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Variation, overproduction, natural
selection
Evolution
The Darwin-Wallace theory
Species and speciation
Artificial selection
Classification
5 kingdom classification
Round up
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Variation – genetic and environmental
Genetic variation arises through:
Independent assortment of chromosomes during meiosis
Crossing over of homologous chromosomes during meiosis
prophase I
Allele
reshuffling
Random mating between organisms within a species
Random fertilisation of gametes
Mutation
New allele
production
These are all changes in the genes that can be inherited by the offspring
Environmental variation e.g. body mass due to food availability, do not
involve genetic changes and are thus not passed on to the offspring.
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Overproduction
numbers
Populations have the potential for
exponential growth but are normally
checked by environmental pressures or
‘factors’.
Biotic factors:
e.g. predation, food supply, infection
Abiotic factors:
e.g. water supply, soil nutrient levels.
If the pressure of environmental factors is
sufficiently great the population may fall
before later possibly rising again.
Thus populations may oscillate about a
mean over a period of time.
myxomatosis
Denser populations increase the
probability of disease
transmission. Why?
Time (months)
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Overpopulation
Lemmings
The chart below outlines the lemming population changes over a decade.
The populations of most species show a far
less spectacular oscillation, but the pattern is
similar – more young are produced than can
survive into adulthood.
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Natural selection e.g. rabbits
What determines which rabbits will survive the environmental or ‘selection’
pressures?
Successful
allele
Alleles
Selection
pressure
Brown (agouti)
/ white colour
Fox predation
brown
Blends better
with background
Long / short
ears
Fox predation
long
Better sound
detection
Thick / thin
tooth enamel
Limiting food
thick
Teeth last longer
Advantage
Natural selection increases the frequency of advantageous alleles.
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Evolution Stabilising and directional selection
Stabilising selection
The environment is not changing and only
extremes of the population are selected
against
New Zealand
tuatara – no
change in 200
million years
Stabilizing selection
Selective pressures select against
the two extremes of a trait. For
example, plant height might be
acted on by stabilizing selection. A
plant that is too short may not be
able to compete with other plants
for sunlight. However, extremely tall
plants may be more susceptible to
wind damage. Combined, these two
selection pressures select to
maintain plants of medium height.
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Evolution Stabilising and directional selection
Directional selection
The environment changes shifting the range of variation in a particular
direction. The population shift is an adaptation to the new environment
In directional selection, one extreme
of the trait distribution experiences
selection against it. Thus the
population's trait distribution shifts
toward the other extreme and the
mean of the population graph shifts.
Using the familiar example of giraffe
necks, there was a selection pressure
against short necks, since individuals
with short necks could not reach as
many leaves on which to feed. As a
result, the distribution of neck length
shifted to favour individuals with long
necks.
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Evolution
New alleles
New alleles are produced by mutation.
Nearly all mutations are harmful, conferring a
disadvantage on the individual.
Very occasionally advantageous mutations arise,
e.g. here, white coat colour
Hares possessing the allele are more difficult to
see and are less likely to be predated.
They survive to pass the allele on and over many
generations almost all hairs will inherit the allele.
Such changes in allele frequency are the basis of evolution
Mountain hare
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Evolution
Antibiotic resistance
C
Amp
P
Strep
Tet
Petri dish with lawn of
Staphylococcus bacteria and
antibiotic discs
Which is the most effective
antibiotic against this sample
of Staphylococcus?
Some Staphylococcus bacteria produce an
enzyme called penicillinase which renders
them immune to penicillin.
1 resistant bacterium can produce
10000000000 descendants in 24 hours under
favourable conditions.
Similar antibiotic resistant strains of bacteria
are constantly emerging. The selection
pressure causing this is the presence of the
antibiotic.
The more antibiotics are used, the greater the
selection pressure to evolve resistance.
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Evolution
Industrial melanism
Peppered moths
During the day the moth rests underneath the
branches of trees and relies on camouflage to
protect it from insect-eating birds
Until 1849 only the speckled variety was
observed. From 1849 (industrial pollution
getting worse) until the late 1960’s the black
variety was reported in ever-increasing
numbers
Speckled form
A single gene controls appearance.
The alleles are - C: black, c: speckled
Trees in unpolluted regions often had growths of
speckled lichen. This lichen was killed off in
polluted areas, causing bark to look much
blacker
Black form
Selection pressure: Bird predation.
Successful allele?
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Evolution
Industrial melanism
The advent of clean air acts from
the 1960s began to reduce the
level of industrial pollution
Edinburgh
Newcastle
Distribution of the
pale and dark
forms of the
peppered moth,
Biston betularia in
the early ’60s
Manchester
Birmingham
Cardiff
London
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Evolution
Sickle cell anaemia
HS HS Most people die from
anaemia before adulthood
HN HS Most people show no
symptoms. Also, reduces the chance
of death during malaria by about 20x
2 selection pressures on the sickle
cell allele:
Selection against those who
are homozygous
Selection for heterozygous
individuals – malaria
resistant
The frequency of the HS allele in
an area depends on a balance
between these 2 pressures
The malarial parasite, Plasmodium,
cannot live in sickle cells. Thus sickle
cell confers immunity to the carrier.
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Evolution
Sickle cell anaemia
The frequency of the
HS allele in an area
depends on a
balance between :
Selection FOR
(malaria rersistance
in heterozygotes)
Selection AGAINST
(death due to
anaemia in
homozygotes)
There is selection pressure for both alleles
to remain in malaria-affected areas
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Evolution
Artificial selection
The application by humans of selection pressures to populations
For cattle desired features may include: docility, fast growth rates
and high milk yields.
Chillingham White Cattle breed.
Similar to the original wild cattle which
once roamed the forests of Britain from
earliest times.
Guernseys are bred for the
production of large quantities
of fat-rich milk
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Evolution
Fossil Evidence
Richard Dawkins – explains the evidence for evolution
http://www.youtube.com/watch?v=IQWv_0Mjq0&feature=player_embedded
Fossils show
the
development of
legs from fins
over 20 million
years
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Evolution
The Darwin-Wallace theory of evolution by natural selection
Charles Darwin
Observation 1
Organisms produce more offspring
than are needed to replace the
parents
Observation 2
Natural populations tend to remain
stable in size over long periods
Deduction 1
Observation 3
Deduction 2
Alfred Russel
Wallace
There is a competition for survival
– a struggle for existence
There is variation amongst
individuals of a given species
The best adapted variants are the
fittest and survive to reproduce
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Speciation
Introduction
The Darwin Wallace theory did not attempt to explain how new species
could arise.
The mechanisms discussed in this unit explain how new genes / alleles
arise by mutation, and how selection pressures determine the spread of
these through the population
An important question is ‘how did different species arise?’
A species is: A group of organisms with similar morphological, biochemical,
physiological and behavioural features which can interbreed to produce fertile
offspring, and which are reproductively isolated from other species.
The feature that really decides whether 2 organisms belong to the same
species is their ability to interbreed successfully
(A problem is that the 2 organisms may be dead, fossils, the same sex etc!)
How does a new species arise (speciation)? Here are 2 models:
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Classification
The study of the classification of organisms is called taxonomy.
A key element in classification is the identification of shared or homologous
features
In vertebrates one such feature is the pentadactyl limb.
frog
reptile
horse
human
bat
Each has the same basic design, suggesting a shared or common ancestor.
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Classification
Species which share many homologous features are grouped into the
same genus.
e.g. horse, donkeys and zebras all belong to the genus Equus, probably
evolving from what was a single species along time ago.
Every species is given a 2-word Latin name called a binomial
Genus
Species
e.g. zebra: Equus burchelli . Note the italics (if hand written, underlined)
Kingdom
Animalia
Phylum
Chordata
Class
Mammalia
Order
Perissodactyla
Family
Equidae
Genus
Equus
Species
This classification reflects the evolutionary
history of the zebra and its relationship with
other organisms.
The evolutionary history of living organisms
is called phylogeny.
burchelli
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Classification - the five kingdoms
The system was proposed in 1988 by Margulis and Schwartz.
The kingdoms are: Prokaryotae, Proctista, Fungi, Plantae and Animalia
Prokaryotae
Proctista
Fungi
Plantae
Animalia
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Classification - the five kingdoms
Prokaryotae (Prokaryotes)
Escherichia coli. A rodshapes gram negative
bacterium. Scanning electron
microscope
Prokaryotes
Light micrograph of
filamentous cyanobacterium
Cylindrospermum
Generalised bacterial cell
Eukaryotes
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Classification - the five kingdoms
Protoctista
Amoeba . A unicellular heterotrophic proctist.
Some species live in the sea, others in fresh
water. Some are parasites, e.g. causing the
disease amoebic dysentery
Spirogyra. A multicellular
photosynthetic protoctist
found in ponds and streams
Fucus vesiculosis. A
multicellular photosynthetic
protoctist. A seaweed!
Protoctists are simple eukaryotes. Some are single cells, others
multicellular. Some are photosynthetic, others heterotrophic.
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Classification - the five kingdoms
Fungi
Saccharomyces yeast cells
Fungal hyphae with inset drawing
based on EM studies.
Light microscopy photo of yeast
cells, Saccharomyces
Some fungi are unicellular, e.g. yeasts. Others
are multicellular containing mycelium
composed of hyphae. They feed
heterotrophically, some are saprophytes,
others parasites (athlete’s foot). They don’t
have chlorophyll and although they have cell
walls they are not made from cellulose (usually
chitin)
Boletus. Growing
on leaf litter
Different species
of Aspergillus
growing on a
lemon
Spores of of
Aspergillus niger
on decaying fruit
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Classification - the five kingdoms
Plantae
A mixture of moss species e.g.
Eurhychium crassinervium
A fern Polystichium
setiferum
A flowering plant Ranunculus
Plants are multicellular eukaryotes that feed using photosynthesis.
The kingdom includes mosses, liverworts, ferns, conifers and
flowering plants. Plants have cell walls containing cellulose
(bacteria have cell walls but not made of cellulose). Many plant cells
have chloroplasts and often large vacuoles.
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Classification - the five kingdoms
Animalia
Orang-utan. Pongo pygmaeus. A
mammal.
The common house spider
Tegenaria domestica. An
arthropod.
A jellyfish
Animals relatively simple organisms such as worms as well as more
complex ones such as birds. Their cells do not have cell walls and
they don’t contain chloroplasts. They feed heterotrophically
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AT THE END OF THIS
UNIT YOU SHOULD BE
ABLE TO:
describe one example of artificial selection
Explain how natural selection may
bring about evolution
explain why variation is important in selection
explain how all organisms can potentially overproduce;
describe the processes that affect allele frequencies
in populations with reference to the global
distribution of malaria and sickle cell anaemia
explain the role of isolating mechanisms in
the evolution of new species
use the knowledge gained in this section in new
situations or to solve related problems.
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PROCTISTA
Amoeba
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Fungi
Penicillium spores and hyphae
Aspergillus niger spores and hyphae
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Fungi
Fungal hypha
Yeast – drawing from EM
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Migrations
In Scandinavia, lemmings become restless in years when their populations are high. In the
mountainous terrain of Norway, for example, when lemmings begin to move they tend to
go downhill and get funneled into valleys. The result is that large numbers eventually reach
the sea or a large lake. They may proceed onto sea or lake ice or jump into the water,
which has given rise to the popular conception that they are committing mass suicide to
relieve a problem of overpopulation.
The Inuit have no legends about migrating lemmings and it is difficult to believe that they
would have overlooked such an event, especially if it occurred repeatedly.
Spring is a time of social upheaval caused by the environmental changes associated with
snow melt, and the physiological changes associated with onset of the breeding season.
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