Download Hardy Weinberg topic

20.5 Evolution
Specification reference: 6.1.2
Demonstrate and
apply knowledge and
understanding of:
➔ factors that can affect the
evolution of a species
➔ the use of the Hardy–
Weinberg principle to
calculate allele frequencies in
populations.
Evolution, the change in inherited characteristics of a group of
organisms over time, occurs due to changes in the frequency of
different alleles within a population.
Population genetics
Population genetics investigates how allele frequencies within
populations change over time. The sum total of all the genes in a
population at any given time is known as the gene pool. The gene
pool of a population includes millions of genes, but you will look at
the variation in the different alleles of a single gene within the gene
pool. The relative frequency of a particular allele in a population is the
allele frequency.
The frequency with which an allele occurs in a population is not
linked to whether it codes for a dominant or a recessive characteristic,
and it is not fixed. It can change over time in response to changing
conditions. Evolution involves a long-term change in the allele
frequencies of a population, for example, alleles for antibiotic
resistance have increased in many bacteria populations over time.
Biologists have developed ways of determining allele frequencies and
use them in models to determine whether evolution is taking place.
Calculating allele frequency
Imagine a population of 100 diploid organisms that can all breed
successfully. You are going to look at a gene that has two possible
alleles, A and a. The frequency of allele A in the population is
represented by the letter p. The frequency of allele a in the population
is represented by q. If every individual in your population of 100 is a
heterozygote (Aa), then the frequency of each allele is 100/200 or 0.5
(50%) so p + q = 1
In a diploid breeding population with two potential alleles, the
frequency of the dominant allele plus the frequency of the recessive
allele will always equal 1. This simple formula is very important when
using the Hardy–Weinberg principle.
The Hardy–Weinberg principle
The Hardy–Weinberg principle models the mathematical relationship
between the frequencies of alleles and genotypes in a theoretical
population that is stable and not evolving. The Hardy–Weinberg
principle states: in a stable population with no disturbing
factors, the allele frequencies will remain constant from
one generation to the next and there will be no evolution. A
completely stable population is not common in the real world, but this
is still a useful tool. The Hardy–Weinberg principle provides a simple
model of a theoretical stable population that allows us to measure and
study evolutionary changes when they occur.
538
835192_OCR_A Level Biology_SB_Ch20.indd 538
06/07/2016 10:52
Pat t e r n s o f i n h e r i ta n c e a n d va r i at i o n
20
The Hardy–Weinberg principle is expressed as:
p2 + 2pq + q2 = 1
where
p2 = frequency of homozygous dominant genotype in the
population
2pq = frequency of heterozygous genotype in the population
q2 = frequency of homozygous recessive genotype in the
population
How do you use this information? Recessive phenotypes are often
easy to observe. As a result you can find the frequency of the recessive
genotype and use it to measure the equivalent allele frequency.
Frequency of the recessive genotype = q2
So, the frequency of the recessive allele is √q2 = q
You can then use this to find p because you know p + q = 1
Finally, you can substitute these values back into the equation of the
Hardy–Weinberg principle to find the frequencies of the three different
genotypes.
Hardy–Weinberg worked example
The peppered moth, Biston betularia, comes in two forms, light coloured
and dark coloured. The light colour is inherited through a recessive allele.
Students investigated a population in an area of woodland and found that
48 of the 50 peppered moths they captured were light in colour.
This gives the frequency of the homozygous recessive genotype (q2)
that results in a light colouration as 48/50, or 0.96 (96%). Now you can
calculate the value of q, the frequency of the allele in the population.
q2 = 0.96
so q = √0.96 = 0.98 (98%) (2 s.f.)
You know that p + q = 1, so p + 0.98 = 1, so p = 1 – 0.98 = 0.02 (2%)
Now substitute these values into the equation for the Hardy–Weinberg
principle to work out the frequency of the homozygous dominant
genotype and the heterozygous genotype in this population of Biston
betularia.
Frequency of homozygous dominant genotype (p2) = 0.022 = 0.0004
(0.04%)
Frequency of the heterozygous genotype (2pq) = 2 × 0.02 × 0.98 = 0.039
(3.9%) (2 s.f.)
This gives you the frequencies for the three main genotypes of the
Biston betularia population in the woodland studied. Around 96% of the
moths are homozygous recessive and therefore light coloured, 3.9%
are heterozygous and so dark coloured, and 0.04% are homozygous
dominant and dark in colour.
Remember allele frequencies must add up to 1 and population
percentages to 100% (allowing for rounding numbers up or down).
539
835192_OCR_A Level Biology_SB_Ch20.indd 539
06/07/2016 10:52
20
20.5 evolution
Disturbing the equilibrium
The Hardy–Weinberg principle assumes a theoretical breeding
population of diploid organisms that is large and isolated, with random
mating, no mutations, and no selection pressure of any type. In a
natural environment these conditions virtually never occur. Species
are continuously changing. In the peppered moths of the worked
example, the light alleles were dominant historically but the allele
frequencies changed dramatically after the Industrial Revolution,
when the dark alleles gave individuals an advantage. Now the allele
frequencies have changed again as cities and woodlands have become
cleaner again. These changes in allele frequencies can be illustrated
using the Hardy–Weinberg principle and upsetting the equilibrium
may eventually result in evolution.
factors affecting evolution
There are a number of factors that lead to changes in the frequency of
alleles within a population and so they affect the rate of evolution:
•
Mutation is necessary for the existence of different alleles in
the first place, and the formation of new alleles leads to genetic
variation.
•
Sexual selection leads to an increase in frequency of alleles which
code for characteristics that improve mating success.
•
Gene flow is the movement of alleles between populations.
Immigration and emigration result in changes of allele frequency
within a population.
•
Genetic drift occurs in small populations. This is a change in allele
frequency due to the random nature of mutation. The appearance
of a new allele will have a greater impact (is more likely to
increase in number) in a smaller population than in a much larger
population where there is a greater number of alleles present in
the gene pool.
•
Natural selection leads to an increase in the number of individuals
that have characteristics that improve their chances of survival.
Reproduction rates of these individuals will increase as will the
frequency of the alleles coding for the characteristics. This is how
changes in the environment can lead to evolution.
the impact of small populations
The gene pool of a large population ensures lots of genetic diversity
owing to the presence of many different genes and alleles. Genetic
diversity leads to variation within a population which is essential in
the process of natural selection. Selection pressures such as changes in
the environment, the presence of new diseases, prey, competitors, or
even human influences lead to evolution. The population can adapt to
change over time.
540
835192_OCR_A Level Biology_SB_Ch20.indd 540
01/07/2016 11:47
Pat t e r n s o f i n h e r i ta n c e a n d va r i at i o n
20
Small populations with limited genetic diversity cannot adapt to
change as easily and are more likely to become extinct. A new strain
of pathogen could wipe out a whole population.
The size of a population can be affected by many factors. Factors
which limit or decrease the size of a population are called limiting
factors. There are two types of limiting factors:
1
Density-dependent factors are dependent on population size and
include competition, predation, parasitism, and communicable
disease.
2
Density-independent factors affect populations of all sizes in
the same way including – climate change, natural disasters,
seasonal change, and human activities (for example,
deforestation).
Large reductions in population size which last for at least one
generation are called population bottlenecks (Figure 1). The
gene pool, along with genetic diversity, is greatly reduced and
the effects will be seen in future generations. It takes thousands
of years for genetic diversity to develop in a population through
the slow accumulation of mutations.
time
catastrophic event
▲ Figure 1 A natural disaster or epidemic
can drastically reduce a population. The gene
pool will be greatly reduced and the remaining
individuals may not be representative of the
original population as some rarer alleles may not
have been present in any of the survivors. The
‘founder effect’ and genetic drift will influence
genetic variation as the population grows again
Northern elephant seals were almost hunted to extinction in the
19th century. There were probably only about 20 seals left by
the time hunting stopped. They now have a population of about
30 000 but show much less genetic diversity than southern elephant
seals that did not experience a genetic bottleneck.
Cheetahs are thought to have experienced an initial population
bottleneck about 10 000 years ago with other bottlenecks happening
more recently. The species now shows low genetic diversity. Cheetahs
face the same threats as many other African animals such as habitat
loss and poaching, but while the population sizes of other animals are
increasing thanks to the efforts of conservationists, cheetahs are not
recovering as quickly. They are, in fact, close to extinction.
The reduced genetic diversity of cheetahs means that they share around
99% of their alleles with other members of the species, more than we
share with members of our own family. Mammals usually share about
80% of their alleles with other members of a species. As a result they
are showing problems of inbreeding including reduced fertility.
▲ Figure 2 Cheetah mother and cubs,
Acinonyx jubatus, Masai Mara Reserve,
Kenya
Humans and chimpanzees split from a common ancestor about six
million years ago. A small group of chimpanzees are likely to show
more genetic diversity than all the humans alive today. It is believed
that humans have experienced at least one genetic bottleneck, reducing
our genetic diversity, as we have evolved into our present form.
A positive aspect of a genetic bottleneck is that a beneficial mutation
will have a much greater impact and lead to the quicker development
of a new species. This is thought to have a played a role in the
evolution of early humans.
541
835192_OCR_A Level Biology_SB_Ch20.indd 541
01/07/2016 11:47
20
20.5 evolution
sample of
original population
founding population A
descendants
founding population B
Founder effect
Small populations can arise due to
the establishment of new colonies
by a few isolated individuals,
leading to the founder effect.
The founder effect is an extreme
example of genetic drift.
These small populations have much
smaller gene pools than the original
population and display less genetic
variation. If carried to the new
population, the frequency of any
alleles that were rare in the original
▲ Figure 3 Diagram illustrating how small samples from a population can lead to
populations with very different, and reduced, gene pools
population will be much higher in
the new, smaller population and so
they will have a much bigger impact during natural selection.
The Afrikaner population in South Africa is descended mainly from
a few Dutch settlers. The population today has an unusually high
frequency of the allele that causes Huntington’s disease. It is thought
that just one of the original settlers carried the disease-causing allele.
The Amish people of America have descended from 200 Germans who
settled in Pennsylvania in the 18th century. They rarely marry and
have children outside their own religion and are therefore a closed
community. The Amish have unusually high frequencies of alleles that
cause the normally rare genetic disorder Ellis–van Creveld syndrome.
People with the syndrome are short, they often have polydactyly (extra
fingers or toes), abnormalities of nails and teeth, and a hole between
the two upper chambers of the heart. Ellis–van Creveld syndrome is an
example of founder effect caused by one couple, Samuel King and his
wife, who settled in the area in 1744.
Evolutionary forces
▲ Figure 4 Close-up of a baby’s hand
showing an extra finger. This condition is
called polydactylism
The traits or characteristics of all living organisms show variation
within populations. The distribution of the different variants will take
the form of a bell-shaped curve if plotted on a graph. This is known in
statistics as a normal distribution.
Stabilising selection
Taking the birth weight of babies as an example,
babies with an average birth weight will be the
most common and therefore form the peak of the
graph. Babies with very low birth weight are more
prone to infections and very large babies result in
difficult births. Both of these extremes in weight
reduce the survival chances of babies so the
numbers of survivals of very small or very large
babies remains low forming the tails on Figure 5.
99.7%
95%
68%
3.5
4.5
5.4
6.3
7.3
8.2
birth weights (pounds)
9.1
▲ Figure 5 Birth weights of baby girls in Europe
10.0
11.0
This is natural selection, or survival of the fittest,
at work. Babies with average birth weights
are more likely to survive and reproduce than
542
835192_OCR_A Level Biology_SB_Ch20.indd 542
01/07/2016 11:47
Pat t e r n s o f i n h e r i ta n c e a n d va r i at i o n
20
underweight or overweight babies. It is an example of stabilising
selection because the norm or average is selected for (positive selection)
and the extremes are selected against (negative selection). Stabilising
selection therefore results in a reduction in the frequency of alleles at
the extremes, and an increase in the frequency of ‘average’ alleles.
Directional selection
Directional selection occurs when there is change in the environment
and the normal (most common) phenotype is no longer the most
advantageous. Organisms which are less common and have more
extreme phenotypes are positively selected. The allele frequency then
shifts towards the extreme phenotypes and evolution occurs.
▲ Figure 6 Light and dark-coloured
peppered moths
The changes seen in peppered moths during the industrial revolution
are a good example of directional selection. During this period of
time a lot of smoke was released from factories, which killed lichens
growing on barks of trees, and the soot made the bark black. Peppered
moths were originally light coloured meaning they were camouflaged
by the lichen from predation by birds. There were always a few darker
moths present, due to variation, but these were quickly eaten and the
allele frequency maintained.
When the lichens died and the trees became black the situation
was reversed. The light-coloured moths were very visible and were
eaten and the darker moths were camouflaged. Over time the allele
frequency shifted due to natural selection and the majority of the
peppered moths had the darker colour. The allele frequency had been
shifted towards an extreme (less common) phenotype.
As pollution has decreased again the allele frequency of the lighter
coloured moths has increased.
Disruptive selection
number of individuals
In disruptive selection the extremes are selected for and the norm
selected against. The finches observed by Darwin in the Galapagos
Islands had been subjected to disruptive selection. This is opposite to
stabilising selection when the norm is positively selected.
evolved
original
population population
stabilising selection
original population
phenotypes (shell color)
directional selection
▲ Figure 8 Top: lazuli bird with bright,
blue plumage, middle: lazuli bird with
intermediate plumage, and bottom:
lazuli bird with dull, brown plumage
diversifying selection
▲ Figure 7 Graphs showing the different forms of selection. The arrows indicate a
selection pressure
543
835192_OCR_A Level Biology_SB_Ch20.indd 543
01/07/2016 11:47
20
20.5 evolution
Although examples of disruptive selection are relatively rare, a welldocumented example involves feather colour in male lazuli buntings
(Passerina amoena), birds which are native to North America. The feather
colour of young males can range from bright blue to dull brown.
There are limited nesting sites in their habitat
and so there is a lot of competition between
male birds to establish territories and attract
female birds. Dull, brown males are seen as
non-threatening and bright, blue males too
threatening by adult males. Both the brown
and blue birds are therefore left alone but
birds of intermediate colour are attacked by
adult birds and so fail to mate or establish
territories.
frequency
mean
dull
bright
▲ Figure 9 The distribution of phenotypes as a result of disruptive
selection pressures on lazuli buntings
Synoptic link
You learnt about natural selection in
Topic 10.4, Evidence for evolution.
The extremes are selected for and the
distribution of phenotypes shows two peaks
as in Figure 9.
summary questions
1 Explain why evolution does not occur within single organisms but
groups of organisms.
(3 marks)
2 Around the world, humans choose their partners for a wide variety of
reasons. Explain why this might affect any conclusions about human
evolution drawn using the Hardy–Weinberg principle.
(3 marks)
3 In cats, the short-haired allele L is dominant to the long-haired allele l.
In a population of feral cats, 10 out of 90 animals had long hair. Give the
expected frequency for the homozygous recessive, homozygous dominant,
and heterozygote genotypes in this population of cats.
(6 marks)
4 Explain why the allele frequency is changing so quickly in Figure 10.
(4 marks)
generation
1
2
3
frequency of
green trait
17%
25%
Figure 10 An example
of genetic drift happening
rapidly over three
44% generations
5 Eukaryotic organisms have large quantities of non-coding DNA whereas
most prokaryotic organisms have very little.
Suggest, with reference to the different forms of reproduction in
eukaryotes and prokaryotes, why eukaryotes may have evolved to have
more non-coding DNA.
(4 marks)
544
835192_OCR_A Level Biology_SB_Ch20.indd 544
01/07/2016 11:47