Nature encourages no looseness, pardons no errors
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05/07/2009 |
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.
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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 continuously to reach them. This habit sustained for long, has had the result in all members of its race that the forelegs have grown longer than the hind legs and that its neck has become so stretched, that the giraffe, without standing on its hind legs, lifts its head to a height of six meters."
In essence, this says that the necks of Giraffes became long as a result of continually stretching to reach high foliage. Larmarck was incorrect in the hypothesized mechanism, of course, but his example makes clear that naturalists were thinking about the possibility of evolutionary change in the early 1800's.
- Darwin was influenced by observations made during his
youthful voyage as naturalist on the survey ship Beagle. On the
Galapagos Islands he noticed the slight variations that made tortoises from
different islands recognizably distinct. He also observed a whole array
of unique finches, the famous "Darwin's finches," that
exhibited slight differences from island to island. In addition, they
all appeared to resemble, but differ from, the common finch on the
mainland of Ecuador, 600 miles to the east. Patterns in the distribution
and similarity of organisms had an important influence of Darwin's
thinking. The picture at the top of this page is of Darwin's own
sketches of finches in his Journal of Researches.
- In 1858, Darwin published his famous On the Origin
of Species by Means of Natural Selection, a tome of over 500 pages
that marshalled extensive evidence for his theory. Publication of the
book caused a furor - every copy of the book was sold the day that it
was released. Members of the religious community, as well as some
scientific peers, were outraged by Darwin's ideas and protested. Most
scientists, however, recognized the power of Darwin's arguments. Today,
school boards still debate the validity and suitability of Darwin's
theory in science curricula, and a whole body of debate has grown up
around the controversy (see the WWW site Talk.Origins for an ongoing
dialogue). We do not have time to cover all of Darwin's evidence and
arguments, but we can examine the core ideas. What does this theory
of evolution say?
Darwin's Theory
Darwin's
theory of evolution has four main parts:
- Organisms have changed over time, and the ones
living today are different from those that lived in the past.
Furthermore, many organisms that once lived are now extinct. The world
is not constant, but changing. The fossil record provided ample
evidence for this view.
- All organisms are derived from common ancestors by a
process of branching. Over time, populations split into different
species, which are related because they are descended from a common
ancestor. Thus, if one goes far enough back in time, any pair of
organisms has a common ancestor. This explained the similarities of
organisms that were classified together -- they were similar because of
shared traits inherited from their common ancestor. It also explained
why similar species tended to occur in the same geographic region.
- Change is gradual and slow, taking place over a long
time. This was supported by the fossil record, and was consistent with
the fact that no naturalist had observed the sudden appearance of a new
species. [This is now contested by a view of episodes of rapid change
and long periods of stasis, known as punctuated equilibrium].
- The mechanism of evolutionary change was natural
selection. This was the most important and revolutionary part of
Darwin's theory, and it deserves to be considered in greater detail.
The Process of Natural Selection
Natural
selection is a process that occurs over successive generations. The following
is a summary of Darwin's line of reasoning for how it works (see Figure 2).
- If all the offspring that organisms can produce were
to survive and reproduce, they would soon overrun the earth. Darwin
illustrated this point by a calculation using elephants. He wrote:
"The elephant is reckoned the slowest breeder of all
known animals, and I have taken some pains to estimate its probable minimum
rate of natural increase; it will be safest to assume that it begins breeding
when 30 years old and goes on breeding until 90 years old; if this be so,
after a period from 740 to 750 years there would be nearly 19 million
elephants descended from this first pair."
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Figure 2: The Process of Natural
Selection |
- This unbounded population growth resembles a simple
geometric series (2-4-8-16-32-64..) and quickly reaches enormous
numbers.
- As a consequence, there is a "struggle"
(metaphorically) to survive and reproduce, in which only a few
individuals succeed in leaving progeny.
- Organisms show variation in characters that influence
their success in this struggle for existence. Individuals within a
population vary from one another in many traits. (Animal behavioralists
making long-term studies of chimps or elephants soon recognize every
individual by its size, coloration, and distinctive markings.)
- Offspring tend to resemble parents, including in
characters that influence success in the struggle to survive and
reproduce.
- Parents possessing certain traits that enable them to
survive and reproduce will contribute disproportionately to the
offspring that make up the next generation.
To the extent that offspring
resemble their parents, the population in the next generation will consist of
a higher proportion of individuals that possess whatever adaptation enabled
their parents to survive and reproduce.
The well-known example of camouflage coloration in an insect makes for a very powerful, logical argument for adaptation by natural selection. Development of such coloration, which differs according to the insect's environment, requires variation. The variation must influence survival and reproduction (fitness), and it must be inherited.
During
the early and middle 20th Century, genetics became incorporated into
evolution, allowing us to define natural selection this way:
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Natural Selection is the differential reproduction of genotypes. |
Natural Selection Requires...
For
natural selection to occur, two requirements are essential:
- There must be heritable variation for some
trait. Examples: beak size, color pattern, thickness of skin,
fleetness.
- There must be differential survival and
reproduction associated with the possession of that trait.
Unless
both these requirements are met, adaptation by natural selection cannot occur.
Some examples:
- If some plants grow taller than others and so are
better able to avoid shading by others, they will produce more
offspring. However, if the reason they grow tall is because of the soil
in which their seeds happened to land, and not because they have the
genes to grow tall, than no evolution will occur.
- If some individuals are fleeter than others because
of differences in their genes, but the predator is so much faster that
it does not matter, then no evolution will occur (e.g. if cheetahs ate
snails).
In
addition, natural selection can only choose among existing varieties in a
population. It might be very useful for polar bears to have white noses, and
then they wouldn't have to cover their noses with their paws when they stalk
their prey. The panda could have a much nicer thumb than the clumsy device
that it does have.
When
we incorporate genetics into our story, it becomes more obvious why the
generation of new variations is a chance process. Variants do not arise
because they are needed. They arise by random processes governed by the laws
of genetics. For today, the central point is the chance occurrence of
variation, some of which is adaptive, and the weeding out by natural
selection of the best adapted varieties.
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 3). Dark colored or melanic moths were rare and were therefore collectors' items.
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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
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light moth |
dark moth |
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non-industrial woods |
14.6 % |
4.7 % |
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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 4). 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 4: 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 5). 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 5
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?
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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 6a-c capture these three major categories of natural selection.
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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 6a). 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 7).
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 6b). This is what we usually think of as natural selection. Industrial melanism was such an example.
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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 8).
Under diversifying (disruptive) selection, both extremes are favored at the expense of intermediate varieties (see Figure 6c). 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. 1958. 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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