1. Evolution: drift and selection
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution

2. Unit 2- Organisms Drift and Selection Rate of evolution
Co-evolution and the Red Queen Hypothesis

3. a) Drift and Selection key areas
Define the terms evolution, natural selection, sexual selection and genetic drift.
Describe the processes of evolution, natural selection, sexual selection and genetic drift.
Identify and describe the affect of mutations.
Identify the effect on variation.
Define the term absolute fitness.
Define the term relative fitness.

4. Starter Questions What is evolution?
Evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits.
It occurs through different processes: natural selection, genetic drift and sexual selection.
Can you describe an example of organisms evolutionary journey?
Humans- Homo sapiens?

5. What can you remember from Higher?
On the boards, try to remember as much as you can about natural selection, genetic drift and sexual selection.

6. Evolution- famous views
John Maynard Smith ( ) – (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” Charles Darwin ( ). ‘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”.

7. Example- Human evolution
When was the last time we shared the planet with other human species?
Roughly 200,000 years ago- Homo floresiensis
We share between 1-3% of our DNA with another human species. Which other?
Homo neanderthalensis

8. Example- Human evolution
Human evolutionary timeline
A few Twig videos.
Evidence
Origin of species
Chimps: Our closest relative?
Mechanisms for evolution
Natural selection

9. Evolution
The process of evolution can be very complex and can take many years to occur.
Evolution occurs as a result of two main factors:
The random process of genetic drift.
The non-random processes of natural selection and sexual selection.

10. Genetic drift What is it?
It is the random change in frequency of alleles varying from one generation to the next due to random events such as disease, predation and abiotic factors.
This can have a greater effect on small populations. Why?
The smaller number of individuals mean alleles are more likely to be lost from the gene pool.
The diagram opposite shows the effect of population size on genetic drift.
Genetic drift changes the frequency of alleles in a population as a result of random events.

11. Genetic drift
The graphs show ten simulations, each of random change in the frequency distribution of a single hypothetical allele over 50 generations for different sized populations.
The higher the population number, the fewer alleles that are lost.

12. 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.

13. Genetic drift
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 (random process) to be beneficial to the organism, they may be harmful but still increase in frequency in the population.

14. Read the example in your notes booklet and discuss it in small groups.

15. Natural Selection Revision from Higher.
What can you remember about natural selection?
Selection pressure, variation, survival of the fittest, mutations, inheritance, beneficial characteristics, phenotype, genotype, Darwin, finches, peppered moths????

16. 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.
‘I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.’

17. What was proposed by natural selection?
Darwin observed that individuals in a population show variation.
This can be caused by mutations creating novel alleles.
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 (selection pressure).
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.

18. Some slow ones get eaten by predators
Some lose out in competition for food
Some with thinner coats die of cold
Some weaker ones die of disease
Some die of thirst
A population of organisms will produce more young than the environment can support
Example
There is variety among
these offspring.(mutations)
Those best adapted to the environment survive to breed and pass on their genes
This happens generation after generation.
The genes which aid survival build up in the gene pool. Those which are detrimental to survival become scarce

19. Key concepts
Natural Selection is a non-random increase in allele frequency that favours survival.
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.

20. Sexual Selection
Sexual selection also has an effect on the evolution of a species, where allele frequency is determined through the selection of mating partners.
The phenotypes needed to attract and keep a mate do not always match the phenotypes that are best suited to survival.
For example in birds of paradise the lavish, colourful plumage of a bird can attract a mate but when it comes to survival it can slow down an organism and make them more visible to predators.
However, if the colourful plumage increases the birds chance of mating then that allele will be favoured.

21. What is sexual selection?
Sexual selection is a mode of natural selection where typically members of one gender choose mates of the other gender.
This occurs in two ways:-
- Intersexual selection
- Intrasexual selection

22. Intersexual selection
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 courtship displays or behaviour by males influence a female’s choice. e.g. Peacocks.
Bowerbird
Females are often plainer in colour or have less plumage etc.
This difference, also shown in other forms of adornment such as antlers in deer, is called sexual dimorphism.

23. Intrasexual selection
Ideas- What is it?
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.
However, it has also been seen in females e.g. Ring Tailed Lemurs.
Elephant Seals
Attribution: RadioFan at en.wikipedia

24. Comparing mechanisms for evolution
Effect on gene pool
Random ?
Genetic drift
Mutation
Sexual selection
Natural selection

25. Comparing mechanisms for evolution
Effect on gene pool/ genetic variation
Random?
Genetic drift
Has more impact on small populations (bottleneck effect or founder effect). Many alleles are lost from the population, less genetic variation.
Yes
Mutation
Increases or decreases allele frequencies, may introduce new allele forms. Can be beneficial, neutral or harmful to organism.
Sexual selection
Will increase DNA sequences that will aid sexual reproduction. Helps the best genes to remain in the population. Sexual selection increases genetic variation. No
Natural selection
Will reduce deleterious sequences and increase DNA sequencies that aid survival.

26. 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 process of genetic drift or the non-random processes of natural selection and sexual selection.
Genetic drift is the change in frequency of alleles varying from one generation to the next due to unpredictable events such as disease, predation and abiotic factors.
Genetic drift is more important in small populations, as alleles are more likely to be lost from the gene pool.

27. 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 (novel) alleles. (eg, blue eyes)
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.

28. What is fitness in evolution?
Fitness is an indication of an individuals ability to be successful 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 there is variation of alleles of genes.
We know that frequencies of alleles change through many generations and that there would be an expectation that alleles with the highest fitness would become more common in a population.

29. 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.
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.

30. Relative fitness example

31. Key concepts 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.
Fitness is an indication of an individuals ability to be successful both at surviving and reproducing.
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.

32. Past paper questions

35. Consolidation tasks…..
Complete a glossary for all of the new terms. You can use the scholar book to help you.
Taboo cards- make 3 taboo cards for this key area using the glossary that you have made. You will be giving this to another pair, so don’t make it too easy for them!