Natural & Artificial Selection (Cambridge (CIE) A Level Biology): Flashcards
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- FrontNatural Selection
Define natural selection.
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Define natural selection.
Natural selection is the process by which organisms best adapted to their environment survive and reproduce, passing on their advantageous alleles to the next generation.
Outline the process of natural selection.
Step 1: Populations produce more offspring than the environment can support.
Step 2: There is variation, and individuals compete for resources.
Step 3: Individuals with advantageous alleles are better adapted and more likely to survive.
Step 4: Survivors reproduce and pass on the advantageous alleles.
Step 5: The frequency of the advantageous allele increases over generations.
What is meant by the 'struggle for existence'?
Because more offspring are produced than can survive, individuals must compete for limited resources such as food, space and mates.
Why is variation essential for natural selection?
It provides a range of phenotypes, some better adapted than others.
Selection then acts on this variation, favouring the best-adapted individuals.
Individuals best adapted to their environment are most likely to survive and .
Individuals best adapted to their environment are most likely to survive and reproduce.
True or False?
Natural selection acts directly on an organism's genotype.
False.
Selection acts on the phenotype; individuals with advantageous phenotypes survive, which then changes allele frequencies.
Define stabilising selection.
Stabilising selection favours intermediate phenotypes and selects against the extremes, reducing variation (e.g. human birth mass).
Define directional selection.
Directional selection favours one extreme phenotype, shifting the population mean in that direction (e.g. antibiotic resistance).
Define disruptive selection.
Disruptive selection favours both extreme phenotypes and selects against the intermediate, which can increase variation.
Which type of selection acts in a stable, unchanging environment?
Stabilising selection.
The intermediate phenotype is already well adapted, so extremes are selected against.
Give an example of stabilising selection.
Human birth mass.
Very low and very high birth masses have higher mortality, so intermediate masses are favoured.
Give an example of directional selection.
Antibiotic resistance in bacteria (or the peppered moth).
A change in the environment favours one extreme phenotype.
Selection that favours the two extreme phenotypes is called selection.
Selection that favours the two extreme phenotypes is called disruptive selection.
True or False?
Directional selection favours the intermediate phenotype.
False.
Directional selection favours one extreme; it is stabilising selection that favours the intermediate.
Define genetic drift.
Genetic drift is the random change in allele frequencies from one generation to the next, most significant in small populations.
Define the founder effect.
The founder effect is the loss of genetic variation when a new population is started by a small number of individuals from a larger population.
Define the bottleneck effect.
The bottleneck effect is a sharp reduction in population size (e.g. by disease or disaster) that reduces genetic diversity, leaving a non-representative set of alleles.
How does selection change allele frequencies?
Advantageous alleles increase in frequency.
Disadvantageous alleles decrease in frequency, over successive generations.
Why is genetic drift more significant in small populations?
Chance events have a proportionally larger effect on allele frequencies when only a few individuals are present.
Why do the founder effect and bottleneck effect reduce genetic diversity?
Both leave a small, non-representative sample of alleles.
The gene pool of the new or surviving population is therefore less varied.
A random change in allele frequency, important in small populations, is called genetic .
A random change in allele frequency, important in small populations, is called genetic drift.
True or False?
The founder effect increases genetic variation in the new population.
False.
The founder effect reduces variation, because only a small sample of alleles founds the new population.
Outline how bacteria become resistant to an antibiotic.
Step 1: A random mutation gives a few bacteria a resistance allele.
Step 2: When the antibiotic is present, non-resistant bacteria are killed.
Step 3: Resistant bacteria survive and reproduce.
Step 4: The resistance allele increases in frequency in the population.
Where does the resistance allele originally come from?
From a random, spontaneous mutation.
It is not caused by the antibiotic itself.
Why is antibiotic resistance an example of directional natural selection?
The antibiotic acts as a selection pressure favouring one extreme (resistant) phenotype.
This shifts the population towards resistance.
How can resistance alleles spread between bacteria?
By transfer of plasmids carrying the resistance allele from one bacterium to another (conjugation).
A resistance allele first arises in a bacterium by a random .
A resistance allele first arises in a bacterium by a random mutation.
True or False?
Exposure to an antibiotic causes bacteria to mutate and become resistant.
False.
Mutations occur randomly beforehand; the antibiotic simply selects for bacteria that are already resistant.
What are the two Hardy-Weinberg equations?
p + q = 1 (allele frequencies)
p^2^ + 2pq + q^2^ = 1 (genotype frequencies)
In the Hardy-Weinberg equations, what do p and q represent?
p = frequency of the dominant allele.
q = frequency of the recessive allele.
What do p^2^, 2pq and q^2^ represent?
p^2^ = frequency of homozygous dominant genotype.
2pq = frequency of heterozygous genotype.
q^2^ = frequency of homozygous recessive genotype.
State the conditions required for the Hardy-Weinberg principle to apply.
Large population.
Random mating.
No mutations.
No natural selection.
No migration (no gene flow in or out).
1 in 100 individuals shows a recessive condition. What is q?
q2 = 1/100 = 0.01
q = √0.01 = 0.1
If q = 0.1, what is the frequency of heterozygous carriers?
p = 1 − 0.1 = 0.9
2pq = 2 × 0.9 × 0.1 = 0.18 (18%)
In the Hardy-Weinberg equation, the frequency of heterozygotes is given by the term .
In the Hardy-Weinberg equation, the frequency of heterozygotes is given by the term 2pq.
True or False?
The Hardy-Weinberg principle assumes that natural selection is acting on the population.
False.
The principle assumes there is no natural selection (allele frequencies stay constant).
Define artificial selection (selective breeding).
Artificial selection is the process by which humans choose organisms with desirable characteristics to breed together, so those traits become more common over generations.
Outline the principles of selective breeding.
Step 1: Select individuals with the desired characteristics.
Step 2: Breed these individuals together.
Step 3: Select the offspring with the best characteristics.
Step 4: Breed these selected offspring together.
Step 5: Repeat over many generations.
How is selective breeding similar to natural selection?
In both, only some individuals reproduce and pass on their alleles.
This changes allele frequencies over generations.
How does selective breeding differ from natural selection?
In selective breeding, humans (not the environment) choose which individuals reproduce, based on characteristics useful to humans.
Why can selective breeding reduce genetic diversity?
Repeated breeding from a small number of selected, often related, individuals reduces the variety of alleles.
This can cause inbreeding depression.
In selective breeding, the organisms that are allowed to reproduce are chosen by .
In selective breeding, the organisms that are allowed to reproduce are chosen by humans.
True or False?
Artificial selection increases the frequency of desirable alleles over many generations.
True.
Only individuals with the desired traits are bred, so their alleles become more common.
How is selective breeding used to improve wheat and rice?
Varieties with disease resistance are bred with high-yielding varieties.
This introduces resistance alleles, producing disease-resistant crops.
Define inbreeding, and state one problem it can cause.
Inbreeding is breeding between closely related individuals.
It can cause inbreeding depression — reduced vigour and more harmful recessive alleles being expressed.
How are vigorous, uniform maize varieties produced?
Inbreeding produces uniform (homozygous) lines.
Two different inbred lines are then crossed (hybridised) to give vigorous, uniform hybrid offspring.
Define hybrid vigour.
Hybrid vigour is the increased vigour, size or yield shown by hybrid offspring compared with their inbred parents.
How is the milk yield of dairy cattle improved by selective breeding?
Cows with high milk yields are bred with bulls whose female relatives had high yields.
The best offspring are then selected and bred over generations.
Why are bulls selected using data from their female relatives?
Bulls do not produce milk.
So their breeding value for milk yield is judged from the yields of female relatives such as their mother and daughters.
Crossing two different inbred lines of maize produces offspring that show hybrid .
Crossing two different inbred lines of maize produces offspring that show hybrid vigour.
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