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Chapter 23: The Evolution of Populations

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1 Chapter 23: The Evolution of Populations

2 Population Genetics microevolution – change in genetic makeup of a population from generation to generation

3 macroevolution – evolutionary change above the species level

4 population – group of individuals of the same species living in the same area

5 gene pool – all the genes in a given population at a given time

6 allele frequency – proportion of an allele in a gene pool
p = dominant allele q = recessive allele f (p) = frequency of the dominant allele f (q) = frequency of the recessive allele


8 Calculating allele frequency:

9 Genotype # of Individuals MM 1787 MN 3039 NN 1303 Total 6129
Genotypic frequencies MM 1787 MM = 1787/6129 = 29% MN 3039 MN = 3039/6129 = 50% NN 1303 NN = 1303/6129 = 21% Total 6129

10 Hardy-Weinberg Theorem
helps measure changes in allele frequencies over time provides an “ideal” population to use as a basis of comparison

11 Conditions for Hardy-Weinberg Equilibrium:
Large population No gene flow No mutations Random mating No natural selection – hypothetical population that is not evolving – rarely met in nature

12 Mutation and sexual recombination only sources of new variations
mutation – changes in nucleotide sequence in DNA

13 point mutations – change in one nucleotide

14 gene duplication – duplication of a chromosome segment

15 sexual recombination – crossing over, shuffling of genes during meiosis

16 Genetic Drift – change in allele frequencies due to chance
usually in smaller populations reduces genetic variation

17 bottleneck effect –when a population has been dramatically reduced, and the gene pool is no longer reflective of the original population’s

18 Human actions can create a genetic bottleneck

19 founder effect – when a small number of individuals colonize a new area; new gene pool not reflective of original population

20 The Fugate family Kentucky's Troublesome Creek

21 gene flow – when a population gains or loses alleles
a movement of fertile individuals leaving/arriving – a reduces differences between populations

22 genetic variation – heritable variations in a population

23 discrete characteristics – are all one discrete variety

24 quantitative characteristics – vary along a continuum, usually due to influence of two or more genes


26 average heterozygosity – measure of polymorphism in a population

27 geographic variation – difference in variation between population subgroups in different areas

28 cline – a graded change in a trait along a geographic axis

29 (the more offspring that you have that survive = more fit you are)
Evolutionary Fitness fitness – contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals (the more offspring that you have that survive = more fit you are)

30 relative fitness – fitness of a particular genotype

31 Types of selection directional selection – shift toward a favorable variation

32 disruptive selection – favors the extremes

33 stabilizing selection
– favors the mean

34 Heterozygous Advantage – when individuals heterozygous
Recessive allele is maintained in the population

35 Example: sickle-cell anemia
prevelence of malaria sickle-cell disease

36 Sexual selection – a natural selection for mating success

37 Sexual dimorphism – differences between the sexes in secondary sexual characteristics

38 Not necessarily better adaptations; example – mane on lion very hot, feathers on peacock very “expensive” to make

39 Common misconceptions:
Natural selection acts on phenotype, not genotype! Natural selection does not create more perfect organisms! (what is perfect in one environment may not be perfect in another)

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