1. Ch. 16- Genes and Variation

2. What you will learn today…
How do we measure genetic variation in a population?
What are the sources of genetic variation in a population?
Why is genetic variation in a population important?

3. What determines a heritable trait?
DNA
(gene)
Observed trait
mRNA
protein
translation
transcription
protein function
(enzyme activity)
Therefore, if traits vary in a population, then the
genes (alleles) must vary in the population!

4. How do we measure genetic variation in a population?
Gene Pool- Total genetic information available in a population (all the alleles that are present). Allele (Relative) Frequency- The percentage of an allele in the gene pool. Tells you whether a given allele is common or rare (%)

5. New Vocab
Population – group of individuals of the same species that interbreed
Gene Pool – all genes (including all alleles) present in a population
Relative Frequency – number of times an allele occurs in a gene pool
Because members of a population interbreed, they share a common group of genes called a gene pool

6. A population with variation in traits…
Grey
White
Tall ears
Short ears

7. ...is caused by variation in alleles
Grey allele = G
White allele = g
Tall ear allele = T
Short ear allele = t
T t
G G
t t
G g
T T
G g
t t
g g
t t
G g
t t
G G
T t
g g
t t
G g

8. How do we measure genetic variation in a population?
Grey allele = G
White allele = g
Tall ear allele = T
Short ear allele = t
8 / 16 = 50% G
T t
G G
t t
G g
T T
G g
t t
g g
8 / 16 = 50% g
t t
G g
t t
G G
4 / 16 = 25% T
T t
g g
t t
G g
12 / 16 = 75% t
“Gene Pool”

9. Why is genetic variation in a population important?
A gene pool without much variation limits a species’ ability to further evolve.
Evolution- change over time in the gene pools of a species
If populations do not change (adapt) to their environment, they may become extinct.

10. Sources of genetic variation
SEXUAL REPRODUCTION
Meiosis – one allele is passed on from each parent (recall that sperm and eggs are haploid cells, each containing half the necessary genetic information).
Random fertilization – only one of the millions of sperm involved in mating will fertilize the egg.
The randomness of sexual reproduction explains why siblings can look so different.

11. Crossing over during meiosis

12. Sources of genetic variation
MUTATION
A change in DNA sequence.
New DNA sequence = new allele of a gene.
Many mutations produce genes that are harmful (e.g. Huntington’s disease)
Some mutations produce genes that are neutral (neither helpful nor harmful)
Very, very few mutations produce genes that are advantageous, beneficial

13. Mutations add new alleles to the gene pool
Mutations add new alleles to the gene pool. That is, they increase the variety of alleles in the population.

14. Deck of Cards Analogy
Deck is Gene Pool – It contains all possible alleles for the next generation.
Drawing cards picks the alleles that are inherited by the next generation.
Shuffling of the deck is sexual reproduction.
Adding new cards to the deck is mutation.
(Mutation is rare, but shuffling happens each time a new generation is produced)

15. Natural Selection
Individuals with advantageous genes survive to reproduce and pass on these genes to their offspring.
Individuals without advantages genes do not survive to reproduce, and these genes do not get passed on in the population.

16. Kerosene Karl- Changes in Allelic Frequency

17. Allele Frequency example
Warmup:
Allele Frequency example
Figure 16-2, pg work out the frequency of each allele.
Sample Population
Frequency of Alleles
allele for brown fur, b
allele for black fur, B
48% heterozygous black, Bb
16% homozygous black, BB
36% homozygous brown, bb
Population = mice
Gene Pool – all possible genes/alleles for coat color
Relative frequency of each allele
20 alleles are black (40%), 30 alleles are brown (60%).

18. Sources of Genetic Variation
Mutations
Mistakes in replication
Radiation or chemicals in environment
Gene Shuffling
Assortment of chromosomes
Crossing over
Mutation – change in DNA sequence
don’t always affect phenotype
some affect fitness; other’s don’t
Gene Shuffling – during meiosis
independent assortment of chromosomes from each parent
23 pairs of chromosomes can produce 8.4 million different combinations of genes
sexual reproduction is major source of variation within population – can produce many different phenotypes but does NOT change relative frequency of alleles

19. Single-Gene Trait
Variation in single gene traits lead to only two distinct phenotypes
Frequency of phenotype determined by frequency of alleles
100 80 60 40 20
Frequency of Phenotype (%)
The number of phenotypes produced for a given trait depends on how many genes control the trait
Widow’s peak
No widow’s peak
Phenotype

20. Frequency of Phenotype
Polygenic Trait
Trait controlled by 2 or more genes
Many possible genotype and phenotype possibilities
Bell shaped curve typical of polygenic traits
Frequency of Phenotype
Phenotype (height)

21. CHECK POINT How do we measure genetic variation in a population?
What are the sources of genetic variation in a population?
Why is genetic variation important in a population?

22. Homework Read section 16-1 in textbook (pages 393-396)
Complete worksheet 16-1: Genes and Variations

23. Natural Selection on Polygenic Traits
Directional Selection
Low mortality,
high fitness
High mortality, low fitness
Food becomes scarce
Individuals at one end of curve have higher fitness
Range of phenotypes shifts

24. Natural Selection on Polygenic Traits
Individuals near center of curve have highest fitness
Keeps center of curve at same position and narrows graph
Stabilizing Selection
Low mortality,
high fitness
High mortality,
low fitness
Selection against both extremes keeps curve narrow and in same place
Percentage of Population
Birth Weight

25. Natural Selection on Polygenic Traits
Disruptive Selection
Largest and smallest seeds become more common
Population splits into two subgroups specializing in different
seeds.
Low mortality,
high fitness
Number of Birds
in Population
Number of Birds
in Population
High mortality,
Low fitness
Beak Size
Beak Size
Individuals at upper and lower ends of the curve have higher fitness than individuals in the middle
Selection acts strongly against individuals of the intermediate type

26. Genetic Drift Random change in allele frequency by chance
Occurs in small populations
Founder effect – allele frequencies change as a result of migration of subgroup of population A a
1st (26)
.62
.38
3rd (28)
.30
.70
4th (30)
.37
.63
5th (32)
.52
.48
6th (28)
.34
.66
In small populations, an allele can become more or less common simply by chance
Genetics controlled by laws of probability – works in large population not always in small population
In small populations, individuals that carry a particular allele may leave more descendents than others just by chance. Over time a series of chance occurrences of this type can cause an allele to become common in a population
May occur when a small group of individuals colonize new habitat

27. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

28. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

29. Genetic Drift Section 16-2 Sample of Original Population Descendants
Founding Population A
Founding Population B

30. Hardy-Weinberg Principle- Are there any conditions when evolution DOES NOT happen?
States that allele frequencies remain constant (genetic equilibrium) unless one or more factors cause them to change
No change in allele frequency of population = no evolution in population
To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place.
Are there any conditions under which evolution will not occur?

31. Mechanisms for Evolution
Random genetic drift
Gene flow
Non-random mating
Mutation
Natural selection

32. 5 Conditions of Genetic Equilibrium- 1 or more of these must happen in order for evolution to occur
Random Mating
Equal chance of passing on alleles to offspring
Large Populations
Genetic drift less likely to occur
No Movement In or Out of Population
New members might bring new alleles
Random mating ensures that everyone has an equal chance of passing on their alleles. In natural populations, mating is rarely completely random- species select mates with traits they want. Nonrandom mating means genes for those selected traits are NOT in equilibrium, but are under strong selection pressure.
Genes drifting “in and out” of a large population will have less of an effect on the gene pool of a small population.
In new individuals come n, new alleles will too- the gene pool will change.

33. 5 Conditions of Genetic Equilibrium
No Mutations
New alleles may be introduced
No Natural Selection
All genotypes must have equal chance of survival and reproduction
4. Genes mutate = allele frequencies will change.
5. No phenotype must have selective advantage over the others.

34. Speciation Speciation = formation of new species
Species = group of organisms that breed with one another and produce fertile offspring
As new species evolve, populations become reproductively isolated from each other
Members of same species share same gene pool
Genetic change that occurs in one individual can spread through the population
Gene pools must become separated for speciation to occur
When two population cannot interbreed and produce fertile offspring, reproductive isolation has occurred

35. Isolating Mechanisms (lacewing interactive)
Behavioral Isolation
Differences in courtship rituals or other reproductive strategies
Geographic Isolation
Two populations separated by geographic barrier such as rivers, mountains or bodies of water
Temporal Isolation
Two or more species reproduce at different times of the day or year

36. Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Temporal isolation
Geographic isolation
produced by
produced by
produced by
Behavioral differences
Different mating times
Physical separation
which result in
Independently evolving populations
which result in
Formation of new species

37. Speciation of Darwin’s Finches
Founders Arrive
Separation of Populations
Changes in the Gene Pool
Reproductive Isolation
Ecological Competition
Continued Evolution

38. Speciation in the Andes (Ecuador)
Hummingbird video
Explain the hypothesis presented by the scientists profiled in this segment to explain the process of speciation in hummingbirds and possibly other species.
How does this hypothesis differ from the traditional view that speciation often requires geographic separation of populations?
Why were the researchers collecting blood from the populations they studied? Discuss at least two possible analyses that could be performed on those samples and, identify at least two different questions that might be answered with sufficient data.