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Evolution
AP Biology
Unit 4
History of Evolutionary Theory
• Linnaeus
– Developed a system of
classification based on body
structures (morphology)
• Lamarck
– Said species could “will”
themselves to change
Images taken without permission from http://65.107.211.206/victorian/science/lamarck.jpg
and http://www.necsi.org/projects/evolution/lamarck/lamarck/giraffes.jpg
History of Evolutionary Theory
• Darwin
– Main idea: Natural Selection
causes species to change over
time
• Mendel
– Genetics – now used as another
way to look at evolution
(similarities between DNA,
RNA sequences)
Img Src: http://www.panspermia.org/darwin.jpg
Natural Selection
5 parts:
1. Population growth has the ability
to be exponential (many
offspring).
2. Populations tend to be stable in
size (despite #1).
3. Resources are limited.
4. Individuals vary in phenotype
(variations are random).
5. Much phenotypic variation is
heritable.
Img Src: http://www.abc.net.au/nature/parer/img/galapagos.gif
Natural Selection (in summary)
• There are more individuals produced than
the environment can support  leads to
competition for resources.
• Individuals with advantageous traits in this
competition will survive to reproduce and
pass on those advantageous traits to the
next generation.
Artificial Selection
• Breeding animals or plants to have the
desired traits
• Ex. Dog & Cat breeds
Great Dane
Shar-Pei
American curl cat
Images taken without permission from http://www.caine.de.rasa.ham.8m.com/images/shar_pei.jpg,
http://www.petsmart.com/aspca/images/dogs/great_dane.jpg, http://www.cfainc.org/breeds/profiles/articles/acurl.html
Newer developments in Artificial Selection
• Cocker spaniel + poodle
= Cockapoo
• Labrador + Poodle =
Labradoodle
• Why mix with poodles?
– Their fur tends to shed
less– they were originally
bred in an effort to create
a guide dog for people
who were allergic to dogs.
Bay Area Link… Luther Burbank
• Developed MANY new (now
famous) plants using artificial
selection
Evidence for Evolution
•
•
•
•
Fossil record
Biogeography
Genetics
Similarities between organisms
– Chemical
– Anatomical
Vestigial Structures
• Structures that are no
longer used in an
organism
– Ex. Appendix, tail bone
• Existence suggest
common ancestry with
other organisms
• Many structures present in
embryos are also used to
show common ancestry -developmental biology
Img Src: http://www.vh.org/adult/provider/anatomy/AnatomicVariants/SkeletalSystem/Images/19.html
A fictional vestigial structure 
Analogous Structures
– Similar functions, different structures
– Indicates different ancestors
– Ex. Bird wing, butterfly wing
– Result from convergent evolution:
similar selection pressures cause similar
structures to evolve.
Homologous structures
– Similar structure,
different functions
– Indicate a common
ancestor
– Ex. Human hand,
seal flipper, bat
wing
Biochemical comparisons
• Common ancestry is also evident through
similarities in biochemistry and genetic
sequences
– DNA nucleotide/gene
– Protein sequences
– Metabolic processes
Fossil Record
• Fossils = preserved remains of ancient
organisms
• The fossil record allows scientists to
observe changes on earth over time.
• Fossils are dated using a variety of
methods:
– Age of rock fossil is found in
– Decay of isotopes like C-14
– Relationships within phylogenetic trees
Biogeography
• Study of distribution of
organisms (past and present)
• Demonstrate that organisms can
evolve similarly to one another in
similar environments from
different ancestors
• Ex. Marsupials in Australia –
they are only found in Australia
but some share similar
characteristics with other
organisms
How Evolution Occurs:
Genetic Drift
• Changes in the gene pool caused by
random events
• Examples: Bottleneck Effect, Founder
Effect
Bottleneck Effect
• drastic reduction in population (due to natural
disasters, predators, etc.)
Example of Bottleneck Effect:
Pingapalese People
Img Src:
http://www.genomenewsnetwork.org/articles/06_00/pingelapese_colorblind.shtml
Img Src: http://www.7blueseas.com/destinations/images/map_southpacific.jpg
Example of Bottleneck Effect:
Cheetahs
Img Src: http://www.meerkats.com/images/cheetah-cub.jpg
Img Src: http://easyweb.easynet.co.uk/~iany/patterns/images/cheetah.jpg
Founder Effect
• A few individuals become isolated from the
original population
• The resulting population is NOT
representative of the original population
Example of
Founder Effect
Another Example of Founder Effect: Ellis-van Creveld Syndrome
Img src: http://www.emedicine.com/ped/images/296682EVC-polydactylya.jpg
Img Src: http://www.emedicine.com/ped/topic660.htm
Geographic Isolation
• When physical
separation/isolation results in
the formation of new species
• Is a cause of founder effect
• Adaptive radiation = many
species evolving from one
original ancestor
Example of Geographic Isolation
Img Src: http://www.desertfishes.org/na/cyprinod/cyprinod/cssalinu/cssalin1.jpg
Question…
• What is the difference between bottleneck
effect and founder’s effect?
– Bottleneck effect and founders effect are both
examples of genetic drift but what causes the
drastic change in population and allelic
frequencies is different.
Gene Flow
 Migration (immigration, emigration)
• Breeding across prior barriers (cultural
barriers, for instance, as in human
populations)
• Changes allelic frequencies
• Ex. Human migration
Phenotypic Polymorphism
• Defined as variety of physical traits
• Due to:
- genetic variation (more general– an
individual can have a variety of alleles)
- genetic polymorphism (2 or more alleles
exist for a gene)
- nonheritable variation
Fitness
 fitness = contribution of individual to gene
pool, relative to others’ contributions
 relative fitness = contribution of a genotype
(all individuals alike)
 fitness of 1 means you (or your genotype)
contribute at a maximum (100%)
• fitness of 0 means you (or your genotype)
do not reproduce at all
Types of Selection
• Selection = Environment chooses those with
the best adaptations to survive
Stabilizing Selection = “middle” phenotype is selected
for
Disruptive Selection
• The two extremes are selected for
• Ex. Snails
Directional Selection = one phenotype is selected for
Img Src: http://web.nmsu.edu/~wboeckle/pepper_moth2.JPG
Industrial melanism
Heterozygote Advantage
• When being heterozygous
gives you an advantage
• Explains why a harmful
recessive allele does not
get eliminated from a
population
• Ex. Sickle cell anemia –
heterozygotes are resistant
to malaria
Img Src: http://www.nidcd.nih.gov/news/releases/02/malaria.jpg
Img Src: http://www.unomaha.edu/~swick/images/sickel.jpg
Sexual Selection
• Genders appear different due
to Sexual Dimorphism
• Competition is intergender
(compete for the opposite
gender) and intragender
(compete with each other)
specific
• Ultimately has to do with
competition for higher fitness
Constraints on Natural Selection
• Populations are not perfectly suited to their
environments because
– Environments constantly change (Ex. A storm
destroys all of the fruit trees in an area.
Animals and plants that previously were
successful at eating these fruits are now at a
disadvantage)
Constraints on Natural Selection
• Populations are not perfectly suited to their
environments because
– Evolution is slow and new structures rarely
form (Ex. Birds have 2 wings & 2 legs rather
than 4 wings, (even though more wings could
help with flight), because they evolved from a
4-appendage reptile; 2 wings and 2 legs)
Constraints on Natural Selection
• Populations are not perfectly suited to their
environments because
– Adaptations are compromised (Ex. seal legs
would help in land motion, but swimming with
legs is less efficient than with flippers)
Constraints on Natural Selection
• Populations are not perfectly suited to their
environments because
– Natural selection can only select for the best
variation—it can’t create variations