Evolution (A Level Only)

Học bằng cách chơi

Trả lời những câu hỏi này để kiếm năng lượng, rồi câu cá và khám phá. Không cần tài khoản.

Dành cho nhà giáo dục: slide bài học, ghi chú ôn tập sẵn dùng cho Evolution (A Level Only) (Biology, AQA) — dùng trong bài giảng của bạn, hoặc chạy chủ đề như một hoạt động lớp học tương tác để người học chơi như một trò chơi trực tiếp.

Ghi chú bài học

Genetic Variation

Phenotypic variation refers to differences in observable characteristics among individuals of the same species.
Variation can arise from genetic factors (e.g., human blood groups from combinations of three alleles at the ABO gene), environmental factors (e.g., genetically identical plants growing to different heights in different conditions), or both (e.g., the sickle cell allele is maintained because heterozygotes are resistant to malaria).
Genetic variation arises mainly from mutations, which create new alleles; these alleles may be beneficial, harmful, or have no effect, and some may remain unexpressed for generations.
Sexual reproduction also generates genetic variation through crossing over of non-sister chromatids during prophase I, independent assortment of chromosomes during metaphase I, and random fertilisation of gametes.
Crossing over exchanges genetic material between non-sister chromatids, creating new allele combinations and breaking linkage between genes.
Independent assortment results in different combinations of chromosomes and alleles in each gamete.
Random fertilisation means any male gamete can fuse with any female gamete, creating genetic variation between zygotes and resulting individuals.
Phenotypic variation = genetic variation + environmental influence.

Natural Selection

Selection pressures are environmental factors that affect an individual's chance of survival; they can be biotic (living), such as predation, competition, and disease, or abiotic (non-living), such as light, temperature, water, and soil pH.
In any population, genetic variation means some individuals have phenotypes better adapted to survive, giving them a selective advantage.
Individuals with a selective advantage are more likely to survive and reproduce, passing on their favourable alleles to the next generation.
Favourable alleles increase in frequency in the gene pool, while unfavourable alleles decrease in frequency.
This differential survival and reproduction is the basis of natural selection.
Over generations, differential reproductive success leads to an increase in advantageous allele and phenotype frequencies.
When allele frequencies change significantly over time, the characteristics of the population change; if changes are great enough, a new species may form.
Evolution is defined as 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.

Types of Selection

Stabilising selection favours the average phenotype and removes extremes, maintaining average values (e.g., human birth weight).
Directional selection favours individuals with a phenotype at one extreme, causing a shift in a population's traits over time (e.g., antibiotic resistance in bacteria).
Disruptive selection maintains high frequencies of two different sets of alleles, selecting against intermediate phenotypes.
Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in a species.
Disruptive selection can occur in environments that show variation; for example, birds on the Galapagos Islands with either small or large beaks are favoured because different-sized seeds are more efficiently foraged by a shorter or longer beak, while medium-sized beaks are selected against.

Reproductive Isolation

For evolution to occur, the new species population must be genetically and reproductively separated from the pre-existing species population, so no exchange of genes occurs.
Reproductive isolation occurs when changes in alleles or phenotypes prevent certain individuals from breeding successfully with others in the same population, reducing gene flow and leading to speciation.
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.
When two populations become reproductively isolated, they stop exchanging genes, and allele frequencies change independently in each group over time.
If genetic differences accumulate to the point where individuals can no longer interbreed and produce fertile offspring, speciation has occurred.

Allopatric and Sympatric Speciation

Allopatric speciation happens when a population is split by a geographical barrier (e.g., mountains, rivers, roads).
The separated groups are reproductively isolated, so no gene flow occurs; different selection pressures act on each group, changing allele frequencies through natural selection.
Over time, these changes lead to differences in phenotypes (physical, behavioural, and physiological traits), and eventually the two populations become genetically distinct and can no longer interbreed.
Sympatric speciation happens without a geographical barrier; the population lives in the same area but splits into two groups with no gene flow.
Sympatric separation can be caused by ecological separation (groups live in different environments within the same area, e.g., soil pH affects flowering time in plants) or behavioural separation (groups develop different behaviours, e.g., courtship, feeding, or communication, preventing mating).
Example: sympatric speciation in fish—some feed at the bottom, others in open water; selection favours long jaws for bottom-feeders and short jaws for mid-water feeders, and different courtship behaviours evolve, eventually preventing interbreeding.
Genetic drift is when chance (instead of environmental selection pressures) affects which individuals survive, breed, and pass on their alleles; it has a greater impact in small populations.
In large populations, genetic drift is less likely to have an effect because chance variations in allele frequencies usually even out; natural selection has a much more influential role.

Investigating Allele Frequencies

Investigations using coloured beads, marbles, or sweets can mimic the effects of random sampling on allele frequencies in a population.
Method: place 24 red beads (dominant allele R) and 24 white beads (recessive allele r) in a container; shake, then draw two beads without looking to represent fusion of sex cells; carry out 24 random 'matings', returning beads after each draw.
The theoretical offspring genotype ratio, if thousands of draws are made and there is no genetic drift, should be 1:2:1 (RR = 25%, Rr = 50%, rr = 25%).
However, the chances of drawing exactly this ratio in 24 matings are very small; the effects of chance mean the offspring genotype ratio is likely to differ from the theoretical ratio—this change is genetic drift.
Example results: after 24 matings, genotype frequencies might be RR = 20.8%, Rr = 29.2%, rr = 50%; the number of red alleles = (5×2) + 7 = 17, white alleles = (12×2) + 7 = 31, showing genetic drift has occurred.
If another round of 24 matings is carried out from the new population (17 red, 31 white), genetic drift can become even greater; e.g., red alleles = (4×2) + 5 = 13, white alleles = (15×2) + 5 = 35.
Percentage allele frequencies can be calculated: original population = 50% red, 50% white; after first round = 35.4% red, 64.6% white; after second round = 27.1% red, 72.9% white.
Computer programs can simulate natural selection and genetic drift, allowing users to change factors such as selection pressures, mutation rates, dominance of alleles, environmental changes, and adaptations; evolutionary time can be sped up to observe long-term effects quickly.

Slide

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

ĐĂNG KÝ — MIỄN PHÍ ĐĂNG NHẬP

Câu hỏi luyện tập

Xem trước miễn phí — 8 trên 60 câu hỏi. Đăng ký để xem tất cả.

1.Which of the following is the best definition of evolution?
Medium
- AThe formation of new species from pre-existing species over time as a result of changes to gene pools and allele frequencies
- BA change in an individual's phenotype during its lifetime
- CThe sudden appearance of a new allele in a population
- DThe movement of individuals between populations
2.Which type of selection favours individuals with phenotypes at both extremes and selects against intermediate phenotypes?
Medium
- AStabilising selection
- BDirectional selection
- CDisruptive selection
- DArtificial selection
3.Which of the following is an example of a behavioural barrier that can lead to reproductive isolation?
Medium
- AChanges in courtship behaviour preventing mate attraction
- BA mountain range separating two populations
- CDifferences in soil pH affecting flowering time
- DChanges in genitalia preventing successful mating
4.Which of the following statements about genetic drift is correct?
Medium
- AIt has a greater impact in large populations
- BIt is a result of natural selection
- CIt has a greater impact in small populations
- DIt always increases genetic diversity
5.In the bead modelling investigation, 24 red beads and 24 white beads represent alleles. After 24 random matings, a student obtains 5 RR, 7 Rr and 12 rr. What is the new frequency of the red allele?
Medium
- A35.4%
- B50%
- C27.1%
- D64.6%
6.Which of the following are examples of reproductive isolation mechanisms? (Select all that apply)
Medium
- ASeasonal isolation
- BMechanical isolation
- CBehavioural isolation
- DGeographical isolation
- EGenetic drift
7.Which of the following factors increase genetic variation in a gene pool? (Select all that apply)
Medium
- AMutation
- BCrossing over during meiosis
- CIndependent assortment during meiosis
- DRandom fertilisation
- EGenetic drift
8.Stabilising selection favours individuals with extreme phenotypes.
Easy
True or false?

Thêm câu hỏi

9.Allopatric speciation occurs without a geographical barrier.
Easy
True or false?
10.Genetic drift is more likely to have a significant effect in a large population than in a small population.
Medium
True or false?
11.Match each type of selection with its correct description.
Medium
- Stabilising selection
- Directional selection
- Disruptive selection
- Favours average phenotype, removes extremes
- Favours one extreme phenotype, shifts population traits
- Favours both extreme phenotypes, selects against intermediates
12.Place the following steps of natural selection in the correct order.
Medium
- Variation exists in the population
- Some individuals have a selective advantage
- These individuals survive and reproduce more
- Advantageous alleles increase in frequency over generations
13.Which of the following best defines a mutation?
Medium
- AA change in the DNA base sequence
- BA change in the phenotype due to the environment
- CThe movement of alleles between populations
- DThe fusion of gametes during fertilisation
14.In the bead modelling investigation, what does the genotype ratio 1:2:1 represent?
Medium
- AThe expected ratio of RR:Rr:rr if thousands of draws are made
- BThe observed ratio after 24 draws
- CThe ratio of red to white alleles
- DThe ratio of dominant to recessive phenotypes
15.Genetic drift is a result of natural selection.
Easy
True or false?
16.Which of the following is an abiotic selection pressure?
Medium
- APredation
- BCompetition for mates
- CDisease
- DTemperature
17.In the bead modelling investigation, what does genetic drift refer to?
Medium
- AThe change in expected genotype ratio due to chance in a small breeding population
- BThe change in allele frequency due to natural selection
- CThe movement of alleles between populations
- DThe random fusion of gametes during fertilisation
18.Genetic drift can cause alleles to be lost from a population by chance.
Easy
True or false?
19.Which of the following processes generate new allele combinations in offspring, increasing genetic diversity within a population? (Select all that apply)
Medium
- ACrossing over of non-sister chromatids during prophase I of meiosis
- BIndependent assortment of chromosomes during metaphase I of meiosis
- CRandom fertilisation of gametes during fertilisation
- DMutation in a body cell
- EMitosis in gamete-producing cells
20.Match each type of selection to its correct description.
Medium
- Stabilising selection
- Directional selection
- Disruptive selection
- Favours the average phenotype and removes extremes
- Favours individuals with a phenotype at one extreme, causing a shift over time
- Maintains high frequencies of two different sets of alleles and selects against intermediates
21.Which of the following are examples of reproductive isolation mechanisms that can lead to speciation? (Select all that apply)
Hard
- ASeasonal isolation: individuals reproduce at different times of year
- BMechanical isolation: changes in genitalia prevent successful mating
- CBehavioural isolation: changes in courtship behaviour prevent attraction of mates
- DGeographical isolation: a mountain range splits a population
- ETemporal isolation: individuals are active at different times of day
22.Place the following steps of allopatric speciation in the correct order.
Hard
- A geographical barrier splits a population into two groups
- The two groups are reproductively isolated, so no gene flow occurs
- Different selection pressures act on each group, changing allele frequencies through natural selection
- Over time, genetic differences accumulate and the groups can no longer interbreed
23.Which of the following statements best describes genetic drift?
Medium
- AA change in allele frequencies due to chance events, with a greater effect in small populations
- BA change in allele frequencies due to natural selection favouring advantageous alleles
- CThe movement of individuals between populations, introducing new alleles
- DA random change in the DNA base sequence that creates a new allele
24.Genetic drift is more likely to have a significant effect in a small population than in a large population.
Easy
True or false?
25.Match each term related to allele frequencies to its correct definition or example.
Medium
- Gene pool
- Allele frequency
- Genetic drift
- The total number of alleles of all the genes in a population
- The proportion of a particular allele within a population
- A change in allele frequencies due to chance events, especially in small populations
26.In the bead modelling investigation, 24 red beads and 24 white beads represent alleles. After 24 random matings, a student obtains 5 RR, 7 Rr and 12 rr. Which of the following statements about this result are correct? (Select all that apply)
Hard
- AThe frequency of the red allele has decreased from 50% to approximately 35.4%
- BThe frequency of the white allele has increased to approximately 64.6%
- CGenetic drift has occurred because the observed genotype ratio differs from the expected 1:2:1
- DThe result shows that natural selection has favoured the white allele
- EThe expected genotype ratio if there were no genetic drift is 1:2:1
27.In the bead modelling investigation, after one round of 24 random matings, a student obtained 5 RR, 7 Rr and 12 rr. Which of the following correctly shows the number of red and white alleles in the new population?
Medium
- ARed = 17, White = 31
- BRed = 12, White = 12
- CRed = 10, White = 14
- DRed = 24, White = 24
28.In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype (aa). What is the frequency of the recessive allele (q)?
Medium
- A0.4
- B0.16
- C0.6
- D0.84
29.In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele (q) is 0.3. What percentage of the population is expected to be heterozygous?
Medium
- A42%
- B21%
- C9%
- D49%
30.A population of beetles has two alleles for colour: G (green) and g (brown). In a sample of 200 beetles, 32 are brown (gg). Assuming Hardy-Weinberg equilibrium, what is the frequency of the green allele (G)?
Medium
- A0.6
- B0.4
- C0.8
- D0.2
31.In the bead modelling investigation, after two rounds of 24 random matings, a student obtained 4 RR, 5 Rr and 15 rr. What is the percentage frequency of the red allele in this third generation?
Medium
- A27.1%
- B16.7%
- C37.5%
- D35.4%
32.Which of the following are valid conclusions from the bead modelling investigation? (Select all that apply)
Medium
- AChance can cause allele frequencies to change in a small population.
- BGenetic drift is more likely to have a significant effect in small populations.
- CNatural selection is the main cause of allele frequency change in the investigation.
- DThe observed genotype ratio always matches the theoretical 1:2:1 ratio.
- EGenetic drift can lead to the loss of an allele from a population.
33.A scientist is investigating a population of insects where the mean body size has shifted towards larger individuals over many generations. Which of the following could explain this observation? (Select all that apply)
Medium
- ADirectional selection favouring larger individuals.
- BStabilising selection favouring average-sized individuals.
- CDisruptive selection favouring both extremes.
- DA change in the environment that makes larger size advantageous.
- EGenetic drift causing random changes in allele frequencies.
34.In a population at Hardy-Weinberg equilibrium, the frequency of the dominant allele (p) plus the frequency of the recessive allele (q) always equals 1.
Easy
True or false?
35.Genetic drift is more likely to cause significant changes in allele frequencies in a large population than in a small population.
Medium
True or false?
36.Match each scenario with the type of selection it describes.
Medium
- A population of birds has a range of beak sizes; after a drought, only birds with very large or very small beaks survive.
- Human birth weight is maintained around an average because very small and very large babies have lower survival rates.
- A population of moths becomes darker over time because dark moths are better camouflaged on polluted trees.
- Disruptive selection
- Stabilising selection
- Directional selection
37.What type of selection is acting on claw length in the mice?
Medium
- ADisruptive selection
- BDirectional selection
- CStabilising selection
- DArtificial selection
38.Which of the following best explains why the medium claw length became less common?
Medium
- AMice with medium claws were less able to escape predators or climb trees.
- BMice with medium claws were more likely to be eaten because they were slower.
- CMice with medium claws had a lower chance of survival and reproduction.
- DMice with medium claws were selected against because they could not find food.
39.In the bead modelling investigation, why must each pair of beads be returned to the container after every draw?
Medium
- ATo keep the allele frequencies in the population constant between draws
- BTo prevent the beads from becoming contaminated
- CTo increase the rate at which genetic drift occurs
- DTo make sure each offspring has a different genotype
40.A student sets up a computer simulation of a small population to model evolution. Which of the following are valid advantages of using a computer model rather than the bead investigation? (Select all that apply)
Hard
- AEvolutionary time can be sped up so long-term effects are observed quickly
- BSpecific mutations and selection pressures can be introduced and their effects observed
- CIt produces real alleles that can be passed to living offspring
- DIt allows many generations to be simulated in a short time
- EIt removes the need for any understanding of allele frequencies
41.In a population of 100 individuals, 16 show the recessive phenotype (aa). Assuming Hardy-Weinberg equilibrium, calculate the frequency of the recessive allele (a). Give your answer as a decimal.
Medium
42.Complete the sentence about types of speciation.
Medium
When a population is split by a geographical barrier, ____ speciation occurs. When reproductive isolation occurs without a geographical barrier, ____ speciation occurs.
43.Name the type of selection that maintains high frequencies of two different sets of alleles.
Medium
44.What term describes the continued existence of two or more distinct phenotypes in a species?
Medium
45.Name the type of speciation that occurs when a population is split by a geographical barrier.
Medium
46.Complete the sentence about selection pressures.
Medium
Factors that affect an organism's chance of survival are called ____ pressures. These can be biotic, such as ____, or abiotic, such as ____.
47.Complete the sentence about types of selection.
Medium
____ selection favours the average phenotype, while ____ selection favours one extreme phenotype.
48.What term describes the total number of alleles of all the genes in a population?
Medium
49.Name the type of selection that favours the average phenotype and removes extremes.
Medium
50.Name the type of speciation that occurs without a geographical barrier.
Medium
51.In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype (aa). Calculate the frequency of the recessive allele (q). Give your answer as a decimal to two decimal places.
Hard
52.In a population at Hardy-Weinberg equilibrium, 9% of individuals show the recessive phenotype. Calculate the frequency of the recessive allele (q). Give your answer to 2 decimal places.
Medium
53.Define the term gene pool.
Medium
54.Define the term differential reproduction.
Medium
55.Describe the role that differential reproduction plays in altering allele frequencies within a gene pool.
Medium
56.Define the term polygenic and give an example of a characteristic which is polygenic.
Medium
57.Charles Darwin used the phrase ‘Survival of the fittest’ to support his theory of evolution through natural selection. State the meaning of this phrase.
Medium
58.In a population of 100 individuals, 16 show the recessive phenotype (aa). Assuming Hardy-Weinberg equilibrium, calculate the frequency of the recessive allele (a). Give your answer as a decimal.
Medium
- A0.4
- B-0.6
- C1.4
- D2.4
59.In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype (aa). Calculate the frequency of the recessive allele (q). Give your answer as a decimal to two decimal places.
Hard
- A1.4
- B0.4
- C2.4
- D-0.6
60.In a population at Hardy-Weinberg equilibrium, 9% of individuals show the recessive phenotype. Calculate the frequency of the recessive allele (q). Give your answer to 2 decimal places.
Medium
- A2.3
- B0.3
- C1.3
- D-0.7

Ghi chú ôn tập

## Genetic Variation - **Phenotype** = observable characteristics; **phenotypic variation** = differences among individuals of the same species. - Arises from **genetic factors** (e.g. ABO blood groups), **environmental factors** (e.g. plant height), or **both** (e.g. sickle cell allele maintained because heterozygotes are resistant to malaria). - **Phenotypic variation = genetic variation + environmental influence**. - Genetic variation arises mainly from **mutations** (new alleles; must occur in gametes to be passed on). - Also from sexual reproduction: - **Crossing over** of non-sister chromatids during prophase I → new allele combinations, breaks linkage. - **Independent assortment** of homologous chromosomes during metaphase I → different allele combinations in gametes. - **Random fertilisation** of gametes → genetic variation between zygotes. ## Natural Selection - **Selection pressures** are environmental factors affecting survival: **biotic** (predation, competition, disease) and **abiotic** (light, temperature, water, soil pH). - Individuals with phenotypes better adapted to survive have a **selective advantage** → more likely to survive and reproduce → pass on favourable alleles. - Favourable alleles **increase in frequency** in the gene pool; unfavourable alleles decrease. - This **differential survival and reproduction** is natural selection. - Over generations, allele frequencies change; if great enough, a new species may form. - **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*. - **Speciation** = formation of a new species from an existing one, usually when populations become reproductively isolated and genetically distinct. - Avoid saying well-adapted organisms are “guaranteed” to survive — say they are **more likely** to survive and reproduce. ## Types of Selection - **Stabilising selection**: favours average phenotype, removes extremes (e.g. human birth weight). - **Directional selection**: favours one extreme phenotype, causes shift over time (e.g. antibiotic resistance). - **Disruptive selection**: maintains high frequencies of two different sets of alleles; intermediate phenotypes selected against → **polymorphism** (e.g. Galapagos bird beak size: small or large beaks favoured, medium selected against). ## Types of Speciation - **Reproductive isolation**: changes in alleles/phenotypes prevent breeding; reduces gene flow. - **Seasonal isolation**: reproduce at different times. - **Mechanical isolation**: changes in genitalia prevent mating. - **Behavioural isolation**: changes in courtship prevent attraction. - **Allopatric speciation**: geographical barrier (natural e.g. mountains, rivers; man-made e.g. roads) splits population → no gene flow → different selection pressures → genetic differences accumulate → cannot interbreed. - **Sympatric speciation**: no geographical barrier; same area but splits into two groups with no gene flow. - **Ecological separation**: different environments in same area (e.g. soil pH affects flowering time). - **Behavioural separation**: different behaviours (courtship, feeding, communication). - **Genetic drift**: chance (not selection) affects which individuals survive, breed and pass on alleles; greater impact in **small populations** because chance events significantly alter allele frequencies. In large populations, effects even out; natural selection more influential. ## Investigating Allele Frequencies - Bead model: 24 red (R) + 24 white (r) beads; random mating by drawing two beads; expected genotype ratio **1:2:1** (RR 25%, Rr 50%, rr 25%) if thousands of draws and no genetic drift. - Actual small samples deviate → genetic drift. - Allele frequency calculation: (number of allele ÷ total alleles) × 100. - Computer simulations can model natural selection and genetic drift, speeding up evolutionary time.

Gene pool: The total collection of alleles present in a population.
Phenotypic variation: Differences in observable characteristics among individuals of the same species.
Mutation: A random change in the DNA base sequence that results in the generation of a new allele; must occur in gametes to be passed on.
Selection pressure: An environmental factor that affects an individual's chance of survival; can be biotic (living) or abiotic (non-living).
Selective advantage: A phenotype that makes an individual better adapted to survive, so it is more likely to reproduce and pass on favourable alleles.
Differential reproduction: The process where individuals with favourable alleles reproduce more successfully, increasing those alleles' frequency in the gene pool.
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.
Speciation: The formation of a new species from an existing one, usually when populations become reproductively isolated and genetically distinct.
Stabilising selection: Selection that favours the average phenotype and removes extremes, maintaining average values.
Directional selection: Selection that favours individuals with a phenotype at one extreme, causing a shift in a population's traits over time.
Disruptive selection: Selection that maintains high frequencies of two different sets of alleles; intermediate phenotypes are selected against, causing polymorphism.
Polymorphism: The continued existence of two or more distinct phenotypes in a species.
Reproductive isolation: Changes in alleles or phenotypes that prevent certain individuals from breeding successfully with others in the same population, reducing gene flow.
Allopatric speciation: Speciation that occurs when a population is split by a geographical barrier, leading to reproductive isolation and genetic divergence.
Sympatric speciation: Speciation that occurs without a geographical barrier, caused by ecological or behavioural separation within the same area.
Genetic drift: A gradual change in allele frequencies in a small population due to chance, not natural selection.
Polygenic: A characteristic controlled by multiple genes, resulting in continuous variation (e.g. height).

Unlock all 60 questions & more

Tạo tài khoản miễn phí để xem mọi câu hỏi, slide, thẻ ghi nhớ và ghi chú ôn tập cho chủ đề này.

ĐĂNG KÝ — MIỄN PHÍ ĐĂNG NHẬP

Đề thi cũ

Luyện đề thi cũ cho chủ đề này sắp ra mắt.

Sắp ra mắt