1. Essentials of Biology Sylvia S. Mader
Chapter 15
Lecture Outline
Prepared by: Dr. Stephen Ebbs Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 15.1 Microevolution
A population is defined as all the members of a single species occupying a particular area and reproducing with one another.
• Microevolution involves the evolutionary changes within a population.
While variation within a population is important to evolution, it is not the only factor.

3. 15.1 Microevolution (cont.)

4. 15.1 Microevolution (cont.)

5. Evolution in a Genetic Context
Evolution at the population level can be studied using population genetics.
In population genetics, the various alleles at all the gene loci in all the individuals make up the gene pool of that population.
The gene pool of a population can be described in terms of gene frequencies.

6. Evolution in a Genetic Context (cont.)
Consider the following example for a population of 100 Drosophila fruit flies.
36% of flies are homozygous dominant for long (L) wings (36 flies)
16% are homozygous recessive for short (l) wings (16 flies)
48% are heterozgous (48 flies)

7. Evolution in a Genetic Context (cont.)
So how many L and l alleles are in the population?
Number of L alleles
Number of l alleles LL
(2 L x 36)
= 72
(0 l)
= 0 Ll
(1 L x 48)
= 48
(1 l x 48) ll
(0 L)
(2 l x 16)
= 32
120 L
80 l

8. Evolution in a Genetic Context (cont.)
To determine the frequency of an allele in the population, calculate the percentage of that allele from the total number of alleles in the population.
For the dominant allele, L = 120/200 = 0.6
For the recessive allele, l = 80/200 = 0.4

9. Evolution in a Genetic Context (cont.)
These percentages represent the frequency of each allele in the gametes of this population.
If the mating in this population is assumed to be random, then the genotypes in the subsequent generation can be determined using a Punnett square.

10. Evolution in a Genetic Context (cont.)
In this Punnett square, the cross indicates the possible alleles contributed by the population, not an individual.
sperm
0.6 L
0.4 l
Genotype frequencies
0.36 LL Ll ll
eggs
0.36 LL
0.24 Ll
0.16 ll
Note that the frequencies of L and l remain the same in the subsequent generation.

11. Evolution in a Genetic Context (cont.)
Hardy-Weinberg equilibrium is expressed as a simple binomial equation.
p2 + 2 pq + q2
The letters p and q are used to represent the frequency of the two alleles in the population.

12. Evolution in a Genetic Context (cont.)

13. Evolution in a Genetic Context (cont.)

14. Evolution in a Genetic Context (cont.)
Hardy-Weinberg equilibrium is maintained in a population of sexual reproducing individuals if five conditions are met.
No net change in frequency due to mutations
No gene flow (migration of alleles in or out of the population)
Random mating must occur
No genetic drift
No natural selection

15. Evolution in a Genetic Context (cont.)
These conditions are rarely if ever met in the real world.
Thus allele frequencies continually change and microevolution occurs.
The value of the Hardy-Weinberg principle is that it describes the factors that cause evolution.

16. Evolution in a Genetic Context (cont.)
In order for natural selection to act on allele frequencies, the change must affect the phenotype associated with the gene.
A classic example of microevolution is industrial melanism.

17. Evolution in a Genetic Context (cont.)

18. Evolution in a Genetic Context (cont.)

19. Causes of Microevolution
Deviations from the conditions of Hardy-Weinberg equilibrium cause the allelic changes associated with microevolution.
– Mutations
– Gene flow
– Nonrandom mating
– Genetic drift
– Natural selection

20. Genetic Mutations
• Mutations are the raw material of evolutionary change.
Mutation introduces new variation into a population.
This variation is adaptive if it helps members of a population adjust to specific environmental conditions.

21. Gene Flow
• Gene flow, or gene migration, occurs when breeding members of a population leave a population or new members enter.
Gene migration can introduce new alleles to populations.
However continual gene flow between populations decreases differences in allele frequencies, preventing speciation.

22. Gene Flow (cont.)

23. Nonrandom Mating
When males and females reproduce together strictly by chance it is called random mating.
Any behavioral activity that fosters the selection of specific mates is nonrandom mating.
– Assortive mating occurs when organisms select mates with a similar phenotype.
– Sexual selection favors traits that increase the likelihood of securing a mate.

24. Genetic Drift
Chance events that cause the allele frequency to change is called genetic drift.
The effect of genetic drift becomes increasingly important as the size of the population decreases.

25. Genetic Drift (cont.)

26. Genetic Drift (cont.)
Another example of genetic drift is the bottleneck effect.
A bottleneck occurs when an event or a catastrophe drastically reduces the number of organisms in a population.
The variation in that population may also be reduced, changing the allele frequencies within the population.

27. Genetic Drift (cont.)

28. Genetic Drift (cont.)
The founder effect is another example of genetic drift.
The founder effect occurs when combinations of alleles occur at a higher frequency in a population that has been isolated from a larger population.

29. Genetic Drift (cont.)

30. 15.2 Natural Selection
Natural selection is the process that adapts populations to the environment.
Some aspects of the environment can involve biotic (living) components.
Competition for limiting resources
Predation
Parasitism

31. 15.2 Natural Selection (cont.)
Some aspects of the environment can involve abiotic (nonliving) components.
Weather and climate
Temperature
Moisture

32. 15.2 Natural Selection (cont.)

33. Types of Selection
The variation within a population creates different phenotypes for a given trait.
The distribution of those phenotypes typically forms a normal distribution.
The effect of the three types of natural selection have different effects on this normal distribution.

34. Directional Selection
When one extreme phenotype is favored by natural selection, the distribution of the phenotype shifts in that direction.
This type of selection is therefore called directional selection.

35. Directional Selection (cont.)

36. Directional Selection (cont.)

37. Stabilizing Selection
• Stabilizing selection occurs when the intermediate, or most common, phenotype is favored.
This type of selection tends to narrow the variation in the phenotype over time.
This is the most common type of selection because it is associated with the adaptation of an organism to the environment.

38. Stabilizing Selection (cont.)

39. Disruptive Selection
In disruptive selection, natural selection acts upon both extremes of the phenotype.
This creates a increasing division within the population which may ultimately lead to two different phenotypes.
Disruptive selection is the process that leads to speciation.

40. Disruptive Selection (cont.)

41. Disruptive Selection (cont.)

42. Maintenance of Variations
The preservation of variation in a population is important because it provides a foundation on which natural selection can act.
Variation is preserved by a variety of processes.
Mutations and genetic recombination
Gene flow
Natural selection
Polymorphisms (differences in form)
Diploidy and the heterozygotes

43. Diploidy and the Heterozygote
Natural selection can only cause evolution if the different alleles produce different phenotypes.
Because many organisms are diploid, heterozygotes are carriers of recessive alleles, preserving them in the population.
This also creates another phenotype that may contribute to ratio of balanced polymorphisms.

44. Diploidy and the Heterozygote (cont.)
Sickle cell disease is an example of balanced polymorphism.
Under normal conditions, the different phenotypes provide different fitness levels.
HbSHbS genotypes suffer from sickle cell and usually die young.
HbAHbA genotypes are normal and usually the most fit.
HbAHbS genotypes are affected by sickle cell disease only when oxygen levels are low.

45. Diploidy and the Heterozygote (cont.)
Surprisingly, the recessive allele (HbS) occurs at a higher than expected frequency in regions where malaria is present, such as in Africa.
This occurs because the heterozygote phenotype is favored under these conditions and homozygotes are selected against.

46. Diploidy and the Heterozygote (cont.)

47. Diploidy and the Heterozygote (cont.)