IB Biology SL D4.1.4 Abiotic Factors As Selection Pressures Questions

PanI is a gene in cod fish that codes for an integral membrane protein called pantophysin. Two alleles of the gene, $\operatorname{PanI}{ }^{\mathrm{A}}$ and $\operatorname{PanI}{ }^{\mathrm{B}}$, code for versions of pantophysin, that differ by four amino acids in one region of the protein. Samples of cod fish were collected from 23 locations in the north Atlantic and were tested to find the proportions of PanI ${ }^{\mathrm{A}}$ and PanI ${ }^{\mathrm{B}}$ alleles in each population. The results are shown in pie charts, numbered 1-23, on the map below. The proportions of alleles in a population are called the allele frequencies. The frequency of an allele can vary from 0.0 to 1.0 . The light grey sectors of the pie charts show the allele frequency of PanI ${ }^{\mathrm{A}}$ and the black sectors show the allele frequency of PanI ${ }^{\mathrm{B}}$.

The sites close to Iceland, at a latitude of $60-65^{\circ}$, had very varied allele frequencies, with both PanI ${ }^{\mathrm{A}}$ and PanI ${ }^{\mathrm{B}}$ occurring. The water at these sample sites was highly stratified, with much warmer water at the surface and much colder water below. Suggest reasons for both $\operatorname{PanI}^{\mathrm{A}}$ and $\operatorname{PanI}^{\mathrm{B}}$ alleles occurring at these sites.