1. Evolution of Populations
Chapter 17
How can populations evolve
to form new species?

2. How do genes make evolution possible? To be added at a later date….
Genes & Variation
Section 17-1
How do genes make evolution possible?
To be added at a later date….

3. Evolution as Genetic Change in Populations
Section 17-2
How does natural selection affect single-gene and polygenic traits?

4. Evolution of insecticide resistance in insect populations

5. Evolution of Populations
Populations, not individuals, evolve
Individuals are selected (natural selection), populations evolve
Population = a group of individuals of the same species that live in the same area

6. Natural Selection on Polygenic Traits
Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce three types of selection:
Directional
Diversifying
Stabilizing

7. Modes of Natural Selection
Directional Selection
Shifts the overall makeup of the population by favoring one variant or extreme
Diversifying Selection
Favors variants of two opposite extremes
Stabilizing Selection
Acts on extreme phenotypes and favors the more common intermediate

8. Figure 23.12 Modes of selection

9. Besides natural selection, is there anything else that may influence allele frequencies?
Yes, chance occurrences in a small population can cause an allele to become more or less common in a population

10. Genetic Drift
Related to small population sizes that may not be representative of the original population
Two examples of genetic drift
Bottleneck Effect
natural disaster drastically reduces the size of a population
Potential loss of individual variation and adaptability
Founder Effect
A few individuals from a population colonize a new habitat

11. Figure 23.5 The bottleneck effect: an analogy

12. Can populations remain stable and not evolve?
The answer is yes, some populations may be so stable they are considered to be in equilibrium (which means no net change)
To describe a population that is in equilibrium and test to see whether things have changed over time, we use the Hardy-Weinberg Equilibrium Theory

13. Hardy Weinberg Equilibrium Theory
Describes the genetics of a nonevolving population
States that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination
Remember! Meiosis and random fertilization do not change the overall genetic structure of a population

14. Hardy Weinberg EquilibriumTheory
To maintain equilibrium the following conditions must be met:
Very large population size
No migration
No net mutations
Random mating (= NO sexual selection!)
No natural selection

15. Hardy Weinberg Equilibrium, cont’d
p + q = 1
p2 + 2pq + q2 = 1
Allows us to calculate the frequencies of alleles if we know the frequencies of genotypes and vice versa
Note that p and q represent the frequencies of two different alleles and that p2 and q2 represent homozygous individuals for each of the alleles and pq represents the heterozygous individuals

16. Figure 23.3a The Hardy-Weinberg theorem

17. Practice Problems, cont’d
In a population that is in Hardy-Weinberg equilibrium, 16% of the individuals show the recessive trait. What is the frequency of the dominant allele in the population?
q2 = .16 so q = 0.4
therefore, p = 0.6

18. The Process of Speciation
Section 17-3
How do new species form?

19. How do new species
form?

20. Biological Species Concept
What defines a species?
Biological Species Concept
Defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to provide viable, fertile offspring

21. How do new species form?
In order for a new species to form, some type of reproductive isolation must occur, in other words, there is a failure to produce viable, fertile offspring
Reproductive Isolation can develop in a variety of ways:
Behavioral Isolation
Geographic/Habitat Isolation
Temporal Isolation

22. Reproductive Isolation cont’d
Geographic/Habitat Isolation
Live in different habitats, encounter each other rarely
Ex: two species of garter snakes, one predominantly aquatic, one living predominantly terrestrial
Kaibab & Abert’s Squirrels

23. Reproductive Isolation cont’d
Important note
Although geographic isolation often leads to reproductive isolation and the development of new species, it is not a guarantee.
It may be possible for geographically separated populations to be reunited and still interbreed and produce fertile offspring. Should this occur, no new species developed during the period of separation.

24. Reproductive Isolation cont’d
Temporal Isolation
Two species breed at different times of the day or year
Ex: skunks late spring vs late summer or flowers bloom at different times of the year
Behavioral Isolation
Signals that attract mates are very different between species
Ex: songs of birds, firefly lighting pattern, courtship dances

25. Speciation in Darwin’s Finches
What is the current hypothesis about the development of the various finch species on the Galapagos Islands?

26. Speciation in Darwin’s Finches
The steps proposed include:
Founders Arrive
Geographic Isolation
Changes in the Gene Pool
Reproductive Isolation
Ecological Competition
Continued Evolution

27. Adaptive Radiation A proposed mechanism by which different finch species may have evolved .

28. Molecular Evolution Section 17-4 What are molecular clocks?
Where do new genes come from?
How may Hox genes play a role in evolution?

29. What are molecular clocks?
Molecular clocks use mutation rates in DNA to estimate the time that two species have been evolving independently

30. What are molecular clocks?
Neutral Mutations as Ticks
Neutral mutations must be used for the clock because they are not selected for or against. They simply mark time.
The more similar the series neutral mutations, the more recently two species are thought to have shared a common ancestor
Distantly related organisms are expected to have very few similarities in their accumulated neutral mutations

31. Where do new genes come from?
One way in which new genes evolve is through the duplication and modification of existing genes
Once genes are duplicated, they may accumulate different mutations over time, thereby changing their function
Several different “gene families” are present in our genomes as a result of duplication/mutation cycles
Ex: the globin gene family (makes hemoglobin), ribosomal RNA gene family, etc.

32. How may Hox genes be involved in evolutionary change?
Hox genes are involved in the development of body plans
Ex: How many legs do you have? Where are they placed on your body? How long or short will they be?
Small changes in Hox gene activity during embryonic development can produce large differences in adult animals.