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Evolution and Natural Selection
Nature encourages no looseness, pardons no errors
- Ralph Waldo Emerson
I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.
- Charles Darwin, The Origin of Species
10 Oct 2010 Format for printing
In this lesson, we wish to ask:
How did observations in nature lead to the formulation of the theory of evolution? What are the main points of Darwin's theory of evolution? How does the process of natural selection work? What evidence do we have for local adaptation? How can natural selection affect the frequency of traits over successive generations?
The (R)Evolution of Theory
The theory of evolution is one of the great intellectual revolutions of human history, drastically changing our perception of the world and of our place in it. Charles Darwin put forth a coherent theory of evolution and amassed a great body of evidence in support of this theory. In Darwin's time, most scientists fully believed that each organism and each adaptation was the work of the creator. Linneaus established the system of biological classification that we use today, and did so in the spirit of cataloguing God's creations.
In other words, all of the similarities and dissimilarities among groups of organisms that are the result of the branching process creating the great tree of life ( see Figure 1 ), were viewed by early 19th century philosophers and scientists as a consequence of omnipotent design.
Figure 1: A phylogenetic "tree of life" constructed by computer analysis of cyochrome c molecules in the organisms shown; there are as many different trees of life as there are methods of analysis for constructing them.
However, by the 19th Century, a number of natural historians were beginning to think of evolutionary change as an explanation for patterns observed in nature. The following ideas were part of the intellectual climate of Darwin's time.
No one knew how old the earth was, but geologists were beginning to make estimates that the earth was considerably older than explained by biblical creation. Geologists were learning more about strata , or layers formed by successive periods of the deposition of sediments. This suggested a time sequence, with younger strata overlying older strata. A concept called uniformitarianism , due largely to the influential geologist Charles Lyell, undertook to decipher earth history under the working hypothesis that present conditions and processes are the key to the past, by investigating ongoing, observable processes such as erosion and the deposition of sediments. Discoveries of fossils were accumulating during the 18th and 19th centuries. At first naturalists thought they were finding remains of unknown but still living species. As fossil finds continued, however, it became apparent that nothing like giant dinosaurs was known from anywhere on the planet. Furthermore, as early as 1800, Cuvier pointed out that the deeper the strata, the less similar fossils were to existing species. Similarities among groups of organisms were considered evidence of relatedness, which in turn suggested evolutionary change. Darwin's intellectual predecessors accepted the idea of evolutionary relationships among organisms, but they could not provide a satisfactory explanation for how evolution occurred. Lamarck is the most famous of these. In 1801, he proposed organic evolution as the explanation for the physical similarity among groups of organisms, and proposed a mechanism for adaptive change based on the inheritance of acquired characteristics. He wrote of the giraffe:
"We know that this animal, the tallest of mammals, dwells in the interior of Africa, in places where the soil, almost always arid and without herbage, obliges it to browse on trees and to strain itself
Figure 2. Image of Peppered Moth
- Variation. Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring. On the other hand, some traits show little to no variation among individuals—for example, number of eyes in vertebrates.
- Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.
- High rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality.
4. Differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources will
contribute more offspring to the next generation.
From one generation to the next, the struggle for resources (what Darwin called the “struggle for existence”) will favor individuals with some variations over others and thereby change the frequency of traits within the population. This process is natural selection. The traits that confer an advantage to those individuals who leave more offspring are called adaptations.
In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection. (We now know that such traits may change by other evolutionary mechanisms that have been discovered since Darwin’s time.)
Natural selection operates by comparative advantage, not an absolute standard of design. “… as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates ” (Charles Darwin, On the Origin of Species, 1859).
During the twentieth century, genetics was integrated with Darwin’s mechanism, allowing us to evaluate natural selection as the differential survival and reproduction of genotypes, corresponding to particular phenotypes. Natural selection can only work on existing variation within a population. Such variations arise by mutation, a change in some part of the genetic code for a trait. Mutations arise by chance and without foresight for the potential advantage or disadvantage of the mutation. In other words, variations do not arise because they are needed.
Evidence of Natural Selection
Let's look at an example to help make natural selection clear.
Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth, Biston betularia. Prior to 1800, the typical moth of the species had a light pattern ( see Figure 2 ). Dark colored or melanic moths were rare and were therefore collectors' items.
During the Industrial Revolution, soot and other industrial wastes darkened tree trunks and killed off lichens. The light-colored morph of the moth became rare and the dark morph became abundant. In 1819, the first melanic morph was seen; by 1886, it was far more common -- illustrating rapid evolutionary change.
Eventually light morphs were common in only a few locales, far from industrial areas. The cause of this change was thought to be selective predation by birds, which favored camouflage coloration in the moth.
In the 1950's, the biologist Kettlewell did release-recapture experiments using both morphs. A brief summary of his results are shown below. By observing bird predation from blinds, he could confirm that conspicuousness of moth greatly influenced the chance it would be eaten.
Recapture Success
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 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.
Evolution and Natural Selection Self Test
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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