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Biology - Variation and Selection

Grade 10IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Variation is defined as the differences between individuals of the same species. It is divided into two types: continuous variation (e.g., height measured in cmcm) and discontinuous variation (e.g., blood groups A,B,AB,OA, B, AB, O).

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Mutation is a spontaneous genetic change that results in the formation of a new allele. The rate of mutation can be increased by environmental factors such as XX-rays, γ\gamma-rays, and certain mutagenic chemicals.

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Natural Selection is the process by which individuals with advantageous phenotypes are more likely to survive and reproduce. This leads to the passing on of favorable alleles to the next generation.

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Evolution is the change in adaptive features of a population over time as a result of natural selection. It describes how species change over many generations.

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Selective Breeding (Artificial Selection) is used by humans to produce varieties of animals and plants with increased economic importance, such as high-yield TriticumTriticum aestivumaestivum (wheat) or domestic dogs with specific traits.

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Adaptation is the process, resulting from natural selection, by which populations become suited to their environment. For example, xerophytes adapting to low H2OH_2O availability.

📐Formulae

VP=VG+VEV_P = V_G + V_E

Mutation Frequency=Number of mutationsTotal number of gene copies\text{Mutation Frequency} = \frac{\text{Number of mutations}}{\text{Total number of gene copies}}

Fitness (w)=Number of offspring of a genotypeNumber of offspring of the most fit genotype\text{Fitness (w)} = \frac{\text{Number of offspring of a genotype}}{\text{Number of offspring of the most fit genotype}}

💡Examples

Problem 1:

Explain how the development of antibiotic resistance in a population of bacteria like StaphylococcusStaphylococcus aureusaureus is an example of natural selection.

Solution:

  1. Mutation: A random mutation occurs in the DNADNA of a bacterium, providing resistance to an antibiotic (e.g., methicillin). 2. Selection pressure: The population is exposed to the antibiotic. 3. Survival: Non-resistant bacteria die, while the resistant bacterium survives. 4. Reproduction: The survivor reproduces asexually, passing the resistance allele to its offspring. 5. Frequency increase: Over time, the proportion of resistant bacteria in the population increases.

Explanation:

This demonstrates the shift from a susceptible population to a resistant one due to the survival of the 'fittest' individuals in a specific environment.

Problem 2:

Distinguish between the inheritance of Sickle-cell anaemia and its relationship with Malaria.

Solution:

Individuals with the heterozygous genotype HbAHbSHb^A Hb^S (Sickle-cell trait) have a selective advantage in regions where Malaria (caused by PlasmodiumPlasmodium parasites) is endemic.

Explanation:

The HbSHb^S allele causes hemoglobin to deform. PlasmodiumPlasmodium cannot easily infect these cells. Thus, natural selection maintains the HbSHb^S allele in the population because heterozygotes survive both Malaria and severe Sickle-cell anaemia better than homozygous individuals (HbAHbAHb^A Hb^A or HbSHbSHb^S Hb^S).

Variation and Selection - Revision Notes & Key Formulas | IGCSE Grade 10 Science