1. Population Genetics and Speciation
Chapter 16
Population Genetics and Speciation

2. Section 1 Vocabulary Pretest
Population Genetics
Microevolution
Gene Pool
Allele Frequency
Phenotype Frequency
Total genetic information in a population
Portion of gene copies of a given allele
Study of the frequency and interaction of alleles and genes in populations
Change in the collective genetic material of a population
Ratio of individuals with a given phenotype to the total population

3. Answer Key Population Genetics C Microevolution D Gene Pool A
Allele Frequency B
Phenotype Frequency E

4. Population Genetics
Population Genetics is the study of evolution from a genetic point of view (it is the study of microevolution)
Microevolution—a change in the collective genetic material of a population
Population–members of the same species that can interbreed. It is the smallest unit in which evolution occurs.

5. Variation Populations show natural variety within a species.
Many quantitative traits (height and weight etc.) follow a bell shaped curve.

6. Causes of Variation
Environmental factors—amount of food, quality of food, etc.
Genetic factors
Mutations—random changes in genes
Recombination—reshuffling of genes
Random pairing of gametes

7. Gene Pool
Gene pool —total genetic information available in a population

8. Allele and Phenotypic Frequency
Allele frequency—expressed as a percent: it is determined by dividing the number of a certain allele by the total number of alleles of all types in the population
Phenotypic frequency—expressed as a percent: it is the number of individual with a particular phenotype divided by the total number of individuals in the population.

9. Hardy-Weinberg Genetic Equilibrium
Developed by Wilhelm Weinberg (German physician) and Godfrey Hardy (British mathematician)
Godfrey Hardy
Wilhelm Weinberg

10. It is based on a “hypothetical population” that is not evolving.
States that genetic frequencies in a population tend to remain the same from generation to generation unless acted on by outside influences.
It is based on a “hypothetical population” that is not evolving.

11. Conditions of Hardy-Weinberg Equilibrium
In Genetic Equilibrium:
No net mutations occur
Population size remains constant
The population is infinitely large
Individuals mate randomly
Selection does not occur
This flock of mallards probably
violates some or all of the
conditions necessary for the
Hardy-Weinberg genetic
equilibrium

12. It is highly unlikely that all five of the conditions in the Hardy-Weinberg Model will happen in the real world.
Therefore, Genetic Equilibrium is impossible in nature.
It is a theoretical state that allows us to consider what forces could disrupt such balance (equilibrium).

13. Section 2 Vocabulary Pretest
Immigration
Emigration
Gene Flow
Genetic Drift
Sexual Selection
Stabilizing Selection
Disruptive Selection
Directional Selection
Individuals move out
Individuals move in
Choice of mates based on favorable traits
Genes move from one population to another
Average trait is selected
One extreme trait is selected
Two extreme traits are selected
Allele frequencies change randomly

14. Answer Key Immigration B Emigration A Gene Flow D Genetic Drift H
Sexual Selection C
Stabilizing Selection E
Disruptive Selection G
Directional Selection F

15. Disruption of Genetic Equilibrium
Disruptions to the Hardy-Weinberg equilibrium can result in evolution.
The five requirements for genetic equilibrium can be disrupted by the following outside forces:
Mutation
Gene Flow
Genetic Drift
Nonrandom Mating
Natural Selection

16. Requirement #1: No Net Mutations Occur
Mutations occur constantly at very low rates under normal conditions.
Exposure to mutagens (mutation-causing agents, i.e. radiation and chemicals) can increase mutations rates.
Mutations produce new alleles for a trait
They can be harmful, harmless or helpful
Helpful mutations are a vital part of evolution.

17. Requirement #2: Population Size Remains Constant
Individuals enter and leave populations constantly. Their “genes” move with them. This is called Gene Flow.
Factors influencing gene flow include:
Immigration—movement of individuals into a population
Emigration—movement of individuals out of a population
Migration and dispersal patterns can also influence the movement of individuals into new populations
Birth and Death Rates also remove or add genes from individuals to a population.

18. Requirement #3 Population is Infinitely Large
In nature, population sizes are restricted rather than infinitely large.
Genetic Drift can occur in small populations of organisms
Genetic Drift—the random change in allele frequency in a population
Significant changes can happen in small populations if even a single organism either fails to reproduce or reproduces too much.

19.
If the frequency of an allele reaches zero in a population, then (assuming you started with two alleles), there is only one left.
All individuals will be homozygous for that trait---creating no variations.
This weakens a species.
Ex: Northern Elephant Seal
Homozygous for every gene tested

20. Bottleneck Effect
Genetic Drift can lead to a bottleneck effect in which variations are reduced overtime.

21. Requirement #4: Random Matings
Organisms do not mate randomly in nature.
Mate selection is influence by:
Geographic proximity— choose mates nearby: can result in kinship mating
Assortative Mating— choose mates with similar traits: reduces variation
Sexual Selection—choose mates based on favorable traits

22. Requirement #5: Selection Does Not Occur
Natural Selection—organisms with favorable traits are more likely to survive and reproduce, passing on their favorable genes to the next generation.
It is an ongoing process in nature and an important disruption to equilibrium.
Three patterns of Natural Selection:

23. Stabilizing Selection: individuals with the average form of a trait have the highest fitness.
Ex: Lizard body size

24. Ex: Shell Color of Limpets
Disruptive Selection: individuals with either extreme variation of a trait have the highest fitness.
Ex: Shell Color of Limpets

25. Ex: Nose and tongue lengths of anteaters
Directional Selection: individuals with one extreme of a trait have the highest fitness
Ex: Nose and tongue lengths of anteaters

26. Section 3 Vocabulary Pretest
Speciation
Geographic Isolation
Allopatric Speciation
Reproductive Isolation
Sympatric Speciation
Gradualism
Punctuated Equilibrium
A slow change in a species over millions of years
Bursts of rapid change
Formation of a new species
Physical separation of populations
Inability to mate or produce offspring
Speciation resulting from geographic isolation
Speciation resulting from reproductive isolation

27. Answer Key Speciation C Geographic Isolation D Allopatric Speciation F
Reproductive Isolation E
Sympatric Speciation G
Gradualism A
Punctuated Equilibrium B

28. Speciation Speciation—formation of a new species
Species: a single kind of organism whose members are morphologically similar and can interbreed to produce fully fertile offspring.
Two types of speciation: Allopatric Speciation and Sympatric Speciation

29. Allopatric Speciation
Allopatric Speciation: species arise as a result of geographic isolation. (Allopatric = different homelands)
Geographic Isolation—physical separation of members of a population
Gene flow between the new subpopulations stops and the two begin to diverge
Eventually, they become incompatible for mating, creating new species.
Debate exists as to whether or not allopatric species are different enough to be considered new species.

30. Examples of Geographic Isolation

31. Sympatric Speciation
Sympatric Speciation —occurs when two subpopulations become reproductively isolated within the same geographic area.
Reproductive Isolation—the inability of members of the same species to mate
Can be caused by disruptive selection
Two types: prezygotic isolation and postzygotic isolation

32. Prezygotic (premating) isolation: (different mating seasons, different mating calls, etc.)
Behavioral Isolation

33. Habitat
Isolation
Other Prezygotic Isolation Mechanisms
include
Mechanical
Isolation
Gametic
Isolation
Temporal
Isolation

34. Postzygotic (postmating) isolation: offspring do not fully develop, die, or are infertile.
Hybrid is weak and likely to die
Hybrid is sterile

36. Rates of Speciation Two Theories:
Gradualism —slow change over millions of years
Punctuated Equilibrium—short bursts of rapid change

37. Evidence exists that suggests that both have taken place over time.

38. The Hardy-Weinberg Equation
Hardy and Weinberg went on to develop an equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.
The equation is: p2+2pq+q2 = 1
p= frequency of the dominant allele
q = frequency of the recessive allele
See handout

39. Significance of the Hardy-Weinberg Equation
Punnett Squares allow geneticists to predict the probability of offspring genotypes for particular traits based on the known genotypes of their two parents
The Hardy-Weinberg equation essentially allowed geneticists to do the same thing for entire populations.

40. Before Hardy and Weinberg, it was thought that dominant alleles must, over time, wipe out recessive alleles (genophagy = “gene eating”)
According to this wrong idea, dominant alleles always increase in frequency from generation to generation.
Hardy and Weinberg demonstrated that dominant alleles can just as easily decrease in frequency.