Variation & Natural Selection (Cambridge (CIE) IGCSE Biology)

True.

Mutations can result in new alleles.

Factors that increase the rate of mutation include:

• ionising radiation

• exposure to some chemicals

False.

Mutations are random changes to the base sequence of a DNA molecule.

Sources of genetic variation in populations include:

• mutation

• meiosis

• random mating

• random fertilisation

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.

Adaptive features of the dolphin include:

• streamlined body shape to allow easy forward movement through the water

• flippers and tail fins to propel the dolphin forwards

• the ability to hold its breath for prolonged periods under water

• blubber under the skin provides insulation

Adaptive features of a venus flytrap include:

• spines on leaves that prevent flies escaping

• red colour that attracts flies

• sensitivity to movement that allows them to close when a fly lands

• the ability to produce digestive juices to break down the bodies of flies

Hydrophytes are plants adapted to live in very wet environments, e.g. aquatic plants.

Hydrophyte adaptations of the water lily include:

• stomata on the upper leaf surface, allowing gas exchange with the air

• large air spaces in the leaves, allowing them to float

• a reduced root system due to the high availability of water

Xerophytes are plants that are adapted to live in very dry environments.

Xerophyte adaptations shown by the cactus include:

• leaves are reduced to spines to reduce water loss by transpiration

• thick waxy cuticle reduces water loss by evaporation

• extensive roots maximise water absorption from the soil

• a thickened stem allows water storage

False.

Natural selection can only occur when genetic variation is present. This is why populations with no, or limited, genetic variation are unable to adapt to changes in their environment.

The process of natural selection states that:

• individuals within a species exhibit variation due to genetic differences

• reproduction in populations produces many offspring

• there is competition for resources within populations, resulting in a struggle for survival

• those with advantageous traits are more likely to survive and reproduce, passing on their advantageous alleles to the next generation

Adaptation is the process by which populations become more suited to their environment over many generations. It occurs as the result of natural selection.

Antibiotic resistance is the ability of bacteria to withstand the effects of an antibiotic; bacteria with resistance to a particular antibiotic are more likely to survive when they are exposed to the drug.

True.

Random mutations in bacterial DNA can lead to antibiotic resistance, enabling some bacteria to survive antibiotic treatment.

Antibiotic resistant strains of bacteria can arise by natural selection as follows:

• mutation gives rise to an allele that codes for antibiotic resistance in one individual

• the resistant individual survives when treated with the antibiotic

• the individual reproduces and passes on the resistance allele

• this occurs over many generations until only resistant individuals remain

Antibiotic use can increase the probability that a resistant population of bacteria will arise because the antibiotics kill any non-resistant individuals, meaning that any randomly occurring alleles for resistance become advantageous. The resistance alleles are therefore more likely to be passed on.

The selective breeding process involves:

• the selection of individuals with characteristics that are desired by humans

• crossing together individuals with desired characteristics

• selection of offspring with desired characteristics and crossing them together

• repetition of the process over many generations

The purpose of selective breeding is to improve the yield from crops and domesticated animals.

Features that might be selectively bred for in plants include:

• increased yield

• disease resistance

• pest resistance

• frost resistance

Features that might be selectively bred for in animals include:

• faster growth

• disease resistance

• gentle temperament

• increased milk yield

Natural and artificial selection are different as follows: