Microevolution
Chapter 18 contined
Microevolution
 Generation to generation
 Changes in allele frequencies within a
population
 Causes:
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Nonrandom mating
Mutations
Genetic drift
Gene flow
Natural selection
Nonrandom mating
 Inbreeding
 Impacts the entire genome, not just certain
alleles
 Individuals are more likely to mate with
neighbors than distant members of the
population  increases chances of genetically
similar individuals mating
 Common in plants (self-fertilization an extreme
example)
 In animals it often causes inbreeding
depression, where offspring are less fit
Nonrandom mating con’t…
 Assortative mating
 Mates are chosen based on a particular
phenotype
 Only impacts the genes involved in the
selected phenotype
 Positive  select mate with the same
phenotype
 Negative  select mate with the opposite
phenotype
Mutations
 Changes in:
 Nucleotide base pairs
 Arrangement of genes on a chromosome
 Chromosome structure
 Only mutations in gametes are inherited
 Most mutations are silent:
 Only a small % of the DNA is expressed
 Mutations that are expressed are usually
harmful
 Mutations do NOT cause evolution, but natural
selection needs the variations mutations create
Genetic Drift
 It is ‘easier’ to lose a rare allele in a
small population due to chance
 This may produce random
evolutionary changes
 It can decrease genetic variation
within a population
 It can also increase genetic variation
between different populations
Genetic Drift con’t…
 Bottleneck
 Can occur if the population decreases
suddenly – causes a dramatic change in
allele frequencies
 Founder effect
 Decreased variation in a small population
that has broken off from the parent
population
Gene Flow
 Due to migration of breeding individuals
from one population to another
 Isolated populations tend to be different
from surrounding populations – increased
gene flow changes this:
 Makes the population internally more varied
 Makes the population less varied from other
populations
Natural Selection
Occurs over time
Increases the frequency of favorable, adaptive traits
‘weeds out’ less adapted traits
Only operates on the phenotype, not the genotype
Phenotypes are usually due to interactions of
genotypes and the environment
 Most polygenic phenotypes show a normal distribution
– most of the population is in the middle, with fewer
at either extreme
 Natural selection can only ‘work’ if there is preexisting variation within the population
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Types of Natural Selection:
 Stabilizing Selection
 Population is well-adapted to its
environment
 Selection is against phenotypic extremes
Types of Natural Selection…
 Directional Selection
 Changes in the environment cause
selection of a particular extreme
phenotype so that one phenotype
gradually replaces another
Types of Natural Selection…
 Disruptive Selection
 Extreme changes in the environment
may favor more than one phenotype
Genetic Variation in Populations
 Sources:
 Mutation
 Sexual reproduction
 crossing-over
 independent assortment of chromosomes
 random union of gametes
Genetic Polymorphism:
 The presence of 2 or more alleles in a
population
 May not be evident if it does not
produce distinct phenotypes
 Balanced polymorphism
 2 or more alleles persist due to natural
selection
 May be due to heterozygous advantage
 Sickle cell anemia is actually selected for in
area where malaria exists