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L4: 5-4
Microevolution cont…
 Variation: differences in species due to mixtures of alleles
 Variation is inheritable AND due to environmental
– Natural selection can only have an effect on the
genetic component of variation.
 Environmental effects:
– Hydrangea = blue in acidic soil; pink in alkaline soil
Flowering time in relation to global climate change
Lung cancer due to smoking
Gaining weight due to diet
Color in flamingos due to diet
Siamese cats = darker color in cold weather
L4: 5-4
 between individuals within a population
 between populations in different geographical areas
 polymorphism: subpopulations of different forms
– genetic polymorphism = ex. blood types in humans & coat
color in Labrador retrievers
– phenotypic polymorphism = ex. shells of snail populations
Phenotypic polymorphism
Genetic polymorphism
 Types of Selection:
 Natural selection: some individuals will produce more offspring
 environment limits population number
 fitness: relative ability to survive and reproduce; ability to
pass genes onto the next generation
 selection operates on a phenotypic range of variation; not
directly on genotype
L4: 5-4
 disruptive: favors variants at both ends of range; intermediate
phenotype is less favorable.
 stabilizing: trends toward narrow range of phenotype; favors
intermediate area of phenotypic range.
 directional: shifts to variation at edges of variation range;
shifts away from average phenotype.
L4: 5-4
 sexual selection: certain traits lead to preference in mates
• Intrasexual selection = within same sex
Ex. Male behavior – competition for female
• Intersexual selection = mate choice; female choice
Ex. Male’s ornaments; eg. Lion’s mane, peacock tail
Note: Advantage of passing on genes (getting picked) outweighs
the risk of “ornament” increasing chance of predator attack.
Examples of Natural Selection
 peppered moth: black became more prevalent after
- due to soot on trees and
- selection against light forms by predators (birds)
 drug resistance: mutations create enzymes to neutralize
- antibiotics kill non–mutants but
- drug resistant mutants free to multiply
- leads to tolerance
L4: 5-4
 sickle
cell: recessive hemoglobin gene (afflicts mainly blacks)
usually fatal in homozygous recessive condition
but heterozygous individuals keep allele in population
freq in America: 0.001
freq in central Africa is about 0.045 (heterozygous
- selection advantage for heterozygote to malaria
Heterozygous Advantage:
Ex. gene that codes for the oxygen carrying protein in blood
• aa = sickle cell (cannot carry
oxygen efficiently or not at
• Aa = heterozygous advantage
- protection against
malaria, but not
- no sickle cell
• AA = very susceptible to
malaria / no sickle cell
L4: 5-4
Can you see the advantage of the heterozygote?
 Individuals who are heterozygous at a particular gene locus
have greater fitness than the homozygotes.
 Natural Selection will tend to favor these different alleles ( A
and a) at that locus.