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CHAPTER 22
Descent With Modification:
A Darwinian View of Life
REALLY Looking at Organisms
• Differences from other organisms
• Features that show the relationship of an
organism to where it lives, what it does
Ex: marine iguanas to land iguanas (claws,
tail = diet)
Who Thought What Prior to Darwin
• Early Greek: (2500 years
ago) life began in water
w/simpler forms then
more complex forms
• Aristotle Greek: (2000 +
years ago) life was fixed •Lamarck:
(French, early
and didn’t evolve
1800s) supported
• Judeo-Christian: all
evolution but
species were created in a believed in
single act of creation
acquired
characteristics as
about 6000 years ago
Con. 22.1
mechanism for
change
Con. 22.2
Darwin & His Boat Trip
• Spent 4 ½ years traveling to
different places
• Had awareness of how each
organism’s adaptations allow it to
fit into its environment
• Came to see how species change
over time & that living species
arose from earlier species
• Based on ideas of interrelatedness
dating back to over 2500 years ago
The Man of Controversy
• Published The Origin
of Species by Means
of Natural Selection
or the Preservation of
Favoured Races in
the Struggle For Life
Charles Darwin
in 1859 based on
ideas of those before
him, evidence he had
accumulated, &
publishing pressure
Natural Selection
“As many more individuals of each species
are born than can possibly survive; and as,
consequently, there is a frequently recurring
struggle for existence, it follows that any
being, if it vary however slightly in any
manner profitable to itself, under the
complex and sometimes varying conditions
of life, will have a better chance of
surviving, and thus be naturally selected.” –
Charles Darwin, The Origin of Species
Natural Selection
•In a varied population, individuals
whose inherited characteristics adapt
them best to their environments are
more likely to survive and reproduce
leaving more offspring than less fit
individuals do
Natural Selection = Differential
Reproduction
• Indivduals in populations vary
• Some are more suitable to a given
environment & reproduce more easily and
abundantly
• Favored characteristics are passed on to
next generation (less-favored are not)
• Gradual accumulation of favored
characteristics among individuals in a
population occurs over vast amounts of
time
Natural Selection: The Prominent
Force in Nature
• Regional and timely
• Populations tend to adapt to local
environments during one time period
• These same adaptations may be
pointless in other locales or times
Sect 13.5
2 Examples: Cepaea nemoralis (land
snail) & Biston betularia (peppered
moth)
• European land
•
Light
&
dark
variety
snail’s shell
• Light was predominant
patterns
before Industrial
camouflage in
Revolution
different areas
(striped for well- • Darkened trees
lit, dark for
w/pollution allowed for
shady areas)
dark variety to become
predominant
Con. 22.3
Fossil Evidence for Evolution
• Hard parts (skeletons, shells) fossilize
easy
• Some fossils retain organic matter
(leaves) that can be analyzed
molecularly
• Fossilized tree amber trapped
organisms (protecting them from
decomposition)
•Fossil record is an array of fossils w/in
strata of sedimentary rock
- each stratum has deposits of
material that has accumulated
- younger strata sit on top of older
strata
•Fossil record shows historical
sequence of organisms from earliest
prokaryotes (3.5 billion years ago) to
eukaryotes through many intermediate
forms to modern forms
Mass Evidence for Evolution
1. Biogeography-observations about
distribution of different but related
forms around the world & in
neighboring geographical regions
• Island forms are more similar to
forms found on the closest mainland
than those found on similar but more
distant islands
• Can give rise to a new species found
only there (endemic)
2. Molecular Biology- demonstrates
universality of genetic code
•Conservation of amino acid sequences
in proteins (hemoglobin) are compared
•Some processes virtually the same in
all organisms (glycolysis)
3. Comparative
Anatomycomparison of
homologous
structures
• All mammals have
same basic limb
structures
• Structure not
function
• Leads to descent
w/modification
• Vestigial organs
4. Comparative
Embryologycomparison of
embryonic stages
of different
organisms
•All vertebrates
have pharyngeal
pouches in their
throat regions
Chapter 21: Genomes & Their
Evolution
Bioinformatics – computer geek meets
massive amounts of biology data
Genome Sequencing
1. recombination frequencies using RFLPs
to get genetic markers
2. DNA fragments given physical
measurements using YACs
3. sequence DNA using dideoxy chaintermination & a supercomputer
- now that background work is complete,
skip to stage 3 to use whole-genome
shotgun approach
Con. 21.1
Genbank: Holy nucleotide Batman!
- compare DNA, aa, or protein
sequences
- redefining categories & stages of
cancer
Con. 21.2-21.3
Why can’t we determine type, cause,
treatment or cure for viruses or cancer
very easily?
> genome size
> # of genes
> gene density
& noncoding
parts
Noncoding DNA
Mutated sections no longer used
Repetitive sections
Transposable elements (genes that
move on the DNA strand either by
bending or RNA
- transposons
- retrotransposons
Multigene families
Con. 21.4
Why are
noncoding parts
important?
+ structure
+ replicating
faster for
function
Root Cause of Evolution – Mutation
Duplication of Chromosome Sets –
polyploidy (mostly advantageous in plants)
Alterations of Chromosome Structure:
(name them )
Exon Shuffling
Con. 21.5
Genome Sequences & Divergence
comparing distantly related species
(evolutionary trees)
comparing closely related species
(classification)
comparing within a species (diseases,
immunity)
Chapter 23:
The Evolution of Populations
• Population – group of individuals of
the same species living in the same
place at the same time
• Evolution is measured by the
frequency of a given characteristic
w/in a population over successive
generations not an individual
•Darwin realized this but didn’t know
the genetic mechanism – which is??
•population genetics
(1920s)- rediscovery of
Mendel’s genetic
principles & Darwin’s
natural selection (how
populations change
genetically over time)
Modern synthesis (1940s) – incorporates
population genetics & ideas from
paleontology, taxonomy, biogeography
•Focuses on populations as the units of
evolution & that central role of natural
selection
biological species – a group of populations
whose individuals have the same potential
to interbreed & produce fertile (viable)
offspring
•Opportunities for breeding among
populations will vary, depending on species
and extent of isolation of populations
Microevolution
• A change in a population’s gene pool
– all of its alleles in all individuals
making up a population that can be
inherited by the next generation
• Most gene loci have 2 or more alleles
in a population
• During microevolution, the relative
frequencies of alleles can change
(light to dark peppered moths)
Con.
23.2
Hardy-Weinberg Principle
• In a large population, the gene pool
will remain constant unless acted
upon by some outside agent
• Separation of alleles during meiosis,
recombining during fertilization has no
effect on frequency of alleles in a
population
Is this the case in nature?
5 Conditions Required by HardyWeinberg
1. Very large population
2. Isolated population (no migrations into
or out)
3. Mutations do not alter gene pool
4. Mating is random
5. All individuals are equal in reproductive
success (no natural selection)
Then why use it?
Public health agencies:
•estimate the frequency of
harmful alleles in a population
•establish programs to deal
w/genetic diseases
•real populations are not the
ideal (static) Hardy-Weinberg,
stats must be constantly reevaluated.
4 Potential Causes of Microevolution
1. Genetic Drift – change in a gene pool of
a small population due to chance (effect
of losing a few individuals is much
greater than in a large population)
Bottleneck effect: an event that drastically
reduces population size (earthquake,
fire, flood, humans)
Founder effect: small group moves
locations, genes not representative
Con. 23.3
2. Gene Flow – gain or loss of alleles in a
population due to immigration or
emigration of individuals or gametes
- must consider average heterozygosity (2
different alleles)
3. Mutations – rare events that it changes
the whole population beneficially
4. Nonrandom Mating – more often the
cause, particularly among animals (choosing
mates w/particular traits, ex: healthiest)
-differential success in
reproduction is probably
always the case in natural
populations (leads to
adaptive evolution)
Adaptive Evolution
• results when natural selection
upsets genetic equilibrium
• degree of adaptation that can
occur is limited only by amount
and kind of genetic variation in
a population
•endangered species often have less
variation because of a small population
•not all genetic variation is due to
natural selection:
- some neutral variation
shows no apparent selective
advantage (fingerprints)
- frequency of
characteristics may be due
to genetic drift
- difficult to show an allele
benefits in one environment
but not in another
Extensive Variation in Most
Populations
• a single characteristic can be caused
by one or more genes or by
environmental action causing a
phenotypic change
• population is polymorphic for a
characteristic if 2 or more morphs
(contrasting forms) are present
•populations can exhibit
geographic variation
(preference of flower color
by the local pollinator)
Cline- graded change
in an inherited
characteristic due to
geographic location
2 Random Processes Generate
Variation
• mutations normally are harmful but
could be adaptive in a changing
environment
• sexual recombination shuffles mixture
of alleles (such as?)
Independent assortment, crossing over,
random fertilization
How does Natural Selection Affect
Variation?
Con 23.4
• starts w/varied ancestral population
• over successive generations, those
w/characteristics best suited for
environment leave more offspring
behind than ill-suited
• recessive alleles (and their effects)
can remain in population over long
periods of time
Overall Effects of Natural Selection
• Perpetuation of Genes Defines
Evolutionary Fitness
• Natural Selection’s 3 Modes of Action
• Sexual Selection & Sexual Dimorphism
• Natural Selection Acts on Whole
Organisms
• Resistant Populations of Pests &
Parasites
Perpetuation of Genes Defines
Evolutionary Fitness
• emphasis is survival of genes over
time, not individual organisms
• Relative fitness - the relative
combination that an individual makes
to the gene pool of the next
generation (passes on the greatest
number of genes)
Natural Selection’s 3
Modes of Action
• Stabilizing
Selection - tends to
narrow ranges in
population
variability toward
some intermediate
form (in stable
environments,
most common)
(1)
• Directional
Selection - tends to
move the modal
form toward one of
the extremes (most
common during
times of
environmental
change)
(2)
(3)
• Disruptive
Selection - occurs
when
environmental
factors are varied
in a way that
favors extremes
over the
intermediate form
disruptive
Sexual Selection & Sexual Dimorphism
• Secondary sexual characteristics (size,
male adornment)
• Differences between male & female
• Intrasexual selection – 1 lion w/pride
• Intersexual selection - peacocks
Natural Selection Acts on Whole
Organisms
• acts on phenotypes
• each phenotype is the sum of the
organism’s genotype
• there is no way for natural selection
to select individual gene loci (it favors
the whole organism)
Resistant Populations of Pests &
Parasites
• directional selection is the
mode for pesticide, antibiotic,
and drug resistant pests and disease
causing organisms
• these life forms are adaptable
because they are numerous, multiply
rapidly, and have short generation
times
So Why Aren’t We Perfect?
1. Selection w/Variations – populations can
only use what is available
2. Historical Constraints – we can only use
what our ancestors gave us (see # 1)
3. Adaptations & Compromises – no rain, no
rainbows
4. Chance & Natural Selection – nature can
change things, sometimes good,
sometimes bad
“The key is man’s power of
accumulative selection: nature gives
successive variations; man adds them
up in certain directions useful to him.
In this sense he may be said to have
made for himself useful breeds....for
hardly any one is so careless as to
breed from his worst animals.”
- Charles Darwin The Origin of Species