I.
Microevolution
D.
Genetic Drift
1.
Bottleneck effect
•
•
2.
Ex: Elevated frequency of Tay-Sachs Disease in
Ashkenazi Jews
Ex: Genetic homogeneity in populations of African
cheetahs
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
• 43% of central & 13% of eastern first-time-breeding females immigrated from mainland
• Mainland females survive and reproduce poorly
Fig. 23.12
• 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
Sources
1)
2)
3)
4)
Mutation
Crossing over
Independent assortment (Meiosis)
Random fertilization
II.
Genetic Variation
A.
Within Populations
•
Variation in
•
•
1.
Discrete characters
•
Ex: Color in some flowers (pink or white)
Quantitative characters
•
Ex: Skin color in humans
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 Populations
1.
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
•
1.
Can alter frequency distribution of heritable traits
Directional selection
•
•
•
2.
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
Disruptive selection
•
•
•
3.
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
Stabilizing selection
•
•
•
Favors mean at expense of extremes
Reduces variation
Ex: Birth weight in humans
Fig. 23.13
II.
Genetic Variation
C.
Natural Selection
•
1.
Can alter frequency distribution of heritable traits
Directional selection
•
•
•
2.
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
Disruptive selection
•
•
•
3.
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
Stabilizing selection
•
•
•
Favors mean at expense of extremes
Reduces variation
Ex: Birth weight in humans
Fig. 23.13
II.
Genetic Variation
C.
Natural Selection
•
1.
Can alter frequency distribution of heritable traits
Directional selection
•
•
•
2.
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
Disruptive selection
•
•
•
3.
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
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
Heterozygote advantage
•
1.
•
•
2.
Heterozygotes more fit than homozygotes
Ex: Sickle-cell disease
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
Heterozygote advantage
•
1.
•
•
2.
Heterozygotes more fit than homozygotes
Ex: Sickle-cell disease
Frequency-dependent selection
•
•
Phenotypic fitness depends on rarity in population
Ex: Non-selective predation
III. Development of New Species
A.
Anagenesis (Phyletic Evolution)
•
•
B.
Accumulated changes transform one species
into another
Same number of species at beginning and
end
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
•
B.
Prevent fertilization
Postzygotic barriers
•
Act after fertilization has occurred
Time of Day
Time of Year
Flowers
Snails
Courtship
Sounds/Songs
Bullfrog x
Leopard Frog
Plants
Broadcast Spawners
Horse (2n=64) x
Donkey (2n=62) 
Mule (2n=63)
Fig. 24.3
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)