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Natural
Selection
Chapter 17
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Evolution
• Change over time in the genetic
composition of a population
– Individuals do not evolve
• 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
– All modern species descend from
ancestral species that were different
• Natural selection
– Population will change from
generation to generation, those who
are best fit for their environment will
go on to live and reproduce
Precursors to Darwin
• Aristotle – species were fixed and unchanging
• Linnaeus – classified diversity – taxonomy
– Binomial nomenclature – genus and species
• Hutton – gradualism – slow and continuous change
• 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 22_03GrandCanyon_SV.mpg
• Lamarck – Acquired characteristics
– Characteristics gained during an organisms lifetime are
passed to offspring: Incorrect, your scars don’t show up
in your children
– Ex: giraffe
• Wallace – developed a similar theory of natural
selection as Darwin
Darwin’s Studies
• HMS Beagle
• South American
coastline
– Noticed organisms
resembled European
species in similar
climates
• Galapagos Islands
– Studied finches and
noticed many similarities
with key differences in
many species of animals
– Common ancestor
Origin of the species
• 1. Descent with
modification
– All organisms are related
from an 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.
• Protists:
– Our knowledge of the Eukarya
domain is changing rapidly.
– Protists are mostly unicellular
prokaryotes
• Grouped into 5
supergroups w/
tremendous variation
Trypanosoma: Protist that causes
sleeping sickness (fatal w/out
treatment)
Fig. 28-03a
Parabasalids
Euglenozoans
Excavata
Diplomonads
Dinoflagellates
Diatoms
Stramenopiles
Golden algae
Chromalveolata
Alveolate
s
Apicomplexan
s
Ciliates
Brown algae
Oomycetes
Forams
Radiolarians
Chlorophytes
Charophyceans
Land plants
Archaeplastida
Red algae
Rhizaria
Chlorarachniophytes
Slime molds
Gymnamoebas
Nucleariids
Fungi
Choanoflagellates
Animals
Unikonta
Entamoebas
Origin of the Species
• 2. Natural Selection
– All species would increase exponentially in size 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
may arise through mutations or previous
characteristics become advantageous in the
new enironment: possibility of speciation
• Acts on populations NOT individuals
• Only affects heritable traits
• Environmental factors vary from place to
place  not all adaptations are favorable in
all environments.
(a) A flower mantid
in Malaysia
(b) A stick mantid
in Africa
Figure 22.11
Fig. 28-03l
Ameoba: single celled protist
Endocytosis
100 µm
Types of Selection
• 1. Disruptive – extreme
phenotypes are favored
and the average is
selected against
• 2. Directional – One
extreme phenotype only
is favored
• 3. Stabilizing – Average
individuals are selected
for and extremes are
selected against.
Types of Selection
• 4. Sexual Selection – mating selection
– Intrasexual – within the same sex
• Ex: males competing
– Intersexual – between the sexes
• Ex: Females choosing the males
• 5. Artificial Selection - Modification of
species due to selective breeding to
produce organisms with desired traits
– Ex: pets
Evidences for Evolution
• 1. Paleontology
• Fossils – remains or traces of
organisms from the 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 organism are found
in the same geographic regions
Evidences for Evolution
• 2. Homologous Structures – comparison
of similar structures that have different
functions between species.
– Ex: human arm and bat wing
• Same bone structure with different functions
Human
Cat
Whale
Bat
Evidences for Evolution
• 3. Embryology – comparison of the early
stages of development
– Ex: all vertebrate embryos have a tail and
pharyngeal pouches during embryonic
development
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Human embryo
Evidences for Evolution
• 4. Vestigial Organs - structures with little
significance in a modern species, yet
present and functional in ancestors.
– Ex: snakes have small leg bones
– Ex: humans have an appendix
Evidences for Evolution
• 5. Molecular –
comparison of
organisms at the
molecular level
– Ex: DNA, RNA or protein
sequences
– The more similar the
molecular sequences the
more similar the
organisms
– The 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%