Background Essay: Drift Seeds And Drift Fruits: Seeds That Ride The Ocean Currents
When it comes to evolution, reproduction is more important than life itself. Even if an organism lives a very long life, it contributes nothing to the evolution or the long-term survival of its species unless it passes its genes on to the next generation.
Many plants reproduce through the process of sexual reproduction. Pollen grains from the flowers of one plant fertilize the eggs of another plant's flowers. These fertilized eggs, or zygotes, then develop into embryos, the nascent offspring inside a plant's seeds.
Unlike animals, plants can't just get up and move to a better location. If conditions are crowded or the soil is poor, individual plants must simply endure. The same is not necessarily true for their seeds, however. Plants can disperse their seeds far and wide, a process that prevents offspring from having to compete with the parent plant for light, water, and nutrients, and one that in some cases helps to establish new populations of the species elsewhere in the world.
Wind and water are probably the two most effective modes of transport used by plants to disperse their seeds. A single gust of wind can carry hundreds of thousands of dandelion seeds, for example, from one field to the next. Similarly, ocean currents can transport plants thousands of miles, from one continent to another. Other types of plants use animals to transport their seeds to new locations -- either in their digestive tracts or on their hair or fur.
Like many traits that prove to be advantageous to an organism, most physical adaptations that allow seeds to float thousands of miles on ocean currents probably began as fortunate accidents. Perhaps one individual plant had a mutation that resulted in a tougher or waxier protective covering (seed coat) that made the seed less permeable to water. This slightly altered seed coat may have allowed some of that plant's seeds to drift short distances -- a few miles downstream or a few miles offshore to nearby islands. Gradually, over thousands of plant generations, if the advantage of water dispersal was great enough, some plants would have evolved seeds with increasingly tough or waxy seed coats -- coverings so resistant to water that they would have the potential to drift around the world.