A population of the marine snail Littorina littorea\textit{Littorina littorea}Littorina littorea exhibits phenotypic variation in shell thickness, which is a polygenic trait. In a rocky shore habitat, two distinct selective forces act on this population:
- Predatory crabs (Carcinus maenas\textit{Carcinus maenas}Carcinus maenas) crush and eat snails with thin shells.
- The high energetic cost of producing very thick shells reduces the resources available for egg production, lowering the fecundity (reproductive output) of thick-shelled snails.
The graph below shows the distribution of shell thickness in this snail population before and after several generations under these selective forces.
Which of the following statements correctly identifies the type of selection occurring and describes its effect on the genetics of the population?
Disruptive selection is occurring. Because two separate selection pressures (crab predation and energetic cost) are acting simultaneously, the population is driven to split into two distinct phenotypes.
Directional selection is occurring. In response to the dual environmental pressures, individual snails adaptively alter their gene expression during their lifetimes to produce intermediate shells, shifting the population mean.
Stabilising selection is occurring. Over generations, the frequency of alleles contributing to extreme phenotypes decreases, reducing phenotypic variation while the mean phenotype remains unchanged.
Stabilising selection is occurring. To ensure survival of the species under dual selection pressures, snails with extreme phenotypes undergo directed mutations to produce offspring with intermediate shell thickness.