Observe on Your Own


The clues to evolution are in every backyard, every biome. Try this activity. Find 100 of anything (well, just about anything*) of the same species—leaves, shells, beetle carapaces, cat footprints, human handprints, snakes or snails or puppy dog tails. Measure any single parameter (characteristic) of the collection. Then group the measurements into categories (about 9 usually works well) to create a frequency histogram.


Suppose I collected 100 pine cones, and categorized them by length (to the nearest centimeter.) I could find a few very tiny cones. (Maybe the tree had some genetic defect!) A few may be huge. Most pretty average—about 5 cm (~2 inches). Grouped to the nearest centimeter, the graph looks like a “bell.”


Now suppose there had been a parasitic insect that infected and destroyed pine cones as they grew. Imagine the insect was relatively large, and had preferentially destroyed the largest cones. Then if I collected 100, I might get a skewed graph like this.  If the factors that determined the size of cones were genetic, then I might expect the average cone size in a decade to be smaller.


These are very much like the observations of Charles Darwin (1809-1882), who sailed round the world on H.M.S. Beagle from 1831 to 1836. He saw variation, and made the assumption that whatever caused this variation could be passed on to offspring. When he looked at a population, he imagined a collection of “messages” for various traits; when a variation reproduced that message survived. Die (or fail to reproduce) and it did not. He assumed overreproduction, competition, and survival of the fittest. You’ll learn more about Darwin, and particularly his observations in South America, in the next section. But you can probably already see that the key to defending his model of evolution by natural selection was understanding how traits were passed on. And, in fact, he didn’t!

 *To make this work you can’t pick a trait that’s determined by only one gene. But you’ll see in the next section that’s very, very rare. And you shouldn’t pick an organism that reproduces asexually like a dandelion or a hydra. You’ll see why sex is so important later on, as well. This is part of the week 2 assignment for this course.


Mendel Mania?


Gregor Mendel (1822-1884) was a monk. For a relatively short period of his life, he studied the genetics of garden peas and took (we assume) copious notes on his research. He presented a single paper to the scientific society of Brünn, (in what was then Austria)  in 1866. Then he became abbot and stopped studying peas, and when he died, every single record of his work was destroyed except the paper in the library at Brünn. His work remained undiscovered and unnoticed until 1896.


No biologist underestimates the revolutionary influence that Gregor Mendel’s single scientific paper had on our understanding of how traits are passed on through generations. But it’s important to remember that his seminal work was done almost 150 years ago, and our understanding is far greater today. Before Mendel, most people believed that traits were passed “in the blood.” Some cultures believed that only the male influenced the offspring, some that miniature adults were contained in the sperm. They also believed that different traits (like blue and brown eyes) could “blend” in the body. Mendel’s most remarkable discovery was that heredity was a function of “unit characters” (genes), which were found in equal measure in the gametes (egg and sperm) of both sexes. He imagined that a population of organisms carried a collection of these “units” and that those that reproduced passed them on. He proposed that some of the units had stronger (more dominant) messages, and could mask other messages for a generation but didn’t blend. His work formed the basis for the science of genetics and that of population biology, in which probabilities are used to predict future characteristics of populations based upon which organisms reproduce successfully.


It’s important not to limit your understanding of population genetics to Mendel’s model, though. Mendel culled his data to find examples of traits that were determined by only one pair of traits. We now know that’s very, very rare. You might have studied attached earlobes or bent little fingers in high school. But most traits are determined by combinations of genes, and influenced by both genetic control mechanisms and the environment.


Think of the graphs above. If there were only two forms of a trait (as Mendel studied), or even three (including a blended form) the graph couldn’t form a “bell” or “normal curve.” At very most, it would have three categories. Instead, most of the traits we measure are pretty much continuous, or at least display a wide range of variations. Think of human skin color. Some researchers think that at least ten sets of genes contribute to the appearance of our skin. So the range of skin colors (even without tanning or vitamins) is very broad. The range of colors in many flowers may once have been broad, as well. But in the bug-eat-plant world of the rainforest, some colors are far more likely to do the job of attracting pollinators. Selection occurs in the individual genes that contribute to this complex trait. Change can occur—but usually, a little at a time, and in response to changes in the number and types of insects or predators, a phenomenon called co-evolution.