Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Natural selection can only occur if there is variation among members of the same species. This variation is primarily generated by mutation, meiosis, and sexual reproduction.
Mutation is the original source of variation; new alleles are produced by replication errors or environmental factors, enlarging the gene pool of a population.
Meiosis allows for a huge variety of combinations of alleles in a diploid cell by crossing over (Prophase I) and the independent assortment of bivalents (Metaphase I).
Adaptations are characteristics that make an individual suited to its environment and way of life. These traits develop over time through the selection of beneficial alleles.
Species tend to produce more offspring than the environment can support, leading to a struggle for survival where individuals with favorable variations are more likely to reach reproductive age.
Individuals that reproduce pass on their characteristics to their offspring. This results in an increase in the frequency of favorable alleles in the population over generations, while the frequency of deleterious alleles decreases.
Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species (Evolution).
Example of evolution in response to environmental change: The increase in mean beak size of (Medium Ground Finch) on Daphne Major following the drought.
Evolution of antibiotic resistance: In a bacterial population, a mutation may confer resistance to an antibiotic (e.g., ). When the population is exposed to , non-resistant bacteria die, and resistant ones survive and replicate via binary fission, passing the resistance gene to the next generation.
📐Formulae
💡Examples
Problem 1:
In a population of finches on the Galapagos Islands, a drought reduced the availability of small, soft seeds. Finches with a beak depth of or greater were able to crack larger, harder seeds, while those with smaller beaks could not. If the initial population had a mean beak depth of , explain the shift in the population mean after the drought using the principles of natural selection.
Solution:
- Variation: There was existing variation in beak depth (ranging from roughly to ) due to genetic differences. 2. Overproduction/Struggle: The drought created a shortage of food, leading to competition for resources. 3. Differential Survival: Finches with beak depths had a selective advantage. 4. Inheritance: Survivors reproduced and passed the alleles for larger beaks to their offspring. 5. Evolution: The mean beak depth of the next generation increased to approximately .
Explanation:
This is a classic example of directional selection where an environmental pressure (drought/seed hardness) favors one extreme of the phenotypic range.
Problem 2:
Explain the role of mutation in the development of antibiotic resistance in ().
Solution:
A random mutation occurs in the of a single bacterium, such as the gene, which provides resistance to -lactam antibiotics like . When the environment is treated with the antibiotic, the selective pressure kills the susceptible bacteria. The resistant bacterium survives and reproduces rapidly ( growth). Because the resistance trait is heritable, the proportion of resistant individuals in the population increases over time.
Explanation:
Natural selection acts on the individual (the bacterium), but the effect is seen in the population's gene pool over time. The antibiotic does not 'cause' the mutation; the mutation is random and precedes the exposure.