How Populations Evolve. Chapter 15

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1 How Populations Evolve Chapter 15

2 Populations Evolve Biological evolution does not change individuals It changes a population Traits in a population vary among individuals Evolution is change in frequency of traits

3 Variation in Populations All individuals have the same genes that specify the same assortment of traits Most genes occur in different forms (alleles) that produce different phenotypes Some phenotypes compete better than others

4 The Gene Pool All of the genes in the population Genetic resource that is shared (in theory) by all members of population

5 Variation in Phenotype Each kind of gene in gene pool may have two or more alleles Individuals inherit different allele combinations This leads to variation in phenotype Offspring inherit genes, not phenotypes

6 Change over Time Over time, the alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Change leads to increased fitness Increased adaptation to environment

7 What Determines Alleles in New Individual? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure

8 Genetic Equilibrium Allele frequencies at a locus are not changing Population is not evolving

9 Five Conditions for No Evolution No mutation Random mating Gene doesn t affect survival or reproduction Large population No immigration/emigration

10 Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p 2 + 2pq + q 2 = 1 p 2 AA + 2pq Aa + q 2 aa = 1 Frequency of allele A = p Frequency of allele a = q

11 Punnett Square p A q a p A AA(p 2 ) Aa(pq) q a Aa(pq) aa(q 2 )

12 If traits show up in different proportions from one generation to the next, (one or more of the 5 assumptions are not being met (No mutation,random mating,gene doesn t affect survival or reproduction,large population,no immigration/emigration) We can then begin the hunt for the evolutionary force.

13 No change through generations thus population is not evolving STARTING POPULATION THE NEXT GENERATION THE NEXT GENERATION 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies white wings 90 aa butterflies white wings 90 aa butterflies white wings

14 Frequencies in Gametes 490 =.49, 420 =.42, 90 =.09 F 1 genotypes: Gametes: 0.49 AA 0.42 Aa 0.09 aa A A A a a a A 0.3a

15 We must determine the frequency of the gene first There is a total of 2000 gametes 490 AA individuals makes 980 A gametes 420 Aa individuals makes 420 A and 420 a gametes 90 aa individuals makes 180 a gametes

16 We must determine the frequency of the gene first alleles = 0.7 = p alleles = 0.3 = q p + q = 1

17 Microevolutionary Processes Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: Natural selection Gene flow Genetic drift

18 Gene Mutations Infrequent but inevitable Each gene has own mutation rate Lethal mutations Neutral mutations Advantageous mutations

19 Number of individuals in population Number of individuals in population Number of individuals in population Directional Selection Allele frequencies shift in one direction Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3

20 Pesticide Resistance Pesticides kill susceptible insects Resistant insects survive and reproduce If resistance has heritable basis, it becomes more common with each generation

21 Antibiotic Resistance First came into use in the 1940s Overuse has led to increase in resistant forms Most susceptible cells died out and were replaced by resistant forms

22 Natural Selection A difference in the survival and reproductive success of different phenotypes Acts directly on phenotypes and indirectly on genotypes

23 Number of individuals in population Number of individuals in population Number of individuals in population Stabilizing Selection Range of values for wing-color trait at time 1 Intermediate forms are favored and extremes are eliminated Range of values of values for wing-color for the trait trait at time at time 2 2 Range of values for wing-color trait at time 3

24 Number of individuals in population Number of individuals in population Number of individuals in population Range of values for wing-color trait at time 1 Disruptive Selection Forms at both ends of the range of variation are favored Intermediate forms are selected against Range of values for wing-color trait at time 2 Range of values for wing-color trait at time 3

25 Stabilizing Selection: Another Example Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve)

26 Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Disruption of an existing range of values

27 Sexual Selection Selection favors certain secondary sexual characteristics Through nonrandom mating, alleles for preferred traits increase Leads to increased sexual dimorphism

28 Balanced Polymorphism Polymorphism - having many forms Occurs when two or more alleles are maintained at frequencies greater than 1 percent

29 Sickle-Cell Trait: Heterozygote Advantage Allele Hb S causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle cell trait less than 1 in 1,600 1 in 400-1,600 1 in in in more than 1 in 64

30 Genetic Drift Random change in allele frequencies brought about by chance Effect is most pronounced in small populations Organisms killed by a fire or volcanoi n outcome

31 Bottleneck A severe reduction in population size Causes pronounced drift Example Elephant seal population hunted down to just 20 individuals Population rebounded to 30,000 Electrophoresis revealed there is now no allele variation at 24 genes

32 Bottleneck

33 Founder Effect Effect of drift when a small number of individuals start a new population By chance, allele frequencies of founders may not be same as those in original population Effect is pronounced on isolated islands Amish population and Ellis van Creveld syndrome (short arms and legs, heart condition)

34 Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Amish, cheetahs

35 Gene Flow Physical flow of alleles into a population Tends to keep the gene pools of populations similar Counters the differences that result from mutation, natural selection, and genetic drift