This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Previously - Evolution's Tempo, Movement I: Adagio
Small changes can have big effects. The mood of a piece is set by is tempo. You can play the same notes, in the same style but change the speed at which you play it and it takes on an entirely different meaning. Still, some changes are so small that you cannot perceive it to have any meaningful impact. Slow down the Allegro from Mozart's Eine Kleine Nachtmusik by one beat per second and you will not likely notice any difference - the feeling still remains vivace.
Now repeat this change with each listen. You may gradually become more accustomed to the slower tempos and may not even notice how these small, imperceptible changes affect your mood. As the tempo decreases a song once vivace turns moderato, perhaps larghissimo. Over time, the summation of the changes can be dramatic.
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One of the key characteristics in the Great Song of Evolution is differential reproductive success. Some live and some die. Of those that live, some reproduce while some don't. It is not so much of a struggle for existence as a fact of life - nothing lasts forever. Births are the crescendo of populations. Kept unchecked they increase al forte, but population growth is never unchecked. The decrescendo, or deaths, attempts to balance out the population's volume.
Ecology has often taken a very simplified approach: counting the notes without paying attention to the key the music is played in. Adding evolution to the ecology is akin to introducing a world class composer to an eighth grade orchestra. It works the other way, too. In each passing generation, only a subset of all living things confers its most prized possessions - the genes. The confluence of evolution and ecology is powerful and masterful. The works of humankind merely pass through the wake of the selective maestro.
The intimate relationship of organisms' form and function over only a few generations is shaped by a wide variety of interactions - many of which are out of the control of the organism. One must remember, though, that while soloists live and die by the song, the orchestra moves on. Since populations are groups of organisms that interbreed and exchange genes, any arrangement of genotypes is available in a population at any given time. Potentially adaptive genes may reside in low abundance until conditions favor their phenotype, like the french horn section waiting for its ripieno moment in a baroque march.
For example, take the unseemly stickleback (drawing above), which recently has emerged as a model organism for studying evolution at an allegro pace. They reside both in the ocean (e.g. top fish: larger, with spines and plates) and in freshwater lakes and streams (e.g. bottom fish: not as heavily armored). The differences between the two are stark. Yet, in only a handful of generations can one turn into the other.
Sticklebacks are anadromous fish, meaning they spend part of their lives in both freshwater and in the ocean. More familiar examples of this are salmon, who swim upstream in freshwater rivers to spawn their young in its shallow, protective confines. If, per chance, the salmon stayed in the ocean to spawn then filter-feeding fish and whales would swim fin over tail to gulp the egg masses down before the larvae's heart had beat even a quarter-note.
That is not to say that shallow headwaters of freshwater rivers and lakes are without their dangers, but their chances improve greatly enough that this life cycle is favored. For some ancient populations of the stickleback, though, they stuck around the lake or managed to get cut off from the route to the ocean. The young smelt made their living in the freshwater, eventually establishing stable populations.
Life in the lake is much different than life in sea. To an oceanic fish, freshwater predators must seem quite ornery. Dragonfly larvae attacking from the lake bottom by grabbing onto the spines under belly, versus the larger fish that gave chase in the open sea. It makes sense then those fish in lakes with smaller belly spines survived attacks more often.
Remarkably, what we see is that the transition is astonishingly staccato! An abrupt change in frequency of fossil with more armor and spines to less (see video above). But it is not a complete and permanent change since the genes for this adaptation are still prevalent in the population and eventually we see various hybrid morphs (see image at top and video). That is, the gene variants for each phenotype exist in the population irrespective of their frequencies. This genetic variation is key to a population's ability to rapidly evolve.
Astonishingly, in just half a century a reversal occurred just outside of Seattle! In the 1960s it was realized that Lake Washington was literally a cesspool of filth and pollution from decades of neglect, runoff and outright pollution. Once called "Lake Stinko", the city, University of Washington and advocacy groups spent $140 million to clean it up in what University of Michigan professor John Lehman hailed in a NRC report as "an example of creative interaction between the scientific community and the political arena".
Fast forward to 2006 and we have a very striking example of rapid reverse in evolution to the armored phenotype (figure on right)! It is thought that the much improved water clarity resulted in the necessity for better armor and larger size against pelagic lake predators, such as cutthroat trout.
Rapid evolution is not isolated only to sticklebacks. We have inferred, and even directly observed, changes in populations over very short periods in a wide variety of organisms. For instance,
Periwinkle snails (Littorina obtusata) on the northeastern seaboard in 1870s were were significantly taller and had thicker shells, yet those same population in 1980s were much flatter and thicker. In the early part of the 1900s, the intertidal crab (Carcinus maenas) expanded it's range northwards. Clearly, those snails whose shells were more difficult to handle lived.
Aphids, a common commercial crop pest, were shown to evolve rapidly even in a single experimental planting season. Experimental populations containing two genetically distinct clonal lines with different growth rates (i.e. potentially evolving) were shown to grow up to 42% faster and attained up to 67% more density than populations consisting of single clonal lines of aphids (not evolving) in the presence of competitors or predators. With even minimal genetic variation, evolution - as measured by changes in allele frequencies in a population - still proceeded by only means of natural selection.
Plants and pollinators form a highly specialized bond, often strengthened through thousands of years of coevolution. But, what happens when a plant loses its pollinator? Does it wither away, reproduce with itself? In the wild Mimulus plants, experiments show that when pollinators are removed the plant initially suffers in terms of it's fitness but quickly rebounds within 5 generations. There are also morphological changes as well as changes in genes associated with improved self-fertilization abilities.
In addition to ecological examples, there are many more from rapidly evolving reproductive proteins, antibacterial and pesticide resistance, to pathogens evolving resistance to host defenses. This is the hallmark of a star performer: the ability to improvise in a world of change. There are no better performers than the diversity of life embedded within the natural orchestra that surrounds us.
Bodbyl Roels, S., & Kelly, J. (2011). RAPID EVOLUTION CAUSED BY POLLINATOR LOSS IN MIMULUS GUTTATUS Evolution, 65 (9), 2541-2552 DOI: 10.1111/j.1558-5646.2011.01326.x
Kitano, J., Bolnick, D., Beauchamp, D., Mazur, M., Mori, S., Nakano, T., & Peichel, C. (2008). Reverse Evolution of Armor Plates in the Threespine Stickleback Current Biology, 18 (10), 769-774 DOI: 10.1016/j.cub.2008.04.027
Seeley, R. (1986). Intense Natural Selection Caused a Rapid Morphological Transition in a Living Marine Snail Proceedings of the National Academy of Sciences, 83 (18), 6897-6901 DOI: 10.1073/pnas.83.18.6897
Turcotte, M., Reznick, D., & Hare, J. (2011). The impact of rapid evolution on population dynamics in the wild: experimental test of eco-evolutionary dynamics Ecology Letters, 14 (11), 1084-1092 DOI: 10.1111/j.1461-0248.2011.01676.x