Download Selection Drift Isolating mechanisms

Stabilising and Directional Selection
Mention the word selection and it is usually associated with change and evolution – this is not
necessarily the case. Sometimes selection can operate to maintain a characteristic. It is only when there
is a change in the selection pressure that change and evolution happens. There are three types of
selection recognised: stabilising, directional and disruptive (which is really directional selection in two or
more directions).
y
This type of selection selects for the norm, i.e. it selects against the illadapted individuals (those at the extremes of the range for the
characteristic), essentially maintaining the status quo.
Examples include:
 wing length in birds
 birth weight in humans
frequency
Stabilising Selection
For either of these examples, explain how the stabilising selection
operates.
character
Example:
Example:
SJWMS Biology
y
An environmental factor changes so that some
individuals have a selective advantage
Examples include:
 milk yield in cattle
 grain yield in cereals
 heavy metal tolerance in grasses
 industrial melanism in insects (monogenic, dominant
allele codes for melanism)
For one of these examples, explain how directional
selection operates.
frequency
Directional Selection
character
character
Genetic Drift
Genetic drift is seen in small populations and is a change in allele frequency caused by chance. It has
important consequences for genetic variation and the potential ability of a species to adapt and survive.
There are two causes of genetic drift:
Founder Effect
In large populations (or smaller populations with good gene flow) even alleles with low frequency tend
to persist from generation to generation. In small populations, with few breeding individuals, it is
possible that rare alleles may not be passed to the next generation, simply by chance. In which case,
the variation will be lost. Alternatively, it is possible that a variant or possibly a mutation could be passed
on, and its frequency increase as a result of inbreeding.
So, with the founder effect, unusual genes are often found in small populations. This happens when a
population starts from just a few individuals and there is reproductive isolation from the original
population. This may occur when a new area is colonised (e.g. an isolated island) or there are
religious/social reasons for segregation.
 Pitcairn Island and the descendants of Fletcher Christian and the Bounty mutineers.
The mutineers and Tahitians that travelled with them settled on Pitcairn. There were nine mutineers,
nine Tahitian females and nine Tahitian males.
 Amish and the incidence of Ellis–van Creveld syndrome.
In this condition, the limbs are short and there are extra digits (polydactyly). It usually causes early
death.
 Tay–Sachs syndrome in Ashkenazi Jews.
The condition is an autosomal recessive and approximately 1 in 30 Ashkenazi Jews is a recessive
carrier. Tay–Sachs disease can affect infants, juveniles and adults. There is a progressive mental and
physical decline and early death in infantile and juvenile TS syndrome.
Bottlenecks
In this situation, a catastrophic natural event may reduce the population to a small number, which then
recovers. Only the genes present in the relic population are available, so there is little variation in the
gene pool. As the population recovers, variation does not increase and there may be insufficient to allow
adaptation to changing conditions.
It was noticed that in the 1980s the breeding success of cheetahs, both in the wild
and zoos was very low, with many cubs failing to survive. Genetic fingerprinting
revealed that there was little genetic variation amongst the cheetah population.
Indeed, genetic fingerprints were so similar that they could only have been
produced by intensive inbreeding, the sort between siblings. Consequently, some
population geneticists suggested that a catastrophe might have wiped out most
cheetahs, apart perhaps from a single pregnant female. In this scenario, it is little
wonder that there is little variation and a very real evolutionary bottleneck exists.
Q. Why are conservationists concerned about small population size and the viability
of captive breeding programmes of animals such as the scimitar-horned oryx, wild Mongolian horse
and the Asian elephant?
Rhetorical question: Was taking the animals into the ark, two by two a sensible approach to
conservation?
SJWMS Biology
Isolating Mechanisms in Evolution
In order for natural selection and evolution to operate, there must be isolation. Without isolation, gene
flow between populations would occur and differences would not accumulate. Isolation mechanisms are
essential and they may be ecological, seasonal or reproductive.
Ecological (Geographic) Isolation
These include barriers such as mountain ranges, hostile environments (desert) and large expanses of
water (the sea and wide rivers). Organisms cannot migrate, populations are isolated and different
selection pressures result in the accumulation of genetic differences in the isolated gene pool. When
sufficient differences have accumulated, it may no longer be possible for successful interbreeding to take
place, even if the barrier is removed. Speciation resulting from geographic isolation is allopatric
speciation. Refer back to your AS notes and review the section on the Galapagos Islands in this unit.
Seasonal (Temporal) Isolation
Season (climate changes throughout the year)
may determine the breeding status of organisms
and so prevent breeding between populations. Red
and white campion are closely related species, but
they are in flower or reproduce at different times
of the year. Red campion flowers mainly in late
spring whilst white campion flowers mainly in early
summer. They are reproductively isolated by
season. Note: there is some overlap in flowering
and some overlap in preferred environment (red
campion prefers shade, white campion prefers
sun). So, hybridisation is possible.
Although red campion (left) and white campion (middle) are
isolated by season and preferred habitat, overlap can occur
and hybrids form (right).
Reproductive Isolation
It is possible that members of different populations can no longer mate and reproduce for a variety of
reasons. Some geneticists categorise these as pre- and post-fertilisation barriers.
 Courtship behaviour
This is important in many animal species. If the courtship ritual is not recognised by either partner it
can prevent mating. In Galapagos finches, male plumage is very similar, so the females depend upon
courtship display and beak size (food may be presented during courtship) to recognise the correct
species.
 Mechanical barriers
Differently sized genitalia may create incompatibility and prevent mating.
 Physiological barriers
Many species of flowering plant are in flower at the same time. Cross pollination between different
species often happens, but the pollen tube does not grow and there is no fertilisation.
 Hybrid not viable
There is fertilisation, but the embryo fails to develop and aborts.
 Hybrid is viable but is sterile
The horse and donkey can hybridise and produce a mule. However, the mule is sterile because
pairing is not possible during Meiosis 1 and so they cannot make gametes. Note: if the same sort of
thing happened in a plant, the sterile hybrid can often survive vegetatively. This can happen
indefinitely or until such time that a chromosome mutation causes the chromosome number to double
(polyploidy), which then allows pairing of chromosomes during meiosis (see evolution of wheat).
SJWMS Biology
Concept of Species
From the AS year you will be familiar with the idea that a species is a group of organisms that share a
similar morphology, physiology, behaviour and genetics that are potentially capable of interbreeding and
producing fertile offspring. Importantly, they are reproductively isolated from other groups (species).
This is the biological species concept. Unfortunately there are some problems with it, e.g. not all species
reproduce sexually, and variation means that some members of the same species look very different
from each other.
Phylogenetic or Cladistic/Evolutionary Species Concept
This relies upon evolutionary relationships to classify organisms rather than using similarities in
morphology/physiology. The evidence comes from DNA, RNA and protein sequencing and computer
programmes are used to produce the evolutionary relationships. Such analysis has resulted in the
proposed reclassification of living things into three domains – the Eubacteria (Bacteria), Eukaryotae
(Eukarya) and Archaebacteria (Archaea).
EUBACTERIA
EUKARYOTAE
Ciliiates
Gram positive
bacteria
Purple bacteria
Animal
Green non-sulphur
bacteria
Fungi
Plants
Flagellates
Cyanobacteria
Flavobacteria
Thermologia
Extreme halophiles
Microsporidia
Methanogens
Extreme thermophiles
ARCHAEBACTERIA
The three domain system, proposed by Carl Woese.
SJWMS Biology