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Unit 9: Change in Organisms
Over Time
Honors Biology
Honors Biology
In historical context
 Other people’s ideas paved the
path for Darwin’s thinking
struggle for survival
population growth
exceeds food supply
land masses change over
immeasurable time
Some Important Contributions
 Carolus Linneaus (1707-1778) was a
Swedish naturalist in the field of
developed a binomial system of
nomenclature (two-part names for each
species [e.g., Homo sapiens]).
 believed in the ideas of
 special creation-- each species had
an "ideal" structure and function
 fixity of species -- each species had a
place in the scala naturae, a
sequential ladder of life.
Jean-Baptiste de Lamarck
Organisms adapted to
their environments by
acquiring traits
change in their life time
 Disuse
organisms lost parts because they did not use them
— like the missing eyes & digestive system of the
 Perfection with Use & Need
the constant use of an organ leads that organ to
increase in size — like the muscles of a blacksmith
or the large ears of a night-flying bat
transmit acquired characteristics to next
Charles Darwin
 1809-1882
 British clergyman
and naturalist
Proposed the idea
of evolution by
natural selection
Collected clear
evidence to
support his ideas
 Who Was Darwin?
LaMarckian vs. Darwinian view
 LaMarck
in reaching higher
vegetation giraffes
stretch their necks &
transmits the acquired
longer neck to offspring
 Darwin
giraffes born with longer
necks survive better & leave
more offspring who inherit
their long necks
Darwin’s study of geology and Fossils
 Darwin took only a few books with him on the
5 year journey (no TV; no computer; no radio.
 In Principles of Geology, Charles Lyell
presented arguments to support a theory
of slow but continuous geological change
proposed by James Hutton.
 Darwin took Lyell's book on the voyage of
the HMS Beagle. And the Bible, being a
clergyman. And Milton’s Paradise Lost.
 Some of Darwin’s fossil
Ocean shells occurred far inland and at
great heights (12,000 feet) in the Andes.
 Fossils of huge sloths and armadillolike animals suggested modern forms
were descended from extinct forms with
change over time.
Succession of types
Armadillos are native to the
Americas, with most species
found in South America.
Why should extinct
armadillo-like species
& living armadillos be
found on the same
Glyptodont fossils are also
unique to South America.
Mylodon (left)
Giant ground
sloth (extinct)
Modern sloth (right)
“This wonderful relationship
in the same continent between
the dead and the living will…throw more light
on the appearance of organic beings on our earth,
and their disappearance from it,
than any other class of facts.”
The Galápagos Islands
 Volcanic islands off the South
American coast
 Fewer types of organisms than
Island species varied from the mainland
species, and from island-to-island.
 Each island had a variation of tortoise;
long and short necked tortoises
correlated with different vegetation.
More observations…
Correlation of species
to food source
Turtles, too!
Darwin's Finches
 Finches on the Galapagos Islands
resembled a mainland finch but there were
more types.
 Galapagos finch species varied by
nesting site, beak size, and eating habits
The finches posed questions to Darwin:
 did they descend from one mainland
ancestor ?
 did islands allow isolated populations to
evolve independently ?
 could present-day species have resulted
from changes occurring in each isolated
Natural Selection and Adaptation
 Darwin decided adaptations develop
over time, and sought their
 Natural selection was proposed by
both Alfred Russel Wallace and
Darwin as a driving mechanism of
evolution caused by environmental
selection of organisms most fit to
reproduce, resulting in adaptation
 Because the environment is always
changing, there is no perfectlyadapted organism.
Why Does Evolution Matter Today?
Preconditions for natural selection
 members of a population have random
but heritable variations
in a population, many more individuals
are produced each generation than an
environment can support (Malthus)
some individuals have adaptive
characteristics that enable them to
survive and reproduce better
Consequences of natural selection
 More individuals in succeeding
generations have the adaptive
 The result of natural selection is a
population more adapted to its local
 Natural selection can only utilize
variations that are randomly
provided; therefore, there is no
directedness or anticipation of future
Organisms Differ in Fitness
 Organisms whose traits enable
them to reproduce to a greater
degree have a greater fitness.
 Fitness is a measure of an
organism’s reproductive
ex: Black pocket-mice are more
likely to survive on dark lava flows;
lighter-colored pocket-mice are
more likely to survive on desert
Pocket Mice
Artificial Selection
 Darwin noted that humans carry out
artificial selection
Humans selectively bred many varieties of
pigeon, dog, chicken, cattle, horse, etc.
 In nature, interactions with the
environment and predators determine
which members reproduce more.
 Evolution by artificial or natural
selection occurs when more fit
organisms reproduce and leave more
offspring than the less fit
Artificial selection
This is not just a
process of the
It is all
us today
Selective breeding
the raw genetic
material (variation)
is hidden there
Selective breeding
Hidden variation
can be exposed
through selection!
Organisms Become Adapted
 An adaptation is a trait that helps an
organism be more suited to its
Because of differential reproduction,
adaptive traits increase in each succeeding
 (in other words, those with the traits best
suited to an environment will live and
reproduce more successfully; therefore,
their traits are passed on to the next
generation in a higher proportion than
poorly adapted members of the same
Essence of Darwin’s ideas
(1) Variation exists in natural populations
(2) Many more offspring are born each season than can
possibly survive to maturity (from Malthus)
(3) As a result, there is a struggle for existence
- competition
(4) Characteristics beneficial in the struggle
for existence will tend to become more common in the
population, changing the average characteristics of the
- adaptations
(5) Over long periods of time, and given a steady input of new variation into a
population, these processes lead to the emergence of new species
Adaptation Vs. Acclimation
 A mutation or combination of mutations
that causes an adaptation may occur
tens of thousands of years before the
trait becomes adaptive
 When environmental conditions change
within the range of tolerance for an
organism, the organism may acclimate
to new surroundings or conditions
within its lifetime.
Evidence supporting evolution
 Fossil record
transitional species
 Anatomical record
homologous & vestigial structures
 embryology & development
 Molecular record
protein & DNA sequence
 Artificial selection
human-caused evolution
Fossil Evidence
 The fossil record is the history of life recorded by
remains from the past.
 Fossils include skeletons, shells, seeds,
insects trapped in amber, and imprints of
Fossils can be dated in a wide variety of ways;
location in rock strata; several radiometric
measurements, etc.
Living organisms resemble the most recent
fossils in the line of descent; underlying
similarities allow us to trace a line of descent
over time.
Fossil evidence supports the common descent
hypothesis; fossils can be linked over time
because they reveal a similarity in form, despite
observed changes.
Fossil Record
Comparing Relative and Absolute Dating of Fossils
Can determine
Is performed by
Relative Dating
Absolute Dating
Age of fossil with respect to
another rock or fossil (that is,
older or younger)
Age of a fossil in years
Comparing depth of a fossil’s
source stratum to the position
of a reference fossil or rock
Determining the relative
amounts of a radioactive
isotope and nonradioactive
isotope in a specimen
Imprecision and limitations of
age data
Difficulty of radioassay
laboratory methods
Water carries small rock
particles to lakes and seas.
Dead organisms are buried
by layers of sediment, which
forms new rock.
The preserved remains
may later be discovered
and studied.
Fossil record
 A record showing us that today’s organisms
share common ancestors with ancestral species
Evidence for Evolution: The fossil record
Archaeopteryx and modern pigeon
Transitional Species
 Transitional forms between fossil remains
and /or modern day species reveal links
between groups.
 Caudipteryx is between dinosaurs and
 This Chinese fossil shows some
dinosaurs had feathers on arms, tail and
probably body.
 Advantages during running and escape
gave rise to birds once lift-off occurred.
 Eustheopteron is an amphibious fish.
 Seymouria is a reptile-like amphibian.
 Therapsids were mammal-like reptiles.
 Many mollusks and fish have transitional
fossil forms
2006 Fossil Discovery of Early Tetrapod
 Tiktaalik
“missing link” from sea to land animals
Land Mammal
 Biogeography is the study of the
geographic distribution of life forms on
Darwin's comparison of the animals of
South America and the Galápagos Islands
caused him to conclude that adaptation to
the environment can cause
diversification, including speciation.
 The location of continents influence
where a population can spread.
 cacti are restricted to North American
deserts and euphorbia grow in African
 marsupials arose when South America,
Antarctica, and Australia were joined;
Australia separated before placental
mammals arose, so only marsupials
diversified in Australia.
Evidence for Evolution: Biogeography
Toucan (Neotropics)
Hornbill (SE Asia)
Anatomical Evidence
 Organisms have anatomical similarities when
they are closely related because of common
 Homologous structures in different
organisms are inherited from a common
 Vertebrate forelimbs contain the same
sets of bones organized in similar ways,
despite their dissimilar functions.
 Analogous structures have come to
resemble each other because they serve a
similar function (not related to common
 wings on bats, birds, butterflies
Homologous structures
succulent leaves
colored leaves
Convergent evolution
 Fish: aquatic vertebrates
 Dolphins: aquatic mammals
similar adaptations to
life in the sea
 not closely related
Vestigial Structures
 Remains of a structure that was functional in
some ancestor but is no longer functional in the
organism in question.
 Most birds have well-developed wings; some
bird species have reduced wings and do not
 Humans have a tailbone but no tail
 Whales have a pelvis with attachment sites to
support lower limbs
Presence of vestigial structures is explained by
the common descent hypothesis; these are
traces of an organism's evolutionary history.
 During development, all vertebrates (this
would include YOU) have a post-anal tail
and paired pharyngeal pouches.
In fishes and amphibian larvae, the pouches
become gills
In humans, few are born with visible gill slits,
 The first pair of pouches becomes a cavity
of middle ear and auditory tube
 second pair becomes tonsils
 third and fourth pairs become thymus and
parathyroid glands.
 Above features are explained if fishes are
ancestral to other vertebrate groups.
Biochemical Evidence
 Most organisms use the same basic biochemical
molecules, e.g., DNA, ATP, and many identical or
nearly identical enzymes
 Organisms utilize the same DNA triplet code and the
same 20 amino acids in their proteins.
 Early example:
 analysis of degree of similarity in amino acids for
cytochrome c among organisms.
 cytochrome c was originally chosen because it is
a mitochondrial protein present in all organisms
that use aerobic respiration; this protein is
essential to life on Earth for most living
 Life's vast diversity has come about by only slight
differences in the same genes.
 Genes for the same characteristics are found
in organisms as diverse as yeast and
 DNA from two species can be cut and the
degree to which the two reattach can be
measured and quantified
Animals, including roundworms and insects
are used for molecular studies in part
because they use the same molecules we do
Many disorders are more easily studied in
non-human animals
 they can be selectively bred
 they have genes for the same
 they have a shorter generation time
 In science, the term “theory” is reserved for
those conceptual schemes that are
supported by a large amount of experimental
evidence over a long period of time and has
not been found lacking.
 Much different from the way the word is
used in common language
 A scientific theory is never a hunch,
guess, or idea
 Theories never become laws; (laws are
very narrow mathematical truths) the
details of all theories are modified over
 Because it is supported by so many lines of
evidence, evolution is no longer considered
a hypothesis; it is a theory
Evolution is one of the great unifying
theories of biology.
Equivalent scientific theories:
 Cell theory: all organisms are made of
 Gene theory: organisms contain coded
information that determines their form and
 Atomic theory: all matter is composed of
 Kevin Padian: Paleontologist UC
Population Genetics
 Population genetics studies the genetic
variation in a population.
The gene pool is the total of all the alleles
in a population, described in terms of
gene frequencies.
Neither allele dominance nor sexual
reproduction changes allele frequencies.
The Hardy-Weinberg Law
 This is a tool used by population genetics to determine
whether a change in a population (evolution) has occurred
 This law states an equilibrium of allele frequencies in a
gene pool (using the formula p2 + 2pq + q2 ) remains in
effect in each succeeding generation of a sexually
reproducing population if five conditions are met.
 No mutation: no allelic changes occur.
 No gene flow: migration of alleles into or out of the
population does not occur.
 Random mating: individuals pair by chance and not
according to the genotypes or phenotypes.
 No genetic drift: the population is large so changes in
allele frequencies due to chance are insignificant.
 No selection: no selective force favors one genotype
over another.
 Note that the converse is also true: if equilibrium is not
maintained, then evolution has occurred by one of those
five conditions
 In reality, conditions of the Hardy-Weinberg law
are rarely, if ever met, and allele frequencies in
the gene pool of a population do change from one
generation to the next, (which is evolution)
Any change of allele frequencies in a gene pool of
a population signifies that evolution has
The Hardy-Weinberg law tells us what factors
cause evolution-- those that violate the
conditions listed.
The Hardy-Weinberg equilibrium provides a
baseline by which to judge whether evolution has
+ 2pq +
 Yes, you have to know this and be able to use it.
 Yes, you will need calculators. Get over it.
 Going back to genetics, this equation is looking
at allele frequency
 Homozygous dominant individuals, AA = p
 Heterozygous individuals, Aa = 2pq
 Homozygous recessive individuals, aa = q2
 In a population of 10,000 rats, there are
36 albinos
 How many total alleles at the albinism
locus are in the population?
A. 36,000
B. 20,000
C. 10,000
 What are the possible genotypes for
the albinism locus in this population?
A. AA, Aa, aa
C. AA and Aa only
B. AA only
D. Aa and aa only
 Which part of the Hardy – Weinberg
equation goes with which genotype?
AA =
A. q2
 Aa =
A. q2
 aa =
A. q2
B. p2
C. 2pq
B. p2
C. 2pq
B. p2
C. 2pq
 What is the frequency of homozygous
recessive individuals in this
 A. 0.36
B. 0.6
C. 0.0072
D. 0.0036
36 / 10,000 (36 per 10,000) = 0.0036
 What is the frequency of the albinism
allele in this population?
 A. 0.0036 B. 0.072
C. 0.06
q2 = 0.0036
= 0.06
 That means 6% of the alleles in this population are a.
 What are the possible genotypes in the gametes of
this population?
 A and a ; p + q = 1
What is the frequency of the A allele in this
A. 0.06
B. 0.94
C. 0.8836
D. None of these
 p = 1 – q; so 1 – 0.06 = 0.94
So 94% of the alleles in this population are A
What is the frequency for homozygous dominant
individuals in this population?
A. 0.94
B. 0.36
C. 0.8836 D. None of these
 AA = p2
 p2 = 0.942 = 0.8836 ; so 88.36% of this population
is homozygous dominant, AA
 What is the frequency of carriers, Aa in this
 A. 0.0564
B. 0.0282
C. 0.0032
D. None of these
 Aa = 2pq
 2(0.94 x 0.06) = 0.1128; so 11.28% of this
population are heterozygotes
 What is the frequency of aa in this population?
A. 0.094
B. 0.0036 C. 0.06
D. none of these
 q = 0.06 so
 q2 = 0.062 = 0.0036
 Reality check: Does this make sense?
 Yes, the original question said the number albinos
in the population was 36/10,000, or .36%
 Detailed explanations of each of the
Hardy-Weinberg conditions follow:
#1 Genetic Mutations
 Natural populations contain high levels of
allele variation
 Analysis of Drosophila (fruit fly) enzymes
indicates have at least 30% of gene loci
with multiple alleles
 Similar results with other species indicates
that allele variation is the rule in natural
Gene mutations provide new alleles, and
therefore are the ultimate source of variation.
 A gene mutation is any change in the DNA
 Mutations can be beneficial, neutral, or
harmful ex: a seemingly harmful mutation
that requires Daphnia to live at higher
temperatures becomes an advantage
when the environment changes.
 a change that occurs in an area of DNA
that does not code for a gene is a
neutral mutation
 a change that occurs in a gene, but still
codes for the same amino acid is a
neutral mutation
#2 Gene Flow
 Is the sharing of genes between two populations
though interbreeding
Gene flow moves alleles among populations by
migration of breeding individuals.
Gene flow can increase variation within a
population by introducing new alleles produced
by mutations from another population.
Continued gene flow decreases diversity among
populations, causing gene pools to become
Gene flow among populations can prevent
speciation from occurring.
#3 Nonrandom Mating
 Random mating involves individuals pairing by
chance, not according to genotype or phenotype.
Nonrandom mating involves individuals inbreeding,
assortative mating, and sexual selection
Inbreeding is mating between relatives to a greater
extent than by chance.
 Inbreeding decreases the proportion of
heterozygotes and increases homozygosity at all
gene loci.
 Inbreeding increases the frequency of recessive
genes, many of which are abnormalities.
 Note that these were always in the population,
they are just being uncovered
 If there were no recessive abnormalities; none
would show up by inbreeding
 Assortative mating occurs when
individuals mate with those that have
the same phenotype.
Assortative mating divides a population
into two or more phenotypic classes with
reduced gene exchange
 Homozygotes for gene loci that control a
trait increase , and heterozygotes for these
loci decrease
 Sexual selection occurs when males
compete for the right to reproduce and
the female selects.
#4 Genetic Drift
 Genetic drift refers to changes in allele
frequencies of a gene pool due to chance
Genetic drift causes isolated gene pools to
become dissimilar; some alleles are lost and
others are fixed
 If a tsunami were to split an island in two, pink
haired yaks may have just happened to be in
abundance on the north side of the island,
while green haired yaks happened to be mostly
on the south side
 Over time, if there is no migration between
these two islands, the gene frequencies will
become more dissimilar, even though there
may have been an equal number of each color
morph of yak on the original island
 Genetic drift occurs when founders start new
population, or after a genetic bottleneck with
 The bottleneck effect prevents most genotypes
from participating in production of the next
 Bottleneck effect is caused by a severe
reduction in population size due to natural
disaster, predation, or habitat loss
 Bottleneck effect causes severe reductions in
total genetic diversity of the original gene pool.
 The cheetah bottleneck causes relative infertility
because of the intense interbreeding when
populations were reduced in earlier times.
 The founder effect is genetic drift where rare alleles
or combinations occur in higher frequency in a
population isolated from the general population.
 This is due to founding individuals containing a
fraction of total genetic diversity of the original
 Which particular alleles are carried by the
founders is dictated by chance alone
So, a boy and girl yak get lost at sea, but manage to
grab hold of a piece of floating debris. They hold on
until they reach an island uninhabited by yaks, but
with plenty of preferred food, water and shelter.
They can only mate with each other, and later the
yaklettes can only mate with each other. The alleles
available to this population are due to chance.
#5 Natural
 Natural selection is the process that results in
adaptation of a population to the environment.
Natural selection requires
 variation
 inheritance of the variable traits
 differential adaptedness
 differential reproduction
Relative fitness compares the fitness of one
phenotype to another.
Types of Selection: Directional
 Directional selection occurs when extreme
phenotype is favored; the distribution curve
shifts that direction.
 Increases in insecticide-resistant
mosquitoes and resistance of malaria
agent to medications are examples of
directional selection.
 The gradual increase in the size of the
modern horse, Equus, correlates with a
change in the environment from forest-like
conditions to grassland conditions.
Stabilizing selection
 occurs when extreme phenotypes are
eliminated and an optimum phenotype
is favored.
The average human birth
weight is near optimum birth
weight for survival.
 The death rate is highest for
infants at the extremes of the
ranges of birth weights.
Disruptive selection
 occurs when extreme phenotypes are
favored and can lead to more than one
distinct form
 British snails (Cepaea nemoralis) vary
because a wide range causes natural
selection to vary.
 In forest areas, thrushes (a bird) feed on
snails with light bands.
 In low-vegetation areas, thrushes feed
on snails with dark shells that lack light
 the splitting of one species into two or more
species or the transformation of one species into
a new species over time; speciation is the final
result of changes in gene pool allele and
genotypic frequencies.
A biological species is a category whose
members are reproductively isolated from all
other such groups.
Reproductive isolation occurs when members of
one species can only breed successfully with
each other
Reproductive Isolating Mechanisms
 For two species to be separate, gene flow
must not occur between them.
A reproductive isolating mechanism is any
structural, functional, or behavioral
characteristic that prevents successful
reproduction from occurring.
 Two types:
 Prezygotic and Postzygotic
Prezygotic Isolating Mechanisms
 anatomical or behavioral differences between the
members of two species that prevent mating or make
it unlikely fertilization will take place if mating occurs.
 Habitat isolation occurs when two species occupy
different habitats, even within the same
geographic range, so that they are less likely to
meet and to attempt to reproduce.
 Temporal isolation occurs when two species live
in the same location, but each reproduces at a
different time of year, and so they do not attempt
to mate.
 Behavioral isolation results from differences in
mating behavior between two species.
 Mechanical isolation is the result of differences
between two species in reproductive structures or
other body parts, so that mating is prevented.
Postzygotic Isolating Mechanisms
 prevent successful development after mating has
taken place.
 Gamete isolation includes incompatibility of
gametes of two different species so they
cannot fuse to form a zygote; an egg may have
receptors only for the sperm of its own species
or a plant stigma prevents completion of
 zygote mortality is when hybrids (offspring of
parents of two different species) do not live to
 hybrid sterility occurs when the hybrid
offspring are sterile (e.g., mules).