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Evolution continued
Phylogenetic trees
• When we recognise that organisms share
homologous features, we group them
together to indicate that they are related.
• The pattern of how organisms are related
through evolutionary descent from a common
ancestor is termed phylogeny.
• Based on these shared homologous features,
we can build a picture of the relationships of
organisms in the form of a branching diagram.
• Such diagrams are called phylogenetic trees.
Consider three vertebrates: fish, bat
and seal
• Theoretically, there are three possible ways
that these organisms are related, and thus
three possible phylogenetic trees.
• Tree 1 suggests that the seal and fish are most closely
related relative to the bat, based on their similar body
shape, which suits their aquatic lifestyle.
• The second tree indicates that the seal and bat are the
two most closely related. They have similar bones in
their forelimbs, have hair, suckle their young, are
endotherms, and share many other features.
• The third alternative tree has little to support it
because the bat and fish have few features in common
(other than the fact that they are vertebrates as is the
seal).
How do we choose between these
three possibilities?
• We base it on evidence of shared homologous
features.
• Tree 2 is the one in which we can have most
confidence because seals and bats share
homologous features (e.g. hair)—evidence of
them being part of the group, mammals.
Phylogentic trees revisited
• Anatomically characterizing an organism
involves two main approaches: studying the
morphology of animals and analyzing the
fossil record.
• Molecularly characterizing an organism uses
various sequencing techniques to identify
similarities in genetic information between
organisms as expressed in nucleic acids or
proteins.
• Phylogenetic trees are constructed to record
the hypothesized classifications of organisms.
• If a group of organisms is hypothesized to
share a common ancestor, the group is
referred to as monophyletic.
• If members of a group did not all evolve from
a common ancestor, the group is referred to
as polyphyletic.
UNDERSTANDING RELATEDNESS USING
ANATOMICAL CHARACTERIZATION
• Morphology
– The morphology of an organism is simply a description of
its physical characteristics. If the organism under study is
extinct or impossible to resolve with modern microscopy
techniques, observing morphology is unfeasible.
• The fossil record
– The fossil record contains fossilized remains and imprints
whose age is estimated by the age of the surrounded rock.
The oldest known fossils are believed to be approximately
3.5 billion years old and represent the existence of
bacterium-like life. An example of a dating technique used
to determine the ages of rocks and fossils on a scale of
absolute time is radiometric dating.
How to create a phylogenetic tree
Carefully look at the list of cars and trucks below:
• car with three wheels and one seat
• car with four wheels, four doors , front and back seat
• car with four wheels, two doors, front seat only
• car with four wheels, two doors, front and back seat, no top
• truck with four wheels, two doors, one seat and a short bed
• truck with four wheels, two doors, one seat and a long bed
• truck with six wheels, two doors and only one seat
• truck with six wheels, four doors, front and back seat
In this activity we are going to consider these vehicles to be
organisms that are all related in an evolutionary way.
• As you study this list of vehicles think about
the characteristics that can be used to show
relationships. The number of wheels and the
number of doors are two that come
immediately to mind, but there are others as
well.
• Using a pencil, draw a branched phylogenetic
tree starting at the bottom of a sheet of paper.
• Start with what you think is the most primitive
vehicle.
• Remember that scientific knowledge grows by
trial and error. All theories or interpretations
are open to revision.
• There will be a few questions that follow.
Vehicles:
•
•
•
•
•
•
•
•
car with three wheels and one seat
car with four wheels, four doors , front and back seat
car with four wheels, two doors, front seat only
car with four wheels, two doors, front and back seat,
no top
truck with four wheels, two doors, one seat and a short
bed
truck with four wheels, two doors, one seat and a long
bed
truck with six wheels, two doors and only one seat
truck with six wheels, four doors, front and back seat
Questions
• Explain several of the branching points on
your tree.
• Which vehicle seems to be the most
primitive? Justify your answer.
• Which vehicles seem to be the most
advanced? Justify your answer.
Something a little more realistic:
• Hypothesize the appearance of the part of the
morphological tree that shows the
relationships between gorillas, chimpanzees,
and humans.
• On a sheet of notebook paper, they make a
diagram of their hypotheses by drawing lines
from Point A to each of the three organisms
(G = gorilla, C = chimpanzee, H = human,
A = common ancestor).
• Modern research techniques allow biologists
to compare the DNA that codes for certain
proteins and to make predictions about the
relatedness of the organisms from which they
took the DNA.
• You will use models of these techniques to
test their hypotheses and determine which
one is best supported by the data they
develop.
Copy out the following strands of DNA:
Label this strand "human DNA." This strand
represents a small section of the gene that
codes for human hemoglobin protein.
Position 1 Position 20
A-G-G-C-A-T-A-A-A-C-C-A-A-C-C-G-A-T-T-A
Copy out the following strands of DNA:
• Label this strand "chimpanzee DNA." This
strand represents a small section of the gene
that codes for chimpanzee hemoglobin
protein.
Position 1 Position 20
A-G-G-C-C-C-C-T-T-C-C-A-A-C-C-G-A-T-T-A
Copy out the following strands of DNA:
• Label this strand "gorilla DNA." This strand
represents a small section of the gene that
codes for gorilla hemoglobin protein.
Position 1 Position 20
A-G-G-C-C-C-C-T-T-C-C-A-A-C-C-A-G-G-C-C
Copy out the following strands of DNA:
• Label this strand "common ancestor DNA."
This DNA strand represents a small section of
the gene that codes for the hemoglobin
protein of a common ancestor of the gorilla,
chimpanzee, and human.
Position 1 Position 20
A-G-G-C-C-G-G-C-T-C-C-A-A-C-C-A-G-G-C-C
• Compare the human DNA to the chimpanzee
DNA by matching the strands base by base.
• Count the number of bases that are not the
same.
• Record the data in a table.
• Repeat these steps with the human DNA and
the gorilla DNA.
Genetic comparisons
Genetic linkage groups
• Humans and cats are both mammals, although
they belong to different groups.
• Because they need many of the same gene
products (enzymes) to function, cats and
humans have many of the same genes.
Genetic linkage groups
• One method of analysing how similar humans
are to cats is to look at the linkage groups in
both species.
• A genetic linkage group is a group of genetic
loci close together on the same chromosome.
Genetic linkage groups
• Because they so close together, the genes
rarely segregate.
• The linkage groups of a number of genes are
known for humans.
• If groups of linked genes in humans are
compared with the same genes in cats, they
are also generally found to be linked.
Examples of the linkage relationship of
genes in humans and the domestic cat.
• Similarly, when genes of similar function in the
vinegar fly Drosophila melanogaster and the
Australian sheep blowfly Lucilia cuprina are
analysed, they are found to be linked in both
species.
• The simplest explanation for the similarity of
linkage groups in humans and cats or among
flies is that these organisms have a common
ancestry.
In summary
• Homologous features between different organisms are
evidence of evolution from a common ancestor.
• Analogous features are features with the same function but
have evolved independently in different groups of
organisms. Comparative anatomy can reveal that they are
structured differently.
• Embryonic comparisons show that general features of large
groups of organisms appear early in development. More
specialised features, which distinguish the members of a
group, appear later in development.
• The construction of a phylogenetic tree based on the
sharing of homologous characteristics by organisms is
consistent with the theory of evolution.
In summary
• A group of genes may be tightly linked and
rarely segregate. They are inherited as a
group.
• The similarity of genetic linkage groups
between species provides evidence that the
species have a common ancestry.
Evolution—genetic change over
time
Major players in modern evolution
theory
• Jean Baptiste Lamarck (1744–1829):
– A French naturalist who was first to publish a
reasoned theory of evolution. In France, he is
regarded as the ‘father of evolution’, but he died
in poverty and was scorned because of his theory
of the inheritance of acquired characteristics.
Major players in modern evolution
theory
• Charles Darwin (1809–1882)
– An Englishman who started training in medicine.
He sailed as a naturalist on the HMS Beagle,
collecting information that led him to his theory of
evolution by natural selection. Under the
influence of natural selection, those individuals in
a variable population that are best suited to the
environment have the greatest chance of surviving
and reproducing.
Major players in modern evolution
theory
• Alfred Russel Wallace (1823–1913)
– An Englishman who travelled and collected
specimens in the Amazon and the region of IndoMalaya. While there, he independently came up
with the same idea of evolution by natural
selection as Darwin (which spurred Darwin on to
publish his work On the Origin of Species).
• At the time Lamarck, Darwin and Wallace were
writing, people believed that species were fixed
and did not change.
• Lamarck was the first to challenge this idea and
publish a modern theory of evolution.
• His theory stated that characteristics were
inherited by subsequent generations and that
characteristics within populations change over
time.
• However, he proposed that characteristics were
acquired by organisms as the need arose—the
neck of the giraffe got longer as the giraffe
stretched its neck to reach higher and higher
leaves on trees!
• Darwin on to publish his work On the Origin of
Species).
• Darwin also believed in the inheritance of
acquired characteristics because in his time no
one knew about genes and chromosomes.
• Mendel’s genetic theory provided the explanation
that segregation of alleles allows variation to be
passed from generation to generation.
• While Mendel’s theory overcame the puzzle of
Darwin and Wallace, it took almost threequarters of a century before the contributions of
Darwin, Wallace and Mendel were moulded
together as the theory of evolution that is
currently accepted.
Evolution
• All the genes and their allelic forms in a
population constitute a gene pool.
• Evolution is the genetic change in the gene
pool of a population over time.
• During the course of evolution, organisms
respond to environmental changes, some
surviving and leaving offspring, others not.
Evolution
• Thus, the particular allele carried by the most
successful individuals in a population will
increase in frequency over time. (A frequency
is a percentage expressed as a decimal, e.g.,
100% = 0.1).
• Shifting allele frequencies in a population are
what drive evolutionary change.
Why evolution?
• Understanding how evolution occurs—the
underlying mechanisms— gives us tools, for
example, to make the most effective use of
reduced levels of pesticides.
• The evolution of insecticide resistance in
heliothis moths is just one example.
The modern theory of evolution
• Can be summarised in the following seven points:
• Reproduction: Reproduction of organisms in a
population produces descendant populations.
• Excess of potential offspring: Parents have the
potential to produce many more offspring than
actually survive.
• Variation: Members of a population vary.
Variation that is genetically based (heritable) is
passed on to offspring.
• Selection: Environmental resources, such as food and
nest sites, are limited, so there is competition between
individuals. Individuals that can compete successfully
will leave a greater proportion of offspring than less
successful individuals. In this way their characteristics
are selected. The limiting factor acts as a selection
pressure.
• Adaptation over time: Environments change over time.
Heritable characteristics that suit a particular
environment will be selected. Populations diverge over
time and become adapted to new conditions.
• Chance effects: In small populations, shifts in the
frequency of certain characteristics can also occur by
chance.
• Divergence and speciation: When populations are
geographically isolated and thus cannot interbreed,
divergence over time may result in them becoming
different species.
Genetic variation as a basis of
evolution
• Nearly all populations show variation between
individuals for particular traits.
• The human population, for example, shows
considerable variation in hair, skin and eye
colour (look around you at your classmates).
• When members of a population show
variation in a trait, such as flower or feather
colour, the population is described as being
polymorphic (poly meaning many, morph
meaning form).
How does polymorphism (variation)
arise in populations?
• Mutations
• Sexual Reproduction
Mutations
• Mutations produce genotypic and
consequently phenotypic differences between
individuals.
• The probability that a mutation occurs in any
given generation is low. Therefore, while of
critical importance in producing new
genotypes, mutation alone does not account
for significant changes in the genetic make-up
of a population. birds and the burning of their
habitat too often.
Sexual Reproduction
• Sexual reproduction, on the other hand,
generates significant amounts of variation in
every generation through genetic recombination,
acting on the variation that already exists in a
population.
• Recombination occurs either between
chromosomes (independent assortment) or
within chromosomes (crossing over) during
meiosis.
• More new genotypic combinations are produced
by recombination than are possible by mutation
alone.
Asexual reproduction
• In contrast to sexually reproducing organisms,
some organisms reproduce asexually and form
clones, with no genetic variation in a
population.
• Organisms that reproduce asexually are often
found in environments that do not change
very much over time.
Selection
• It is because populations are variable that
evolutionary mechanisms can bring about
change over time.
• The main mechanism is by selection.
• Selection occurs because some individuals,
with particular favourable features, have a
greater chance than others of leaving fertile
offspring.
Survival of the fittest
• You have probably heard the expression ‘survival
of the fittest’.
• When selection occurs, the evolutionary success
of an individual depends on how fit he or she is.
• The ‘fittest’ are those phenotypes that are best
suited to the environment, relative to other
phenotypes present.
• In evolutionary terms, biological fitness is
measured as the relative proportion of offspring
that an individual leaves to the next generation.
Survival of the fittest
• The inherited characteristic that allows the
individual to survive and reproduce is called
an adaptation.
• The adaptation of the resistant heliothis
moths is their biochemical resistance to
insecticide.
• Fit individuals are said to be adapted to an
environment in which pyrethroids are used.
Survival of the fitest
• Fitest is always relative because the impact of
natural selection depends on both the
environment and the genotypes present in a
population.
• The relative fitness of a phenotype in one
environment may be different from the
relative fitness of that phenotype in different
environmental conditions.
Natural and artificial selection
• If the fitness of an individual is determined by
the natural environment (e.g., succulent
plants in coastal salty environments),
evolutionary change occurs as a result of
natural selection.
• If humans decide which animals or plants
should leave offspring to the next generation,
as is the case in animal and plant breeding
programs, changes in the phenotypic and
genotypic make- up of the population occur
because of artificial selection.
• Based on a common ancestral population of
horses, artificial selection by horse breeders
has produced a number of breeds of very
different phenotype. Examples of breeds are
• Humans have deliberately selected horses
with particular characteristics for breeding,
resulting in a number of different breeds
arising.
• Based on a common ancestral population of
horses, artificial selection by horse breeders
has produced a number of breeds of very
different phenotype.
Artificial selection in dogs
• ‘Man’s best friend’, the dog, shows an amazing
range of variation, probably more than any other
domestic animal.
• The domestic dog is descended from the wolf
Canis lupus. Archaeological evidence indicates
that dogs have been domesticated for at least 14
000 years.
• The many breeds of dogs are an indication of the
considerable genetic variation inherent in Canis
lupus.
Defects in dogs
• Boxers have been selected for their muscular
body and flattened face. Their short nose and jaw
led to respiratory and dental problems in many
cases.
• The breed also suffers a higher than normal
incidence of cancer.
• Cocker spaniels have sore eyes due to extra
lashes on the inside of the eyelid, and commonly
have severe bacterial ear infections resulting
from lack of air circulation and high humidity
caused by floppy and hairy ears.
Selection does not generate variation
• It is important to remember that selection
acts on the variation already present in a
population, largely due to genetic
recombination.
• Variation initially comes from random
mutations and is not acquired through some
need.
Selection in action
• The ideas discussed so far—that evolutionary
change results from selection acting on
genetic variation in populations over time—
can be illustrated by the following two case
studies:
– Case study 1: Drunken Drosophila
– Case study 2: The Australian sheep blowfly
Case study 1: Drunken Drosophila
• As far as winemakers are concerned, the
vinegar fly Drosophila melanogaster is
inappropriately named as it is often found in
winery cellars and in surrounding vineyards.
• Laboratory tests show that concentrations of
ethanol (6% by volume) do not greatly affect
D. melanogaster but kill other species such as
the closely related D. simulans.
• In seepage from casks in the cellar, where D.
melanogaster feeds and breeds, ethanol
concentrations can be in excess of 6% by
volume.
• Tolerance of ethanol is under genetic control.
Drosophila melanogaster carries the genes for
alcohol tolerance and D. simulans does not.
• Field experiments show that D. melanogaster
is particularly attracted towards the cellars
during the vintage period when grapes are
crushed and fermentation of grape sugars to
alcohol occurs.
• There is also evidence that tolerance of
ethanol varies between different populations
of D. melanogaster.
• Flies found in the cellar can tolerate higher
concentrations of ethanol than those collected
in the nearby vineyard.
• In contrast, populations of D. melanogaster
collected from areas 100 kilometres from the
vineyard have the lowest levels of tolerance to
ethanol.
• This result shows that tolerance to alcohol can
be selected for in D. melanogaster.
This result shows Alcohol tolerance, a
genetically inherited trait, increases in
populations of Drosophila melanogaster the
closer the population is to an alcohol source in
wine cellars and vineyards.
Case study 2: The Australian sheep
blowfly
• The Australian sheep blowfly (Lucilia cuprina) is a
significant pest of the wool and sheep meat
industries.
• The blowfly is the primary cause of flystrike in
Australia.
• During flystrike, larvae develop on sheep, causing
open wounds and influencing sheep survival,
fleece production and weight gain.
• The damage is done by the pest during the larval
stage of its lifestyle.
Chemical control of the pest
• Various management programs have been
used to control the blowfly, but the major
form of control is through the use of chemical
insecticides.
• These are applied to the sheep by dipping or
jetting (spraying) to provide protection from
flystrike.
• In the case of the use of dieldrin, control was
excellent between 1955 and 1958.
• When it was first used, this insecticide gave
approximately twelve weeks protection from
flystrike.
• However, the evolution of insecticide
resistance by the blowfly reduced the
protection period and effective control of the
pest ceased.
• Within two years the protection period had
decreased to two weeks, too short a period
for dieldrin to be useful in blowfly control.
• This required the introduction of a new
insecticide to achieve satisfactory protection
from the pest.
Genetic resistance to dieldrin
• Resistance to dieldrin is determined by a
single gene, with two alleles RR and RS.
• Genotype RRRR specifies a highly resistant
phenotype that survives concentrations of
dieldrin that kill the other blowfly phenotypes.
• Genotype RSRS specifies the sensitive,
susceptible phenotype (killed by the
insecticide).
• It is reasonable to assume that the RR allele
was at a very low frequency in the sheep
blowfly population before the introduction of
dieldrin for blowfly control.
Natural selection at work
• The effect of natural selection on the pattern of
dieldrin resistance is easy to explain.
• Susceptible individuals are poisoned by the
insecticide and either die or, at the very least,
have their normal development severely
disrupted.
• In either case, susceptible blowflies have little
chance of becoming adults and of leaving fertile
offspring in the next generation; their fitness is
greatly reduced relative to resistant phenotypes.
• The fl exibility of a species to adapt to
environmental change depends on a number
of characteristics such as population size and
generation time. The Australian sheep blowfl y
has shown this fl exibility. Evolution occurs
when natural selection acts on genetically
based phenotypic variation present in natural
populations.
Selection pressures
• Selection pressures include all the abiotic and
biotic components of the external and internal
environment of an organism that influence
population change over time.
• Selection factors include not only pesticides
but also climate, soil, fire, availability of food
resources, predators, parasites and so on.