1. Bellwork: What indicates that a population is evolving
Bellwork: What indicates that a population is evolving? What are some of the key sources of genetic variation?

2. Genes and Variation
Section 17.1

3. How are genetics and evolution linked?
When Mendel’s work was rediscovered around 1900, genetic research took off!
Discovered that heritable traits are controlled by genes found on chromosomes
Changes to genes and chromosome generate variation
Variation is key to natural selection
Modern genetics allows us to understand evolution better than Darwin ever could

4. Genotype and Phenotype in Evolution
Plants and animals typically have one gene from each parent
Specific forms of this gene (alleles) will vary from individual to individual
Genotype = combination of alleles
Genotype and environmental conditions give an organism it’s phenotype
Natural selection acts on phenotype not genotype
Some phenotypes are better suited to the environment than others
Organisms with this phenotype will be fitter, and produce more offspring
Natural selection refers to an entire organism, not an individual gene

5. What is a population?
A group of individuals of the same species that mate and produce offspring
Members of a population interbreed - they contain a common group of genes – a gene pool
A gene pool is all of the genes, including every allele present for each gene, in a population
Researches study gene pools by investigating the numbers of different alleles they contain
Allele frequency is the number of times an allele occurs in a gene pool
Has nothing to do with whether an allele is dominant or recessive

7. Evolution in genetic terms
Evolution is the change in the frequency of alleles over time
It is populations not individuals the evolve
Natural selection operates on individual organisms,
BUT the changes it causes on allele frequency will show up in the population as a whole

8. What are sources of genetic variation?
Heritable variation can be produces in one of three ways:
Mutations
Genetic recombination in sexual reproduction
Lateral gene transfer

9. Mutation A mutation is any change in the genetic material of a cell
Some can involved changes of an individual gene, others large pieces of a chromosome
Not all mutations will change an organisms phenotype (neutral mutations)
Not all mutations that affect phenotype will affect fitness
Some can have positive impacts, some very negative and serious
Mutations are pretty common
Each of us is thought to have 300 million mutations that make our DNA different from our parents
Most of these are neutral
One or two may potentially be harmful
Mutations only matter if the can be passed from generation to generation – Germ line cells.
Cells that go on to produce egg or sperm cells

10. Genetic Recombination in Sexual reproduction
Most heritable differences are not due to mutations, but due to genetic recombination during sexual reproduction
During Meiosis, each chromosome will move independently
Give rise to 8.4 million possible gene combinations
Crossing over during meiosis produces genetic variation
Explains why siblings are not identical (apart from identical twins)
Explains why individual members of a species differ from one another

11. Lateral gene transfer
In Eukaryotic organisms, genes are passed from parent to offspring
In some organisms however, this is not the case
Bacteria can swap genes on plasmids as if they are trading cards
Passing of genes from one organism to another that is not it’s offspring is lateral gene transfer
Can occur between organisms of the same or different species
Important for antibiotic resistance in bacteria
Most applicable to single celled organisms

12. Single-gene and polygenic traits
Sometime one genes controls a trait, other times, multiple genes can control a trait
The number of phenotypes produced for a trait depends on how many genes control the trait
A single gene trait, is a trait controlled by only one gene
Phenotypes are distnict
In this situation, in a population a recessive allele can be more dominant
Polygenic traits are controlled by two or more genes
Multiple possible genotypes and phenotypes
Phenotypes are not distinct
Form a normal distribution – example of height

13. Single gene and polygenic traits

14. Evolution as genetic change in population
Section 17.2

15. Why don’t pesticides always work?
At first virtually all of the targeted insects are killed off
BUT, a few individuals will survive
These will survive and reproduce
Gives rise to a population of insects with a natural resistance to this insecticide

16. How does natural selection work?
Evolutionary fitness is success in passing on genes to it’s offspring
But, there is a difference between single trait and polygenic genes
Single trait genes
Natural selection can lead to changes in allele frequencies and changes in phenotype frequency

17. Natural selection of polygenic traits
Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selection
Directional selection
Stabilizing selection
Disruptive Selection

18. Directional selection
If individuals at one end of the curve are fitter than the other end directional selection occurs
Phenotypes shift, because some individuals are better at surviving and reproducing than others
Example – Darwin’s finches beak size, peppered moths

19. Stabilizing selection
When individuals near the center of the curve have a higher fitness, stabilizing selection occurs
Curve stays in the same place, but the overall curve will narrow
Example – birth weight of infant babies – those with an average mass are more likely to survive

20. Disruptive selection
When individuals at the two extremes of the curve have a higher fitness, disruptive selection occurs
Acts against individuals of an intermediate type
This can eventually lead to two distinct phenotypes
Example – are where medium size seeds become less common

21. What is genetic drift?
In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance
A series of chance occurrences can cause an allele to become less common in a population
This random change in allele frequency is called genetic drift

22. Genetic bottlenecks
Sometimes a disaster can kill off many individuals in a population
By chance, this may change the allele frequency in the gene pool
Bottleneck effect : change in allele frequency following a dramatic reduction in the size of the population
A severe bottleneck can sharply reduce a population’s genetic diversity

23. The founder effect
Genetic drift may occur when a few individuals colonize a new habitat
The alleles in these founders may differ in relative frequencies from those of the main population
Again this would occur by chance
When allele frequencies change as a result of migration of a small subgroup is called the founder effect

24. Evolution vs genetic equilibrium
If a population is not evolving then, allele frequencies in it’s gene pool will not change, and it is in genetic equilibrium
Sexual reproduction and allele frequency
Gene shuffling during sexual reproduction produces many gene combinations, but meiosis and fertilization by themselves do not change allele frequencies
Hardy-Weinberg Principle
Allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change
Like making a Punnett square for a population, not an individual

25. Hardy-Weinberg principle
Using these equations, it is possible to predict phenotype frequencies
If a population doesn’t show these predicted frequencies, the evolution is taking place

26. Exception to Hardy – Weinberg principle
There are five predicted conditions that can disturb genetic equilibrium, and result in evolution occurring
Normally at least one of them is happening and evolution is occurring
Non random mating
In practice not common – organisms choose a partner on the basis of heritable traits – size, strength, color. Forms basis of sexual selection
Small population size
Evolutionary change due to genetic drift is more common in small populations
Immigration or emigration
New individuals may bring in new alleles, people who leave may remove alleles
Mutations
New alleles can be introduced into the gene pool
Natural selection
If genotypes result in different fitness, genetic equilibrium will be disrupted and evolution may occur