I. Microevolution

D. Genetic Drift1. Bottleneck effect

• Ex: Elevated frequency of Tay-Sachs Disease in Ashkenazi Jews

• Ex: Genetic homogeneity in populations of African cheetahs

2. Founder effect• Allele frequencies in small populations may reflect

genotypes of founding individuals• Common in isolated populations• Ex: Finns descended from small group of people

~4000 years ago; genetically distinct from other Europeans

I. Microevolution

E. Gene Flow• Movement of fertile individuals or gametes

among populations• Tends to

• Increase diversity within populations• Decrease diversity among populations

• Elevated gene flow can amalgamate separate populations into a single population

Fig. 23.12

• 43% of central & 13% of eastern first-time-breeding females immigrated from mainland• Mainland females survive and reproduce poorly• Gene flow from mainland reduces fitness of central vs. eastern females

II. Genetic Variation

• Provides raw material for natural selection

• Homogeneous population – little opportunity for differential fitness

• Sources1) Mutation

2) Crossing over

3) Independent assortment (Meiosis)

4) Random fertilization

II. Genetic Variation

A. Within Populations• Variation in

• Discrete characters• Ex: Color in some flowers (pink or white)

• Quantitative characters• Ex: Skin color in humans

1. Polymorphism• Two or more alleles at a single locus• Extensive in most populations• Phenotypic – Different morphs (body forms)• Genotypic – May not produce discrete phenotypes• Measurement

• Drosophila – 14% heterozygosity, ~1% nucleotide variability

• Homo sapiens – ~0.1% nucleotide variability

II. Genetic Variation

B. Between Populations1. Geographic Variation

• Differences among genetically distinct populations within a species• Differences may be due to random variation

• Differences may occur over a geographic range• Cline – Graded variation in phenotype and genotype

over a geographic range• Common among species with continuous ranges

over large areas• Higher latitudes: Smaller individuals (plants)• Higher latitudes: Larger individuals (animals)• Why?

II. Genetic Variation

C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection

• Environmental change over time favors phenotypes at one extreme

• Possible only if population contains multiple alleles, at least one of which is favored

• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials

2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks

able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful

3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans

Fig. 23.13

II. Genetic Variation

C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection

• Environmental change over time favors phenotypes at one extreme

• Possible only if population contains multiple alleles, at least one of which is favored

• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials

2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks

able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful

3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans

Fig. 23.13

II. Genetic Variation

C. Natural Selection• Can alter frequency distribution of heritable traits1. Directional selection

• Environmental change over time favors phenotypes at one extreme

• Possible only if population contains multiple alleles, at least one of which is favored

• Ex: Black bears in Europe larger during glacial periods, smaller during interglacials

2. Disruptive selection• Favors extremes at expense of mean• Also called diversifying selection• Ex: During a drought, Galápagos finches with long beaks

able to open cactus fruits, birds with wide beaks stripped off tree bark to expose insects, intermediate beaks less useful

3. Stabilizing selection• Favors mean at expense of extremes• Reduces variation• Ex: Birth weight in humans

Fig. 23.13

Stabilizing Selection

Fig. 23.13

II. Genetic Variation

D. Preservation of Variation• Why aren’t we all homozygous for the most

favorable alleles?• Balancing selection occurs when natural

selection maintains two or more phenotypes in a population = balanced polymorphism

1. Heterozygote advantage• Heterozygotes more fit than homozygotes• Ex: Sickle-cell disease

2. Frequency-dependent selection• Phenotypic fitness depends on rarity in population• Ex: Non-selective predation

Fig. 23.17

http://www.cdc.gov/malaria/about/biology/sickle_cell.html

II. Genetic Variation

D. Preservation of Variation• Why aren’t we all homozygous for the most

favorable alleles?• Balancing selection occurs when natural

selection maintains two or more phenotypes in a population = balanced polymorphism

1. Heterozygote advantage• Heterozygotes more fit than homozygotes• Ex: Sickle-cell disease

2. Frequency-dependent selection• Phenotypic fitness depends on rarity in population• Ex: Non-selective predation

III. Development of New Species

A. Anagenesis (Phyletic Evolution)• Accumulated changes transform one species

into another• Same number of species at beginning and

end

B. Cladogenesis (Branching Evolution)• Formation of new species, with parental

species continuing to exist (potentially altered)• Increased number of species

III. Development of New Species

• Biological Species Concept• Developed by Ernst Mayr• “Population or group of populations whose

members have the potential to interbreed in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species”

• Why don’t individuals from different species interbreed?

IV. Reproductive Isolation

A. Prezygotic barriers• Prevent fertilization

B. Postzygotic barriers• Act after fertilization has occurred

Fig. 24.3

Time of Day

Time of Year

Courtship

Sounds/Songs

Flowers

Snails

Plants

Broadcast Spawners

Bullfrog x

Leopard Frog

Horse (2n=64) x

Donkey (2n=62) Mule (2n=63)

IV. Reproductive Isolation

C. Limitations of Biological Species Concept• Mayr’s definition emphasizes reproductive

isolation; may not work in all situations• Ex: Classifying fossil organisms• Ex: Species that reproduce asexually [prokaryotes,

some protists, fungi, plants (e.g. bananas), animals (e.g. fishes, lizards)]

• Ex: Multiple species are inter-fertile but remain distinct (e.g. orchids)