Reduced Frequencies
The founder effect leads to a population with a new, smaller, gene pool
We can expect less variation within the new population

22. Founder Effect

23. Founder Effect

24. Founder Effect

25. Founder Effect

26. Founder Effect

27. The Galapagos Tortoise
The entire population is the result of as few as one pregnant female tortoise from South America
Her genes, and those of her offspring, formed the entire gene pool for the new Galapagos Tortoise population

28. The Amish Community
A closed-off religious community that shuns the modern world
Isolation has created a smaller gene pool within the Amish community

29. Bottlenecks vs. Founder Effect
Effect on Gene Pool
Smaller
Results in Genetic Drift?
Yes
How does it Happen?
Catastrophe
Some individuals leave the population to start their own
Examples
Cheetahs, Elephant Seals
Galapagos Tortoise, Mennonites

30. A Mathematical Model for Evolution
We can think of evolution as a change in the genetic makeup of a population
We can also think about evolution from a math perspective:
The frequency of each allele in the population
Can we predict the relationship between allele frequency and the chances of those frequencies remaining constant?

31. The Hardy-Weinberg Principle
The Hardy-Weinberg Principle: states that in large populations where only random chance is at work, alleles should stay constant between generations

32. The Hardy-Weinberg Principle
Other factors will result in evolution, as these will lead to changes in variant frequencies
Natural selection
Small population size
Mutation
Immigration or Emigration
Transfer of new alleles from a different species

33. Predicting Change
By considering these factors, we can predict which populations are most likely to undergo the most evolutionary change

34. Applying the Principle to Different Species
Some species will undergo evolutionary change faster than others
A species with
High genetic diversity
Fast reproduction
will respond to natural selection more quickly
This is why bacteria develop resistance to antibiotics so quickly

35. Homework Questions
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