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AP Biology Notes
Evolution
Unit 4 - Evolution
 Overview
 multiple alleles exist in a population with certain frequencies (or %)
 certain factors can change these frequencies
 populations change in response to environmental change
 species: organisms that can freely interbreed in nature to produce fertile offspring.
 population: members of a species sharing the same geographical area and ecological niches in the
same period of time
 microevolution: changes in the relative frequencies of alleles in a population over successive
generations leading to new species
 macroevolution: the sum of microevolutionary changes over a long period of time to produce new
structures and evolutionary trends
Natural Selection (Charles Darwin)
o variations exist within populations
o these variations can be inherited
o populations have the ability to increase exponentially
o resources are finite
o resulting in competition for these resources
o those with best fit adaptations will preferentially survive
o producing more offspring
o leading to gradual changes in the population

Evidence for Evolution
 fossil record: fossils are found in rocks which can be dated
• when arranged sequentially time-wise
- fossils show steps of development and
- origins of common ancestry
 molecular record: evolution of genes
• most animals (plants) share many genes
• these genes have nucleotide substitutions in different species
- the more difference, the more substitutions
- agrees well with fossil record
• also applies to protein similarities
 homology: (“divergent evolution”)
• many animal (plant) species have similar structures
• appear to derive from a common ancestor
• but maybe adapted to different function
e.g.: hand, paw, hoof, flipper, wing
- all have same bones
- all have similar function (propulsion)
e.g.: mammals all have 13 cervical vertebra
- giraffe’s are elongated
- cetacean’s are very short and fused (not required)
Nature usually adapts what she's already developed
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AP Biology Notes



Evolution
development: “ontogeny recapitulates phylogeny”
• embryos exhibit traits during development
- disappear later in development
- homologous with traits in other animals that fully develop
(gill slits, tail, lanugo: disappear in humans before birth)
• vestigial structures: developed in adult with no function
biogeography:
• suggests common ancestry and geographical isolation
- thought Australia connected to mainland 50 m.y. ago
- S.A.—Africa: jaguar—leopard
• organisms most closely related to closest populations
– even if different habitat (tropical vs. temperate)
– than to those in similar habitats on other continents
• organisms on a continent more similar to fossils found there
• parallel adaptation: (“convergent evolution”)
— in differing geographical areas
– organisms in similar niches
– have similar appearance and relation to environment
e.g.: placental mammals vs. marsupials
– mouse - marsupial mouse
– wolf - Tasmanian wolf
– anteater - numbat , etc.
– but still appear more closely related to organisms on same continent
Probability:
2 penny toss: 1/2 chance shinny heads, 1/2 chance dull heads
chance of both heads = 1/2 x 1/2 = 1/4
(25%)
Heterozygous cross: Ss x Ss
1/2 chance of s gamete from mom, 1/2 chance of s gamete from dad
chance of both s gametes in offspring = 1/2 x 1/2 = 1/4 (25% or 3:1)
What is chance for heterozygote?
1/2 S x 1/2 s = 1/4 Ss
and 1/2 s x 1/2 S = 1/4 sS
two ways of getting Ss: 1/4 + 1/4 = 1/2 chance for Ss
chance for SS = 1/4, ss = 1/4, Ss = 1/2 (all combinations)
TOTAL = 1/4 + 1/2 + 1/4 = 1
Double heterozygous cross: (e.g.. SsYy x SsYy)
How many double recessives in F2? (ssyy)
1/4 x 1/4 = 1/16
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AP Biology Notes

Evolution
Hardy–Weinberg Principle
 frequency: the proportion (or percent) of an allele in a population.
• p and q represent 2 alleles of a certain gene (p dominant allele; q = recessive allele)
• total of proportions of alleles = 1 (100%)
p + q = 1
• for populations— cross alleles by frequency:
p2 + 2pq + q2 = 1
New population will consist of (total = 1):
EXAMPLE: if q = .3 (or 30%)
What % of individuals will be p2 (homozygous recessive)?
q2 = q x q = .3 x .3 = .09
(= 9%)
What % will be homozygous dominant?
1) What is p?
p+q=1
If q = .3 then = .7
(.3 + p = 1)
2
2) p = (.7)2 = .49 = 49%
Check: pq = .7 x .3 = .21
2pq = 2(.21) = .42
2
2
p + 2pq + q = .49 + .42 + .9 = 1
(100%)
Problem: If 25% of a population are wrinkled, what is the freq. of S?
p2 = .25 so
p = .5
q + .5 = 1
q = .5
The Hardy-Weinberg Law = Frequencies of alleles in a given population will remain constant
unless acted on by an outside factor.

Factors changing allele frequencies:
 genetic drift: random loss of alleles by chance
• frequencies are statistical: more constant in large populations
• drift happens in small populations
— isolation through founder effect
— or bottlenecking
 migration: emigration or immigration of genes in a population
• new genes can enter a population
• certain genes may leave a population (non–randomly)
 mutation: alleles may be changed to another form
• some genes mutate at a higher rates than others
- may keep an allele freq. higher than expected
• new genes may be produced
- most are fatal or neutral
- some function better than the old allele
Mutations are the building block of evolution.
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AP Biology Notes
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
Evolution
non–random mating: selective choosing of mates
• inbreeding: mating with relatives
- changes number of homozygous individuals
• dominant breeding members of herds, packs etc.
selection: the favoring of certain traits by environmental factors
• breeding: artificial selection for traits by humans
• natural selection: selection by the environment
• selection must be for traits that help individuals:
- reach reproductive age and/or
- produce more or better fit offspring
Selection is the tool of evolution.
Natural selection acts on variation within populations culminating in evolution.
Variation: differences in species due to mixtures of alleles
• between individuals within a population
• between populations in different geographical areas
• polymorphism: subpopulations of different forms
• sources of variation:
— mutation (source of all new alleles)
— recombination
Natural selection: some individuals will produce more offspring
• environment limits population number
• fitness: relative ability to survive and reproduce
• selection operates on a phenotypic range of variation
• types of selection:
— stabilizing: trends toward narrow range of phenotype
— directional: shifts to variation at edges of variation range
— disruptive: favors variants at both ends of range
• sexual selection: certain traits lead to preference in mates

Examples of Natural Selection
 peppered moth: black became more prevalent after industrialization
- due to soot on trees and
- selection against light forms by predators (birds)
 drug resistance: mutations create enzymes to neutralize antibiotics
- antibiotics kill non–mutants but
- drug resistant mutants free to multiply
 lead tolerance: bent grass growing in pastures in Wales
- varieties growing on nearby lead mine tailings
- e.g.: plant pasture varieties on tailings
- obs: 3/60 showed some ability to grow
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AP Biology Notes

Evolution
sickle cell: recessive hemoglobin gene (afflicts mainly blacks)
- usually fatal in homozygous recessive condition
– but heterozygous individuals keep allele in population
– freq in America: 0.001
- freq in central Africa is about 0.045 (heterozygous advantage)
– selection advantage for heterozygote to malaria
Speciation : the formation of new species
 divergence: the varieties separate into different local populations
• these groups adjust to local differences in environment
• if groups do not interbreed:
- divergence occurs
- which leads to speciation
 isolating mechanisms: lead to divergence by separating a population
• geographical: separation in space [..English oak, valley oak, scrub oak]
• ecological: separation in habitat or niche [...lion and tiger]
• temporal: e.g. : different mating or fertility times
• behavioral: e.g. : divergent mating signals [... fruit flies in Hawaii]
• mechanical: e.g. : incompatible copulatory organs
• infertility: failure of hybrid zygote formation or development
- when enough genetic changes accumulate chromosomes can't
function together in development
 Mechanisms of Speciation
 allopatry: geographical separation [“different” + “country”]
• a population can become separated geographically
• populations then diverge in response to different environments
Platyfish [Xiphophoorus species] of Mexico and South America
- three similar species in different stream systems cannot interbreed
- but: hybrids produced from distinct populations within a system
 sympatry: speciation without geographical isolation
• reproductive isolation can separate a population
— polyploidy: more than two homologues of each chromosome
- develops spontaneously in many pants (47% of angiosperms)
- cannot breed with diploid individuals (infertility)
• disruptive selection: selection for extremes in the bell curve
 adaptive radiation: new species form (allopatrically or sympatrically)
• by changes in sub–populations
• taking advantage of new opportunities
- which change as environment or other organisms change
e.g.: Darwin’s finches on the different islands of the Galapagos
different opportunities/ selected different variations/ leading to different species
 gradualism: slow, gradual evolutionary change
punctuated equilibrium: spurts of relatively rapid change
interspersed with periods of little change
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AP Biology Notes
Evolution
 Origins of Life:
• Earth approx. 4.5 billion years old
• fossil evidence of life
— first organisms (prokaryotes) ~ 3.5 billion years ago
- stromatolites: fossilized bacterial mats
— first eukaryotes ~ 2.5 b.y. ago
— multicellular organisms ~ 1 b.y. ago
• early atmosphere:
— lack of oxygen (a “reducing atmosphere”)
— intense u.v. radiation (no ozone) and lightening (energy sources)
— much volcanism: H2O, CO2, NH3, CH4 prevalent in atmosphere
• organic synthesis:
— Harold Urey & Stanley Miller (1953)
- assembled hypothesized early atmosphere with water
- added energy (electrical sparks = lightning)
- formation of organic molecules including amino acids
— similar experiments have produced
- all amino acids, lipids, sugars, purines, pyrimidines and ATP
- polymerization
• suspected lineage [very tentative]
— polymerization using clay as catalyst
— first information molecule = RNA
— incorporation into membranes (liposomes, coascervates)
— anaerobic bacteria, methane producing from CO2 + H2
— cyanobacteria, photosynthesizing
— biotic production of oxygen
— eukaryotes (endosymbiont hypothesis?)
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AP Biology Notes
Evolution
Teacher’s Notes – Use with Natural Selection
Examples of isolating mechanisms:
geographical:
lions x tigers = fertile offspring
valley oak x scrub oak = fertile offspring
ecological:
lions & tigers in India untill 150 years ago but no hybrids observed
lions: open grassland and prides
tigers: forests and solitary
oaks in California (hybribs rare)
valley oak: fertile soils, open grassland
scrub oak: less fertile soils, slopes
toads
Bufo woodhousi breeds in streams
Bufo americanus breeds in rainpools
temporal:
wild lettuce: occur together but flower at different times
Lactuca graminifolia flowers in early spring
Lactuca canadensis flowers in summer
frogs: 5 species occur together in eastern U.S.
different breeding times = hybrids rare
termites
mating flights at different times (spring vs fall)
behavioral:
500 species of Drosophila in Hawaii
[Pg 411, Raven & Johnson]
different sounds, (& smells??) [mating attractants]
12 species of fidler crab in Panama
distinctive mating dances
mechanical:
some arthropods (esp. insects) classified by sex organs
milkweed: pollen must be inserted into slits in stigma
diff. shapes = diff. species (even though hybrids possible)
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