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Evolution of Populations CHAPTER 16

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1 Evolution of Populations CHAPTER 16

2 Intro Darwin didn't know how heredity worked!
Darwin didn't know how heredity worked! How do you think heredity relates to evolution?

3 Gene Pools Gene pool-Combined genetic information of all member s of a particular population Relative frequency- number of times that allele occur in a gene pool

4 Gene Pool

5 Relative frequencies of alleles Number of times alleles show up in gene pool

6 Sources of Genetic Variation
Mutations-any change in DNA sequence Gene shuffling- Occurs during production of Gametes Gene pools are important to evolutionary theory, because evolution involves changes in populations over time.  

7 In genetic terms, evolution is any change in the relative frequency of alleles in a population

8 Single gene trait- controlled by a single gene that has two alleles
widow's peak—is a single-gene trait.

9 Single gene trait The allele for a widow's peak is dominant over the allele for a hairline with no peak. As a result, variation in this gene leads to only two distinct phenotypes,

10 Polygenic trait Polygenic trait has two or more genes
Each gene of a polygenic trait often has two or more alleles. One polygenic trait can have many possible genotypes and phenotypes. Ex.) Human Height The symmetrical bell shape of this curve is typical of polygenic traits. A bell-shaped curve is also called a normal distribution.


12 Natural Selection on Single Gene Traits
Natural selection on single gene traits can lead to a changes in allele frequencies and thus to evolution


14 Natural Selection on Polygenic Traits
Directional Selection Stabilizing selection Disruptive Selection

15 Natural Selection on Polygenic Traits
Directional Selection-one end has higher fitness

16 Natural Selection on Polygenic Traits
Stabilizing selection- middle has higher fitness

17 Natural Selection on Polygenic Traits
Disruptive Selection-both ends have higher fitness

18 Natural Selection on Polygenic traits
Directional Selection- Individuals at one end of the graph have higher fitness than individuals in the middle or at other end Stabilizing Selection-Individuals near the center of the curve have higher fitness than individuals at either end of the curves Disruptive Selection-Individuals at upper and lower ends have higher fitness than individuals near the middle

19 Which type of selection?

20 Which Type of Selection?

21 Which Type of Selection?

22 Genetic Drift Genetic Drift- random change in allele frequency
In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common


24 Genetic Drift Founder effect- allele frequencies change as a result of migration of small subgroups of population

25 Evolution vs. Genetic Equilibrium
To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. Biologists ask: Are there any conditions under which evolution will not occur? Is there any way to recognize when that is the case?

26 Evolution vs. Genetic Equilibrium
The answers to those questions are provided by the Hardy-Weinberg principle, named after two researchers who independently proposed it in 1908.

27 Hardy-Weinberg Equilibrium
Random Mating Large Population No Migration in or out of Population No Mutations No Natural Selection


29 Formation of new species is called speciation

30 Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

31 Reproduction Isolation
Reproductive isolation can develop in a variety of ways, including behavioral isolation geographic isolation temporal isolation.

32 Behavioral Isolation Behavorial isolation-two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. For example, the eastern and western meadowlarks are very similar birds whose habitats overlap in the center of the United States.

33 Behavioral Isolation Members of the two species will not mate with each other, however, partly because they use different songs to attract mates. Eastern meadowlarks will not respond to western meadowlark songs, and vice versa.

34 Behavioral Isolation

35 Geographic Isolation Geographic isolation - two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. The Abert squirrel lives in the Southwest. About 10,000 years ago, the Colorado River split the species into two separate populations.

36 Geographic Isolation Two separate gene pools formed. Genetic changes that appeared in one group were not passed to the other. Led to the formation of a distinct subspecies, the Kaibab squirrel.


38 Albert Squirrel

39 Kaibab Squirrel

40 Geographic Isolation

41 Temporal isolation A third isolating mechanism is temporal isolation, in which two or more species reproduce at different times. For example, three similar species of orchid all live in the same rain forest. Each species releases pollen only on a single day. Because the three species release pollen on different days, they cannot pollinate one another.

42 Temporal isolation


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