The rapid evolution of urban hawksbeard (Crepis sancta) was examined by Cheptou et al. (2008). Pierre-Olivier Cheptou has a webpage at CEFE, including a description of Crepis sancta as a model system.
Seed head of wall lettuce (Mycelis muralis) with five achenes remaining
C olonization is a filter: Only certain individuals may have the right combination of traits to cross a significant barrier and inhabit a new place. But the traits that enable individuals to make such crossings are not always very good for staying in the new place.
Brain-body allometry revisited across mammals
Cheptou and his coworkers were able to go farther than Cody and Overton: They planted the seeds from urban patches in a greenhouse alongside seeds from the rural areas outside of town. The urban/rural differences were still present in the greenhouse plants, showing that growing in a small patch is not enough to change the seeds. Instead, the differences are caused by differences in genes, the result of selection on the urban plants.
Wall lettuce populations that manage to stick around on an island make a change. After ten years, they are producing achenes just as big as the mainland populations. But instead of big, fluffy parachutes, they grow smaller, stunted ones. Like green army men tied to smaller and smaller postage stamps, these seeds aren’t made for dispersal. They drop fast, right next door to their parents.
Many readers may notice that there is another factor besides low dispersal potential by which selection might favor large achenes in these plant species. Larger seeds may be advantageous because, with more stored energy, they may tend to germinate more readily or grow more quickly, thus making them more competitive. In this circumstance, selection would pertain to energy allocation or reproductive effort, rather than dispersal. I’ll be discussing that form of selection in a later essay. In the present cases, dispersal is indicated as the target of selection, but seed size and energy allocation may also be involved.
Cox G. 2004. Alien Species and Evolution. Island Press, Washington.
Enjoying warm locations, dandelions (Taraxacum officinale) grow roots as deep as 15 feet into the soil, if they remain undisturbed. Often considered a weed in lawns and flowerbeds, these rapidly growing plants are hardy in U.S. Department of Agriculture plant hardiness zones 3 through10. The white floaties that the dandelion produces are specialized seeds that are highly successful for widespread reproduction.
The white floaties provide widespread dandelion populations since they fly far distances, especially if the wind is strong enough. In fact, successfully grown dandelion roots help your soil remain aerated. As the roots grow deeply, they reduce soil compaction by creating air and moisture pockets underground. As a result, other tender plant roots have a chance to move into the aerated soil for ample foliage and stem growth. The dandelion taproot also increases nutrients in the shallow topsoil by moving critical elements, like calcium, from the deeper ground regions. Overall, successful dandelion seeds and seedlings create a fertile environment for all plant growth.
Because many dandelions find a good growing location in lawn areas, wind gusts often disperse the seed parachutes throughout the area. The umbrella hairs lift the seed from the head and float along the breeze. The extremely lightweight seed can float as far as the wind allows. Once dropped into another soil location, these seeds do not have extensive dormant periods like other plant species. In fact, the seed germinates quickly to establish itself in the new location before plant competition takes over for natural resources, such as moisture and sunlight.
The white floaties originate from a densely packed seed head that resembles a fuzzy ball. If you look closely, each seed head has dozens of umbrella-like extensions. Located at the seed head’s center are the seeds — each seed has this umbrella structure attached to them. The umbrella’s canopy consists of hairs formed much like a chimney sweep brush. Combining both a tall stem and airy seed head, dandelions keep their seeds upright and available to wind vectors for successful distribution in the region.
Before the specialized seeds appear, dandelions generate a yellow to orange flower on a stem that can rise up to 18 inches from the ground. This flower appears bright and fluffy against its green background, but is not a large or particularly appealing blossom for insect attraction. Requiring no pollinators, dandelions are self-pollinating and often change from flower to seed head over several days. This rapid seeding ability makes dandelions extremely successful at populating a widespread area — gardeners cannot keep up with the constant growth.