Question

Lizards in the Cold

Data Point Educator Materials

IMME

Caption: Figure 1A shows the locations of the five anole lizard populations in the study. Figure 2A shows the mean CTmin (critical thermal minimum, the temperature at which lizards lose their coordination) over time for two of the populations. Figure 2B compares the mean CTmin values for all five populations in the summers of 2013 (closed circles) and 2014 (open circles). Asterisks indicate that the CTmin for a population was significantly lower in 2014. Error bars represent the standard error of the mean (SEM).

BACKGROUND INFORMATION

Extreme climate events, such as droughts or storms, can drive evolutionary changes in populations. One such event was the winter storms of 2013 to 2014 in the southern United States, which caused some of the area’s coldest temperatures in the last 15 years. Scientists investigated how these extremely cold temperatures affected local populations of anole lizards. These lizards’ ability to tolerate cold is an inherited trait. The scientists thought that the extremely cold winter could cause natural selection on this trait.

The scientists studied five anole lizard populations in different locations: four in Texas and one in Oklahoma. In the past, the southern locations usually had warmer, milder winters than the northern ones. Before the winter storms, the scientists sampled lizards from each population. They determined how well these lizards could tolerate cold by measuring their critical thermal minimum (CTmin), the temperature at which the lizards lost their coordination. (A lizard with a lower CTmin may be better able to move when it is cold.) After the winter storms, the scientists sampled the surviving lizards from each population. They compared the mean CTmin values of the samples taken before the storms (from the initial populations) to those of the samples taken after the storms (from the winter survivors).

INTERPRETING THE FIGURES

Figure 1A shows that the five populations of anoles (Anolis carolinensis) in the study were located along a latitudinal transect. The scientists had previously found that cold tolerance in this species varies with latitude (the anoles located farther north, where winters are typically colder, have greater cold tolerance), and that this variation may have a genetic component. They hypothesized that the extreme cold snap during the winter of 2013 to 2014 selected for more cold-tolerant lizards, and that selection depended on how much cold stress the lizards experienced.

Figure 2A compares two of the five populations: the southernmost population in Brownsville, Texas (abbreviated BRO), and a more northern population in Austin, Texas (abbreviated AUS). This figure shows the cold tolerance of each population, as represented by the critical thermal minimum (CTmin), at three different time points:

1. Summer 2013: At this point, the BRO population had a significantly higher CTmin, indicating a lower cold tolerance, than the AUS population did. During the following winter, minimum daily temperatures dropped throughout the study area. The BRO population experienced considerably more cold stress than the AUS population did, as there were many more days with temperatures below the BRO CTmin (see Figure 1c in the original paper for more data).
2. Spring 2014: By this point, the CTmin of the BRO population had significantly decreased, indicating increased cold tolerance. The CTmin of the AUS population, on the other hand, did not change significantly.
3. Summer 2014: The CTmin of the BRO population remained low, suggesting that its decrease was due to evolutionary change through natural selection rather than phenotypic plasticity (the same genotype producing different phenotypes based on environmental conditions). The BRO and AUS error bars also overlap at this point, suggesting that the two populations had become statistically similar with regard to cold tolerance.

A common misconception is that evolution involves individual organisms changing over time, so it is important to note that the lizards sampled after the cold snap were most likely not the same lizards that were sampled before. The figures reflect comparisons between individuals from the initial populations and individuals that survived the winter storms, not comparisons of the same individuals over time.

Figure 2B compares the cold tolerance of all five populations in summer 2013 (before the cold snap) and summer 2014 (after the cold snap). As shown, only the two southernmost populations, BRO and VIC, experienced a significant increase in cold tolerance (decrease in CTmin). The other, more northern populations did not change significantly, presumably because they were already adapted to harsher winters. The overlapping error bars for the populations after the cold snap suggest that all five populations ultimately become more similar with regard to cold tolerance.

As discussed in the original paper, the scientists also analyzed gene expression profiles for the populations in the study. They found that the southernmost population, BRO, had significant differences in 14 genomic regions before and after the cold snap. The BRO lizards sampled after the cold snap (the winter storm survivors) had gene expression levels

more similar to those of the northernmost population, HOD, than the BRO lizards sampled before the cold snap (the initial population) did. (The gene expression profiles of the non-BRO populations did not change as much, suggesting that those populations were less affected by the winter storms.) In addition, some of the same genomic regions that differed between the BRO winter storm survivors and the initial BRO population also differed between the BRO and HOD lizards. These results suggest that winter storms and latitudinal variation in winter cold (i.e., colder winters farther north) may have similar effects on anole populations.

DISCUSSION QUESTIONS

1* Consider the locations of the five populations on the map (Figure 1A). What might have been some of the differences among the lizards in these populations before the winter of 2013 to 2014?

2* Explain in your own words what a critical thermal minimum (CTmin) is. Why do you think the scientists used CTmin to measure the cold tolerance of the lizards? Can you think of other measurements that the scientists could have used instead?

3* If the winter storms of 2013 to 2014 were a rare event that does not occur again, what do you predict might happen to the CTmin values of these populations over time?

Answer 1

1. The main difference of the lizard population from before the winter of 2013 to 2014 are

• There was not much difference in the lizards from northernmost population because they were already cold tolerant.
• But the lizards taken from southernmost population had a difference in their 14 genomic regions after the cold storm. Which made them almost similar to population of anoles from northern region.
• So they have become more cold resistant.

1. Critical thermal minimum - they are the least or minimum temperature that a particular temperature it can withstand and above which disorganisation of the animals body takes place. And thermal minimum is selected because cold tolerance is checked in here and below this threshold the animal may die and without critical thermal minimum the changes in genome will not take place as it doesn't pressurizze the animals for the change.

It's hard to check for other options to check for tolerance.

1. If the winter storms were a rare event then their might not be any changes in the genomic regions and natural selection or genetic drift inorder to adapt into it. So the tolerance decreases so the critical minimum thermal value increases into higher number or becomes gradually increased.