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Variation and Selection - Selection (Natural and Artificial)

Grade 11IGCSEBiology

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

🔑Concepts

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Variation is the differences between individuals of the same species. It can be phenotypic (observable) or genotypic (genetic).

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Discontinuous variation results in a limited number of distinct phenotypes with no intermediates, often caused by a single gene (e.g., blood groups AA, BB, ABAB, and OO).

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Continuous variation results in a range of phenotypes between two extremes, typically caused by the combined effect of many genes and environmental factors (e.g., human height).

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Natural Selection is the process where individuals with favorable alleles are better adapted to their environment, leading to a higher chance of survival and reproduction.

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Selection pressure refers to external factors (e.g., predation, disease, or competition) that affect an organism's ability to survive.

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Antibiotic resistance is a classic example of natural selection: bacteria with a mutation for resistance survive exposure to the drug (AntibioticRAntibiotic^{R}), while others die, leading to the proliferation of the resistant strain.

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Artificial Selection (Selective Breeding) is the process by which humans choose individuals with desirable traits (e.g., high milk yield in cows) and breed them together over many generations.

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Evolution is the change in adaptive features of a population over time as a result of natural selection.

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Fitness is the probability of an organism surviving and reproducing in the environment in which it is found.

📐Formulae

Allele Frequency Change∝Selection Pressure×Heritability\text{Allele Frequency Change} \propto \text{Selection Pressure} \times \text{Heritability}

Fitness (w)=Number of offspring from a genotypeOffspring from most fit genotype\text{Fitness (w)} = \frac{\text{Number of offspring from a genotype}}{\text{Offspring from most fit genotype}}

p+q=1 (Where p and q represent frequencies of two alleles in a population)p + q = 1 \text{ (Where } p \text{ and } q \text{ represent frequencies of two alleles in a population)}

💡Examples

Problem 1:

Describe how natural selection leads to the development of antibiotic-resistant bacteria like MRSAMRSA.

Solution:

  1. Genetic variation exists in a bacterial population due to random mutations in DNA. 2. A mutation may provide resistance to an antibiotic (PenicillinRPenicillin^{R}). 3. When the antibiotic is applied, it acts as a selection pressure, killing non-resistant bacteria. 4. The resistant bacteria survive and reproduce via binary fission. 5. They pass the resistant allele to their offspring, increasing the frequency of the resistant trait in the population.

Explanation:

This demonstrates the shift in the gene pool from a susceptible population to a resistant one through differential survival.

Problem 2:

Compare selective breeding and natural selection in terms of the 'agent' of selection and the outcome for the species.

Solution:

In natural selection, the 'agent' is the environment/nature; in artificial selection, the 'agent' is humans. Natural selection results in better adaptation for survival (FitnessFitness), whereas artificial selection results in traits desirable to humans which may actually decrease the organism's fitness in the wild.

Explanation:

Example: Pugs are selectively bred for their flat faces, but this trait causes breathing difficulties (Pathological anatomyPathological\ anatomy) that would be disadvantageous in a natural environment.

Problem 3:

Explain why continuous variation usually results in a bell-shaped curve (normal distribution) when graphed.

Solution:

Continuous variation is polygenic, meaning it is controlled by many genes (e.g., G1,G2,G3...GnG_1, G_2, G_3...G_n). The combination of many genetic factors and environmental influences (EE) means that most individuals fall near the mean value, with fewer individuals at the extremes.

Explanation:

The graph of frequency against the trait (e.g., mass in kgkg) follows a Gaussian distribution.

Selection (Natural and Artificial) Revision - Grade 11 Biology IGCSE