chapter 23~ microevolution- small changes in the genetics of populations

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Chapter 23~ Microevolution- small changes in the genetics of populations

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Page 1: Chapter 23~ Microevolution- small changes in the genetics of populations

• Chapter 23~ Microevolution- small changes in the genetics of populations

Page 2: Chapter 23~ Microevolution- small changes in the genetics of populations

Populations

• The Smallest Unit of Evolution• One common misconception about

evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes

• Natural selection acts on individuals, but populations evolve

Page 3: Chapter 23~ Microevolution- small changes in the genetics of populations

• Genetic variations in populations– Contribute to evolution

Page 4: Chapter 23~ Microevolution- small changes in the genetics of populations

Individuals are selected, but populations evolve• Population:

– a localized group of individuals belonging to the same species

• Species: – a group of populations whose

individuals have the potential to breed and produce fertile offspring

• Gene pool: – all of the genes in a population

at any one time

Page 5: Chapter 23~ Microevolution- small changes in the genetics of populations

Population genetics

• Is the study of how populations change genetically over time

• Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection

• Focuses on populations as units of evolution

Page 6: Chapter 23~ Microevolution- small changes in the genetics of populations

Traits• Polymorphism -

different forms of traits– Anatomical/Structural

(form)– Physiological

(function) – Behavioral (response)

• Qualitative vs. Quantitative Traits

Page 7: Chapter 23~ Microevolution- small changes in the genetics of populations

• Alleles- versions of genes that are inherited• Genotype is inherited not the phenotype• Mutation and sexual recombination produce

the variation that makes evolution possible• Two processes, mutation (creating alleles)

and sexual recombination (shuffling alleles)– Produce the variation in gene pools that

contributes to differences among individuals

Page 8: Chapter 23~ Microevolution- small changes in the genetics of populations

Stable Gene Pools• Mendelian

inheritance– Preserves

genetic variation in a population

• In a given population where gametes contribute to the next generation randomly, allele frequencies will NOT change= genetic equilibrium

Generation1

CRCR

genotypeCWCW

genotypePlants mate

All CRCW

(all pink flowers)

50% CR

gametes50% CW

gametesCome together at random

Generation2

Generation3

Generation4

25% CRCR 50% CRCW 25% CWCW

50% CR

gametes50% CW

gametesCome together at random

25% CRCR 50% CRCW 25% CWCW

Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions

Page 9: Chapter 23~ Microevolution- small changes in the genetics of populations

• Mutations– Are changes in the nucleotide

sequence of DNA– Cause new genes and alleles to arise

Mutation types:

1. Lethal2. Neutral3. Beneficial

Page 10: Chapter 23~ Microevolution- small changes in the genetics of populations

Variation Creators• Mutation rates

– Tend to be low in animals and plants– Average about one mutation in every

100,000 genes per generation– Are more rapid in microorganisms

• In sexually reproducing populations, sexual recombination– Is far more important than mutation

in producing the genetic differences that make adaptation possible

Page 11: Chapter 23~ Microevolution- small changes in the genetics of populations

Hardy-Weinberg Equation

• p=frequency of one allele (A); • q=frequency of the other allele (a);

p+q=1 (p=1-q &

q=1-p)• p2=frequency of AA genotype; • 2pq=frequency of Aa plus aA genotype;• q2=frequency of aa genotype;

p2 + 2pq + q2 = 1

Page 12: Chapter 23~ Microevolution- small changes in the genetics of populations

Hardy-Weinberg TheoremServes as a model

for a non-evolving population (equilibrium)

5 conditions:• 1- Very large

population size;• 2- No gene flow;• 3- No mutations;• 4- Random

mating;• 5- No natural

selection

Page 13: Chapter 23~ Microevolution- small changes in the genetics of populations

Natural selection

• Natural selection is the primary mechanism of adaptive evolution

• Differential success in reproduction– Results in certain alleles being passed

to the next generation in greater proportions

• Fitness: – contribution an individual makes to

the gene pool of the next generation

Page 14: Chapter 23~ Microevolution- small changes in the genetics of populations

• 3 types of Natural Selection:– A. Directional– B. Disruptive– C. Stabilizing

Page 15: Chapter 23~ Microevolution- small changes in the genetics of populations

Natural Selection Types

• Directional selection– Favors individuals at one 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

Page 16: Chapter 23~ Microevolution- small changes in the genetics of populations

A Closer Look at Selection Types

• From the range of variations available in a population– Natural selection increases the

frequencies of certain genotypes, fitting organisms to their environment over generations

Page 17: Chapter 23~ Microevolution- small changes in the genetics of populations

Sexual selection

• Sexual dimorphism: secondary sex characteristic distinction

• Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Page 18: Chapter 23~ Microevolution- small changes in the genetics of populations

Sexual Selection• Intrasexual selection

– Is a direct competition among individuals of one sex for mates of the opposite sex

• Intersexual selection– Occurs when individuals of one sex

(usually females) are choosy in selecting their mates from individuals of the other sex

– May depend on the showiness of the male’s appearance

Page 19: Chapter 23~ Microevolution- small changes in the genetics of populations

Population variation

• Balanced Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population

• Geographical variation: differences in genetic structure between populations (cline)

Page 20: Chapter 23~ Microevolution- small changes in the genetics of populations

Variation preservation• Prevention of natural

selection’s reduction of variation

• Balanced polymorphism Heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia)

Page 21: Chapter 23~ Microevolution- small changes in the genetics of populations

Microevolution

• Gene Flow: – genetic exchange

due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

– Immigration vs. Emigration

Page 22: Chapter 23~ Microevolution- small changes in the genetics of populations

Genetic drift• changes in the gene pool of a small

population due to chance (usually reduces genetic variability)

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of 10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of 10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

Page 23: Chapter 23~ Microevolution- small changes in the genetics of populations

Microevolution• The Bottleneck Effect:

– type of genetic drift resulting from a reduction in population

– Caused by natural disasters or human activity

– the surviving population is no longer genetically representative of the original population Original

population

Bottle-neckingevent

Survivingpopulation

Page 24: Chapter 23~ Microevolution- small changes in the genetics of populations

• Understanding the bottleneck effect– Can increase understanding of how

human activity affects other speciesBottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

Page 25: Chapter 23~ Microevolution- small changes in the genetics of populations

The Evolutionary Enigma of Sexual Reproduction

• Sexual reproduction– Produces fewer reproductive offspring than

asexual reproduction, a so-called reproductive handicap

Asexual reproduction

Female

Sexual reproduction

Female

Male

Generation 1

Generation 2

Generation 3

Generation 4

Page 26: Chapter 23~ Microevolution- small changes in the genetics of populations

Why Sex?

• If sexual reproduction is a handicap, why has it persisted?– It produces genetic variation that

may aid in disease resistance

Page 27: Chapter 23~ Microevolution- small changes in the genetics of populations

Why Natural Selection Cannot Fashion Perfect Organisms

• Evolution is limited by historical constraints

• Adaptations are often compromises• Chance and natural selection

interact• Selection can only edit existing

variations