Microevolution and Macroevolution
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Transcript Microevolution and Macroevolution
Microevolution
Chapter 18 contined
Microevolution
Generation to generation
Changes in allele frequencies within a
population
Causes:
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
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