Transcript Evolution

Processes and Patterns of
Evolution
Year 13
Evolution
•Evolution is the change in allele frequencies
within a population over time .
•Microevolution: small scale changes within
gene pools over generations.
•Macroevolution: evolutionary changes on a
large scale involving whole groups of species
and genera.
Gene pool
• Sum total of genes in a whole population
Evidence for evolution
• Evidence for evolution comes from a
variety of sources:
Early ideas on evolution
Natural
selection
Natural selection:
1)All organisms have a high reproductive rate
but food supply and other environmental
factors are limiting. This means offspring
struggle to survive.
2)There is variation among offspring. Some
are better adapted to the environment than
others.
3) Those organisms with favourable
variations will live longer and will pass on
favourable characteristics to offspring.
4) Over time, each successive generation will
be better adapted to the environment –
“survival of the fittest”.
5) This leads to a change in the frequency of
alleles within the population and may cause
speciation over a long period of time.
Selection Pressures
• These are pressures that result in survival of
those organisms with favorable alleles.
Selection pressures affect population size
and gene pool.
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Selection pressures include:
1) competition
2) predation
3) climatic factors
4) disease
Three Forms of Natural Selection
Directional Selection
Hominid Brain Size
Beak Depth Changed in a Predictable Way in Response to Natural
Selection
Significantly, beak depth is a
genetically determined trait.
Human Birth Weight Is Under Stabilizing Selection
Modern medicine relaxes this and other forms of selection.
Stabilizing Selection for the Sickle Cell Allele
In heterozygous form, the sickle cell allele of -globin confers resistance to
malaria. Therefore, the allele is maintained, even though it’s harmful in
homozygous form.
Disruptive natural selection
The formation of new species =
speciation
• Definition of species:
A species consists of groups of similar individuals
who can interbreed with each other to produce
fertile offspring but do not naturally interbreed
with members of other species.
A Population is a group of the same species living in
the same area at the same time and who can
interbreed
Demes
• A species usually exists as distinct populations
may be separated geographically. These local
interbreeding populations are called demes.
• Organisms mostly interbreed within the deme
rather than with members of other populations,
therefore, demes often develop slightly different
allele frequencies, giving each different
characteristics.
Species tricky to define
• Boundaries of a species gene pool can be
unclear .
For example: closely related species of the
dog family can interbreed
Also, species can show a gradual change in
phenotype over a geographical area. This
gradual change is called a cline. This often
occurs over the length of a country or
continent.
Species
The boundaries of a species gene pool can be sometimes unclear, such as the genus
to which all dogs, wolves, and related species belong:
Coyote–red wolf hybrids
Coyote
Canis latrans
Red wolf
Canis rufus
Interbreedin
g
Interbreeding
Domestic dog
Canis familiaris
Interbreedin
g
Black-backed jackal
Canis mesomelas
Gray wolf
Canis lupus
No interbreeding
Side-striped jackal
Canis adjustus
No interbreeding
Dingo
Canis familiaris dingo
Golden jackal
Canis aureus
Clines
Species can show a gradual change in phenotype over a geographical
area. This gradual change is called a cline. This often occurs over the
length of a country or continent.
Ring species – a special type of cline
C
B
D
A
E
If a cline forms a ring,
(eg. across a continent)
demes A and E may be
unable to breed when
they meet, although,
the intermediate forms
can still interbreed.
Are A and E still the
same species or two
separate species?
Sub-species
• These arise when populations show
characteristics that are different from nearby
populations. Sub-species can interbreed but
this often occurs less frequently.
Reproductive Isolating
mechanisms
Reproductive isolating mechanisms prevent
successful breeding between different species.
They are barriers to gene flow.
Most species have more than one isolating
mechanism operating to maintain a distinct gene
pool.
Geographical barriers
• Physical barriers such as mountains, rivers,
oceans and deserts prevent gene flow
between two different species.
Prezygotic (before fertilisation)
isolating mechanisms
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These operate prior to mating
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Include:1) habitat preference
2) Timing of mating
3) Behavioural incompatibility
4) Structural incompatibility
5) Gamete mortality
1) Habitat preference
2) Timing of mating
Closely related
species may have
quite different
breeding seasons.
This makes them
sexually active at
different times of
the year.
3) Behavioural incompatibility
These behaviours ensure mating within species.
4) Structural incompatibility
Gamete mortality
Even if mating occurs, the sperm of one species may not be
able to survive in the reproductive tract of another species.
Postzygotic Isolating mechanisms
• These act after fertilisation to prevent successful
reproduction.
Include: 1) hybrid sterility
2) hybrid inviability
3) hybrid breakdown
1) Hybrid sterility
• Even if two species mate and produce
offspring, the offspring may be sterile. This
is common in the horse family.
• One cause of this sterility is the failure of
meiosis to produce normal gametes. This can
occur if parents are different in chromosome
number.
Zebronky
or zedonk
2) Hybrid inviability
• A zygote is formed but fails to develop
properly. Sometimes fail to divide because
of unmatched chromosome numbers from
each gamete.
3) Hybrid breakdown
• The first generation
offspring (F1) are
fertile but the second
generation (F2) are
infertile or inviable.
Review questions
• 1. Would having just one pre-zygotic RIM
between a species/population be enough to prevent
gene flow? Why?
• 2. Would pre-zygotic or post-zygotic RIM have
the best chance of preventing hybrids?
• 3. Which forms of RIM would most likely
establish first? Pre or post? Why?
• 4. At a biological level why are hybrids necessary,
yet also a risk to a species survival?
Types of speciation
• Allopatric speciation
This occurs when species become
geographically separated, each being
subjected to different natural selection
pressures, and finally establishing
reproductive isolating mechanisms.
Sympatric speciation
• Occurs when a population forms a new
species within the same area as the parent
species.
• There is no geographical separation.
• More rare than allopatric speciation.
Two situations where sympatric
speciation thought to occur:
A) Speciation through niche differentiation –
there may be a change in host preference,
food preference or habitat preference. This
will lead to disruptive natural selection.
B) Instant speciation as a result of polyploidy
(particularly in plants).
Page 242 Biozone
New Zealand examples
• In New Zealand, the genus
Melicytus comprises 11 species,
including mahoe. Seven of
these species are diploid (with
2N = 32). They all have
unisexual flowers and grow as
trees or tall shrubs. There are
two tetraploid species (2N =
64), both of which have
hermaphrodite flowers and a
divaricating shrub habit, and
one hexaploid (2N = 96), which
has unisexual flowers but also
grows as a divaricating shrub.
SYMPATRIC SPECIATION
SAME PLACE…
In America 200 years ago there lived a species of Hawthorn
maggot fly, which lived on Hawthorn fruit.
When apples were introduced a new niche opened up, and
some of the Hawthorn flies moves in.
These new flies now were unlikely to breed with Hawthorn flies
as they were not in the same places. Therefore no gene flow.
Our new species:
Apple maggot flies.
Stages in the development of a new
species:
• Do pages 240-245
Patterns of evolution
Divergent evolution
• This is when an ancestral species evolves
into two or more species that occupy
different ecological niches.
• This may be due to the ancestral species
spreading out to occupy new habitats with
differing conditions. The populations then
become genetically isolated.
Adaptive radiation
• This is an example of divergent evolution,
but the ancestral species diverges into a
large number of species.
• Adaptive radiation is more common in
periods of major environmental change eg.
cooling climates. Organisms can develop
new adaptations enabling them to exploit
different habitats.
• Page 254
Adaptive radiation of the ratites
Page 256
biozone
Convergent evolution
• This occurs when unrelated species evolve
similar features .The species DO NOT arise
from a common ancestor but have similar
ecological roles.
• Similarity of form due to convergence is
called analogy.
• http://www.teachersdomain.org/resource/td
c02.sci.life.evo.convergence/
Coevolution
• This is when two (or more) species evolve
in response to each other.
• Over time, the parties in the relationship
become mutually dependent on each other.
• Individuals who have less favorable alleles will be lost via
selection from the other species and therefore only
favorable alleles will be passed to the next generation.
Pollination syndromes
• Flower structure has evolved in response to
many types of animal pollinators.
• Flowers and pollinators have coordinated
traits known as pollination syndromes.
• These improve pollination efficiency.
• See page 252 and 253
Predator and Prey (arms race)
• http://www.teachersdomain.org/resource/td
c02.sci.life.evo.leaf/
• http://www.youtube.com/watch?v=R5piJCy
Hwtw
Phylogeny Tree
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A phylogenetic tree or
evolutionary tree is a tree
showing the evolutionary
relationships among various
biological species or other
entities that are believed to
have a common ancestor. In a
phylogenetic tree, each node
with descendants represents
the most recent common
ancestor of the descendants,
and the edge lengths in some
trees correspond to time
estimates.
Homologous structures
• Structural similarities
between groups of
organisms suggest
they descended from a
common ancestor.
• Homologous
structures are evidence
of adaptive radiation.
The pace of evolution – two theories
1) Punctuated equilibrium
Fossil evidence indicates species stayed the
same for long periods of time, and then
there were short bursts of evolution that
produced new species quite rapidly.
This, is the punctuated equilibrium theory.
2) Gradualism
The theory of gradualism assumes that populations
slowly diverge from one another by accumulating
adaptations in response to different selective
pressures.
If species evolved by gradualism there should be
transitional forms seen in the fossil record, as with
the evolution of the horse.
Extinction
•Extinction is an important process in evolution as
it provides opportunities for new species to develop
in the vacant free niches.
•Extinction is a natural process in the lifecycle of a
species.
•Radiation may follow extinctions but are rarely
the cause of extinctions.
• http://www.teachersdomain.org/resource/td
c02.sci.life.eco.bioinvaders/
Background rate of extinction.
This is the steady rate of extinction of a species within
a taxanomic group. The duration of complex organisms
is estimated to be 1 million years compared with 10-20
million years for simpler organisms.
Mass extinctions
This refers to abrupt increases in extinction rates affecting huge numbers of
species at the same time.
Examples
Modern day (holocene)
Cretaceous-Tertiary (65.5mya)-75% of species extinct
Triassic-jurassic (200mya) -50% of species extinct
Permian-Triassic (250mya) -83% of species extinct .
Late Devonian (360mya) – 70% of species extinct
Mitochondrial DNA mtDNA
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DNA is present inside the nucleus of every cell of our body but it is
the DNA of the cell’s mitochondria that has been most commonly
used to construct evolutionary trees.
Mitochondria have their own genome of about 16,500 base pairs
that exists outside of the cell nucleus.
They are present in large numbers in each cell, so fewer samples
are required.
They have a higher rate of substitution (mutations where one
nucleotide is replaced with another) than nuclear DNA making it
easier to resolve differences between closely related individuals.
They are inherited only from the mother. This means that every
individual within the same maternal lineage will possess the same
mtDNA which allows tracing of a direct genetic line.
They don’t recombine. The process of recombination in nuclear
DNA (except the Y chromosome) mixes sections of DNA from the
mother and the father creating a jumbled genetic history.