Transcript Evolution, drift and selection

Evolution:
drift and selection
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution
SQA mandatory key information
• Processes of evolution, natural selection, sexual selection and
genetic drift. Mutations can be harmful, neutral or beneficial
and give rise to variation.
• 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.
Key concepts
• 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. Genetic drift is more important in small
populations, as alleles are more likely to be lost from the gene
pool.
• 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. Most mutations are harmful
or neutral but in rare cases they may be beneficial to the
fitness of an individual.
• 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.
What is evolution?
It is commonly defined as the change over time in the
proportion of individuals in a population differing in one
or more inherited traits.
Other eminent scientists have also tried to define it....
Evolution
John Maynard Smith (1920- 2004) – (British theoretical evolutionary
biologist and geneticist) wrote in his book ‘The Theory of Evolution’
“any population of entities which has the properties of multiplication,
heredity and variation”
It is impossible to try to define Evolution without reference to Charles
Darwin (1809 -1882). Darwin was a British naturalist whose ground
breaking book ‘Of the Origin of Species’ published in 1859 changed
our thinking about evolution. In the final chapter of the book, Darwin
wrote....
“There is grandeur in this view of life............from so simple a
beginning endless forms most beautiful and most wonderful have
been, and are being, evolved”.
Evolution
Darwin rocked the scientific world with his publication
in 1859....
Today we continue to see evolution in action and it is
important to see what is happening now with reference
to those ground breaking findings in the past.
John Maynard Smith summed this up in his book ‘Did
Darwin get it Right’
"Of course we need to see further than Darwin, but we
shall do so by standing on his shoulders, not by turning
our backs on him."
Further reading
• http://www.newscientist.com/topic/evolution
• Various up to date articles on evolution as well
as some video clips looking at the evidence for
evolution
Evolution
Let’s look at evolution in more detail.
It can occur through:
• The random process of genetic drift.
• The non-random processes of natural selection and
sexual selection.
Genetic drift
What is it?
It is a random process.
In small populations, unpredictable events
(disease, predation, abiotic factors etc)
can result in the frequency of alleles
varying from one generation to the next.
This tends to be masked in larger
populations due to the increased number
of individuals).
The diagram opposite shows the effect of
population size on genetic drift: Ten
simulations each of random change in the
frequency distribution of a single
hypothetical allele over 50 generations for
different sized populations.
Genetic drift – an example
In the original population (Generation 1) due to unpredictable events, only
some individuals of the small population are able to go on and produce fertile
offspring (Generation 2). If this continues then after 5 generations, the
frequencies of the red, purple and green alleles will vary considerably from
the original population. In this example, by the fifth generation, the green
allele is no longer present.
Generation 1
Generation 2
v
Generation 5
Generation 4
Generation 3
Genetic drift continued...
This ongoing variation in allele frequency due to genetic drift is
random. This contrasts with natural selection where the frequency
of an allele in a population is related to the fact they allow an
organism to be more adapted to its environment.
• Due to the random nature of allele fluctuation over time, genetic
drift can result in a reduction or loss of genetic variation as some
alleles are removed from the gene pool over generations (this has a
negative effect on the organisms ability to evolve).
• Although some alleles may be lost, some are retained due to
genetic drift. However, as these alleles are not selected to be
beneficial to the organism, they may be harmful but still increase in
frequency in the population.
Natural selection
Natural selection is a non-random process.
It was proposed by Charles Darwin in his book ‘On the Origin of
Species’ in 1859. He wrote...
‘I have called this principle, by which each slight variation, if
useful, is preserved, by the term Natural Selection.’
What was proposed by natural selection?
• Darwin observed that individuals in a population show variation.
• Genetic characteristics are passed on from one generation to the
next from parents to their offspring.
• All species are capable of producing more offspring than their
environment can support.
• The majority of offspring will not survive due to lack of food or
other resources.
• Individuals who possess variations that make them more suited to
their environment are more likely to survive and pass on these
successful traits to their offspring than less well suited individuals.
• These traits will become more frequent in subsequent
generations.
What is sexual selection?
There are two mechanisms, as mentioned by Charles Darwin in his
book, ‘The Descent of Man and Selection in Relation to Sex’ in 1871:
“The sexual struggle is of two kinds; in the one it is between individuals of the
same sex, generally the males, in order to drive away or kill their rivals, the
females remaining passive; whilst in the other, the struggle is likewise
between the individuals of the same sex, in order to excite or charm those of
the opposite sex, generally the females, which no longer remain passive, but
select the more agreeable partners.”
Sexual selection (1)
Intrasexual selection refers to selection within the same sex (usually males).
Individuals compete with each other with ritualised displays of strength and
stamina to warn off competitors or defend his mate(s) e.g. Red deer.
It is an example of dominance hierarchy.
However, it has also been seen in females e.g. Ring Tailed Lemurs.
Attribution: RadioFan at en.wikipedia
Sexual selection (2)
Intersexual selection refers to individuals (usually female) being
very selective about their choice of mate. It is sometimes called
‘mate choice’.
Often bright plumage and showy courtship displays influence a
female’s choice. e.g. Peacocks. The extreme difference in degree
of plumage shown by the male contrasts with the smaller, much
less showy, Peahen.
This difference between
the sexes, also shown in
other forms of adornment
in males such as antlers in
deer is called sexual
dimorphism.
Variation and mutation
The cornerstone of the theory of natural selection is the fact that individuals
of a population show variation.
Variation in inherited traits arises as a
result of mutation. Mutation is the
original source of new sequences of
DNA. The new DNA sequences can be
new alleles.
Mutations to an organisms DNA are
usually harmful or they may have no
effect at all and be neutral.
On rare occasions, a mutation in an
individual’s DNA can make it better
suited to its environment and increase
the fitness of an individual, increasing
its chances of reproductive success.
What is fitness in evolution?
Fitness is an indication of an individuals ability to be successful
both at surviving and reproducing.
It refers to the contribution that is made to the gene pool of the
next generation by individual genotypes.
As a result of natural selection we
know that there is variation of alleles
of genes.
We know that frequencies of alleles
changes through many generations
and that there would be an
expectation that alleles with the
highest fitness would become more
common in a population.
Fitness
Can be defined in absolute or relative terms:
• Absolute fitness - the ratio of frequencies of a particular
genotype from one generation to the next.
• Relative fitness - the ratio of surviving offspring of one
genotype compared with other genotypes.
It is important to remember that the overall fitness of an
individual is affected by its environment. The fitness of a
phenotype and genotype will differ in different environments.
For example, if a moth is more successful at producing offspring
due to the fact it is more camouflaged from predators, this will
increase the relative fitness of the moth population. However, if
the moth moves to a new environment, this phenotypic adaptive
advantage will no longer apply.
Summary (1)
• Evolution is the change over time in the proportion of
individuals in a population differing in one or more inherited
traits.
• It can occur through the random processes of genetic drift or
non random processes of natural selection and sexual
selection.
• Variation in genetic makeup can arise as a result of mutation.
• Mutation is the original source of new sequences of DNA.
• Most mutations are harmful/neutral but occasionally can be
beneficial to the fitness of an individual.
• Fitness can be defined in absolute or relative terms.
Summary (2)
• 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.
• Organisms produce more offspring that 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.