Evolution Notes
Puzzle of Life’s Diversity
A. Darwin
1. author of “On the Origin of the Species” 1859
2. theory of evolution (‘descent with modification’) and natural
selection
3. evidence was found in:
a. paleontology (fossil record) – shows gradual changes and
mass extinctions
b. biogeography – comparing and contrasting the
distribution of organisms on different continents and
observing their similarities of adaptation
c. comparative morphology –
i. homologous structures: body parts that resemble
each other in form and structure (ex. bones of a whale
fin, bat wing, human arm)
ii. analogous structures: body parts that resemble each
other because they are adaptations to similar
environments (ex. shape of penguin, shark, dolphin)
d. embryology – comparing similar stages of development
to establish evolutionary relationships
(Post-Darwin) e. molecular biology – examining DNA, RNA, amino acids,
and proteins to estimate evolutionary divergences
I.
B. Main factors that increase species diversity:
1. mutations
a. additions, deletions, and substitutions may be neutral,
harmful, or helpful
b. culling won’t eliminate unwanted traits
2. gene shuffling & crossing over during sexual reproduction
3. gene flow –immigration into a population
4. variation increases fitness, or the likelihood that a species will
survive in changing environmental conditions
C. Evolution:
1. Macro – major events changing life on earth as shown by the
fossil record; patterns and rates of change among populations over
time
2. Micro – small changes in genes, chromosome, and allele
frequencies in a population
II. Natural Selection
A. Differences in survival and reproduction among individuals in a
population that differ in heritable traits
B. Individuals with higher fitness (superior phenotypes) will survive,
reproduce, and pass their genes onto their offspring; while those
with inferior traits are selected against.
C. How Selection Acts Upon Populations:
1. Directional Selection: environment favors traits that are at ONE
extreme of a range of traits
a. Ex. peppered moths in Manchester, England
b. humans artificially select crops for agriculture and
animals for breeding which accelerates this
2. Stabilizing Selection: intermediate forms of a trait have high
fitness while the extremes are selected against
Ex. birth weight/size of organisms
3. Disruptive Selection: environment favors both extremes of a trait
(opposite of stabilizing selection)
Ex. finches feeding on hard and soft seeds
4. Sexual Selection: non-random mating where male competition
and female choice lead to traits/behaviors that are selected for.
small
large
III. Speciation – formation of a new species; genetic divergence
A. Species – a group of individuals capable of interbreeding and
producing fertile offspring
B. Reproductive isolation – when members of two populations can no
longer interbreed and produce fertile offspring because of
a. Prezygotic barriers:
V. Factors that affect allele frequencies in the gene pool (all the genes in a
given population):
1. gene flow – emigration (leave) and immigration (move into)
2. genetic drift – random changes in allele frequencies that occur
by chance, usually affecting small populations
a. bottleneck effect
i.
intense pressure reduces population size
eliminating many alleles
ii. few individuals are left to rebuild pop.
iii. remaining allele frequencies might be different
b. founder effect
i.
a few individuals leave a population and
establish a new one elsewhere
ii. allele frequencies of founders might not
represent the original population
VI. The Hardy-Weinberg Principle: allele frequencies within a population
will remain in equilibrium if
1. there are no mutations
2. the population is large
3. no emigration or immigration
3. mating is completely random
4. all individuals are reproducing equally (no nat. select.)
equation: p2 + 2pq + q2 = 1
freq. of allele A
1st generation
freq. of allele a
490 homozygous dominant butterflies
90 homozygous recessive butterflies
420 heterozygous butterflies
th
10 generation
1470 homozygous dominant butterflies
270 homozygous recessive butterflies
1260 heterozygous butterflies
Is the population evolving?