Variation and Natural Selection: Biology 0610 (Cambridge O Level / IGCSE)

A biology class studied a population of garden snails living in a hedge. For each snail, they recorded two features:

• shell length, measured in millimetres, which ranged continuously from the smallest to the largest snail with no natural gaps between values
• shell colour, which fell into only three distinct categories (yellow, pink or brown), with no snails showing an in-between colour

Which statement correctly classifies these two features and identifies what causes each type of variation?

The Saharan silver ant (Cataglyphis bombycina) lives in the Sahara Desert and is one of the most heat-tolerant animals known. Unlike most desert animals, it forages for food above ground during the hottest part of the day, when the sand surface can reach temperatures above 60 degrees Celsius.

Some information about this ant is given below.

• It has unusually long legs, which keep most of its body raised above the hot sand's surface.
• Its body is covered in silvery hairs with a triangular cross-section, which reflect sunlight and radiate heat away from the body.
• It can survive body temperatures that would kill most other desert animals, including the lizards that hunt it.

(a) State what is meant by the term adaptive feature. [1]

(b) Using the information given, describe two adaptive features of the Saharan silver ant and explain how each one helps the ant to survive in its habitat. [4]

(c) Suggest one advantage to the ant of foraging during the hottest part of the day rather than in the cooler early morning. [2]

A laboratory population of red flour beetles (Tribolium castaneum) is normally reddish-brown in colour. A researcher notices that a small number of beetles in one culture are jet black instead.

(a) Describe what is meant by a gene mutation, with reference to the base sequence of DNA, and state whether such changes normally occur in a directed or a random way. [2]

(b) State two factors that can increase the rate at which mutations occur. [2]

(c) Mutation is not the only source of genetic variation in a population. State two processes, other than mutation, that also produce genetic variation between individuals of a species. [2]

Some poultry farms give their chickens a continuous low dose of an antibiotic to prevent bacterial infections. A vet studied the bacterium Escherichia coli living in the gut of these chickens and compared it with E. coli from flocks that had never been given the antibiotic.

• In flocks never given the antibiotic, almost all E. coli were killed by the antibiotic in a laboratory test.
• In flocks given the antibiotic continuously for several years, almost all E. coli survived the same laboratory test.

(a) Use the idea of natural selection to explain how the proportion of antibiotic-resistant E. coli in the treated flocks could have increased over time. In your answer, refer to variation within the bacterial population, the large number of offspring produced by bacteria, competition for survival in the presence of the antibiotic, and the passing on of alleles to offspring. [4]

(b) Explain why the change from a mostly non-resistant population to a mostly resistant population happened gradually, over many bacterial generations, rather than in a single generation. [2]

(c) A farm worker suggests that the bacteria "changed themselves" to resist the antibiotic because they were exposed to it. Explain why this suggestion is incorrect. [2]

A fish farmer keeps a large pond of tilapia, a freshwater fish, for food production. When the farmer weighs six-month-old tilapia from the pond, their body mass forms a continuous range from very small to very large fish, with no natural gaps between individual values. The farmer wants to breed a strain of tilapia that reaches a larger body mass more quickly.

(a) State the three stages the farmer would follow, over one generation, to begin selectively breeding tilapia for faster growth to a larger body mass. [3]

(b) Explain why the farmer needs to repeat this process over many generations, rather than reaching the desired growth rate after selecting and breeding from the fastest-growing fish just once. [2]

(c) State why it was essential that the tilapia in the original pond already showed variation in growth rate before the selective breeding programme could begin. [2]

A student studying a population of ladybird beetles in a garden collected data on two features of 60 individuals.

• Elytra (wing case) pattern: every beetle in the sample belonged to one of three sharply distinct forms (red with black spots, black with two red spots, or all black) with no beetles showing an in-between pattern.
• Body length: the student measured body length, in millimetres, for all 60 beetles and plotted the results as a bar chart. The bar chart formed a single smooth, unbroken range from the shortest to the longest beetle, with the greatest number of beetles found in the middle of the range and progressively fewer beetles towards each end.

The student also read that beetle larvae that feed on more aphids during their larval stage grow into larger adult beetles, regardless of which elytra pattern they display as adults.

(a) Using the description above, state which of the two features shows continuous variation and which shows discontinuous variation. [2]

(b) Explain how the shape of the body-length bar chart supports your classification of body length in part (a). [2]

(c) State what usually causes discontinuous variation of the kind shown by elytra pattern, and explain how the information about larval feeding supports your classification of body length in part (a) as being caused by both genes and the environment. [3]

A species of small, ground-dwelling rodent lives across a region that contains two very different habitats close together: pale, light-coloured sand dunes and outcrops of dark, weathered volcanic rock. In both habitats, individual rodents show natural variation in fur colour, ranging from pale sandy fur to dark, almost black fur. Owls hunt these rodents at night by sight and are their main predator.

Biologists sampling both habitats found that most rodents on the sand dunes have pale fur, with only a small proportion having dark fur, while most rodents on the volcanic rock have dark fur, with only a small proportion having pale fur.

(a) Explain, using the idea of natural selection, why most rodents on the volcanic rock have dark fur while most rodents on the sand dunes have pale fur. In your answer, refer to the variation already present in the rodent population, camouflage against the ground colour, predation by owls, survival to reproduce, and the passing on of alleles. [5]

(b) A farmer suggests that when a pale rodent moves from the sand dunes onto the volcanic rock, its fur gradually darkens during its own lifetime to match its new surroundings. Explain why this suggestion is incorrect. [2]

(c) Suggest what would most likely happen, over many further generations, to the proportion of dark-furred rodents on the volcanic rock if the rock gradually weathered and became covered in pale sand blown from the nearby dunes. [1]

A species of succulent plant grows in a desert region with very low rainfall. Botanists think it evolved from an ancestral, non-succulent plant species that once grew in a much wetter habitat nearby. The present-day desert species has:

• a thick, swollen stem that stores water
• a waxy, thickened outer layer (cuticle) covering its stem
• very small, spine-like leaves instead of broad, flat leaves

(a) Using the information given, describe two adaptive features of the desert succulent and explain how each one helps it to survive periods of low rainfall. [4]

(b) Explain, using the idea of natural selection, how a population of the wetter-habitat ancestral plant could have evolved into a desert species with these features over many generations. In your answer, refer to variation within the ancestral population, water availability as a factor affecting survival, and the passing on of alleles to offspring. [4]

A dog breeder wants to produce a line of dogs with an unusually thick, warm coat, suitable for working in cold climates. Each year, she chooses the dogs in her kennel with the thickest coats and breeds them together, repeating this over several generations.

Wild wolves, the ancestors of domestic dogs, are not bred by any breeder. In a wolf population living in a very cold region, individual wolves vary naturally in coat thickness.

(a) State two ways in which the process used by the dog breeder differs from the process of natural selection that could change coat thickness in the wild wolf population. [2]

(b) Explain how natural selection, rather than a breeder, could cause coat thickness to increase in the wild wolf population over many generations. [4]

A population of a small freshwater fish lives in a lake that was colonised by a predatory fish species several thousand generations ago. Compared with an ancestral population living in a nearby lake that has never had any predatory fish, the present-day population in the colonised lake has much thicker, more extensive bony armour plates along its sides.

Which statement gives the correct natural-selection explanation for this change?