1. The Evolution of Populations Ch. 23
Lecture Objectives
Microevolution & Natural Selection
Genetic Drift
Founder & Bottleneck Effects
Sexual Selection

2. Recall from our lecture on natural selection…..
Charles Darwin presented evidence to support
Descent with Modification (aka evolution)
Natural Selection (driving force behind D w/ M)
Finch Population on Galapagos Islands
Daphne Major Island
During a drought, large-beaked birds were more likely to crack large seeds and survive
The finch population evolved by natural selection
 Of 1200 birds 180 survived

3. 1976 (similar to the prior 3 years)
Figure 23.2 10 9
Average beak depth (mm) 8
Figure 23.2 Evidence of selection by food source
1976 (similar to the prior 3 years)
1978 (after drought)

4. Microevolution
Change in gene frequencies in a population over generations
Population is a localized group of individuals capable of interbreeding & producing fertile offspring
Three mechanisms cause frequency changes
Natural selection
Genetic drift
Gene flow

5. Variation in coat color (PHENOTYPE) is influenced by genes
Figure 23.3
Variation in coat color (PHENOTYPE) is influenced by genes
Figure 23.3 Phenotypic variation in horses

6. Figure 23.5
Not all traits are heritable (although we will focus mainly on heredity
(a)
(b)
Figure 23.5 Nonheritable variation

7. 1. Natural Selection
Individuals in a population exhibit variations in their heritable traits  best suited traits tend to produce more offspring.
Adaptive Evolution: consistently favoring some traits over others (not coincidental)
improvement in the match between organisms and their environment

8. Natural Selection

9. Directional, Disruptive, and Stabilizing Selection
There are three modes of selection
Directional selection favors individuals at one extreme end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes

10. Frequency of individuals
Figure 23.13
Original population
Frequency of individuals
Phenotypes (fur color)
Original population
Evolved population
Figure Modes of selection
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

11. 2. Genetic Drift
Describes how gene frequencies fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation through losses of genes
The smaller a sample, the greater the chance of random deviation from a predicted result

12. Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
Figure 23.9–3
5 plants leave offspring
2 plants leave offspring
CRCR
CRCW
CRCR
CWCW
CRCR
CRCW
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCR
CRCR
CWCW
CWCW
CRCR
CRCW
CWCW
CRCR
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
CRCR
Figure Genetic drift (step 3)
Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3
Generation 2 p = 0.5 q = 0.5
Generation 3 p = 1.0 q = 0.0

13. Ex. Of Genetic Drift

14. The Founder Effect (under umbrella of genetic drift)
Occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in the larger parent population

15. The Bottleneck Effect (under umbrella of genetic drift)
A sudden reduction in population size due to a change in the environment
Resulting gene pool may no longer be reflective of the original population’s gene pool
If the population remains small, it may be further affected by genetic drift
Understanding the bottleneck effect can increase understanding of how human activity affects other species

16. Original population Bottlenecking event Surviving population
Figure 23.10–3
Figure The bottleneck effect (step 3)
Original population
Bottlenecking event
Surviving population

17. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois
The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

18. Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993)
Figure 23.11
Pre-bottleneck (Illinois, 1820)
Post-bottleneck (Illinois, 1993)
Greater prairie chicken
Range of greater prairie chicken
(a)
Number of alleles per locus
Percentage of eggs hatched
Population size
Location
Illinois –1960s
1,000–25,000 <50
93 <50
Figure Genetic drift and loss of genetic variation
Kansas, (no bottleneck)
750,000
5.8 99
Nebraska, (no bottleneck)
75,000– 200,000
5.8 96
(b)

19. Effects of Genetic Drift: A Summary
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful genes to become fixed

20. Gene Flow – AKA Migration
Movement of genes among populations
Genes can be transferred through the movement of fertile individuals or gametes (for example, pollen)
Tends to reduce variation between populations over time
Can increase or decrease the fitness of a population

21. Gene Flow

22. To summarize..
Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between a species and its environment increases
Because the environment can change, adaptive evolution is a continuous process
Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

23. Sexual Selection
Natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
1. Intersexual Selection: Members of the competitive sex show off for mates and the opposite sex chooses the best display. Some examples include dancing, singing, or showing bright colors.

24. Figure 23.15
Figure Sexual dimorphism and sexual selection

25. 2. Intrasexual Selection: Members of the competitive sex fight amongst themselves and the key event determines reproductive success