17.2.317-18 · Selection, Evolution, Classification and Conservation
Natural Selection- Changes in Allele Frequencies — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 11 parts Lesson 9 of 36 1 quiz
Lesson summary
17.2.3 Natural Selection: Changes in Allele Frequencies — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how natural selection and chance processes change allele frequencies in populations.Gene pool — all the alleles of all the genes in a population at a give…
- Allele frequency
- What the symbols mean
- Genetic drift
- Founder effect
- Bottleneck effect
- Heterozygote advantage
Part 1 of 9Allele frequencies and evolution
Gene pool
- Gene pool all the alleles of all the genes in a population at a given time.
Allele frequency
- Allele frequency the proportion of one allele among all alleles of that gene, given as a decimal or percentage.
Evolution
- Evolution change in allele frequencies in a population over successive generations.
Allele frequencies can change by natural selection or by chance.
- Chance processesgenetic drift, the founder effect and the bottleneck effect.
What the symbols mean:
For a gene with two alleles, p + q = 1.
What the symbols mean: p = frequency of the dominant allele · q = frequency of the recessive allele · p2 = homozygous dominant · 2pq = heterozygous · q2 = homozygous recessive
Part 2 of 9Changes caused by natural selection
1
A new allele arises in the population or the environment changes, so directional selection can occur.
2
There is always phenotypic variation, and a selection pressure such as a predator acts.
3
Some individuals have a phenotype produced by particular alleles that aids survival.
4
Those fitter individuals are more likely to reproduce and pass on the advantageous alleles.
5
Individuals without the allele or phenotype are less likely to survive and reproduce.
6
Over several generations the frequency of the advantageous allele increases and others decrease.
Part 3 of 9Genetic drift
Genetic drift
- Genetic drift change in allele frequencies due to chance rather than natural selection.
In a very small population, chance affects which alleles pass to the next generation.
- Meiosis gives haploid gametes, so a fertilisation passes on only half of an individual's alleles.
- Which half is passed on is random, so alleles can be lost or passed on purely by chance.
Drift is more likely in small populations, like a coin tossed 10 times giving no heads more easily than in 100 tosses.
An allele can be lost by chance even though it confers no disadvantage, e.g. pink-flowered plants' seeds landing on an impermeable surface.
In a small population an allele may be lost (frequency 0) or fixed (frequency 1), reducing genetic diversity.
Part 4 of 9Founder effect
Founder effect
- Founder effect a small number of individuals from a large parent population start a new population.
- It can follow a chance event, such as a storm separating a small group from the main population.
Only some of the alleles of the parent gene pool are present in the new population.
The small population is very susceptible to genetic drift.
Anole lizards colonising a smaller Caribbean island may carry only white-scale alleles, so a rare recessive yellow allele can be lost.
High frequencies of some inherited diseases in small isolated human communities, e.g. the Amish, are attributed to founder effects.
Part 5 of 9Bottleneck effect
Bottleneck effect
- Bottleneck effect a previously large population suffers a dramatic fall in numbers.
- A major environmental event kills most individuals.
- Alleles are lost, so genetic diversity falls.
- Survivors breed with close relatives.
Cheetahs today show a lack of genetic variation after most of the population died at the end of the Ice Age about 10 000 years ago.
Northern elephant seals were reduced to about 20 individuals and now have very low genetic variation.
Loss of variation is a problem for conservation because it reduces the ability to respond to environmental change.
Part 6 of 9Comparing the processes
| Process | Result |
|---|---|
| Natural selection | Selection pressures produce a gradual change in allele frequencies over several generations |
| Founder effect | Allele frequencies in the new small population differ from the parent population due to chance |
| Genetic drift | Change in allele frequencies in a small population by chance, not selection |
| Bottleneck effect | Reduction of the gene pool after a dramatic decrease in population size |
Part 7 of 9Other factors that change allele frequency
| Factor | Effect |
|---|---|
| Mutation | Creates new alleles; changes frequency very slowly because mutation rates are low |
| Gene flow (migration) | Alleles move between populations, making them more similar |
| Non-random mating | Changes genotype frequencies; sexual selection changes allele frequencies |
| Heterozygote advantage | Keeps both alleles in the population (balanced polymorphism), e.g. sickle cell allele and malaria |
Part 8 of 9Consequences for populations
1
Loss of diversity from drift reduces the ability of a population to adapt.
2
Inbreeding in small populations raises the frequency of harmful recessive alleles.
3
A harmful recessive allele is removed only slowly by selection because it is hidden in heterozygotes.
4
Conservation programmes keep populations large and exchange individuals between populations.
5
Deviation from Hardy-Weinberg expectations shows that selection, mutation, migration, non-random mating or drift is acting.
Part 9 of 9More from the specification
More from the specification
Natural selection and antibiotic resistance
- Bacterial populations undergo natural selection when exposed to an antibiotic,
- where mutant organisms possessing resistance alleles survive, reproduce,
- and pass these traits to offspring,
- increasing resistance allele frequency.
Directional selection in populations
- Selective pressures in wild species select against susceptible individuals, favoring the survival and growth of resistant phenotypes.
- (such as myxomatosis infection in wild rabbits)
Artificial selection and selective breeding
- In artificial selection, humans selectively breed organisms showing desirable traits over multiple generations of offspring.
- (such as higher milk yield in cattle or disease resistance in crops)
Changes in gene pools
- Natural and artificial selection drive evolutionary changes by altering allele frequencies in the gene pool of a species over successive generations.
Natural selection and antibiotic resistance
- Bacterial populations undergo natural selection when exposed to an antibiotic,
- where mutant organisms possessing resistance alleles survive, reproduce,
- and pass these traits to offspring,
- increasing resistance allele frequency.
Directional selection in populations
- Selective pressures in wild species select against susceptible individuals, favoring the survival and growth of resistant phenotypes.
- (such as myxomatosis infection in wild rabbits)
Artificial selection and selective breeding
- In artificial selection, humans selectively breed organisms showing desirable traits over multiple generations of offspring.
- (such as higher milk yield in cattle or disease resistance in crops)
Changes in gene pools
- Natural and artificial selection drive evolutionary changes by altering allele frequencies in the gene pool of a species over successive generations.
Natural selection and antibiotic resistance
- Bacterial populations undergo natural selection when exposed to an antibiotic,
- where mutant organisms possessing resistance alleles survive, reproduce,
- and pass these traits to offspring,
- increasing resistance allele frequency.
Directional selection in populations
- Selective pressures in wild species select against susceptible individuals, favoring the survival and growth of resistant phenotypes.
- (such as myxomatosis infection in wild rabbits)
Artificial selection and selective breeding
- In artificial selection, humans selectively breed organisms showing desirable traits over multiple generations of offspring.
- (such as higher milk yield in cattle or disease resistance in crops)
Changes in gene pools
- Natural and artificial selection drive evolutionary changes by altering allele frequencies in the gene pool of a species over successive generations.
Disruptive selection
- Selection favouring both extreme phenotypes.
Antibiotic
- A drug that kills bacteria or stops their growth and does not affect viruses.
Resistance
- The ability of bacteria to survive an antibiotic.
Species
- A group of organisms that can reproduce to produce fertile offspring.
Growth
- A permanent increase in size and dry mass.
Organism
- A living thing, such as a plant or an animal.
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
Hardy-Weinberg principle
- Equations predicting allele and genotype frequencies in a non-evolving population.
Stabilising selection
- Selection favouring the average phenotype.
Key terms
| Term | Meaning |
|---|---|
| Gene pool | All the alleles of all the genes in a population. |
| Allele frequency | Proportion of a particular allele among all alleles of that gene in a population. |
| Genetic drift | Random change in allele frequencies, strongest in small populations. |
| Founder effect | Reduced variation when a small group founds a new population. |
| Bottleneck effect | Loss of variation after a population falls drastically in size. |
| Heterozygote advantage | Greater fitness of heterozygotes, which maintains both alleles. |
Avoid these mistakes
1
Saying genetic drift is caused by selection — drift is random and not related to advantage.
2
Saying drift is equally strong in large and small populations — chance averages out in large populations.
3
Confusing founder and bottleneck effects — founders start a new population, while a bottleneck is a fall in an existing population.
4
Saying a harmful recessive allele disappears quickly — heterozygotes hide it from selection.
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