Natural Selection

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Evolution and Natural Selection

Evolution is defined as changes in the heritable characteristics of organisms over generations.
Natural selection is the process by which organisms better adapted to their environment survive, reproduce, and pass on advantageous alleles, causing advantageous characteristics to increase in frequency.
Charles Darwin developed the theory of evolution by natural selection after a five-year expedition observing a variety of organisms.
Darwin published On the Origin of Species in 1859, presenting evidence that favourable characteristics increase in frequency over time.
Unfavourable characteristics decrease in frequency because individuals with them are less likely to survive and reproduce.
Darwin's theory replaced Lamarckism, the idea that organisms could pass on characteristics acquired during their lifetime.
Darwin's theory caused a paradigm shift because it contradicted previous assumptions about how species change.

Natural selection in peppered moths

The Importance of Variation

Variation refers to the differences between organisms of the same species, such as coat colour in mammals or flower colour in plants.
Natural selection can only occur if there is variation in a population; if all individuals are identical, no individual is favoured.
Without variation, a population cannot adapt to environmental changes and its characteristics remain the same over time.
Variation results from small differences in DNA base sequences between individuals.
Sources of genetic variation include mutation, meiosis, and random fertilisation during sexual reproduction.

Distribution of height showing continuous variation

Sources of Genetic Variation

Mutation is the original source of genetic variation; it is a change in the DNA base sequence that results from a copying error during DNA replication.
Mutations in sex organs can produce new alleles in gametes that are passed to the next generation.
A mutation may be advantageous, disadvantageous, or have no effect; advantageous alleles are more likely to be passed on.
Mutations in somatic (body) cells are not passed on and do not affect natural selection.
Meiosis generates variation through crossing over and random orientation (independent assortment).
Crossing over involves homologous chromosomes exchanging alleles, creating new combinations of alleles.
Random orientation occurs during metaphase I when homologous pairs line up independently; the number of combinations is 2ⁿ, where n is the haploid number.
Random fertilisation during sexual reproduction creates genetic variation because any male gamete can fuse with any female gamete.

Overproduction and Competition

Most species produce more offspring than can be supported by the environment, a phenomenon Darwin called overproduction of offspring.
Overproduction leads to competition for limited resources such as food, space, and light.
Environmental factors limit the carrying capacity of a population, so many offspring fail to survive and reproduce.
Intraspecific competition occurs between individuals of the same species and plays a greater role in evolution than interspecific competition.
Individuals of the same species share the same niche and are affected by the same abiotic and biotic factors.
Individuals with characteristics better adapted for survival are more likely to survive, reproduce, and pass on their alleles.
This is sometimes described as 'survival of the fittest'.

Heritable Traits and Evolutionary Change

Characteristics determined by alleles are heritable and can be passed on to offspring.
Heritable characteristics can be physical (e.g., giraffe neck length) or behavioural (e.g., woodlice moving to dark places).
Individuals with advantageous heritable characteristics produce more offspring and pass on the alleles for those characteristics.
Non-heritable characteristics, acquired during an organism's lifetime, are not passed on and do not affect evolution.
Organisms better adapted to their environment are more likely to survive, not guaranteed to survive.

Selection Pressures: Abiotic Factors

Abiotic factors are non-living factors within an ecosystem, such as temperature or rainfall.
Abiotic factors can act as selection pressures, affecting the survival of individuals and causing population size to fluctuate.
Some abiotic factors are density-independent, meaning their effect does not depend on population size.

Adaptations of a fennec fox

Selection Pressures: Sexual Selection

Sexual selection is a form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex.
Sexual selection requires variation within the population; traits act as indicators of overall fitness.
Sexual selection can lead to reproductive isolation and sexual dimorphism.
Reproductive isolation occurs when changes in alleles or phenotypes prevent successful breeding with other individuals.
Sexual dimorphism is the distinct difference in size or appearance between the sexes, such as in Birds of Paradise.
Natural selection occurs due to competition for resources, while sexual selection occurs due to competition for mates.
Sexual selection can result in traits that reduce survival, such as the peacock's long tail feathers.
Sexual selection can be a more prominent evolutionary force than natural selection because variation in mating success can amplify selection and maintain genetic variation.

Selection Pressures: Skills – Guppy Case Study

Guppies in Trinidad and Tobago show variation in colour and pattern, with males brightly coloured and females dull (sexual dimorphism).
John Endler conducted lab and field experiments in the 1970s to investigate natural and sexual selection in guppies.
Endler hypothesised that predation pressure affects spot brightness: low predation increases brightness due to sexual selection, while high predation decreases brightness.
In the lab experiment, guppies were placed in ponds with different gravel types and predation levels; after five months, spot numbers and sizes were measured.
Results showed that high predation decreased mean spot number, while low predation allowed spot number to increase.
Guppies in coarse gravel ponds tended to have larger spots, while those in fine gravel ponds had smaller spots, mimicking gravel size.
In the field experiment, dull male guppies transferred to a low-predation area became more colourful over 15 generations.
The experiments demonstrated a dynamic trade-off between natural selection (predator avoidance) and sexual selection (mate attraction).

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Practice questions

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1.Which process does not generate heritable variation in a population?
Easy
- AMutation
- BMitosis
- CMetaphase I of meiosis
- DFertilisation
2.Which of the following statements correctly explains the process of natural selection? I. Some species produce small numbers of young and provide higher levels of parental care II. Advantageous alleles increase an individual's chance of surviving and reproducing III. Individuals that change to suit their environment pass their alleles on to offspring IV. Advantageous alleles increase in frequency in the population
Medium
- AI and II only
- BI, II, III, and IV
- CII, III, and IV only
- DII and IV only
3.Which of the following applies to the process of evolution by natural selection? I. Changes in the phenotype of organisms in a population II. Selection pressures favouring certain alleles within a population III. Individuals with a certain genetic makeup will not pass on their genes IV. Changes in the allele frequencies within a population over time
Medium
- AII only
- BI, II and III
- CII and IV only
- DI, II, III and IV
4.Which of the following processes generate genetic variation? I. Random assortment II. Mitosis III. DNA replication IV. Random fertilisation
Medium
- AI and III only
- BI and IV only
- CI, III, and IV only
- DI, II, III, and IV
5.Research was carried out on beak size in the island-living finch species Geospiza fortis. G. fortis feeds on seeds, which are plentiful, small, and soft in years when the weather is normal, but which become larger and tougher in drought years. Which of the following statements explains the results in the graph?
Medium
- AFinches adapt to drought years by developing larger beaks, passing on the characteristic to their offspring, and leading to an increase in average beak size.
- BAverage finch beak size increases during years of drought and decreases during wet years.
- CAverage beak size increased from 9.5 mm in 1976 to 9.9 mm in 1977, before slowly decreasing to 9.7 mm by 1979.
- DFinches with larger beaks have an advantage when competing for food in drought years, and are therefore more likely to survive and pass on their alleles, leading to an increase in average beak size.
6.Which of the following does not describe the process of natural selection?
Hard
- AA species of antelope where the population has stronger muscles after the individuals with weak muscles are eaten by predators
- BWhen a bacteria is exposed to a new type of antibiotic the exposure causes the bacteria to become resistant
- CSharks adapting to store more oxygen in their blood because the sharks with less oxygen were more likely to die
- DDark peppered moths increasing in number during the industrial revolution in response to increased predation of the light colour moths
7.Which of the following are sources of genetic variation within a population? (select all that apply)
Medium
- AMutation
- BCrossing over during meiosis
- CRandom fertilisation
- DMitosis
- EIndependent assortment
8.Natural selection can only occur if there is variation within a population.
Easy
True or false?

More questions

9.Mutations that occur in somatic (body) cells can be passed on to offspring and affect natural selection.
Easy
True or false?
10.Match each term with its correct definition.
Medium
- Evolution
- Natural selection
- Sexual selection
- Reproductive isolation
- Changes in the heritable characteristics of organisms over generations
- The process by which organisms better adapted to their environment survive, reproduce, and pass on advantageous alleles
- A form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex
- When changes in alleles and phenotypes prevent individuals from successfully breeding with others
11.Place the steps of natural selection in the correct order.
Medium
- Variation exists within a population
- Some individuals have advantageous characteristics
- Individuals with advantageous characteristics are more likely to survive and reproduce
- Advantageous alleles increase in frequency in the population over generations
12.In Endler's guppy experiment, which statement best explains why the mean number of spots on male guppies decreased in ponds with a high predation rate?
Medium
- APredators caused mutations that reduced spot number in individual guppies.
- BBrightly spotted males were more easily seen by predators, so they survived and reproduced less, and the alleles for many spots decreased in frequency.
- CMale guppies learned to hide their spots when predators were present.
- DFemale guppies preferred males with fewer spots, so sexual selection reduced spot number.
13.Which of the following statements about sexual selection are correct? (select all that apply)
Hard
- AIt occurs due to competition for mates.
- BIt always produces individuals that are better adapted to their environment.
- CIt can lead to sexual dimorphism.
- DIt can cause reproductive isolation.
- EIt only acts on females.
14.Place the following events in the order in which they occur during evolution by natural selection.
Medium
- A mutation creates a new allele in an individual.
- The new allele gives the individual an advantage in its environment.
- The individual survives and reproduces more than others.
- The advantageous allele increases in frequency in the population.
- The population becomes better adapted to its environment.
15.Which of the following are examples of selection pressures that can act on a population? (select all that apply)
Medium
- APredation
- BFood availability
- CTemperature
- DSexual selection
- EMutation rate
16.Sexual selection can result in traits that make an individual more likely to be eaten by predators.
Easy
True or false?
17.In Endler's field experiment, dull male guppies were moved from an area of high predation to an area of low predation. After 15 guppy generations, the males had more colourful patterning. Which statement best explains this?
Hard
- AWith fewer predators, bright colouration was no longer strongly selected against, so sexual selection by females favoured more colourful males.
- BThe guppies needed to become more colourful to survive in the new stream, so they changed colour during their lifetime.
- CThe move to a new area caused new mutations for bright colouration.
- DFemale guppies in the new area preferred dull males, so the population became more colourful over time.
18.Which of the following can be concluded from the table?
Medium
- AThere is no significant difference in the frequency of the -13910×T allele between Neolithic hunter-gatherers and modern day humans in Europe.
- BLactase persistence gave a survival advantage to humans at some stage between Neolithic and modern-day times.
- CLactase persistence gave a survival advantage to European Neolithic hunter-gatherers.
- DAdult European Neolithic hunter-gatherers did not consume milk.
19.A population of mice lives on a beach with light-coloured sand. A new predator that hunts by sight moves into the area. Which statement best predicts what will happen to the fur colour of the mouse population over many generations?
Medium
- AIndividual mice will gradually change their fur to a lighter colour during their lifetime.
- BMice with lighter fur are more likely to survive and reproduce, so the frequency of light-fur alleles will increase.
- CThe predator will cause mutations for lighter fur to appear in the population.
- DAll mice will become lighter because they need to be camouflaged.
20.In a population of beetles, a mutation produces a new allele that gives resistance to a pesticide. The pesticide is sprayed on the field. Which of the following correctly describes what happens?
Medium
- AThe pesticide causes beetles to mutate and become resistant.
- BBeetles with the resistance allele survive and reproduce, so the allele frequency increases.
- CAll beetles become resistant because they need to survive.
- DThe resistance allele is lost because it is new.
21.In a population of guppies, males with brighter spots are more attractive to females but are also more likely to be eaten by predators. In a stream with many predators, what is the most likely outcome?
Hard
- AMales will become brighter because females prefer bright spots.
- BMales will become duller because bright males are eaten before they can reproduce.
- CMales will become brighter because predators cannot see bright spots.
- DMales will stay the same because sexual selection and natural selection cancel out exactly.
22.A population of insects has two alleles for wing colour: B (brown) and b (green). In a forest, brown insects are better camouflaged on tree trunks. Initially, the frequency of B is 0.5. After many generations, the frequency of B is 0.9. Which statement best explains this change?
Hard
- ABrown insects are more likely to survive and reproduce, so the B allele increases in frequency.
- BGreen insects are more likely to survive and reproduce, so the b allele increases in frequency.
- CThe insects change colour during their lifetime to become brown.
- DThe B allele is dominant, so it automatically becomes more common.
23.In a population of snails, shell thickness is determined by a single gene with two alleles: T (thick) and t (thin). Thick shells are better protected against crab predators. In an area with many crabs, the frequency of T increases from 0.2 to 0.8 over 20 generations. What can be concluded?
Challenge
- ACrabs caused mutations for thicker shells.
- BSnails with thick shells had a survival advantage and passed on the T allele.
- CThin-shelled snails learned to grow thicker shells.
- DThe T allele is dominant, so it increased in frequency.
24.Endler's guppy experiment included ponds with different gravel types and predation levels. In ponds with coarse gravel and high predation, what was observed?
Medium
- AGuppies had larger spots.
- BGuppies had smaller spots.
- CGuppies had no spots.
- DGuppies had brighter spots.
25.In a population of guppies, the frequency of allele 1 is 0.6 and allele 2 is 0.3. A new allele 3 is present. Assuming these are the only three alleles, what is the frequency of allele 3?
Medium
- A0.1
- B0.3
- C0.4
- D0.9
26.A population of finches has a gene for beak size with two alleles: L (large) and S (small). In a drought, only large, tough seeds are available. Before the drought, the frequency of L was 0.3. After the drought, the frequency of L was 0.7. What is the change in frequency of the L allele?
Hard
- A0.4
- B0.7
- C0.3
- D1.0
27.In Endler's guppy experiments, which of the following statements are correct? (select all that apply)
Hard
- AIn high predation ponds, the mean number of spots decreased.
- BIn low predation ponds, the number of spots increased.
- CIn coarse gravel ponds, guppies tended to have larger spots.
- DIn fine gravel ponds, guppies tended to have larger spots under high predation.
- EPredators caused new mutations for spot colour.
28.In Endler's field experiment, male guppies transferred from a high-predation area to a low-predation area became less colourful over 15 generations.
Medium
True or false?
29.In Endler's laboratory experiment, guppies were kept in large tanks and allowed to breed freely for six months with no predators before being assigned to the experimental ponds. What was the main purpose of this initial period?
Medium
- ATo allow the guppies to adapt to the greenhouse temperature before the experiment began
- BTo establish a genetically uniform population so that any later changes could be attributed to predation
- CTo allow a wide range of variation in spot size and number to develop in the population
- DTo train the guppies to recognise the predators that would later be introduced
30.In Endler's laboratory experiment, the ponds were set up with either coarse or fine gravel and with no predator, a weak predator (killifish) or a dangerous predator (pike cichlid). Which of the following best describes why both gravel type and predation level were varied?
Medium
- ATo test whether spot size mimics gravel size and whether predation rate affects spot number independently
- BTo give the guppies a choice of habitats so they could select the one they preferred
- CTo ensure that all ponds had the same selection pressures so results could be combined
- DTo increase the mutation rate in the guppy populations so that new alleles appeared
31.In Endler's laboratory guppy experiment, which of the following were observed after predators were introduced? (select all that apply)
Hard
- AIn ponds with a high predation rate the mean number of spots decreased
- BIn ponds with a low predation rate the number of spots continued to increase
- CIn ponds with coarse gravel guppies tended to have larger spots
- DIn ponds with fine gravel guppies tended to have larger spots
- EIn ponds with no predation, guppies always matched the gravel colour exactly
32.In Endler's laboratory experiment, the same gravel type was used in all ten ponds so that only predation level varied.
Easy
True or false?
33.In Endler's field experiment, dull male guppies were moved from a high-predation area to a low-predation area and left for 15 guppy generations. Which conclusion is best supported by the results?
Medium
- APredation is the only selection pressure acting on guppy colouration
- BIn the absence of strong predation, sexual selection favours more colourful males
- CMale guppies deliberately change their colour to attract females
- DLow predation causes males to become duller because they no longer need camouflage
34.A student repeats Endler's laboratory experiment but uses only one pond for each combination of gravel type and predation level. Why does this reduce the strength of the conclusions that can be drawn?
Hard
- ABecause the guppies will not breed in a single pond
- BBecause there is no replication, so any difference could be due to chance rather than the treatment
- CBecause using one pond means the predation level cannot be controlled
- DBecause the gravel type cannot be standardised across a single pond
35.In a population of beetles, elytra colour varies from bright red to brown and green. Birds and rodents hunt the beetles by sight on the forest floor. Over many generations, brown and green beetles become more common. Which statement best explains this change?
Hard
- AThe beetles changed their elytra colour to match the leaf litter during their lifetime
- BBrown and green beetles were better camouflaged, so they survived and reproduced more, passing on their alleles
- CThe predators ate only brown and green beetles, so red beetles increased in number
- DThe allele for red elytra mutated into the allele for brown elytra in every red beetle
36.A scientist wants to test whether a new antibiotic selects for resistant bacteria. Which of the following would be essential features of a valid experiment? (select all that apply)
Hard
- AA control group of bacteria grown without the antibiotic
- BSeveral replicate plates for each antibiotic concentration
- CExposing all bacteria to the antibiotic before counting survivors
- DMeasuring the number of resistant colonies after a set incubation time
- EUsing only one bacterial cell so that variation cannot occur
37.The new allele is dominant and gives a survival advantage. What is the best prediction for the frequencies of the three alleles over time?
Hard
- AThe new allele will increase and allele 1 will decrease, while allele 2 stays exactly the same
- BThe new allele will increase in frequency and the other alleles will decrease, because individuals with the new allele survive and reproduce more
- CAll three allele frequencies will remain constant because the population is already adapted
- DThe new allele will decrease because dominant alleles are always removed by selection
38.Which of the following best defines evolution?
Easy
- AChanges in the heritable characteristics of organisms over generations
- BThe process by which organisms become better adapted to their environment during their lifetime
- CThe formation of new species from a single ancestral species
- DChanges in the phenotype of an individual organism in response to its environment
39.Which of the following is the correct definition of a population?
Easy
- AAll the organisms in a particular area
- BA group of individuals of the same species living in the same area at the same time
- CAll the different species living in a habitat
- DThe number of individuals of a species in a given area
40.Which of the following statements about mutation is correct?
Medium
- AMutations in somatic cells are passed on to offspring and affect natural selection.
- BMutations in sex organs can lead to changes in gametes that are passed on to the next generation.
- CAll mutations are advantageous and increase an individual's chance of survival.
- DMutations only occur during meiosis.
41.What is the original source of genetic variation?
Medium
42.Name the type of competition that occurs between individuals of the same species.
Medium
43.What term describes the distinct difference in size or appearance between the sexes of an animal species?
Medium
44.In a population of guppies, the frequency of allele 1 is 0.81 and the frequency of allele 2 is 0.19. A new allele appears. Assuming these are the only three alleles, what is the frequency of the new allele?
Medium
45.In a population of guppies, the frequency of allele 1 is 0.77 and the frequency of allele 2 is 0.21. A new allele appears. Assuming these are the only three alleles, what is the frequency of the new allele?
Hard
46.Complete the sentence about selection pressures.
Easy
A non-living factor within an ecosystem that can affect the survival of individuals is called an ____ factor.
47.Complete the sentence about sources of genetic variation.
Medium
The original source of genetic variation is ____.
48.Complete the sentence about sexual selection.
Medium
A form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex is known as ____ selection.
49.A population of guppies has allele 1 at a frequency of 0.62 and allele 2 at a frequency of 0.25. A new allele is also present. Assuming these are the only three alleles, what is the frequency of the new allele?
Hard
50.Calculate the increase in the frequency of the -13910×T allele from Neolithic hunter-gatherers (0.03) to modern Europeans (0.74). Give your answer to two decimal places.
Hard
51.In a population of guppies, the frequency of allele 1 is 0.75 and allele 2 is 0.20. A new allele 3 is present. Assuming these are the only three alleles, calculate the frequency of allele 3. Give your answer to two decimal places.
Medium
52.In a population of guppies, the frequency of allele 1 is 0.81 and the frequency of allele 2 is 0.19. A new allele appears. Assuming these are the only three alleles, calculate the frequency of the new allele. Give your answer as a decimal.
Easy
53.Explain how natural selection could enable a population of bacteria to become resistant to an antibiotic.
Hard
54.Define the term population.
Medium
55.Explain how the beetle population could have evolved from having a diverse range of elytra colours to having mostly brown and green.
Hard
56.Explain why Endler's guppy experiments are described as showing a trade-off between natural selection and sexual selection.
Hard
57.In a population of guppies, the frequency of allele 1 is 0.81 and the frequency of allele 2 is 0.19. A new allele appears. Assuming these are the only three alleles, what is the frequency of the new allele?
Medium
- A2
- B1
- C0
- D-1
58.In a population of guppies, the frequency of allele 1 is 0.77 and the frequency of allele 2 is 0.21. A new allele appears. Assuming these are the only three alleles, what is the frequency of the new allele?
Hard
- A0.02
- B-0.98
- C1.02
- D2.02
59.A population of guppies has allele 1 at a frequency of 0.62 and allele 2 at a frequency of 0.25. A new allele is also present. Assuming these are the only three alleles, what is the frequency of the new allele?
Hard
- A2.13
- B0.13
- C-0.87
- D1.13
60.Calculate the increase in the frequency of the -13910×T allele from Neolithic hunter-gatherers (0.03) to modern Europeans (0.74). Give your answer to two decimal places.
Hard
- A0.71
- B1.71
- C2.71
- D-0.29
61.In a population of guppies, the frequency of allele 1 is 0.75 and allele 2 is 0.20. A new allele 3 is present. Assuming these are the only three alleles, calculate the frequency of allele 3. Give your answer to two decimal places.
Medium
- A-0.95
- B2.05
- C0.05
- D1.05

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