The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora? Jezkova and Wiens used a type of graph called a linear regression to find correlations between variables such as the proportion of species per phylum with legs (on the y-axis) and the diversification rate per phylum (on the x-axis). Sketch a graph to show what a strong positive correlation between these two variables would look like and what the absence of a correlation would look like.

Video transcript

Welcome back. Let's look at our next problem, assuming that we graph the proportion of species that have skeletons on the Y axis and the diversification rate per phylum X axis which of the following graph shows a negative correlation. Well let's recall more content videos and negative correlation means that when one variable increases the other decreases. So we see the two variables tending in opposite directions as opposed to size or um direction. So let's look at our four graphs that were provided and evaluate which one shows a negative correlation. Um We'll have to scroll downward a little bit to see them all but we'll start over here with choice A. And we see in this case as the diversification rate per phylum goes upward becomes greater. We also see an increase in the proportion of species that have skeletons. So both values increase at the same time. That's a positive correlation, not a negative correlation. So choice A. Is not correct. Choice B. When we look at the trend of these dots of this data, we see that as we go along the X axis, as the diversification rate for phylum increases, we see that the proportion of species that are skeletons is decreasing. So these two variables are going in opposite directions. One is increasing while the other is decreasing. So this is a negative correlation and therefore we'd expect choice B. Two B are correct answer but just to be thorough we're going to scroll downwards And look at our last two answer choices. Choice C. Here. So as we go along we can see that it's a less steep line, but we can see general trend in these these data points here that as the diversification rate per phylum go increases, the proportion of species that have skeletons is also slightly increasing. So there again we have another positive correlation. And finally in Choice D. It's difficult to tell. Our data points are kind of widely scattered, but you might be able to draw a trend line here in a general upward direction, but that would also show as the diversification rate goes up, the proportion of species with skeletons is also increasing. So another positive correlation here. So Choice B is our graph that shows the two variables going in opposite directions and therefore shows a negative correlation. See you in the next video.