1. Evolution of Populations

2. 23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution possible
1) Microevolution
Change in the allele frequencies of a population over generations
Evolution on the smallest scale

3. 2) Mutations The only source of NEW genes & NEW alleles
Only mutations in cell lines that produce gametes can be passed on to offspring

4. Types of Mutations A) Point Mutation B) Chromosomal Mutation
Change in one base in a gene
Can impact phenotype
Sickle cell anemia
B) Chromosomal Mutation
Delete, disrupt, duplicate, or rearrange many loci at once
Most are harmful, but not always

5. 3) Variations due to sexual reproduction
Rearranges alleles into new combinations in every generation
3 mechanisms for this shuffling:
Next slide

6. 2) Independent assortment
1) Crossing over
During Prophase I of meiosis
2) Independent assortment
During meiosis (223 different combinations possible)
3) Fertilization
223 x 223 for sperm and egg

7. Population genetics Population
23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
Population genetics
Study of how populations change genetically over time
Population
Group of individuals of the same species that live in the same area
Interbreed & produce fertile offspring

9. Gene pool
All of the alleles at all loci in all the members of a population
In diploids, each individual has 2 alleles for a gene & the individual can be heterozygous or homozygous
If all are homozygous for an allele, the allele is FIXED – only one allele exists at the locus in the population
The greater the # of FIXED alleles, the lower the species’ diversity

10. Hardy-Weinberg Used to describe a population that is NOT evolving
Frequencies of alleles & genes in a gene pool will remain constant over generations

12. 5 Conditions for Hardy-Weinberg
1) No mutations
2) Random mating
3) No natural selection
4) The population size must be large (no genetic drift)
5) No gene flow (Emigration, immigration, transfer of pollen, etc.)

13. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2 + 2pq + q2 = 1
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

14. Practice
Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

15. Mutations can alter gene frequency, but are rare
23.3 – Natural Selection, genetic drift, & gene flow can alter allele frequencies in a population
Mutations can alter gene frequency, but are rare
3 major factors alter allelic frequencies
1) Natural selection
Alleles are passed to the next generation in proportions different from their frequencies to the present generation
Those that are better suited produce more offspring than those that are not

16. 2) Genetic Drift
Unpredictable fluctuation in frequencies from one generation to the next
The smaller the population, the greater chance
Random & nonadaptive
A) Founder effect = individuals are isolated and establish a new population – gene pool is not reflective of the source population
B) Bottleneck effect = a sudden change in the environment reduces population size – survivors have a gene pool that no longer reflects original

18. Genetic drift is significant in small populations
Genetic drift causes allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed

19. 3) Gene Flow
Populations loses or gains alleles by genetic additions or subtractions
Results from movement of fertile individuals or gametes
Reduces the genetic differences between populations, makes populations more similar

21. 23.4 Natural Selection is the only mechanism that consistently causes adaptive evolution
Relative fitness
The contribution an organism makes to the gene pool of the next generation relative to the contributions of the other members
Does NOT indicate strength or size
Measured by reproductive success

22. Can alter the frequency distribution of heritable traits in 3 ways:
Natural selection acts more directly on the phenotype and indirectly on the genotype
Can alter the frequency distribution of heritable traits in 3 ways:
1) Directional selection
2) Disruptive selection
3) Stabilizing selection

23. 1) Directional selection
Individuals with one extreme of a phenotypic range are favored, shifting the curve toward this extreme
Example: Large black bears survived periods of extreme cold better than small ones, so they became more common during glacial periods

24. 2) Disruptive Selection
Occurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes
Example: A population has individuals with either large beaks or small beaks, but few with intermediate – apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are available

25. 3) Stabilizing Selection
Acts against both extreme phenotypes and favors intermediate variations
Example: Birth weights of most humans lie in a narrow range, as those babies who are very large or very small have higher mortality rates

27. How is genetic variation preserved in a population?
Diploidy
Capable of hiding genetic variation (recessive alleles) from selection
Heterozygote advantage
Individuals that are heterozygous at a certain locus have an advantage for survival
Sickle cell anemia – homozygous for normal hemoglobin are more susceptible to malaria, homozygous recessive have sickle-cell, but those that are heterozygotes are protected from malaria and sickle-cell

29. Why Natural Selection cannot produce perfect organisms:
1) Selection can only edit existing variations
2) Evolution is limited by historical constraints
3) Adaptations are often compromises
4) Chance, natural selection, & the environment interact