1. Agenda 9/29 Results of Evolution Lecture Sickle cell case study
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3. Three Modes of Natural Selection
Natural selection can alter the frequency distribution of heritable traits in three ways depending on which phenotype is favored:
Directional Selection
Disruptive Selection
Stabilizing Selection
The next few slides deal with a hypothetical deer mouse population with heritable variation in fur coloration from light to dark.
Emphasize to students that the width of the curve corresponds to variance (numerical range of the x-axis) and the “peak” corresponds to the mean (corresponds to the numerical value on the y-axis). This type of curve goes by many names: normal distribution, bell curve, optimum, etc.
When a particular trait confers an advantage to an organism in a given environment, then one would expect a change in the gene pool over time. The next slides explain the three types of selection with examples for each type illustrating how the gene pool changes.

4. Directional Selection
Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range.
Commonly occurs when the population’s environment changes or when members of a population migrate to a new (and different) habitat.
Emphasize that the arrow symbolizes selective pressures AGAINST certain phenotypes. Ask students to interpret the “before and the after” regarding these two graphs. Ask “What’s the scoop?” They should conclude that the environment changed such that the lighter mice were selected against. Perhaps they live among dark rocks, etc. which makes it harder for them to hide from predators. Revisit the peppered moth as an example of directional selection.
Describe a trait in humans that follows this pattern.

5. Possible Effect of Continual Directional Selection
If continued, the variance may decrease.
before
after
before
after
before
after
Frequency
Frequency
Frequency
Ask students to propose possible consequences of continual directional selection (extinction or speciation).
Phenotype (trait)
Phenotype (trait)
Phenotype (trait)

6. Disruptive or Diversifying Selection
Disruptive selection occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.
The “intermediates” in the population have lower relative fitness.
The mice in (b) have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are selected AGAINST as indicated by the arrow.
This art is from the 8th edition of Campbell, revised art from the 9th edition has (b) labeled as “Disruptive selection”. Once again, synonyms are troublesome for students!
Describe a trait in humans that follows this pattern.

7. Stabilizing Selection
Stabilizing selection removes extreme variants from the population and preserves intermediate types.
This reduces variation and tends to maintain the status quo for a particular phenotypic character.
Ask students to explain the significance of the shape change of the graph in (c). They should point out that the value of the mean has increased and the variance of the population has decreased.
If the environment consists of rocks of an intermediate color, both light and dark mice will be selected AGAINST.
Describe a trait in humans that follows this pattern.

8. Sexual Selection
A form of selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.
Can result in sexual dimorphism which is a difference between the two sexes with regard to secondary sexual characteristics.
Ask students to identify “differences in secondary sexual characteristics”. Possible answers include: size, color, ornamentation and behavior.

9. Preserving Genetic Variation
Some of the genetic variation is populations represents neutral variation, differences in DNA sequence that do not confer a selective advantage or disadvantage.
There are several mechanisms that counter the tendency for directional and stabilizing selection to reduce variation:
Diploidy
Balancing Selection
Hererzygote Advantage
Frequency-Dependent Selection
Genetic variation in a gene pool is beneficial. The question is how certain populations can maintain genetic variation. These next slides explain how genetic variation can be maintained.

10. Diploidy
In diploid eukaryotes each organism has two copies of every gene and a considerable amount of genetic variation is hidden from selection in the form of recessive alleles.
By contrast, haploid organisms express every gene that is in their genome. What you see is what you get. It reduces genetic variability.

11. Balancing Selection
Balancing selection occurs when natural selection maintains two or more forms in a population.
This type of selection includes heterozygote advantage and frequency-dependent selection.
Heterozygote advantage involves an individual who is heterozygous at a particular gene locus thus has a greater fitness than a homozygous individual.
Recessive alleles might not be favored under present environmental conditions, but can still bring new benefits if the environment changes.

12. Heterozygote Advantage
A person who inherits the sickle cell gene from one parent, and a normal hemoglobin gene (HgbA) from the other, has a normal life expectancy.
However, these heterozygote individuals, known as carriers of the sickle cell trait
Why are they called carriers?

13. Heterozygote Advantage
The heterozygote is resistant to the malarial parasite which kills a large number of people each year in Africa.
There exists a balancing selection between fierce selection against homozygous sickle-cell sufferers, and selection against the standard HgbA homozygotes by malaria.
The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists.

14. Heterozygote Advantage

15. Frequency-Dependent Selection
The fitness of a phenotype depends on how common it is in the population.
In positive frequency-dependent selection the fitness of a phenotype increases as it becomes more common.
In negative frequency-dependent selection the fitness of a phenotype increases as it becomes less common.
Choose an example of this and explain
Natural selection may favor non-poisonous butterflies that have the same color pattern as poisonous butterflies. This system is called Batesian mimicry. When they are rare, birds will tend to avoid the mimics, because they will have already have encountered a poisonous butterfly of the same appearance. But when the non-poisonous type is common, the previous encounters of birds with butterflies of their appearance are more likely to have been rewarding; the birds will not avoid eating them, and their fitness will be lower. The fitness of the mimics is negatively frequency-dependent.

16. If natural selection is so powerful, why do we not have perfect organisms?

17. Why Natural Selection Cannot Fashion Perfect Organisms
Selection can act only on existing variations.
Natural selection favors only the fittest phenotypes among those in the population, which may not be the ideal traits. New advantageous alleles do not arise on demand.
Evolution is limited by historical constraints.
Each species has a legacy of descent with modification from ancestral forms. Evolution does not scrap the ancestral anatomy. For example in birds and bats, an existing pair of limbs took on new functions for flight as these organisms evolved from nonflying ancestors.
Though natural selection leads to adaptation, nature abounds with examples of organisms that are less than ideally “engineered” for their lifestyles.

18. Why Natural Selection Cannot Fashion Perfect Organisms
3. Adaptations are often compromises.
The loud call that enables a frog to attract mates also attracts predators.
Chance, natural selection and the environment interact.
Chance can affect the subsequent evolutionary history of populations. A storm can blow birds hundreds of kilometers over an ocean to an island, the wind does not necessarily transport those individuals that are best suited to the environment!
With these 4 constraints, evolution does not tend to craft perfect organisms. Natural selection operates on a “better than” basis. As a result, many organisms contain imperfections.