Download Rate of evolution

Evolution:
rate of evolution
CfE Advanced Higher Biology
Unit 2: Organisms and
Evolution
SQA mandatory key information
• Where selection pressures are high, the rate of
evolution can be rapid. The rate of evolution can be
increased by factors such as shorter generation
times, warmer environments, the sharing of
beneficial DNA sequences between different lineages
through sexual reproduction and horizontal gene
transfer.
Selection pressure
What is meant by selection pressure in an evolutionary context?
Selection pressure can be
defined as any factor in
the internal or external
environment that reduces
the reproductive success
of a population and
influences the frequency
of traits in a population. It
is the driving force behind
natural selection.
Rate of evolution
What can affect the rate of evolution?
When selection pressures are high, the rate of evolution can be rapid.
Factors which can increase the rate of evolution include:
 Shorter generation times
 Warmer environments
 Sharing of beneficial DNA sequences between different lineages through
sexual reproduction and horizontal gene transfer.
We will now go on to look at each of these factors which can
increase the rate of evolution in more detail.....
The effect of shorter generation times on
the rate of evolution
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Shorter generation times = more generations in a set period of time.
Each cycle of reproduction requires DNA replication.
Mutations occur during meiosis and DNA replication.
This would lead to an increased rate of mutation which in turn would speed up
the rate of evolution.
e.g. MRSA
antibiotic
resistance in
bacteria
e.g. Insect
resistance to
use of
pesticides.
Warmer Climates
The mean global surface temperature has increased by about 0.5° to 0.7°C since
the late 19th century, and by about 0.3° to 0.4°C over the last 40 years.
Warmer climates continued...
The warming occurred largely during two periods, between 1910
and 1940 and since the mid-1970s. Warming is evident in both
sea surface and land-based surface air temperatures.
What effect do warmer climates have on
the rate of evolution?
• Increasing global temperatures exerts a selection pressure on
all organisms.
• Clearly, if the temperature increases, those heat tolerant
individuals will have an advantage and will leave more
offspring - and those offspring will also carry the genes for
heat tolerance.
• Over many generations, this process produces a population
with adaptations well-suited for the hotter environment.
• So long as the population has different genetic variants in it,
some better able to survive and reproduce in particular
situations than others, the population has the capacity to
evolve when faced with a changing environment.
An example of increased rate of evolution
as a result of warmer climates....
European great tits are evolving
different breeding times. Birds that
are able to adjust egg-laying to
earlier in the spring can time
hatching so that it coincides with
greater caterpillar abundance. With
recent climatic changes, the
caterpillars have been maturing
earlier in the spring. Birds with genes
for more flexible egg-laying times
are more successful than birds with
less flexibility in their egg-laying.
Another example of increased rate of
evolution as a result of warmer climates....
We know that species have actually evolved, experienced a change in gene
frequency in the population, in response to global warming. Interestingly, in
those cases, the species are not necessarily becoming more heat tolerant, but
are adapting to changes in seasonal timing.
Field mustard (Brassica rapa) has evolved in response to
an extreme drought in southern California, which some
sources have linked to global warming. These plants flower
and produce seeds near the end of the rainy season, but
when the rainy season is cut short by a drought, late
blooming plants may wither and die before they can
produce seeds. This form of natural selection favours early
bloomers. Is just four years enough time to see the results
of this evolutionary shift? Researchers compared plants
grown from wild seeds collected before and after the
drought and found that post-drought plants had evolved to
flower much earlier ‒ sometimes by as much as 10 days!
Sharing of beneficial DNA sequences
• The sharing of beneficial DNA sequences between different lineages
through sexual reproduction creates genetic variation. This increase in
variation in a population allows them to adapt to ever changing
environmental conditions. It gives sexually reproducing organisms the
advantage over asexually reproducing genetic clones.
• Horizontal gene transfer has played a
major role in the evolution of bacteria.
Horizontal gene transfer has several
mechanisms but it always involves the
transfer of genetic material (DNA/RNA)
between organisms. It often involves the
use of plasmids.
Barth F. Smets, Ph.D., with permission
Mechanisms of horizontal gene transfer
There are 3 main mechanisms:
 Transformation
 Transduction
 Conjugation.
Transformation
This involves the genetic alteration of a prokaryotic cell due to
the uptake and successful expression of foreign DNA or RNA. The
foreign allele that is taken up by the cell replaces the cell’s
existing allele for a particular characteristic.
Many bacteria have cell-surface proteins that recognise DNA
from closely related species and transport it into the cell. Once
inside the cell, the foreign DNA can be incorporated into the
genome by homologous DNA exchange.
Transduction
1. Bacteriophage infects bacterial cell.
2. Host cell DNA is fragmented, phage DNA and proteins are made, this is the donor cell.
3. Bacterial DNA fragment ( blue) may be packaged in a phage capsid instead of phage DNA
(purple).
4. Phage with bacterial DNA infects a recipient bacterial cell.
5. Recombination occurs between donor DNA and recipient DNA.
6. The genotype of the recombinant cell is different to the donor cell and the original recipient,
Conjugation
Bacterial conjugation involves the transfer of genetic material through cellcell contact.
DNA transfer is one-way only.
1. The donor cell uses sex pili (singular:
pilus) to attach to the recipient.
2. The donor cell, containing an F
plasmid, forms a bridge with the
recipient cell.
3. A single strand of the donor Fplasmid moves into the recipient cell.
DNA replication then begins.
4. DNA replication takes place in both
the donor and the recipient cells.
The plasmid in the recipient cell
forms into a circle and both the
donor and the recipient cell now
contain a complete F-plasmid.
Genetic transfer has taken place and the
recipient cell is now a recombinant.
Summary
Where selection pressures are high, the rate of evolution can be
rapid.
The rate of evolution can be increased by factors such as:
– shorter generation times
– warmer environments
– the sharing of beneficial DNA sequences between different
lineages through sexual reproduction and horizontal gene
transfer.