evolution
Evolution is the change over time in the proportion of individuals in a
population differing in one or more inherited traits.
Evolution can occur through the random processes of genetic drift or
the non-random processes of natural selection and sexual selection.
natural selection
drift
natural selection
Natural selection is the non-random increase in frequency of DNA
sequences that increase survival.
natural selection
As organisms produce more offspring than the environment can
support, those individuals with variations that best fit their
environment are the ones most likely to survive and breed.
Through inheritance, these favoured traits are therefore likely to
become more frequent in subsequent generations.
mutation
Variation in traits arises as a result of mutation.
Mutation is the original source of new sequences of DNA. These new
sequences can be novel alleles.
mutation
Most mutations are harmful or neutral but in rare cases they may be
beneficial to the fitness of an individual.
modes of selection
There are three different modes of selection:
• directional selection favours a single extreme phenotype;
• stabilising selection favours the average phenotype, giving a decrease
in variation as more extreme traits do not survive;
• disruptive selection favours two extreme forms within a population.
modes of selection
activity 1 – modes of selection
Collect a Modes of selection worksheet.
1.Cut out the three examples and the three graphs.
2.Pair up each example with a graph.
3.Stick these into your notes naming the mode of selection shown.
genetic drift
Genetic drift refers to the changes in the frequencies of alleles in a
population that occur by chance, rather than because of natural
selection.
Neutral traits do not seem to have any obvious selection pressures.
Modelling Genetic Drift – The Jar of Marbles Analogy
Imagine a jar containing 20 marbles – 10 blue and 10 red representing 20 organisms
Different colours of marble represent different alleles
The random reproduction of the organisms can be modelled as
follows:
• Select one marble from the first jar, at
random
• Place a marble of the same colour in a
second jar
• Replace the first marble in the original jar and repeat until the
second jar holds 20 marbles
Random sampling error means that the next generation is unlikely to have exactly the same
proportions of red and blue marbles
With each subsequent generation, the red
allele is less likely to be picked for reproduction
Eventually, the red allele may go ‘extinct’
genetic drift
Genetic drift has a greater impact in small, isolated populations, as
alleles are more likely to be lost from the gene pool.
The random loss of rare alleles, without respect to the survival or
reproductive value, reduces the genetic diversity of a population.
activity 2 – founder and bottleneck effects
The founder and bottleneck effects are examples of genetic drift.
Collect a Founder and Bottleneck Effects worksheet.
Research one example of each effect.
Be prepared to share your findings at the next lesson.
founder effect
In population genetics, the founder effect is the loss of genetic
variation that occurs when a new population is established by a very
small number of individuals from a larger population.
bottleneck effect
The bottleneck effect occurs when there is a disaster of some sort that
reduces a population to a small handful, which rarely represents the
actual genetic makeup of the initial population. This leaves smaller
variation among the surviving individuals.
prairie chicken PPQ
fitness
Fitness, from a selection point of view, is a term used to measure how
successful a particular genotype has been in surviving from one
generation to the next.
There are two forms; absolute fitness and relative fitness.
absolute and relative fitness
Absolute fitness is the ratio of frequencies of a particular genotype
from one generation to the next.
Relative fitness is the ratio of surviving offspring of one genotype
compared with other genotypes.
Two hikers are walking in the woods. They round a bend and
see a bear on the trail ahead of them. The bear charges
towards them. They begin to run, but the bear is catching up.
Suddenly, the first hiker sits down and starts changing his
hiking boots for sneakers. The second hiker says, “What are
you doing?! You’ll never outrun that bear, sneakers or no
sneakers!” The first hiker replies, “I don’t have to outrun the
bear, I just have to outrun you.”
The joke is about the difference between absolute and
relative fitness, and the importance of not mixing them up. In
evolution, it’s usually relative fitness that’s of interest, as
that’s all that natural selection cares about. For instance, even
if two alleles at the same locus are both unfit in some
absolute sense, the fitter of the two will still increase in
frequency at the expense of the other. If a species survives
and reproduces well in some absolute sense, it will be
excluded if a competing species survives and reproduces even
better.
activity 3 – absolute and relative fitness
Collect a Absolute and relative fitness worksheet.
Your teacher will explain how to calculate absolute and relative fitness.
Now calculate the absolute and relative fitness for the other
generations.
Original (P)
population
AA
Aa
aa
AA
Aa
aa
AA
Aa
aa
AA
Aa
Aa
20
10
30
15
5
40
15
7
42
7
0.5
1.5
0.75
0.33
1
0.5
20
20
F1 population
F2 population
Absolute Fitness
Relative Fitness
F3 population