Natural and artificial selection — practice questions

1. Selection for specific traits: Farmers identify cows with high milk yield and bulls from high-yielding lineages. Only these individuals are chosen for breeding, discarding those with lower yields.
2. Narrowing the gene pool: Over generations, alleles associated with high milk production become more frequent. However, alleles for other traits (e.g., disease resistance, fertility, longevity) that are not directly selected for, or are negatively correlated with high yield, may be lost or significantly reduced.
3. Reduced genetic diversity: This continuous selection pressure reduces the overall genetic variation within the dairy cattle population. Many different alleles present in the original wild cattle populations are eliminated.
4. Increased susceptibility to disease: A uniform genetic background means if one animal is susceptible to a particular disease, all individuals in the population are likely to be susceptible, making outbreaks more devastating.
5. Inbreeding depression: To maintain specific desirable traits, closely related individuals may be bred. This increases homozygosity, raising the probability of expressing harmful recessive alleles, leading to issues like reduced fertility, lower immune response, and physical deformities.
6. Ethical considerations: The relentless pursuit of extreme milk yields can also lead to welfare issues, such as lameness, mastitis, and metabolic disorders in the cows themselves, which are unintended consequences of the selection process.

Step 1: Calculate initial numbers of susceptible and resistant bacteria.

Total population = 2,000,000 Resistant percentage = 0.05%

Number of resistant bacteria = $2,000,000 \times \frac{0.05}{100} = 1,000$ cells Number of susceptible bacteria = $2,000,000 - 1,000 = 1,999,000$ cells

Step 2: Calculate the number of survivors after antibiotic treatment.

Susceptible bacteria killed = 99.9% Resistant bacteria killed = 0%

Susceptible survivors = $1,999,000 \times (1 - 0.999) = 1,999,000 \times 0.001 = 1,999$ cells Resistant survivors = $1,000 \times (1 - 0) = 1,000$ cells

Step 3: Calculate the total number of surviving bacteria.

Total survivors = Susceptible survivors + Resistant survivors Total survivors = $1,999 + 1,000 = 2,999$ cells

Step 4: Calculate the frequency of resistant bacteria in the surviving population.

This frequency will be the frequency in the next generation, assuming the population size recovers.

Frequency of resistant bacteria = $\frac{\text{Number of resistant survivors}}{\text{Total number of survivors}}$ Frequency of resistant bacteria = $\frac{1,000}{2,999} \approx 0.333$

Final Answer:

The frequency of the resistance allele (and resistant bacteria) in the population after one generation of selection increases from an initial 0.0005 (0.05%) to approximately 0.333 (or 33.3%). This demonstrates the rapid effect of directional selection.