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Natural Selection
Chapter 22
Evolution
• Change over time in the genetic composition
of a population
– NOT individual
• Natural Selection – the mechanism by which
evolution occurs
• Evolutionary adaptation – result of natural
selection – increase in frequency of traits in a
population
Darwin
• Published Origin of the Species in
1859
• Common ancestor
• modern species descended from
ancestral species
• Natural selection
• populations change over generations
• organisms with favorable traits have
more offspring than those less fit
• “survival of the fittest” + reproduction
• More offspring = more genes in the
population
Precursors to Darwin
• Aristotle: species were fixed and unchanging
• Linnaeus: classified diversity “taxonomy”
• binomial nomenclature: Genus and species
• Hutton: gradualism; slow and continuous change
• Lamarck: acquired characteristics
• characteristics gained during an organism’s lifetime are passed to
offspring
• Giraffe’s long neck
• Cuvier: catastrophism; each boundary in layer represents
change
• opposed gradualism
• Lyell: uniformatarianism; changes in the present occur at the
same rate as the past
• modern geology
• Wallace: proposed a theory similar to Darwin’s
Darwin’s Studies
• HMS Beagle
• South American
coastline
• noticed organisms
resembled European
species in similar
climates
• Galapagos Islands
• studied finches; noticed
many similarities with a
few differences in birds
• Common ancestor
Origin of the species
1. Descent with Modification
• organisms related to ancestor in the remote past
• similar to a tree; more similar species have a
common ancestor in the nearer past
• 99% of species that have ever lived are now
extinct  not all common ancestors are still alive.
Origin of the Species
2. Natural Selection
• species would increase exponentially in numbers if all
organisms that were born reproduced and all offspring
survived to reproduce
• populations tend to remain stable in size
• resources are limited
• competition for resources occurs
• organisms within a population demonstrate variation
• most variation is heritable
• Individuals who inherit favorable traits for their environment
are more likely to survive and reproduce
• Unequal reproduction leads to a gradual change in a
population toward favorable traits
Natural Selection
• Differential success and reproduction among
individuals that vary in their heritable traits based
on environment
• Creates adaptations of organisms to their
environment
• If the environment changes new adaptations will
arise and potentially create a new species.
• Acts on populations NOT individuals
• Only effects heritable traits
• Environmental factors vary from place to place
 not all adaptations are favorable in all
environments.
A flower mantid
in Malaysia
A stick mantid
in Africa
Types of Selection
1. Disruptive: extreme
phenotypes favored;
average is not
2. Directional: one
extreme phenotype
only is favored
3. Stabilizing: average
individuals are selected;
extremes are not
Types of Selection
4. Sexual Selection: mating selection
• Intrasexual:within the same sex
• males competing
• Intersexual: between the sexes
• females choosing the males
5. Artificial Selection: modification of species
due to selective breeding to produce
organisms with desired traits
• pets
Evidence for Evolution
1. Paleontology
• Fossils: remains or traces of
organisms from past
• found in sedimentary rock
• Igneous and metamorphic involve heat
so living organisms don’t remain long
enough to preserve
• older fossils found in lower layers
(strata)
• Biogeography: geographic
distribution of species
• closely related organisms found in the
same geographic regions
Evidence for Evolution
2. Homologous Structures: comparison of
similar structures that have different
functions between species.
• human arm and bat wing
• same bone structure with different functions
Human
Cat
Whale
Bat
Evidences for Evolution
3. Embryology: comparison of early stages of
development
• all vertebrate embryos have tail and pharyngeal
pouches during embryonic development
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Human embryo
Evidence for Evolution
4. Vestigial Organs: structures with little
significance in a modern species, yet present
and functional in ancestors.
• snakes have small leg bones
• humans have an appendix
Evidence for Evolution
5. Molecular: comparison
at the molecular level
• DNA, RNA or protein
sequences
• more similar molecular
sequences, more similar
the organisms
• closer the common
ancestor
Species
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
100%
Human
Rhesus monkey
95%
Mouse
87%
Chicken
69%
Frog
Lamprey
54%
14%