The Mechanism for Evolution
Natural Selection is the way evolution
takes place
1.There is naturally variation in the
population
2.More offspring are produced than
can survive.
3.Those individuals best suited to the
environment (FITTEST) will survive
and reproduce (pass on their
successful alleles) better than those
that are not.
4.The subsequent generation has
more of the favored variations.
Natural Selection
Overproduction
Variation
Populations produce too
many young: many must die
Individuals show variation: some
variations are more favourable
than others
Natural selection
Natural selection favours the best
suited phenotypes at the time.
Inheritance
Variations are
inherited. The best
suited variants leave
more offspring.
Sources of Variation
‣ Mutation: some have no effect
some give a selective
advantage or disadvantage.
‣ Meiosis: Unique combinations
of alleles through independent
assortment of homozygous
pair chromatids AND
crossover in gamete formation
‣ Sexual reproduction: Unique
sperm meets unique egg to
give uber-unique individual!!
‣ Variety increases chance that
SOME will survive even in
changed conditions.
Too Many Offspring
‣ Most plants and animals
produce many more offspring
than can survive
• Fish lay hundreds of eggs, Plants
produce thousands of seeds,
Mushrooms produce millions of
spores -more than needed
‣ Too many for the available
food, water, space, etc.
‣ Why? To maximize chance
that at least some will survive.
‣ This leads to struggle for
survival or competition for
resources.
Selection Pressure
‣ Anything that reduces the
reproductive success of a
proportion of the population exerts a
selection pressure.
‣ Ex
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Food preference and selective predation
A new, previously unexploited, food source
Food preference can create selection pressure in
both predator and prey populations
Presence of toxins in the environment
Increased frequency of frosts
Heavy use of insecticides can create an
environment that favours resistance.
Adaptations and Fitness
‣ Fitness
how well suited an
organism is to survive in its
environment and its ability to
maximize the numbers of
offspring surviving to
reproductive age.
‣ Adaptations are
characteristics that are
selected for in a population that
make it more fit to survive and
reproduce.
‣ Ex: camels can release water
by metabolizing fat in their
humps
Adaptations to Climate
‣ The adaptations typical of mammals
living in hot climates include
features to facilitate heat dissipation
and reduce heat gain:
• A small body size, lightweight fur,
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and long ears, legs and nose.
‣ The adaptations typical of mammals
Fennec fox of the Sahara
living in cold climates include
features to reduce heat loss to the
environment:
• Compact body shape with small
ears, short legs and nose, and
dense fur.
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Arctic fox
Ear Length
in Mammals
‣ The external ears of many mammals are
used as important organs to assist in
controlling gain and loss of body heat.
• The ears of rabbits and hares (Lepus)
native to hot, dry climates, such as those
in the south-western USA, are very large
relative to body size.
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Jack rabbit, L. californicus
• In contrast, the ears of the Arctic hare,
which lives in the northern tundra zones,
are relatively short.
‣ A reduction in size of the extremities
(ears, limbs, and noses) is typical of cold
adapted species.
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Arctic hare, L. arcticus
Plant Adaptations
‣ The adaptations found in plants reflect
both the plant’s environment and the
type and extent of predation to which
the plant is subjected.
• Many plant adaptations are
concerned with maintaining water
balance. Terrestrial plant species
show a variety of structural and
physiological adaptations for water
conservation.
• Plants evolve defences, such as
camouflage, spines, thorns, or toxins
in the tissues, to protect themselves
against efficient herbivores.
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Peppered
‣ Transient Polymorphism
‣ The peppered moth, Biston betularia,
Moths
occurs in two forms (or morphs):
•
The mottled or gray form is well camouflaged
and less conspicuous (to predators) against the
lichen-covered bark of trees in unpolluted
regions.
• The dark melanic forms are conspicuous in such
environments as their body shape stands out
against the background.
‣ Air quality declined
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Gray or mottled form of the peppered moth
Biston betularia: camouflaged on lichen
in Industrial
Revolution in England, killing off
lichen and resulting in a marked
increase in the relative frequency of
the dark moths.
• In a polluted environment,
directional selection favoured the
melanic forms.
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Melanic or carbonaria form of the peppered
moth Biston betularia: conspicuous on lichen,
but camouflaged on soot-covered vegetation
Industrial Melanism
Industrial areas
Non-industrial areas
‣ In the 1940s and 1950s,
coal burning was still
intense around the
industrial centres of
Manchester and Liverpool.
Frequency of
peppered moth
forms in 1950
‣ During this time, melanic
forms remained dominant
in these regions.
‣ In the rural areas further
south and west of the
industrial centres, the grey
Key to Frequency Graphs
forms increased
Gray or
speckled form
dramatically.
Melanic or
carbonaria form
Changes in Melanic Morph
‣ With the decline of coal
burning factories and the
Clean Air Acts in cities,
the air quality improved
between 1960 and 1980.
•
Sulfur dioxide and smoke
levels dropped
to a fraction of their previous
levels.
• This caused the proportion of
melanic peppered moths to
plummet.
•
With cleaner air, selection is
increasingly in favour of the
gray form.
Frequency of melanic peppered moth
related to reduced air pollution
Melanic Biston betularia
Summer
smoke
Winter sulfur
dioxide
Antibiotic
Resistance 1
‣ The short generation time of
bacteria (under 30 minutes in
some species) allows for an
enormous number of chances for
mutation.
• A mutation could impart some useful
property in the current selective
environment, e.g. resistance to antibiotics.
‣ Any mutation that provides an
advantage in a particular selective
environment may spread quickly
through a bacterial population.
•
The widespread use of antibiotics to treat
infection has created the selective
environment for increased frequency of
antibiotic resistance genes in bacterial
populations.
Photo: AT
Antibiotic Resistance 2
Susceptible
bacterium
Less susceptible
bacterium
Mutations occur at a
rate of one in every
108 replications
The less susceptible
bacterium survives
Antibiotic resistance
genes can be transferred
to non resistant strains
Fully resistant
bacterium
Any population of bacteria
includes variants with
unusual traits. Mutations
may spontaneously
produce unusual or useful
traits, e.g. resistance to a
particular antibiotic.
Even less
susceptible
bacterium
Under the selective environment
provided by the antibiotic, any
individual with resistance will
have an advantage. They may
survive a certain dose of
antibiotic and proliferate.
Some of the individuals in the growing
population may be able to resist an
even higher dose of the antibiotic. Their
descendants will be favoured as long as
the selective environment continues.
Pesticide
Resistance
‣ The application of pesticide
can provide the selective
environment for the spread
of chemical resistance in
pest insects.
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‣ The pesticide acts as a
selective agent and only
those insects with greater
natural resistance survive
the application to pass their
genes to the next
generation.
The bed bug Cimex lectularius has been
reported to show resistance to the active
pesticide ingredients cyhalothrin, DDT,
deltamethrin, malathion, and various
organophosphates. Resistance is reported in
at least 21 countries.
In bed bugs it has been found the the
resistance to deltamethrin is caused by one of
two mutations, valine to leucine (V419L) and
leucine to isoleucine (L925I) in the voltagegated sodium channel α-subunit gene.
Developing Resistance
Resistant
3 A small proportion of the
population will have the
genetic makeup to survive
the first application of
pesticide. (The application
dose may not have been
sufficient to kill them).
Susceptible
1 In any population, the
combination of genes
provides a range of
phenotypes that will vary in
their suitability to a
particular environment.
Pesticide applied
4 The proportion of resistant
individuals increases
following subsequent
applications of pesticide.
Eventually, almost all the
population is resistant.
2 The genetic
information for
pesticide resistance
is passed to the
next generation.
Pesticide applied
Mechanism of Resistance
‣ Pesticide resistance in insects can
arise through a combination of
mechanisms:
• Increased sensitivity to an
pesticide may cause an
individual to avoid a treated area.
• Certain genes confer stronger
Behaviour
avoidance of contact
Reduced penetration
resistance to entry
lowers uptake levels
physical barriers, decreasing the
rate of pesticide penetration.
• Detoxification by enzymes within
the insect can render a pesticide
harmless.
• Structural changes to the target
enzyme make the pesticide
ineffective.
Destruction
detoxification mechanisms
Hemera
Biochemical resistance
enzymatic changes at points
of action