1. Purposeful Population Genetics
Chapter 23

2. He couldn’t explain the variation! He didn’t know Genetics!
I. Darwin’s Problem
He couldn’t explain the variation! He didn’t know Genetics!

3. Populations evolve (not individuals), natural selection is the most important mechanism of evolution, and gradualism explains how small changes accumulate over time to cause larger changes (like new species)

4. II. Vocabulary
Species – group of individuals that can interbreed and produce fertile offspring
Population – group of individuals of the same species, living in the same place at the same time
Individuals near the population center are more likey to interbreed in the same populations, and are therefore more closely related

5. Vocabulary (cont)
Gene pool – all of the genes in a population at any one time
diploid organisms – have 2 copies of each gene and can be homozygous or heterozygous
if ALL organisms in a population are homozygous for the same allele the allele is fixed in the gene pool

6. III. Hardy-Weinberg Theorem
Gene pool of a NON-EVOLVING population
If a population is not evolving, the frequencies of alleles and genotypes stay constant over time
Hardy-Weinberg Equilibrium – occurs when Mendelian genetics (crossing over/meiosis) do not alter the genetic frequencies of a population – this state does NOT normally exist in nature for a long period of time

7. IV. How do figure out gene frequency?
Frequency of r = # of r Total
Frequency of R= # of R Total

8. Example
In wildflowers – Red color (R) is dominant to white color (r). There are 500 plants in a population. 20 plants are white (rr) are Red are RR, 160 are Rr.
What is the TOTAL number of genes in the population?
1000 (500 x 2)

9. Frequency of r = # of r = 20+20+160 =200 =0.2 Total 1000 1000

10. Predicting Next Generation…
To determine the frequencies of the next generation, use the data above to predict. Assume random mating:
Chance of RR =
.8 x .8 = .64
Chance of Rr =
2 x .8 x .2 = (x2 because Rr or rR)
Chance of rr =
.2 x .2 = .04

11. Allele frequencies remain constant
Freq R = .8 Freq r = .2
So after one generation, this population has remained stable

12. Hardy Weinberg Equation
Scientists use p to represent frequency of one allele, and q to represent the frequency of the other allele
For problem above p = .8 q = .2
p + q = 1
Genotype frequencies p2 + 2pq + q RR Rr rr

13. Practice
One out of babies is born with PKU. The disease is caused by a recessive allele. What is the frequency of carriers in the population (Aa)?

14. Solution q2 = .00001 q = .01 p + q = 1 so p = .99
Frequency of carriers (Aa) = 2pq = 2 x .01 x .99 = = 2%

15. Hardy Weinberg cont
In order for a population to be in Hardy-Weinberg equilibrium (non-evolving), the following conditions must occur:
1. Very large population size
2. No migration
3. No net mutations
4. Random mating
5. No natural selection

16. Hardy Weinberg cont
Populations in nature are not expected to be in equilibrium and because of this, the population is evolving.

17. Microevolution
generation to generation change in a population’s frequency of alleles. Some genotypes may be in equilibrium while others are not. This population is still evolving.

18. There are 4 causes of microevolution:
1. Genetic drift – change in a population’s allele frequency due to chance
Occurs when working with a SMALL population
The gene pool of a small population may not be accurately represented in the next generation

20. Genetic drift (cont) Examples
a. Bottleneck effect: due to disasters such as earthquakes, fires etc reducing the population size dramatically
-the new population may not represent the original gene pool – some alleles may be eliminated totally.
b. Founder effect: occurs when a few individuals from a larger population colonize an isolated island, lake or other new habitat
-may cause a high frequency of inherited diseases in some human populations

22. There are 4 causes of microevolution:
2. Natural Selection – occurs when some variations help organisms survive better in an environment than others. This goes against Hardy-Weinberg.
-Traits that help in survival are more likely to be passed on to the next generation because those organisms are able to reproduce
-This is the only evolutionary factor that adapts a population to it’s environment

23. There are 4 causes of microevolution:
3. Gene Flow – occurs when a population gains or loses alleles due to migration (either in or out)
-can bring neighboring populations together to one

24. There are 4 causes of microevolution:
4. Mutation – change in an organisms DNA
-may change one allele to another and therefore alter the genotype frequency of a population
-overall, mutations are rare events, but they are still important because they can be a source of new variations that may help organisms survive changing conditions in their environment

25. Genetic Variation provides the raw material for Natural Selection
1. Variation within populations:
all populations have variation (easy to see in humans, not always as easy in other organisms
in order for natural selection to occur, the variation MUST be heritable (in the genes)

26. Variation within populations
*Quantitative Characters – vary along a continuum (i.e. height) – usually a result of polygenic inheritance – more than one gene controls the trait
*Discrete characters – either/or (i.e. red or white flower) – usually controlled by only one gene
If two or more distinct characters are seen in high frequency within a population it is said to be Polymorphic

27. Variation within populations
*Measuring diversity
gene diversity – measure of genetic variation in whole genes (humans have about 14% gene diversity)
nucleotide diversity – done by comparing nucleotide sequences of two individuals, and then pooling data from many comparisons (humans only about 0.1% diverse in nucleotides)

28. 2. Variation between populations
-Geographic variation - differences in gene pools between populations or subgroups of populations (due to differences in environments)
*cline – graded change in a trait along a geographic axis (i.e. avg body size of many birds increases with increasing latitude)

30. Generating Genetic Variety
1. Mutation – the ONLY source of new alleles in a population
*most occur in somatic cells and can’t be passed on
*mutations only have an effect if they are in gametes
*most mutations are harmless but some can have a significant impact (usually negative)
*On rare occasions – a mutation actually helps an organism survive in a changing environment
*Mutation can be significant in microorganisms due to their short generation span

31. Generating Genetic Variety
2. Sexual Reproduction – accounts for nearly all of the genetic differences of sexually reproducing organisms
*crossing over during meiosis
*random fertilization of egg/sperm
*independent assortment of alleles

32. Preserving Variation
natural selection might eliminate variety but the variation is preserved by the following methods:
1. Diploidy – most eukaryotes have two copies of each allele (diploid). Because of this, variety (due to recessive alleles) can be hidden in heterozygotes
*Heterozygote protection helps keep the allele frequency variable and this can help if the environment changes

33. Preserving Variation
2. Balanced Polymorphism – ability of natural selection to maintain stable frequencies of two or more phenotypic forms in a population
A. Heterozygote advantage – individuals that are heterozygous survive better and have a higher reproductive success than homozygous individuals. (i.e. sickle cell anemia)

34. Balanced Polymorphism
B. Frequency-dependent selection – survival and reproduction of one morph declines if that phenotype becomes too common in the population (i.e.parasite/host)

35. Preserving Variation
3. Neutral variation – gives no selective advantage for one individual over another (i.e. different fingerprints)

36. Evolutionary Fitness
contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

37. Relative Fitness
contribution of a genotype to the next generation compared to alternative genotypes
*The most reproductively successful variant has a relative fitness of 1. Other genotypes are determined as a percentage of the most successful genotype

38. Effect of Selection on a Population
1. Directional selection – usually seen during periods of environmental change or migration
*the frequency curve for variation is shifted in one direction or another by favoring relatively rare individuals

39. Effect of Selection on a Population
2. Diversifying Selection – occurs when environmental conditions favor individuals on both extremes of a phenotypic range instead of intermediate phenotypes

40. Effect of Selection on a Population
3. Stabilizing selection – favors intermediate phenotypes over the extremes
*reduces variation and maintains status quo

41. Sexual Selection
secondary sex characteristics cause marked differences between males/ females called sexual dimorphism – this can cause differences in size, plumage, adornments etc

42. Sexual Selection
A. Intrasexual selection – direct competition among individuals of one sex for mates of the opposite sex ( i.e. males fighting for a female)

43. Sexual Selection B. Intersexual selection – mate choice
*individuals of one sex are choosy in selecting their mates from individuals of the other sex

44. Why not “perfect” organisms after natural selection?
1. Evolution is limited by historical constraints – organisms are not built from scratch but use existing structures to adapt to new situations (i.e. back pain may be due to skeleton being better adapted for four legged existence)

45. Why not “perfect” organisms after natural selection?
2. Adaptations are often compromises – organisms must do many tasks. An adaptation that helps in one area may hinder another (seals live on rocks and water-legs might be more helpful for rocks, but fins in water outweigh the benefits of legs)

46. Why not “perfect” organisms after natural selection?
3. Not all evolution is adaptive – chance affects the genetic structure of populations (i.e. storms blowing insects/seed to new areas)
4. Selection can only edit existing variations – new alleles do not arise as an organism needs them.