1. Evolution of Populations
Chapter 11

2. Genetic Variation Within Populations
Variation and gene pools
Gene pool – all genes present in a population
Allele Frequency – the number of times an allele occurs in the gene pool, compared to other alleles
Genetic variation is often studied in populations rather than in individuals.
Genetic variation in a population increases the likely hood of some members surviving
A population is a group of individuals of the same species that interbreed, not inbreed.
Because of the interbreeding, the members of the group share a common group of genes among them called the gene pool
Allele frequency = Relative frequency is often expressed as a percentage and has nothing to do with whether the allele is dominant or recessive
For example one study showed that the incidence of polydactyly is 35%

3. Genetic Variation Within Populations
In genetic terms, evolution is any change in the relative frequency of alleles in a population.
This change can be an increase or decrease in the frequency of an allele.

4. Genetic Variation Within Populations
Scientists now recognize two main sources of genetic variation
Mutations
Recombination

5. Genetic Variation Within Populations
Mutations
Any change in a sequence of DNA
Occurs Because:
Problems with replication
Exposure to radiation or chemicals
Mutations do not always affect the phenotype
Changing from GGA to GGU will still code for the amino acid glycine
Some mutations can affect an organisms fitness or its ability to survive and reproduce

6. Genetic Variation Within Populations
Recombination
Parents alleles rearrange when forming gametes
The 23 pairs of chromosomes can produce 8.4 million different combinations!
During meiosis, each chromosome of a homologous pair moves independently.

7. Genetic Variation Within Populations
Crossing Over
Occurs during meiosis
Further increases genetic variation

8. Natural Selection In Populations
Normal distribution
Highest frequency in middle and lowest at the extremes
Also representative for allele freqencies

9. Natural Selection In Populations
Microevolution
Observable changes in allelic frequencies
Occurs in a single population
3 ways
Directional
Stabilizing
Disruptive

10. Natural Selection In Populations
Directional Selection
Shift in phenotype to one extreme
Mean also shifts
Rise of drug resistant bacteria
In 1940’s some bacteria had a resistance to antibiotics, but it was of no use.
After the creation of antibiotics the resistance became an advantage
This has increased the number of bacteria that are resistant to antibiotics to some degree

11. Natural Selection In Populations
Stabilizing Selection
Norm is selected for and becomes more common
Ex. Gall Fly and Goldenrod
Gall flies lay eggs in goldenrod plants
Downy woodpeckers attack large galls and certain wasps attack and lay eggs in small galls
Therefore, the median value is the best
Humans – select against heavy and light infants

12. Natural Selection In Populations
Disruptive Selection
Selection for the extremes
Ex. Peppered moths

13. Natural Selection In Populations
Single-gene and polygenic traits
Widows peak is a single gene trait – a single gene with two alleles
Polygenic Traits are controlled by two or more genes
This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

14. 16-2 Evolution as Genetic Change
Natural Selection on Single-Gene Traits
A few things to keep in mind:
Natural selection does not affect single genes, rather it affects entire organisms. Natural selection determines if the organism survives and reproduces or dies. If an organism dies, then it does not pass on its genes to the next generation.
Genes can influence the gene pool, and this in turn can challenge natural selection.
Individual organisms can not evolve, only populations.
Natural selection on single gene traits can lead to changes in allele frequencies and thus cause evolution.

15. Other Mechanisms of Evolution
Gene Flow
Some animals move once they are able
When the move into new population, their alleles become part of gene pool
Occurs with movement between populations
Increases variation
If less movement, the more different two species become
Bald eagles have been found over 2500 km from their birthing nest when they can fly

16. Other Mechanisms of Evolution
Genetic Drift – Random change in allele frequency
Bottleneck Effect
Founder effect – when allele frequencies change because of migration
Natural selection is not the only source of evolutionary change
With a small population, an allele can become more common just by chance
You can use the laws of probability to predict the overall results for genetic crosses in large populations. This however, does not work for small populations and the results may be farther from what the probabilities predict.
This is called genetic drift.
Genetic drift is random change, natural selection is change due to adaptations
This may happen when a new species colonizes a new habitat because they may carry a certain allele at a higher relative frequency. Natural selection does not cause the change in frequency, just chance.
Bottleneck effect – genetic drift that occurs after some event greatly reduces the population size – once the genes are gone, the are gone.
If two species happen to migrate then you can get what is known as the founder effect

17. Other Mechanisms of Evolution
Sexual Selection
Females the important factor in reproduction
This makes females “picky”
Intrasexual selection
Intersexual selection
Intrasexual selection – involves competition between males – rams butting heads
Intersexual selection – males display certain traits that attract females - peacocks

18. Hardy-Weinberg Equilibrium
Evolution vs. Genetic Equilibrium
Hardy-Weinberg principle – genotype frequencies will stay constant unless some factor changes the frequency
Genetic Equilibrium – when allele frequencies remain constant
Scientists sometimes find it helpful to compare data to a model. This model is the Hardy-Weinberg principle.
This leads to genetic equilibrium where the allele frequency remains the same and evolution does not occur.
There are certain conditions that have to be met

19. Hardy-Weinberg Equilibrium
Five Conditions to Maintain Equilibrium
Random Mating
Large Population
No Movement into or out of the population
No Mutations
No Natural Selection
Since these conditions can’t be met equilibrium is rarely met.
However, biologists can compare real data to what the model indicates

20. Hardy-Weinberg Equilibrium
Biologists can use an equation for comparison purposes
p2 + 2pq + q2 = 1
p = dominant alleles
q = recessive alleles
Population = 3000
Attached earlobes = 2200 (dominant)
Unattached earlobes = 800 (recessive)
Q2 = 800/3000 = 0.27
Q = 0.51
P= 1-q = 0.49
P2 = 0.24
2pq = 0.49

21. Hardy-Weinberg Equilibrium
Five factors leading to evolution
Genetic drift
Gene flow
Sexual selection
Natural selection
Mutations
Genetic drift = change due to chance
Gene flow – movement of alleles from one population changes allele frequencies in that population
Sexual selection – certain traits may improve mating success
Natural selection – certain traits may be of an advantage for survival

22. Speciation Through Isolation
Reproductive Isolation – members of populations can no longer mate
Mules or Hinnies
Speciation - when two species come from one existing species
This could be due to not being able to physically mate with one another or the offspring do not survive
Mules or hinnies are the result of a horse and an ass or a donkey and a mare- differences in chromosome numbers make these animals sterile

23. Speciation Through Isolation
Behavioral Isolation
Two populations will not breed because of differences in courtship
Fireflies
If gene flow stops between two populations, then the populations are said to be isolated – over time the differences between two populations increases
Over 2000 different species of fireflies that differ in blinking patterns

24. Speciation Through Isolation
Geographical Isolation
Rivers, mountains, or bodies of water separate two populations
Snapping Shrimp and placental vs. marsupial mammals
Snapping shrimp became isolated by a geographical barrier in the atlantic ocean – now when the two separate species are placed together males and females snap at each other
When Australia seperated, the marsupials became seperated – because the rest of the world was more condusive to surviving as a placental mammal they displaced them marsupials everywhere but Australia

25. Speciation Through Isolation
Temporal Isolation
Different species mate at different times
Times of day
Times of year
Trees and plants

26. Patterns in Evolution
Convergent Evolution – evolution towards similar characteristics in unrelated species
Analogous structures
Bird and bat wings
Evolution occurs because animals are subjected to similar pressures

27. Patterns in Evolution
Divergent Evolution – closely related species in different directions
Results from adapting to different environments
Kit and Red Fox
Kit fox lives in the desert
Red fox in the forest

28. Patterns in Evolution
Coevolution – two or more species evolve in response to changes in each other
Benefit to both species
Plants and bees

29. Patterns in Evolution
Coevolution can also drive predator-prey relationships
Cheetahs and gazelles

30. Patterns in Evolution Extinctions – species are eliminated
Background extinctions – occur continously, but at very low rates
Mass extinctions – rare, more intense, involve many species

31. Patterns in Evolution
Punctuated equilibrium – bursts of evolutionary activity followed by periods of inactivity
Adaptive radiation – diversification of multiple species from one ancestral species
Mammals