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Transcript
Change in
Populations
Evolutionary Change
• Evolution is the genetic change in a gene pool of a
population over time.
• A gene pool is all the genes (alleles) in a population or
species.
• During the course of evolution, organisms respond to
environmental changes, some survive and pass on
genes, but others do not.
• The particular allele carried by the most successful
individuals will increase in frequency over time. (A
frequency is a percentage expressed as a decimal e.g.
100%=1).
• Shifting allele frequencies in a population are what drive
evolutionary change.
Evolutionary Change
• Nearly all populations show variation between
individuals for particular traits.
• Population variation in a trait is called
polymorphism.
• Mutation and sexual reproduction both increase
genotypic variation.
• Genotypic variation in turn gives rise to
phenotypic variation.
• Evolutionary mechanisms act on this phenotypic
variation resulting in changes in allelic frequency.
Modern Theory of Evolution
The following points summarize the modern theory of evolution:
• 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 is
passed onto 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. 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 divide 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.
Selection
• Selection is the term used for survival of the fittest.
• Selection occurs when some individuals, with particular favourable
features, have a greater chance than others of leaving fertile
offspring.
• Survival of the fittest means that those phenotypes that are best
suited to the environment are more likely to survive.
• Biological fitness is measured by the relative proportion of fertile
offspring left by an individual leaves in the next generation.
• The inherited characteristic that allows the individual to survive and
reproduce is called an adaptation.
• Fitness is a relative characteristic in that a phenotype that suits one
type of environment may not suit another. Human example of this is
predominance of sickle-cell anaemia gene in areas where malaria is
present.
• Selection does not generate variation – it can only
act on the variation already present in the population.
Selection in cattle tick populations
• The cattle tick (Boophilus microplus) is a major pest for graziers in
northern Australia as it carries the protozoan parasite that causes tick
fever in cattle.
• The first cattle dips to
control fleas were
arsenic-based, but
1936, the tick
populations were
showing resistance to
this poison.
• DTT was introduced
by 1946, but by early
1960s, however, the
tick populations had
become resistant to
DDT insecticide.
• Organophosphate pesticides were next to be introduced and by the
1980s, resistance to many of these compounds had developed in tick
populations.
Selection in cattle tick populations
Why did resistance occur?
•
•
•
•
•
•
Resistance to each chemical pesticide involves a different gene.
Before a particular pesticide was widely used, the frequency in the cattle
tick gene pool of the various alleles that confer resistance to these
pesticides was low.
Once a pesticide became widely used, however, it acted as a selecting
agent that produced an increase in the survival and reproduction rates of
resistant ticks relative to those of non-resistant ticks.
As a result of this selection, the proportion of resistant ticks increased and
the frequency of the resistant allele in the population increased.
This re-occurred in turn with different alleles as each new pesticide was
introduced.
The pesticide does not produce the resistant allele, instead it selects for the
allele by creating an environment in which resistant members of the tick
population have a greater genetic fitness than the non-resistant variety.
Natural Selection
• Depends on selection by the environment.
• Factors that can act as agents of natural
selection that can act on populations include:
– physical agents, such as climate change or food
shortages
– biological agents such as an infectious disease or
predation
– chemical agents such as a pollutant in soil or water.
• Natural selection is not directed towards a
particular goal.
Artificial Selection
• Occurs when humans deliberately select particular plants or animals
to breed for a specific purpose.
• When farmers, horticulturalists, horse breeders or geneticists
deliberately select particular organisms from a population to be the
parents of the next generation, a process of artificial selection
occurs.
• Under conditions of artificial selection, the parents that contribute to
the next generation may not be the most fit in a genetic sense.
• For example, certain breeds of dog have been artificially selected to
show phenotypes that in the wild would have low survival rates and
fitness values.
– The dwarf legs and long bodies of Dachshunds whose dwarf legs and
long bodies make them prone to dislocation of the spine, giving them a
lower survival rate in the wild.
– hairless breeds of dogs would also be expected to have lower survival
value in the wild, particularly in low temperatures
Not all evolutionary change is the
result of selection
•
Gene flow is the migration of alleles from one population.
– Genetic variation may result from genetic exchange between populations when
animals migrate between populations or when seed and pollen are dispersed.
– The Duffy blood group is an example of gene flow in the human population. The
Fya allele has steadily increased in frequency in African-Americans in the
northern states where marriages between black and white Americans occur more
often compared to the southern states.
•
Genetic drift is the change in frequency of alleles from generation to
generation due to chance alone.
– It is a random process which in small, isolated populations can cause an
evolutionary change referred to a as the founder effect.
– Perhaps the best example is the atoll of Pingelap in the Pacific Ocean.
– Five percent of the population suffer from a form of colour-blindness called
achromatopsia. This figure is much higher than other populations as
achromatopsia is a recessive autosomal disorder.
– The reason? In 1775 the atoll only had a population of 30 due to famine and a
typhoon. Genealogical analysis shows that one of these people was a chief who
had many children, and who also happened to be heterozygous for
achromatopsia.
Patterns of Evolution
• Through the action of natural selection, gene
flow or chance effects (in small populations),
allele frequencies in populations may change
over time.
• Evolution is defined as a change in the gene
pool of a population over time.
• Patterns of evolution include:
– divergent evolution
– convergent evolution
– parallel evolution.
Divergent Evolution
• Is the evolution of different species (populations) from a common
ancestral species (population).
• The homologous features shared by the species become different
over time due to natural selection or genetic drift.
• Darwin’s finches are a classic example of divergent evolution and
natural selection.
• Adaptive radiation is the rapid divergence of an evolutionary
lineage from a recent common ancestor.
• Darwin’s finches are thought to have undergone this type of
evolution as the adapted to different habitats and different islands in
the absence of competitors.
Divergent Evolution
Convergent Evolution
• Is the independent development of
similar features in unrelated species or
populations. The species become alike
over time (converge).
• Natural selection may lead them to
evolve one or more similar features
which are analogous.
• The eyes of vertebrates and octopuses
are a striking example of convergent
evolution.
Parallel Evolution
• Is similar to convergent evolution except that it occurs
when related species evolve similar features
independently.
• The waxy coating evolved by a number of eucalypt
species to protect themselves from frost damage is an
example of parallel evolution.
• The backward-opening pouches in the Australian native
marsupials, the marsupial mole (Notoryctes typhlops)
and the bilby (Macrotislagotis), which reflect their
burrowing behaviours
Co-evolution
• Refers to changes in two different species that have a
close interaction, such as a parasite and its host, a
predator and its prey or two species that have an
interaction of mutualism.
• For example, bird-pollinated flowering plants and the
birds that pollinate them have evolved features that
favour their mutually beneficial interaction.
– The flowers, for example, have evolved colours, shapes and
other features, such as nectar production, that attract and reward
the birds.
– The birds, in turn, have evolved specialised features such as
long bills that equip them to feed on the nectar while gathering
pollen on their heads.
Speciation
•
•
•
•
Evolution by natural selection can result
in changes within members of one
species and, if sufficient time elapses,
can lead to the formation of new species.
The process of formation of a new
species is known as speciation.
For living, sexually reproducing
organisms, species are recognised as
different when they are not able to
interbreed.
Two patterns of speciation are possible:
– phyletic evolution in which one
population progressively changes over
time to become a new species
– branching evolution in which a
population of one species splits and one
part of the population evolves separately
to form a new species distinct from the
original species
•
A pattern of branching evolution is more
common than phyletic evolution.
Branching Evolution
Branching evolution typically involves:
1. splitting of a smaller group from an original population so that the
small group becomes geographically isolated
2. over many generations, the isolated population is subjected to
different selection pressures because of different environmental
conditions and to other change factors such as genetic drift
3. the isolated population changes over time such that, even if the two
populations were to come together again, their members are
unable to reproduce successfully — they are now two distinct gene
pools and two different species
Two models of speciation
•
There are two general models of speciation: the
gradual and rapid model.:
Gradual Model
•
–
–
–
•
The original population of a species is split by a physical
barrier into two subpopulations of approximately equal size.
Each subpopulation gradually diverges to form a new species.
(This is Darwin’s model).
The two separated species diverge because their
environments differ and they are under different selection
pressure.
Rapid Model
–
–
A small population is isolated at the edge of the range of the
original species by a physical barrier.
Rapid divergence to form a new species is possible in the
small population.
Two models of speciation
(a) Gradual model of speciation
(b) Rapid model of speciation
Microevolution vs Macroevolution
The two models of speciation reflect two different views of evolution:
•
Macroevolution (gradual evolution)
– Given enough time, small micro-evolutionary processes are sufficient to account
for large evolutionary changes
– Over long periods of time new species will give rise to new genera, families,
orders and phyla
•
Microevolution (punctuated equilibrium)
– Natural selection is the force behind permanent gene pool changes and “fine
tunes” organisms to their environment however most morphological changes
occur during abrupt, chance speciation events and once in existence, species
then change very little.
•
There is no doubt that evolution occurs, the debate is only about the relative
importance of each of these mechanisms.
More on Speciation
• Speciation occurs when gene flow has ceased
between populations where it previously existed.
• Speciation is brought about by the development
of reproductive isolating mechanisms which
maintain the integrity of the new gene pool.
• There are two main types of speciation thought
to operate in populations:
– Allopatric speciation
– Sympatric speciation
Allopatric Speciation
• Occurs when one population becomes
geographically separated from the rest of
the species and involves in isolation
• Thought to be the most common form of
speciation and has been the most
important in the evolution of new animal
species
Sympatric Speciation
• Occurs when a population forms a new species
within the same area as the parent species.
• This gene pool divergence is most common in
plants which are not mobile and can form
‘instant’ species through polyploidy.
• May also occur as different groups or members
within a population prefer different microhabitats
and begin to exploit these. Increasing
adaptation to different micro niches results in the
different groups diverging and eventually
becoming reproductively isolated.
Races and Geographic Variation
• A species that is geographically widespread usually
occurs in numerous local populations, each physically
separated from one another.
• Natural selection and genetic drift can lead to divergence
of populations, leading to the evolution of geographic
races or subspecies within a species.
• Differences between races depend on how much gene
flow occurs between them.
• Definition:
– Races or subspecies are populations within a species that show
genetic differences across a geographic range.
Races and Geographic Variation
• Australian eastern rosellas have three separate geographic races
between Tasmania and Cape York.
• Each race has slightly different appearances, however in areas
where the races overlap, birds show intermediate colouration
indicating the races interbreed.
Reproductive isolating mechanisms
• Prevent successful breeding between different species –
they are barriers to gene flow
• A single barrier may not completely isolate a gene pool,
but most species have more than one isolating
mechanism operating to maintain a distinct gene pool.
• Geographical barriers prevent species interbreeding but
are not considered to be RIMs because they are not
operating through the organisms themselves.
• Reproductive isolating mechanism can be categorised
according to when and how they operate:
– Pre-zygotic (pre-fertilization) mechanisms
– Post-zygotic (post-fertilization) mechanisms
Pre-zygotic isolating mechanisms
•
Ecological or habitat
– Different species may occupy different habitats within the same region e.g.
aquatic and terrestrial species
•
Temporal
– Species may have different activity patterns – they may be nocturnal or diurnal,
or breed at different seasons
•
Behavioural
– Species may have specific calls, rituals, postures etc. which enable them to
recognise potential mates
•
Structural
– For successful mating, species must have compatible copulatory apparatus,
appearance and chemical make-up (odour, chemical attractants)
•
Gamete Mortality
– If sperm and egg fail to unity, fertilization will be unsuccessful
Post-zygotic isolating mechanisms
• Zygote mortality
– The fertilized egg may fail to develop properly
• Reduced reproductive success
– Fewer young may be produced and they may have a low viability
(‘survivability’)
• Hybrid sterility
– The hybrid of two species may be viable but sterile – unable to breed
e.g. the mule
• Hybrid breakdown
– The first generation may be fertile but subsequent generations are
infertile or non-viable
Problems with the species
definition
• A biological species is a group of interbreeding (or potentially
interbreeding) individuals, reproductively isolated from other such
groups.
• This does not apply in all situations:
– The concept of a species being able to interbreed cannot apply to
extinct populations because this is unknown – extinct forms must be
classified on purely morphological grounds (unless DNA sequence is
available).
– Asexually reproducing organisms do not breed with each other and so
are assigned to species on the basis of appearance or biochemistry
• Even for sexually reproducing organisms, a species may grade in
phenotype over a geographic area. Such continuous gradual
change is called a cline and often occurs along the length of a
country or continent. All the populations are the same species as
long as interbreeding populations link them.
Extinction
• Extinction is the evolutionary process where
species and groups of species eventually die
out.
• Most species that have ever lived are now
extinct.
• Often the extinction of one group has allowed
another to undergo extensive radiation into free
niches – large scale species changes are
probably opportunistic events.
• Radiations may follow extinctions but are rarely
the cause of extinctions.
Background Extinction
• Is the average rate of natural loss of species due to
environmental change and/or competition.
• The average life of a species varies depending on the
type of organism, but is generally a few million years.
• Generally larger and more complex organism have
higher rates of background extinction than simpler
organisms.
– Average species duration for moss and liverworts is 20+ million
years
– Average species duration for snails is 10-13 million years
– Average species duration for mammals is 1-2+ million years
Mass Extinction
•
•
Superimposed on average rates of background extinction are five events in
the Earth’s history when the rate of extinction rose markedly – called Mass
Extinctions
Mass extinction refers to an abrupt increase in extinction rates affecting
huge numbers of species at the same time.
Mass Extinctions are marked by rapid rate of extinction of groups of
organisms in the fossil record.
During these episodes the diversity of major groups declines.
•
Examples include:
•
•
– The coming together of all continents during the Permian period, resulting in a
harsher climate and loss of coastal seas.
– An asteroid impact that occurred near the Yucatan Peninsula of Mexico is
thought to have caused the extinction of when more than 70 per cent of Earth’s
marine and terrestrial species, including the dinosaurs, 65 million years ago.
Traces of this impact are preserved in the 180-kilometre wide Chicxulub crater
that was caused by an asteroid estimated to be 10 to 20 kilometres in diameter.
Humans as agents of extinction
• Humans have caused an increase in the number
of extinctions.
• Animals that have become extinct as a result of
human activity include the dodo, the passenger
pigeon and the Tasmanian tiger.
• There have been about 500 animal and 600
plant extinctions recorded globally since 1600.
Genetic bottlenecks
Not quite extinction!!!!
• Genetic bottlenecks occur when a species
declines in number and therefore genetic
variation decreases.
• Even if the species recovers, there is less
genetic variation.
• Cheetahs are a good example – genetic
studies suggest all cheetahs descend from
a single pregnant female.
Forces Operating in Evolution
Molecular Level
• Point mutations
• Control of gene expression
• Rate of protein synthesis
Chromosomal Level
• Crossing over
• Block mutations
• Polyploidy
• Aneuploidy
• Independent assortment
• Recombination
Cellular Level
• Gamete viability
• Fertilization
• Genotype expression
Organism Level
• Environmental modification of
phenotype
• Reproductive success of phenotype
• Selection pressures
• ‘Fitness’ of phenotype
Population Level
• Genetic drift and population size
• Natural selection altering gene
frequencies
• Mate selection
• Intra-specific competition
• Founder effect
• Immigration/emigration
Species Level
• Geographical barriers
• Reproductive isolation (pre-zygotic and
post-zygotic)
• Selection pressures
• Inter-specific competition