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Lecture 22
Evolution
Evolution
• three key observations about life
– 1. organisms are suited for life in their
environments
– 2. many forms of life share characteristics
– 3. life is diverse
• 150 yrs ago – Charles Darwin developed
a scientific explanation for these
observation
• published his theory as the Origin of
Species
– from his work categorizing species as a
member of the HMS Beagle
• evolution = descent with modification
Evolution
• can also be defined as a change in the genetic
composition of a population from generation
to generation
• pattern of evolutionary change is revealed by
data taken from biology, geology, physics and
chemistry
History of Evolution
• greek philosophers
suggested that life might
have changed gradually
over time
– but Aristotle – viewed
species as fixed and said that
life-forms could be arranged
on a ladder of increasing
complexity = scalae naturae
History of Evolution
• Carolus Linneaus – developed
the binomial system for naming
species
• developed a nested classification
system in contrast to Aristotle
– grouped animals according to
similar characteristics
– groups known as class, family,
genus
– thought the similarities were due
to God’s creation
Lamarck
• many people proposed that life
evolves as the environment
changes
• Jean Baptiste de Lamarck
proposed how these changes
happened
• published his theory in 1809 –
year Darwin was born
• he was wrong
Lamarck: Use & Disuse
• many people proposed that life evolves as proposed two
principles at work: use and disuse & inheritance of
acquired characteristics
– e.g. the giraffe stretching
his neck to reach the
upper leaves of a tree
produces a longer neck
in subsequent
generations
– also thought animals
evolve because of a drive
to become more
complex
Evolution: Descent with Modification
• Charles Darwin – 1809-1882
• naturalist trained at
Cambridge
• recommended after
graduation to Captain Robert
Fitzroy – captain of the HMS
Beagle
Evolution: Descent with Modification
• Beagle was embarking on a multi-year voyage around the
world to chart coastlines
• Darwin observed plants and animals in temperate regions
of South America resembled species in the South
American tropics more than they did species in the
temperate regions of Europe
• also studied fossils and saw similarities with living species
Galapagos Islands
• group of volcanic islands 900 km
west of South America
• fascinated by the unusual animals
and plants
• collected several kinds of birds
• many were similar to each other
but were different species
• some traits were unique to specific
islands
• saw species on these islands that
resembled the SA mainland but
were seen no where else
• used these specimens to formulate
his theories on adaptations and
descent
Natural Selection
• Darwin observed many examples of
adaptations
– inherited characteristics that enhance their
survival and reproduction in the environment
• linked adaptation to the environment and the
origin of a new species
Descent with Modification
• Darwin never used the term evolution
• used the term descent with modification
• proposed that similarities between organisms
was due to descent from a common ancestor in
the remote past
• the descendants lived in various habitats &
developed adaptations to fit them to their habitat
• Linneaus grouped organisms based on similarities
but never recognized these similarities were due
to descent with modification
Natural Selection
• Darwin made two observations:
– 1. members of a population often vary in their
inherited traits
• SO: individuals whose inherited traits give them a higher
probability of surviving and reproducing will leave more
offspring
– 2. all species can produce more offspring than their
environment can support – many fail to survive
• SO: the ability to survive and reproduce will lead to an
accumulation of favorable inheritable traits
• if these traits make your offspring more successful at
coping with its environment = traits will persist over time
= NATURAL SELECTION
Natural Selection: a recap
• 1. NS is a process in which individuals with
certain heritable traits survive and reproduce at
a higher rate than other individuals who don’t
have those traits
• 2. Over time, NS can increase the match
between organisms and their environment
• 3. if an environment changes (or if an individual
moves to a new environment) - NS may result in
adaptations – sometimes giving rise to new
species.
The science of evolution
•
•
•
•
direct observations
homology
fossil record
biogeography
The science of evolution
• direct observations –
evolution observed using
scientific studies
– soapberry bug – feed on fruits
of various plants using a hollow
“beak”
– beak length must match the
depth at which seeds are found
within their fruit
– introduction of Golden Rain
Tree with flatter pods resulted
in the evolution of shorter
beaks within the bug
population
The science of evolution
• direct observations – evolution observed
using scientific studies
– drug-resistant bacteria – e.g Staphylococcus
aureus
The science of evolution
• homology = similarity resulting from common
ancestry
– several types:
• 1. anatomical
• 2. molecular
The science of evolution
• homology:
• 1. anatomical – closely related species share similar features even
though they may have different functions
– e.g. forelimbs of humans, cats, whales and bats
– comparing early stages of development can reveal additional
anatomical homologies – e.g. pharyngeal pouches  gills in fish and
parts of the ears and throat in mammals
– some “leftover” structures can give us important information about
evolution = vestigial structures
Humerus
Pharyngeal
pouches
Radius
Ulna
Carpals
Metacarpals
Phalanges
Post-anal
tail
Chick embryo (LM)
Human embryo
Human
Cat
Whale
Bat
The science of evolution
• homology:
• 2. molecular –
similarities in DNA,
RNA and protein
sequenes
– many genes have
acquired new
functions
– but other genes – e.g.
ribosomal proteins –
remain remarkably
similar from bacteria
to humans
• although some organisms that are related share
characteristics because of common descent – distantly
related organisms can resemble one another due to
convergent evolution
– the independent evolution of similar features in different lineages
• i.e. different common ancestors!
– marsupials vs. eutherians
– two lineages evolved separately but they experienced similar
environments and underwent similar adaptations
– these shared features are said to be analogous NOT homologous
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel
The science of evolution
• fossil record – documents patterns of evolution
• fossils also show how evolutionary changes have occurred in
various groups of organisms
– e.g. ankle bones of the dog is unrelated to those of the pig & deer
• can also shed light on the origins of new organisms
– e.g. pig and deer are closely related to one another
Cetaceans and even-toed ungulates
(a) Canis (dog)
(c) Sus (pig)
(d) Odocoileus (deer)
The science of evolution
• biogeography – the study of biodiversity over space and
time
• used to understand species distribution
• species distribution – the manner in which a taxon is
geographically distributed
• is influenced by many factors
–
–
–
–
Speciation
Continental drift
Extinction
Glaciation
Species Distribution
• Species distribution can be
affected by continental drift
– 250 MYA – one land mass =
Panagea
– 200 MYA – Panagea began to
break apart
– scientists could predict where
fossils might be found
Genetic Variation in Evolution
• smallest unit of evolution = microevolution
– can result in a change in allele frequencies in a population
over generations
– one of the causes – natural selection
– other causes:
– 1. genetic drift: change in frequency of an allele in a
population
• e.g. disease resulting in the death of several individuals of a population –
can decrease the frequency of an allele in that population
– 2. gene flow: transfer of alleles from one population to
another
• e.g. migration of individuals into a new population
Genetic Variation in Evolution
• genetic variation – seen in individual variations
– genetic variation is the differences in composition of an
individual’s genes or other DNA segments (i.e. junk)
– genetic variation produces variations in phenotypes
– only the genetic component of a phenotype can have
evolutionary consequences
• phenotypes are not necessarily passed on
• e.g. body builder changes his phenotype but doesn’t pass on the bigger
muscles
Genetic Variation in Evolution
• variation within a population
– characters that vary within a population: discrete or
quantitative
– quantitative characters – most heritable variation uses
these characters
• may can be measured – e.g. height, weight, IQ
• vary along a continuum within a population -e.g. hair color, eye
color
• usually results from the influence of 2 or more genes on a single
phenotypic characteristic = known as Polygenic Traits
– discrete characters – classified on an “either-or basis”
• e.g. purple color of a flower or a white color
• usually determined by a single gene with different allele forms
producing distinct phenotypes
Genetic Variation in Evolution
• variation between populations
– species also exhibit geographic variation – differences in the
genetic composition of separate populations
– geographic variation can be observed as a cline – a graded or
continuous change in a character along a geographic axis
– e.g. gradual decrease in plant height as altitude increases
Sources of Genetic Variation
• formation of new alleles – arise through mutation
– must occur in a germ cell to be heritable
– most mutations occur in somatic cells and are loss when the organism dies
• altering gene number or position – chromosomal changes that
delete, duplicate or rearrange genes
– may not necessarily be bad – e.g. crossing over in meiosis
• rapid reproduction – increases the rate of mutations
• sexual reproduction – produces genetic variability due to
combination of gametes
Hardy-Weinberg Principle
• Developed independently by Godfrey Hardy and Wilhelm Weinberg
– Hardy used to play cricket with Reginald Punnett
• used to test whether evolution is occurring in a population
• population = group of individuals of the same species that live in
the same area and interbreed to produce fertile offspring
• the population’s genetic make-up = gene pool
– all copies of every type of allele at every gene locus in all members of the
population
– if only one allele exists for a gene locus = fixed allele within the pool (all
individuals are homozygous – e.g. EE or ee)
– if there is more than one allele – individuals may be homozygous (EE or ee)
or heterozygous (Ee)
Hardy-Weinberg Principle
• each allele has a frequency or proportion in a population
– if a gene has two alleles (i.e. E or e) – p is used for the frequency
of one allele (i.e. the dominant), q is used for the other (i.e. the
recessive allele)
– Hardy-Weinberg principle states that the frequency of these
alleles in a population will remain constant from generation to
generation
– if the gene pool is in equilibrium = p+q = 1 (NO EVOLUTION)
– p2 + 2pq + q2 = 1 (i.e. 100%)
Hardy-Weinberg Principle
•
•
•
•
two alleles = E and e
p = allele frequency for E
q = allele frequency for e
p2 = EE genotype frequency - expected frequency of the
EE genotype in the population
• q2 = ee genotype frequency - expected frequency of the
ee genotype
• 2pq = Ee frequency - expected frequency of the Ee
genotype
• p+q - must equal 1 (i.e. 100% of the population) –
population is NOT evolving
Hardy-Weinberg Principle
• conditions for HW equilibrium:
– 1. no mutations
– 2. random mating
– 3. no natural selection
– 4. extremely large population size
– 5. no gene flow in and out of populations
Applying the HW Principle
• HW principle is used to measure the frequency of the heterozygote
– often hard to measure due to its similarity to the dominant
homozygote
• can be very helpful in diseases because it can predict the frequency
of the carrier (heterozygote)
• disease – PKU; 1 in 10,000 births
– therefore q2 = 0.0001
– This means that 0.01% of the population are homozygous recessive
• frequency of q allele = 0.01 (1% of population - square root of
0.0001)
– This means that 1% of the population has a recessive allele in their genotype
• frequency of p allele = 1.0-0.01 = 0.99 (99% of population)
– This means that 99% of the population has a dominant allele in their genotype
• 2pq = 2x0.99x0.01 = 0.0198 or 2% of the population
Applying the HW Principle
•
•
•
•
•
•
according to the HW principle - p2 + 2pq + q2 must equal 1
does it?
p2 = 0.9801
q2 = 0.0001
2pq = .0198
0.9801 + 0.0198 + 0.0001 = 1
• so the frequency of PKU as a disease is stable within the
population
Applying the HW Principle
• The frequency of the autosomal recessive disease cystic fibrosis
is 1 in 1700 births
• Calculate the allele frequencies of p, q, and the genotype
frequencies of p2, q2 and 2pq and determine if the population is
in HW equilibrium