Did the effects of natural selection occur in a lizard population in response to an extreme cold event?^[1]                                (Google Slides Version)

During the winter of 2013-2014, southeastern Texas and Oklahoma faced an extreme cold event. In these areas, the wild populations of the green anole lizard, Anolis carolinensis live naturally. Cold tolerance in this species naturally varies with latitude. Lizards have an internal thermal temperature (called the CTmin) at which they cannot function and lose coordination. During the cold weather event, lizards in southernmost Texas (BRO) experienced 28 days below their CTmin, while those in northern Texas (AUS) experienced 8 days.

The researchers hypothesized that the extreme cold may exert natural selection on these populations, eliminating less cold-hardy individuals. They  investigated whether survivors of the 2013–2014 winter storms displayed greater cold tolerance than individuals sampled the previous year. They revisited BRO and AUS in April 2014 to measure CTmin of the survivors. 

Figure A (rightmost) shows the critical thermal minimum (CTmin) for anole lizards from Summer 2013 to Summer 2014. The extreme cold event is designated by “2013-2014 winter storms”. The Upper line shows CTmin for southern lizards (BRO) and the lower line for northern lizards (AUS). Error bars are +/- 1 SEM).

Evidence from the study (Figure A - right) showed that BRO populations had a high CTmin  before the cold event (CTmin of 10) and the CTmin  progressively decreased to 8.8 and 8.5 CTmin  in the spring 2014 and summer 2014 respectively. The AUS populations showed little change from the initial CTmin value of 8.4 over the year. In summer 2014, both BRO and AUS populations showed relatively the same CTmin  (8.6).

Scientists also looked at the genes that regulate the CTmin  in anole lizards. They found that the survivors of the cold snap in the BRO location were genetically similar to those in the northernmost population (HOD).

Figures D-F: Bar plots representing the number of genes that shift in expression in the southernmost population (BRO) after the storm in the same direction as gene expression differences between BRO and the northernmost site (HOD).

Question 1:  Did the effects of natural selection occur in a lizard population in response to an extreme cold event?^[2]

CLAIM:  

EVIDENCE:

REASONING:

Question 2:   Does natural selection always lead to evolution? Use the information in this activity to answer that question. What more might we need to know to fully answer it?