Download Chapter 13: How Populations Evolve

How Populations Evolve
Chapter 13
Clown, Fool, or Simply Well Adapted?
 The blue-footed booby
 a type of bird living in the Galápagos Islands
 Excellent example of evolutionary adaptations
Evolutionary adaptations
 Which are inherited traits that
1.
2.
3.
4.
5.
6.


enhance its ability to survive and
reproduce in its particular
environment
Big webbed feet
Streamlined body
Large tail
Nostrils that close
Specialized oil glands
Specialized salt glands
Often adaptations are trade-offs
Ex Big feet good for swimming,
but clumsy on land
DARWIN’S THEORY OF EVOLUTION
 13.1 A sea voyage helped Darwin frame his theory of evolution
 On his visit to the Galápagos Islands
 Charles Darwin observed many unique organisms
 What were Darwin’s Main ideas anyway???
1. Species change over time
2. Living species have arisen from earlier life forms
(descending from a common ancestor)
* Close ties between organisms and their environments*
 Darwin’s main ideas
Can be traced back to the ancient Greeks
 Aristotle and the Judeo-Christian culture
 Believed that species are fixed

 In the century prior to Darwin
 The study of fossils suggested that life forms change
 Others were developing ideas:
 Buffon- proposed fossils may be ancient versions of
similar living species
 Lamark- “big mistake” but was instrumental in early
concept of evolution, before Darwin
 Geologists proposed that a very old Earth
 Is changed by gradual processes
 Much older than 6,000 years
 While on the voyage of the HMS Beagle in the 1830s
 Charles Darwin observed similarities between living
and fossil organisms and the diversity of life on the
Galápagos Islands
North
America
Great
Britain
Europe
Asia
ATLANTIC
OCEAN
PACIFIC
OCEAN
Africa
PACIFIC
OCEAN
Equator
The
Galápagos
Islands
PACIFIC
OCEAN
Pinta
South
America
Genovesa
Equator
Santiago
Pinzón
Fernandina
Isabela
0
0
40 km
Daphne
Islands
Cape of
Good Hope
Tasmania
Santa Santa
Cruz Fe
Florenza
40 miles
Australia
Andes
Marchena
Cape Horn
San
Cristobal
Española
Tierra del Fuego
New
Zealand
Evolution is the greatest unifying theme in biology, and
The Origin of Species fueled an explosion in biological
research and knowledge that continues today.
Evolutionary theory continues to expand beyond
Darwin’s basic ideas.
Nonetheless, few contributions in all of science have
explained so much, withstood as much repeated
testing over the years, and stimulated as much other
research as those of Darwin.
 13.2 Darwin proposed natural selection as the
mechanism of evolution
 Darwin observed that organisms
1. Produce more offspring than the environment
can support
(economist, Malthus)
2. Individuals of a population vary in their
characteristics
3. Many characteristics can be inherited
 Survival in a limited environment depends on the
features the organisms inherit from their parents
 Within a varied population, individuals whose
characteristics adapt them best to the environment
are most likely to survive and reproduce
 Natural selection results in favored traits being
represented more and more and unfavored ones less
and less in ensuing generations of organisms
 Darwin found convincing evidence for his ideas in the results of
artificial selection
 With humans playing the role of the environment
Hundreds to thousands of years
of breeding (artificial selection)
Ancestral dog (wolf)
Evidence of Evolution
1. Fossil Evidence
2. Biogeography
3. Comparative Anatomy
4. Comparative embryology
5. Molecular biology
6. In vitro/ In field observations
 13.3 The study of fossils provides strong evidence for evolution
 Fossils and the fossil record strongly support the theory of evolution
A Skull of Homo
erectus
B Petrified tree
C Ammonite casts
E Fossilized organic
F Insect in amber
matter of a leaf
D Dinosaur tracks
G “Ice Man”
 The fossil record -reveals that organisms have evolved in a
historical sequence
 Strata reveal changes in organisms through time
Transitional
FormsIntermediate
species between
ancient
organisms and
their modern day
equivalents
•Therapsids
•Archaeopteryx
 Many fossils link early extinct species with species living today
 Oldest fossils- 3.5mya
 Prokaryotes
 Various eukaryotes
 Vertebrate Evolution:
 Fish
 Amphibians
 Reptiles
 Mammals
 Birds
 Biogeography
 Biogeography- the geographic distribution of
species
 Suggested to Darwin that organisms evolve from
common ancestors
 Break up of pangea
 Darwin noted that Galápagos animals
 Resembled species of the South American
mainland more than animals on similar but
distant islands
 Comparative anatomy Evolution is a remodeling process
 Is the comparison of body structures in different species
 Anatomical similarities in many species give signs of common descent
 Homology- the similarity in characteristics that result from common ancestry
 Homologous Structures- features that often have different functions but are
structurally similar because of common ancestry
 Vestigial Structures- structures of marginal if any importance to the organism
Human
Cat
Whale
Bat
 Comparative Embryology
 Is the comparison of early stages of development among different
organisms
 Many vertebrates have common embryonic structures
 Pharyngeal pouches- gills in fish, ears/throat in humans
Pharyngeal
pouches
Post-anal
tail
Chick embryo
Human embryo
 Molecular Biology
Comparisons of DNA and amino acid sequences between different organisms
reveal evolutionary relationships
•Anatomical homology isn’t
helpful linking distantly related
organism (plants to animals)
•Strong evidence for Darwin’s
boldest claim: All Life Forms are
Related
•All forms of life use DNA and
RNA with an essentially universal
genetic code
•Passed along through the
branches of evolution
 13.5 Scientists
can observe natural selection in
action
 Camouflage adaptations that evolved in different environments are
examples of the results of natural selection
Madagascan comet orchid
A flower
mantid
in Malaysia
A leaf mantid in Costa Rica
Xanthopan morganii praedicta
 Development of pesticide resistance in insects and antibiotic
resistant bacteria
Chromosome with gene
conferring resistance
to pesticide
Additional
applications of the
same pesticide will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Pesticide application
Survivor
 13.9 In addition to natural selection, genetic
drift and gene flow can contribute to evolution
 Genetic drift

Is a change in the gene pool of a population due to
chance
Can alter allele frequencies in a population
 Has greatest impact on small populations
1. Bottleneck Effect
2. Founder Effect

 Bottleneck Effect: is an event that drastically reduces population size
 Earthquakes, floods, fires
 Produces a small surviving population that is unlikely to have the
same genetic makeup as the original
 Northern elephant seals in California- greatly decreased the
species’ genetic variation
Original
population
Bottleneck
event
Surviving
population
 Founder Effect: colonization of a new location by a small
number of individuals
 The smaller the group- the less likely the genetic makeup of the
founders will represent the gene pool of the population they left
 Darwin’s finches and human Colonists
 Gene flow is the movement of individuals or gametes
between populations
 When fertile individuals move into or out of a population
 Or when gametes (pollen) are transferred between populations
 Can alter allele frequencies in a population
 Reduce differences between populations
 Natural selection
 Leads to differential reproductive success in a population
 Individuals with characteristics that best match them to their
environment have the most reproductive success
 Natural Selection
Can alter allele frequencies in a population
 Results in accumulation and maintenance of
traits that adapt a population it’s environment
 The degree of adaptation is limited by the
amount and kind of genetic variation in a
population

 13.10 Endangered species often have reduced variation
 Low genetic variability -may reduce the capacity of endangered
species to survive as humans continue to alter the environment
 Ex) cheetah- decreased genetic variation due to bottleneck events
during the last ice age
 Human over hunting and encroachment/ breeding programs
 Leads to fewer opportunities for natural selection to lead to traits
best fit for changing environment
VARIATION AND NATURAL SELECTION
 13.11 Variation is extensive in most populations
 VARIATION WITHIN POPULATIONS
 Just like we do as humans: anatomical as well as molecular variation
 Genetics sources as well as environmental
 Many populations exhibit polymorphism (many morphs)- different
forms of a phenotypic character- can relate to behavior
 VARIATION AMONG POPULATIONS
 Populations may also exhibit geographic variation Variation of an inherited characteristic along a geographic continuum
 As environments differ along a geographic continuum, natural
selection will yield changes in individual phenotypes
 Can occur in a cline- a graded change in inherited characteristics
 Ex) Farter away from equator = larger body size
 13.12 Mutation and sexual recombination generate
variation
 Mutations- changes in the nucleotide sequence of DNA
 Nucleotide substitutions
 Chromosomal mutations
 Can create new alleles when they occur in gametes
 Not in somatic cells
 Most are of no effect, Few are deleterious or have a negative effect ,
very, very few are positive
 Rate of mutation varies
 Bacteria- quick
 Plants and animals- slowly (1 in every 100,000 genes per
generation)
 Plants and animals depend more so on sexual
recombination to introduce variation
 Sexual recombination

Generates variation by shuffling alleles during meiosis
Parents
A1
A1
A2
X
A3
Meiosis
Gametes
A2
A1
A3
Fertilization
Offspring,
with new
combinations
of alleles
A1
A2
and
A1
A3
 13.16 Natural selection can alter variation in a population in
three ways
 Stabilizing selection
Favors intermediate phenotypes
 Directional selection
 Acts against individuals at one of the phenotypic
extremes
 Disruptive selection
 Favors individuals at both extremes of the phenotypic
range

Frequency of
individuals
Original
population
Original
population
Evolved
population
Stabilizing selection
Phenotypes (fur color)
Directional selection
Disruptive selection
 13.17 Sexual selection may produce sexual dimorphism
 Sexual selection leads to the evolution of secondary sexual
characteristics- sexual dimorphism
 Which may give individuals an advantage in mating
 Intrasexual Selection- contests
 Intersexual Selection- mate choice
 13.18 Natural selection cannot fashion perfect organisms
 There are at least four reasons why natural selection cannot
produce perfection
1. Organisms are limited by historical
constraints
2. Adaptations are often compromises
3. Chance and natural selection interact
4. Selection can only edit existing variations
The process of EVOLUTION requires the introduction and
assortment of variation at genetic, individual and population
levels
Introduce Variation
mutation
Sort Variation
GENE
genetic recombination
INDIVIDUAL
gene expression
gene flow
POPULATION
Selection
(predictable)
Genetic Drift
(random)