Evolution (A Level only) (AQA A Level Biology): Flashcards
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- FrontGenetic Variation
What is meant by an organism's phenotype?
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What is meant by an organism's phenotype?
An organism's observable characteristics
State the factors that can cause phenotypic variation within a species
Genetic factors
Environmental factors
A combination of both genetic and environmental factors
What is the main source of new alleles in a population?
Mutation (a random change in the DNA base sequence)
State the three processes during sexual reproduction that generate genetic variation
Crossing over of non-sister chromatids during prophase I of meiosis
Independent assortment of chromosomes during metaphase I of meiosis
Random fertilisation of gametes
During which stage of meiosis does crossing over occur, and what is its effect?
Crossing over occurs during prophase I
It involves the exchange of genetic material between non-sister chromatids, producing new combinations of alleles on the chromosomes
How does independent assortment generate genetic variation?
The random alignment of homologous chromosomes during metaphase I results in different combinations of chromosomes, and therefore alleles, in each gamete
True or False?
Phenotypic variation caused by environmental factors is inherited
False.
Environmental factors do not change the DNA of the gametes, so this variation is not inherited
Phenotypic variation = genetic variation + influence.
Phenotypic variation = genetic variation + environmental influence.
A mutation must occur in the in order to be passed on to the next generation.
A mutation must occur in the gametes in order to be passed on to the next generation.
What is a selection pressure?
An environmental factor that affects an organism's chance of survival
State the difference between biotic and abiotic selection pressures, with examples
Biotic (living) factors, e.g. predation, competition and disease
Abiotic (non-living) factors, e.g. temperature, light, water and soil pH
What is meant by a selective advantage?
When an individual has a phenotype that makes it better adapted to survive and reproduce than others in the population
Explain what happens to the frequency of favourable and unfavourable alleles through natural selection
Favourable alleles increase in frequency in the gene pool
Unfavourable alleles decrease in frequency in the gene pool
True or False?
Well-adapted organisms are guaranteed to survive and reproduce
False.
Well-adapted organisms are more likely to survive and reproduce, but survival is never guaranteed
The differential survival and reproduction of individuals is the basis of selection.
The differential survival and reproduction of individuals is the basis of natural selection.
Define evolution
The formation of new species from pre-existing species over time, as a result of changes to gene pools and allele frequencies from generation to generation
State the difference between evolution and speciation
Evolution is the gradual change in the genetic makeup of a population over time
Speciation is the formation of a new species from an existing one, when populations become reproductively isolated and genetically distinct
Evolution occurs when frequencies change significantly over many generations.
Evolution occurs when allele frequencies change significantly over many generations.
State the three types of selection
Stabilising selection
Directional selection
Disruptive selection
What is stabilising selection?
Selection that favours the average phenotype and removes the extremes, maintaining average values, e.g. human birth weight
What is directional selection?
Selection that favours individuals with a phenotype at one extreme, causing a shift in the population's traits over time, e.g. antibiotic resistance in bacteria
What is disruptive selection?
Selection that favours both extremes and selects against the intermediate phenotype, maintaining high frequencies of two different sets of alleles
What is meant by polymorphism?
The continued existence of two or more distinct phenotypes in a species
Explain how disruptive selection acts on beak size in a bird population
Different-sized seeds are foraged more efficiently by small or large beaks
Birds with intermediate (medium) beaks are selected against
This maintains high frequencies of both the small-beak and large-beak alleles
True or False?
Disruptive selection favours individuals with the intermediate phenotype
False.
Disruptive selection favours both extremes and selects against the intermediate phenotype
Stabilising selection favours the average phenotype and can be exemplified by human weight.
Stabilising selection favours the average phenotype and can be exemplified by human birth weight.
Directional selection favours one extreme phenotype, causing a in the population's traits over time.
Directional selection favours one extreme phenotype, causing a shift in the population's traits over time.
Define speciation
The formation of a new species when populations become reproductively isolated, so their gene pools no longer mix
What is reproductive isolation?
When changes in alleles or phenotypes prevent certain individuals from breeding successfully with others in the same population, reducing gene flow
State three ways in which reproductive isolation can occur
Seasonal isolation - individuals reproduce at different times of year
Mechanical isolation - changes in genitalia prevent successful mating
Behavioural isolation - changes in courtship behaviour prevent the attraction of mates
What is allopatric speciation?
Speciation that occurs when a population is split by a geographical barrier, so the groups become reproductively isolated with no gene flow between them
What is sympatric speciation?
Speciation that occurs without a geographical barrier; the population lives in the same area but splits into two groups with no gene flow between them
Explain how allopatric speciation leads to two separate species
A geographical barrier reproductively isolates two groups, so no gene flow occurs
Different selection pressures act on each group, changing allele frequencies through natural selection
Over many generations the populations become genetically distinct and can no longer interbreed
True or False?
Sympatric speciation requires a geographical barrier
False.
Sympatric speciation occurs without a geographical barrier; the groups live in the same area
What is genetic drift?
A change in allele frequencies in a population due to chance, rather than natural selection
Explain why genetic drift has a greater effect in small populations
In small populations there is less genetic diversity to buffer against the random loss of alleles
Chance events can significantly alter allele frequencies
So evolutionary change happens more quickly and unpredictably
In large populations, genetic drift has little effect because chance variations even out, and selection has a more influential role.
In large populations, genetic drift has little effect because chance variations even out, and natural selection has a more influential role.
What everyday materials can be used to model the effect of random sampling on allele frequencies?
Coloured beads, marbles or sweets
In the bead model of allele frequencies, what do the red and white beads represent?
Red beads represent a dominant allele (R)
White beads represent a recessive allele (r)
In the bead model, what does each pair of beads drawn from the container represent?
The fusing of two sex cells (gametes) during random mating, producing an offspring genotype
In the bead model with equal numbers of R and r alleles, what is the theoretical (expected) offspring genotype ratio?
1 : 2 : 1 (25% RR : 50% Rr : 25% rr)
To keep the allele frequencies constant during each draw, the beads must be placed into the container after each 'mating'.
To keep the allele frequencies constant during each draw, the beads must be placed back into the container after each 'mating'.
Explain why the actual offspring genotype ratio in the bead model often differs from the theoretical 1:2:1 ratio
The effects of chance mean the observed ratio differs from the expected ratio
This change in allele frequencies in a small breeding population is known as genetic drift
In the context of the bead model, what is genetic drift?
The change in the expected offspring genotype ratio (and allele frequencies) that occurs in a small breeding population due to chance
True or False?
In the bead model, genetic drift has a greater effect in large breeding populations
False.
Genetic drift has a greater effect in small breeding populations, where chance has a bigger influence on allele frequencies
State factors that can be changed in a computer simulation to investigate natural selection
Selection pressures (e.g. predation, disease, food)
Mutation rates
Dominance of alleles
Environmental changes
State a key benefit of using computer models to investigate natural selection
Evolutionary time can be sped up, allowing long-term effects to be observed quickly
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