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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
Artificial & Natural Selection
• so: Natural selection was proposed as the mechanism
explaining evolution
• Darwin used artificial selection used by humans in
breeding to explain his theory
• 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
– introduced plant species – what happens when
herbivores begin to feed on a plant species with
different characteristics than their usual food source?
• 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
• introduce a plant with fruits closer to the surface –
evolution of shorter beaks within the bug population
– drug-resistant bacteria – e.g Staphylococcus aureus
The science of evolution
• homology – analyzing similarities among different
organisms
– similarity resulting from common ancestry = homology
– several types:
• 1. anatomical
• 2. molecular
• homology:
The science of evolution
• 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
• 2. molecular – similarities in DNA and RNA
– many genes have acquired new functions
– but other genes – e.g. ribosomal proteins – remain remarkably similar from bacteria to
humans
Humerus
Pharyngeal
pouches
Radius
Ulna
Carpals
Metacarpals
Phalanges
Post-anal
tail
Chick embryo (LM)
Human embryo
Human
Cat
Whale
Bat
• 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
• can also shed light on the origins of new organisms
– e.g. cetaceans are closely related to ungulates
Most mammals
(a) Canis (dog)
Cetaceans and even-toed ungulates
(b) Pakicetus
(c) Sus (pig)
(d) Odocoileus (deer)
ankle bones between dogs (unrelated) and an early cetacean (pakicetus) and ungulates
The science of evolution
• biogeography – the geographic distribution of species
• species distribution is influenced by many factors
• including continental drift
–
–
–
–
250 MYA – one land mass = Panagea
200 MYA – Panagea began to break apart
scientists could predict where fossils might be found
horse evolution – based on fossils, present day horses originated about 5
MYA in north america
– at that time north and south america were close together but not connected
– scientist predicted that the oldest horse fossils should be found in north
america – found to be correct
Genetic Variation in Evolution
• smallest unit of evolution = microevolution
–
–
–
–
change in allele frequencies in a population over generations
one of the causes – natural selection
other causes: genetic drift & gene flow
only natural selection improves adaptation to the environment
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 change in a
character along a geographic axis
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
• 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
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two alleles = E and e
p2 = expected frequency of the EE genotype in the population
q2 = expected frequency of the ee genotype
2pq = expected frequency of the Ee genotype
p+q - must equal 1 (i.e. 100% of the population) – population
is NOT evolving
• 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 can be 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
• disease – PKU; 1 in 10,000 births
– therefore q2 = 0.0001
• frequency of q allele = 0.01 (1% of population - square root of
0.0001)
• frequency of p allele = 1.0-0.01 = 0.99 (99% of population)
• 2pq = 2x0.99x0.01 = 0.0198 or 2% of the population
Applying the HW Principle
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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