Chronoclines

kw: evolutionary theory

A cline is a gradual variation in something. For example, the thermocline is a change of temperature in the ocean, and is used specifically to refer to a range of depth in which the temperature change occurs at a greater rate, or in a reverse direction, than in most of the ocean.

A chronocline is a change over time, a change of some measurable character. For example, the average life span in America had changed from about 35 to more than 70, in the past 200 years. Much of this change occurred in the 20^th Century, because of improved public health and the development of effective antibiotics. One could call the past 100 years a cline in American life span.

In evolutionary theory, clines are an important concept. The factors that underlie evolutionary change are

• There is variation within a species and new variations arise continually through mutation,
  
• Some varieties leave more offspring than others, and
  
• Most individuals die without leaving offspring.

Only for humans and for some domestic animals do a majority of the members of a species leave offspring. With the three factors listed above, we would expect that a species will be either well adapted to its environment, or that the mix of varieties will change over a few generations to improve a mismatch. For example, if the average climatic temperature falls by a few degrees (the beginning of an ice age, perhaps), we might expect that the fur of mice or lemmings will get thicker and the animals will tend to get fatter in the Fall (or the animals may die out in that area if the change is too great for them to adapt).

Looked at another way, in any population of mice, some individuals have thicker fur than others due to natural variation. In a stable environment, the "average" amount of fur is the best, and large variations tend to get weeded out. Nonetheless, there is some natural range. Now, suppose we have that drop in temperature. Previously, the mice with the thickest fur were a little more poorly adapted than the "average" mouse. Now, they find themselves with an advantage. They survive winter better, and are the ones leaving the most offspring. Within a few generations, there is a new average, and those whose fur matches the former average are now somewhat disfavored.

If temperature changes gradually over many generations of mice, we might find that, when the environment stabilizes, the range of fur thickness is completely different from what it was earlier. The cline in temperature was followed by a cline in fur (and other characteristics, no doubt). Now, were you to compare a group of mice collected "today" with a group collected before the temperature changed, you might think they are a different species. But if groups of mice were collected from every generation, a gradual change would be seen.

Now, suppose we are talking about mouse fossils, and all this happened thousands or millions of years ago. Fossils are poor samples of the biodiversity of an area. But if we get enough samples, it is sometimes possible to trace variations like that described. However, fur doesn't fossilize except very, very rarely. Bones and shells fossilize, so things that affect the shapes of bones or shells are what we have to work with.

Disclaimer, this is a made-up example, for simplicity. It is based on a real example described by one of my geology professors.

This cliff is like many limestone cliffs, with various layers that weather differently. The limestone in the middle has a more resistant layer above, and a less resistant layer below, so we get an overhang. Within the middle layer, there is also a gradual change in the amount of silt and clay from bottom to top.

If you were to collect fossils from the three spots at the ends of the arrows, you might find shells such as those shown. Ideally, you'd collect a few dozen shells at each level so you could study variation within each population. Then, that population with 19 ribs might be seen to vary between 16 and 22 ribs, though 80% of the shells collected have either 18, 19, or 20.

Side note: the next ten times you eat an orange, first count the segments. Clementines (Mandarin oranges), for example, have an average of 11 segments, but the range, in my experience, is from 8 to 13.

Now, if you stop with three collections, you might conclude that these shells represent three species of Pecten (scallop). However, what do you think you'll find if you collect at two more points, one above the middle and one below; or if you collect a dozen shells at each foot of height up this layer (perhaps twenty collections)?

If this represents a cline, you'd find that the average would vary along a trend. Then instead of three species, there would actually be one clinal species, a chronocline. Now, the question to ask is, how common is such a situation? It is actually rather rare, for two principal reasons. Firstly, it is rare to have a continuous exposure of fossil-bearing rock that encompasses a slow, gradual change in environment. Secondly, what looks like a climate-cline may really represent the sideways shift of an environmental boundary area.

For example, if one beach is mainly sandy and down the beach a mile you have mainly mud, somewhere in the middle there will be a mixed sediment. If a growing delta gradually mixes in one sediment, a vertical section in the middle will appear to be clinal, and in a sense it is. But the shells found in the sediment will probably not form a chronocline. Instead, one kind of shell is best adapted to sand, and another to mud. If mud encroaches, you'll get gradual invasion of the second kind of shell, while the first kind retreats to sandy areas. For a chronocline to form, you need a change of environment that is widespread enough that the critters don't have anywhere to go, and it must be slow enough that the population can shift. Even then, a population will tend to remain stable until a large proportion of its members are overstressed by the change. The smaller a population is, the more rapidly it can generate new varieties and the more quickly favorable new varieties can take over.

To summarize: a chronocline illustrates how one species might gradually change into another, but it is rather arbitrary where to put the boundary. Environments and fossil formation seldom "cooperate" so as to leave a clinal series of fossils. A cline is one set of evidence that evolutionary change is driven by natural selection.