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Microevolution
Levels of Evolutionary Study
• Microevolution: examines changes to the genes
(alleles) within populations
– Population Genetics: studies the changes in the
numbers & types of alleles in a population
• Examines evolution within a species.
• Small changes that do not lead to new species, but can
lead to new variations.
• Macroevolution: examines the evolutionary
changes that create new species
– Speciation: the formation of new species over time
Processes of Microevolution
• Microevolution examines the processes
by which inherited traits change over
time in a population
–Natural Selection
–Migration
–Mate Choice
–Mutation
–Genetic Drift
Processes of Microevolution
• Natural Selection: changes in environmental
pressures can cause an increase or decrease in
certain alleles (traits) in a population
– Favorable alleles stay in population (selected for)
– Unfavorable alleles are eliminated (selected against)
• Migration: as organisms move from one population
to another, their genes move with them
– Causes the numbers and types of alleles within each
population to change.
Processes of Microevolution
• Mate Choice: if parents are selective (picky) or
limited in their choice of mates, only a limited set of
traits will be passed on
• Mutation: can add a new allele to a population
– Mutations are rare, and must occur in egg or sperm to be
passed on
• Genetic Drift: population sizes affect the change in
alleles (traits) in a population
– Large populations experience slower changes in alleles
– Smaller populations can change more quickly
Microevolution: Changes within
Populations
 Populations evolve NOT individuals evolve
Natural selection works on the phenotypes within a
population
Individuals cannot evolve within their lifetime,
because they cannot evolve a new phenotype due to a
change in the environment
Evolution is brought about by breeding that occurs
between organisms within a population. Natural
selection maintains favorable (useful) genotypes, and
eliminates unfavorable (not useful) genotypes.
Natural Selection
- Explains how organisms change over time
1. Over Production: Most organisms produce more
offspring than will survive
2. Variation: Members of the population have
variations
3. Selection: Certain variations will increase the
likelihood of survival
4. Adaptation: Over-time organisms with that
variation make up most of the population and
may look different than their ancestors
Patterns of Natural Selection
Stabilizing
selection:
Directional
selection:
Favors average
individuals; reduces
variation
Favors EXTREME
variations;
evolution is fast
Disruptive selection:
Favors two
extreme variations;
no intermediate
and eventually two
species.
Stabilizing Selection
• “Average” individuals are better able to survive
• Alleles for the “extreme” are eliminated
• A narrow range of
phenotypes is favored
resulting in a narrow
bell shaped curve
Directional Selection
• ONE extreme is favorable, but other is not
• Alleles for other extreme and normal phenotypes
become less common in the population
• Results in FAST
evolution!
• Bell curve is shifted
off of center
Disruptive Selection
• BOTH extremes are favorable!
• Results in increasingly distinct phenotypes,
the “normal” phenotype is selected against
• Bell curve is
‘disrupted’ and
pushed apart
into two peaks