Nature encourages no looseness, pardons no errors |
light moth | dark moth | |
non-industrial woods | 14.6 % | 4.7 % |
industrial woods | 13 % | 27.5 % |
Local Adaptation - More Examples
So far in today's lecture we have emphasized that natural selection is the cornerstone of evolutionary theory. It provides the mechanism for adaptive change. Any change in the environment (such as a change in the background color of the tree trunk that you roost on) is likely to lead to local adaptation. Any widespread population is likely to experience different environmental conditions in different parts of its range. As a consequence it will soon consist of a number of sub-populations that differ slightly, or even considerably.
The following are examples that illustrate the adaptation of populations to local conditions.
- The rat snake, Elaphe obsoleta, has recognizably different populations in different locales of eastern North America (see Figure 3). Whether these should be called geographic "races" or subspecies is debatable. These populations all comprise one species, because mating can occur between adjacent populations, causing the species to share a common gene pool (see the lecture on speciation).
Figure 3: Subspecies of the rat snake Elaphe obsoleta, which interbreed where their ranges meet.
- Galapagos finches are the famous example from Darwin's voyage. Each island of the Galapagos that Darwin visited had its own kind of finch (14 in all), found nowhere else in the world. Some had beaks adapted for eating large seeds, others for small seeds, some had parrot-like beaks for feeding on buds and fruits, and some had slender beaks for feeding on small insects (see Figure 4). One used a thorn to probe for insect larvae in wood, like some woodpeckers do. (Six were ground-dwellers, and eight were tree finches.) (This diversification into different ecological roles, or niches, is thought to be necessary to permit the coexistence of multiple species, a topic we will examined in a later lecture.) To Darwin, it appeared that each was slightly modified from an original colonist, probably the finch on the mainland of South America, some 600 miles to the east. It is probable that adaptive radiation led to the formation of so many species because other birds were few or absent, leaving empty niches to fill; and because the numerous islands of the Galapagos provided ample opportunity for geographic isolation.
Figure 4
Stabilizing, Directional, and Diversifying Selection
Finally, we will look at a statistical way of thinking about selection. Suppose that each population can be portrayed as a frequency distribution for some trait -- beak size, for instance. Note again that variation in a trait is the critical raw material for evolution to occur.
What will the frequency distribution look like in the next generation?
Figures 5a-c
First, the proportion of individuals with each value of the trait (size of beak, or body weight) might be exactly the same. Second, there may be directional change in just one direction. Third (and with such rarity that its existence is debatable), there might be simultaneous change in both directions (e.g. both larger and smaller beaks are favored, at the expense of those of intermediate size). Figures 5a-c capture these three major categories of natural selection.
Figure 6
Under stabilizing selection, extreme varieties from both ends of the frequency distribution are eliminated. The frequency distribution looks exactly as it did in the generation before (see Figure 5a). Probably this is the most common form of natural selection, and we often mistake it for no selection. A real-life example is that of birth weight of human babies (see Figure 6).
Under directional selection, individuals at one end of the distribution of beak sizes do especially well, and so the frequency distribution of the trait in the subsequent generation is shifted from where it was in the parental generation (see Figure 5b). This is what we usually think of as natural selection. Industrial melanism was such an example.
Figure 7
The fossil lineage of the horse provides a remarkable demonstration of directional succession. The full lineage is quite complicated and is not just a simple line from the tiny dawn horse Hyracotherium of the early Eocene, to today's familiar Equus. Overall, though, the horse has evolved from a small-bodied ancestor built for moving through woodlands and thickets to its long- legged descendent built for speed on the open grassland. This evolution has involved well- documented changes in teeth, leg length, and toe structure (see Figure 7).
Under diversifying (disruptive) selection, both extremes are favored at the expense of intermediate varieties (see Figure 5c). This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation (see the lecture on speciation).
Summary
Darwin's theory of evolution fundamentally changed the direction of future scientific thought, though it was built on a growing body of thought that began to question prior ideas about the natural world.
The core of Darwin's theory is natural selection, a process that occurs over successive generations and is defined as the differential reproduction of genotypes.
Natural selection requires heritable variation in a given trait, and differential survival and reproduction associated with possession of that trait.
Examples of natural selection are well-documented, both by observation and through the fossil record.
Selection acts on the frequency of traits, and can take the form of stabilizing, directional, or diversifying selection.
Suggested Readings
o Darwin, C. 1959. On the Origin of Species by Means of Natural Selection, or, the Preservation of Favoured Races in the Struggle for Life. London: J. Murray.
o Futuyma, D.J. 1986. Evolutionary Biology. Sunderland, Mass: Sinauer Associates, Inc.
o Dawkins, R. 1989. The Selfish Gene. Oxford: Oxford University Press.
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