Branching out all over
A page from Darwin's journals shows an early phylogenetic tree.
Combination is only one tactic. Another of life's short cuts is repetition. "Go home and look in the mirror," Weiss said. "Look at your teeth, your hair, your hands. We are built by repetitive patterning. I call this biological programming in installments—repeatedly making the same unit. This is found in all levels of life, from gene to cell to organ. Life is a profusion of modules."
Still another basic strategy for building complexity is branching, which Weiss called "probably the most powerful single metaphor of life." "It's a very simple, logical idea," he acknowledged. "Take one thing, split it into two, and they can then become different."
Darwin was big on branching, of course: How could he not be, when it popped up everywhere he looked? Not just in family trees, but in real ones too, and in fingers and toes, plant roots and candelabras of coral. And the same forms show upinside organisms—in blood vessels, nerves, and the bronchioles of the lungs.
But according to Weiss, it's even more basic than that. "Multicellular organisms begin their lives organized as balls or tubes," he writes. "[They] become more complex through branching"—whether that means splitting off repeat units as in blood vessels and roots, or producing hierarchies, like the digestive organs that branch from the developing gut or the distinctive regions of the brain.
Branching, he continued, allows for yet another developmental gambit: asymmetry. Here a branch of undifferentiated cells is exposed to a quantity of signaling factor at one end which slowly diffuses to the other, the concentration growing less and less as the chemical spreads. These cells, because of the receptors on their membranes, detect the concentration outside, and have mechanisms in their genome that, depending on that concentration, will switch them into different functional behaviors by turning on different genes.
Vertebrates and insects both use this mechanism to turn quantities of signaling factors into qualities of structure, Weiss said—"different limb bones, or wing parts. The same system is used to produce the segmented bodies of insects." Although asymmetry may be less evident than symmetry in the mature organism, its role in development is significant.
Evo-Devo
Ken WeissPhotograph by James Collins
The fresh perspective of developmental biology offers some powerful new generalizations for understanding how life evolves. "It's a lot simpler than having to have a specific gene for every structure, which is what we thought genetics was all about," Weiss said. "At the same time, we can't forget that evolution produces variation, among individuals within a species as well as among species. Development never comes out exactly the same from organism to organism."
He flashed a slide of a complicated flow chart, a dense nest of boxes and connecting lines depicting what is known about the regulation of a single trait—the formation of a single type of cell in a particular variety of sea urchin. "It shows a process made up of multiple gene interactions," Weiss said. "Each of these genes is a target for mutation. Each can accumulate variation. This is the opposite of simplicity, because mutations happen randomly.
"One of the great lessons of early-twentieth-century biology," he added, "was that there can be many genetic pathways to the same result. You being six feet tall and me being six feet tall will not be due to us sharing the exact same genotype, because no two people have the exact same genotype." At the species level, "teeth take similar form in mammals and in fish, but they are made in each case by independently evolved genes.
"Development and evolution are different faces of the same phenomena," Weiss concluded. "Evolution shows how diversity arises among organisms, development how diversity arises within organisms. They work together, but on different timescales: development a few weeks, evolution millions of years."
"With elegant simplicity and symmetry, history and function—evolution and development—are one," he writes. "They are facilitated by genes through the same modular, nested, branching logic of life."
Kenneth M.Weiss, Ph.D., is Evan Pugh professor of biological anthropology and genetics, and can be reached at kenweiss@psu.edu. This article is based on a talk given by Weiss as one of the 2006 Penn State Lectures on the Frontiers of Science. The annual series is sponsored by the pharmaceutical company Pfizer, Inc., and Penn State's Eberly College of Science, and organized by Barbara K. Kennedy, coordinator of college relations.