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Transcript
IV. The Process of Evolution
A. Two types of evolution
1. Macroevolution – any change of a
species over time into another. Any
changes or “long-term” trends at
higher taxonomic levels (i.e. new
genera, families, phyla)
2. Microevolution – A change in gene
frequencies (i.e. alleles) within a pop.
or species over time.
Ex. Overuse of antibiotics has
selected for resistant microbes
Example (microevolution):
 light v. dark colored moths
 frequency due to change in env’t
(i.e. color of tree trunk)
 industrial melanism
B. Causes of microevolution
1.Natural selection
a. gradualism – species evolve at a slow
and constant pace
b. punctuated equilibrium – species
evolve rapidly over short time then
remain the same for long periods
2. Mutations – a change in an allele
 the origin of genetic variation
3. Gene Flow – the mov’t of alleles b/n
populations due to migration of breeding
indiv.
 Results in interbreeding
4. Genetic Drift – allele freq. change due to
chance
 causes alleles to be lost from pop.
 small populations suffer (i.e. greater
chance that rare alleles won’t contribute
to make-up of next generation)
ex. Coin toss:
toss 100x: probability is 50/50 of
getting heads/tails
toss 10x: better chance of getting
8 heads/2 tails
 Genetic drift can be due to:
a. bottleneck effect – dramatic decrease in
alleles b/c of major disaster
ex. Hunting in 1890’s reduced one
elephant seal pop. to ~20 indiv.
 very little genetic variation
 24 exact same proteins
b. Founder’s effect –
1. when a new pop. is started, the pioneers contain
only a fraction of the total genetic diversity of
original gene pool
2. also not likely to have all representations
5. Nonrandom mating
 Examples:
a. assortative mating – mate with
someone w/ same phenotype
(e.g. tall people)
b. sexual selection – mates are chosen
on basis of particular appearance
C. Genetics of evolution (population genetics)
1. Gene pool – all the various alleles at
every locus of every indiv. in a pop.
 the gene pool is defined by allele
frequencies
2. Calculating gene pool frequencies
 the Hardy-Weinburg Principle
Hardy- Weinburg Principle
 states that the frequencies of alleles and
genotypes in a population’s gene pool
will remain constant (i.e. unchanging)
over generations as long as there is:
1. no selection
2. no mutations
3. no gene flow
4. no genetic drift
5. random mating
 If these conditions are met, the pop. is
@ equilibrium
So, what is the final result of changes in gene
pool allele and genotypic frequencies?
V. Speciation – the formation of a new species
A. Due to Isolation – any geographical,
reproductive, or behavioral event
preventing gene flow b/n populations
B. Modes of speciation
1. Allopatric –
geographic
separation
prevents gene
flow
Example of allopatric speciation
Darwin’s finches (Galapagos Islands)
Adaptive Radiation – different species
evolved from one common ancestral species
2. Sympatric –
reproductive
isolation w/o any
geographic barrier
 results in
polyploidy indiv.
 common in
flowering plants
(e.g. sunflowers)
 rare in animal
(but, orcas)
VI. Patterns of Evolution
1.Divergent – two species gradually
become increasingly different
 occurs when related species diversify
to new habitats
 Example: humans and apes
2. Convergent – When diff. species begin to share
traits b/c of shared env’t
example:
whales (mammals), sharks (fish), penguins (birds)
3. Parallel – when two species evolve independently
while maintaining the same level of similarity
 occurs b/n unrelated species that don’t occupy
the same habitat
 example: marsupial v. placental mammals
(give birth to live young)
4. Coevolution – Species that interact closely
often adapt to one another.
Ex. Hawk moth and Orchids