18.1. The World Was Ready (p. 286)
A. Darwin's Voyage
1. In 1831, at the age of 22, Charles Darwin accepted naturalist position aboard the British naval ship HMS Beagle.
2. HMS Beagle began a five-year voyage around the world; it provided Darwin with many observations. (Fig. 18.1)
3. In general, the pre-Darwinian worldview was different from the post-Darwinian worldview.
a. The pre-Darwinian worldview was determined by deep-seated beliefs held to be intractable truths.
1) The earth is young.
2) Each species was specially created and did not change over time.
3) Variations are imperfections varying from a perfectly adapted creation.
4) Observations are to substantiate the prevailing view.
b. However, Darwin lived during a time of great change in scientific and social realms.
c. As a result, the scientific community was ready for Darwin's hypotheses, which received widespread acceptance.
d. Darwin's ideas contributed greatly to development of a post-Darwinian worldview.
B. Mid-Eighteenth-Century Contributions
1. Carolus Linnaeus and Taxonomy (Fig. 18.2)
a. Taxonomy is the science of classifying organisms; a century before Darwin's trip, taxonomy had been a main concern of biology.
b. Carolus Linnaeus (1707-78) was a Swedish naturalist preeminent in the field of taxonomy:
1) Linnaeus developed the binomial system of nomenclature (a two-part name for each species [e.g., Homo sapiens]).
2) He also developed a system of classification for all known plants.
3) Like other taxonomists of his time, Linnaeus believed in the ideas of
a) special creation---each species had an "ideal" structure and function; and
b) fixity of species---each species had a place in the scala naturae, a sequential ladder of life.
c. Linnaeus thought that classification should describe the fixed features of species and reveal God's divine plan.
d. His later work with hybridization suggested species might change with time.
2. Georges-Louis Leclerc
a. Georges-Louis Leclerc, better known by his title, Count Buffon (1707-88), was a French naturalist.
b. He wrote a 44-volume natural history of all known plants and animals.
c. He also provided evidence of descent with modification, and also speculated on various causative mechanisms.
d. Throughout his writings, he mentioned that the following factors could influence evolutionary change: direct influences of the environment, migration, geographical isolation, overcrowding, and the struggle for existence.
e. Buffon vacillated as to whether or not he believed in evolutionary descent, and professed to believe in special creation and the fixity of species.
3. Erasmus Darwin
a. Erasmus Darwin (1731-1802) was Charles Darwin's grandfather.
b. He was a physician and a naturalist whose writings on both botany and zoology contained many comments that suggested the possibility of common descent.
c. He based his conclusions on
1) changes undergone by animals during development,
2) artificial selection by humans, and
3) the presence of vestigial organs (organs that are believed to have been functional in an ancestor but are reduced and nonfunctional in a descendant).
d. However, Erasmus Darwin offered no mechanism by which evolutionary descent might occur.
C. Late-Eighteenth-Century Contributions
1. Cuvier and Catastrophism
a. George Cuvier (1769-1832), a distinguished French vertebrate zoologist, was the first to use comparative anatomy to develop a system of classifying animals.
b. He founded the science of paleontology, the study of fossils, and suggested that a single fossil bone was all he needed to deduce the entire anatomy of an animal. (Fig. 18.3)
c. To explain the fossil record, Cuvier proposed that a whole series of catastrophes (extinctions) and repopulations from other regions had occurred.
d. However, Cuvier was a also a staunch advocate of special creation and fixity of species, which presented him with a serious problem when geological evidence of a particular region showed a succession of life forms in the earth's strata.
e. Catastrophism is the term applied to Cuvier's explanation of fossil history, the belief held by Cuvier that catastrophic extinctions occurred, after which repopulation of surviving species took place, giving an appearance of change through time.
2. Lamarck's Theory of Evolution
a. Lamarck (1744-1829) was the first biologist to clearly state that descent with modification does occur and that organisms do become adapted to their environments.
b. Lamarck was an invertebrate zoologist and held ideas different from Cuvier.
c. Unfortunately, he relied on commonly held but erroneous beliefs (scala naturae and inheritance of acquired characteristics) to substantiate his beliefs and provide a mechanism for evolutionary change.
1) Inheritance of acquired characteristics was the Lamarckian belief that organisms become adapted to their environment during their lifetime and pass on these adaptations to their offspring. (Fig. 18.4)
2) He believed the increasing complexity of life forms in strata is the result of a natural tendency toward perfection inherent in all living things.
d. Experiments have failed to uphold Lamarck's inheritance of acquired characters; the molecular mechanism of inheritance explains why.
18.2. Darwin Develops a Theory (p. 289)
A. Darwin's Background
1. Darwin was a student of nature and collected insects.
2. His nature was too sensitive to pursue medicine; he attended divinity school at Cambridge.
3. He attended biology and geology lectures and was tutored by the Reverend John Henslow, who also arranged his trip on the HMS Beagle.
B. Geology and Fossils Help Darwin Decide
1. His study of geology and fossils caused him to concur with Lyell that the observed massive geological changes were caused by slow, continuous processes. (Fig. 18.5)
a. In his book Principles of Geology, Charles Lyell presented arguments to support a theory of geological change proposed by James Hutton.
b. In contrast to catastrophists, Hutton proposed that the earth was subject to slow but continuous geological processes (e.g., erosion and uplifting; Fig. 18.5) that occur at a uniform rate.
c. Darwin took Lyell's book on the voyage of the HMS Beagle.
2. Fossil Evidence
a. The coast of Argentina presented many raised beaches.
b. Marine shells occur far inland and at great heights in the Andes.
c. Fossils of huge sloths and armadillolike animals suggested modern forms were descended from extinct forms with change over time.
C. Biogeography Also Helps
1. Biogeography is the study of the geographic distribution of life forms on earth.
2. Darwin's comparison of the animals of South America (Fig. 18.7) and the Galápagos Islands (Figs. 18.8 and 18.9) caused him to conclude that adaptation to the environment can cause diversification, including origin of new species.
3. Patagonian hares replaced rabbits in the South American grasslands.
4. The greater rhea found in the north was replaced by the lesser rhea in the south.
5. The Galápagos Islands
a. These volcanic islands off the coast of South America had fewer types of organisms.
b. The island species varied from the mainland species, and from island-to-island.
c. Each island had a variation of tortoise that correlated with different vegetation.
d. Darwin observes finches
1) Finches on the Galápagos Islands resembled a mainland finch but there were more types.
2) Galápagos finch species varied by nesting site, beak size, and eating habits.
3) One unusual finch used a twig or thorn to pry out insects, a job normally done by a woodpecker.
4) The finches posed questions to Darwin: did they descend from one mainland ancestor, did islands allow isolated populations to evolve independently, and could present-day species have resulted from changes occurring in each isolated population?
D. Natural Selection Provides a Mechanism
1. Once Darwin decided that adaptations develop over time, rather than instantaneously, he began to think about a mechanism by which adaptations might arise.
2. Natural selection was proposed by both Alfred Russel Wallace and Darwin as a driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptation.
3. Because the environment is always changing, there is no perfectly adapted organism.
4. Preconditions for natural selection
a. The members of a population have random but heritable variations. (Fig. 18.10)
b. In a population, many more individuals are produced each generation than can survive to reproduce.
c. Some individuals have adaptive characteristics that enable them to survive and reproduce better.
5. Consequences of natural selection
a. An increasing proportion of individuals in succeeding generations have the adaptive characteristics.
b. The result of natural selection is a population adapted to its local environment.
E. Organisms Have Variations
1. In contrast to the previous worldview (Table 18.1), variations are highly significant.
2. Darwin suspected, but did not have today's evidence, that variation is completely random.
3. New variations are as likely to be harmful as helpful.
4. Variations that make adaptation possible are those that are passed on generation to generation.
5. Darwin could not state the cause of variations because genetics was not yet established.
F. Organisms Struggle to Exist
1. Darwin and Wallace both read an essay by Thomas Malthus, a clergyman and socioeconomist.
2. Malthus proposed that human populations outgrow resources and members must compete.
3. Darwin applied this to all organisms; resources were not sufficient for all members to survive.
4. Therefore, there is a constant struggle for existence; only certain members survive and reproduce.
G. Organisms Differ in Fitness
1. Reflecting on all the populations he had observed during his voyage and on how individual members of those populations had varied in size, shape, coloring, and other traits, it occurred to Darwin that some traits could lead to differences in the ability to secure resources.
2. Organisms whose traits enable them to reproduce to a greater degree have a greater fitness.
a. Fitness is a measure of an organism's ability to survive and reproduce.
b. Black rattlesnakes are more likely to survive on lava flows; lighter-colored rattlesnakes are more likely to survive on desert soil.
c. Darwin noted that humans carry out artificial selection with animals (Fig. 18.11) and plants (Fig. 18.12), selecting organisms to reproduce; consequently, desirable traits increase in frequency in subsequent generations.
d. In nature, interactions with the environment determine which members reproduce more.
e. Differential reproduction occurs when more fit organisms reproduce and leave more offspring than the less fit.
H. Organisms Become Adapted
1. An adaptation is a trait that helps an organism be more suited to its environment.
2. Unrelated organisms living in the same environment often display similar characteristics.
3. Because of differential reproduction, adaptive traits increase in each succeeding generation.
I. Darwin Writes a Book
1. After the HMS Beagle returned to England in 1836, Darwin waited over 20 years to publish.
2. During that time, he used the scientific process to test his hypothesis that life forms arose by descent from a common ancestor and that natural selection is a mechanism by which species can change and new species arise.
3. Darwin was forced to publish Origin of Species after reading a similar concept by Alfred Wallace.
18.3. Evidence Accumulates (p. 296)
A. Common Descent
1. The hypothesis of common descent is supported by many lines of evidence.
2. The more varied the evidence, the more certain it becomes.
B. Fossils Tell a Story
1. The fossil record is the history of life recorded by remains from the past.
2. Fossils include skeletons, shells, seeds, insects trapped in amber, imprints of leaves, and tracks of organisms that lived in the distant past; fossils are at least 10,000 years old.
3. The fossil record traces history of life and allows us to study history of particular organisms.
4. Fossil evidence supports the common descent hypothesis; fossils can be linked over time because they reveal a similarity in form, despite observed changes.
5. Transitional forms reveal links between groups: Archaeopteryx is between reptiles and birds (Fig. 18.13); Eustheopteron is an amphibious fish; Seymouria is a reptilelike amphibian; therapsids were mammal-like reptiles.
6. The fossil record allows us to trace the history of the modern-day horse Equus.
a. Earliest fossils show an ancestral Hyracotherium the size of a dog, with cusped low-crowned molars, four toes on each front foot, three on each hind foot--all adaptations for forest living.
b. When forests were replaced by grasslands, the intermediates were selected for durable grinding teeth, speed, etc. with an increase in size and decrease in toes.
c. Living organisms resemble most recent fossils in the line of descent; underlying similarities allow us to trace a line of descent over time.
C. Biogeographical Separations
1. Biogeography is the study of the distribution of plants and animals throughout the world.
2. Distribution of organisms is explained by related forms evolving in one locale and spreading to other accessible areas. (Fig. 18.14)
a. Darwin observed South America had no rabbits; he concluded rabbits originated elsewhere.
b. Biogeography explains why many finch species are on the Galápagos Islands but not mainland.
3. Physical factors, such as the location of continents, determine where a population can spread.
a. Cacti are restricted to North American deserts and euphorbia grow in African deserts.
b. Marsupials arose when South America, Antarctica, and Australia were all joined; Australia separated before placental mammals arose, so only marsupials diversified in Australia.
D. Anatomical Similarities
1. Organisms have anatomical similarities when they are closely related because of common descent, as substantiated by comparative anatomy.
a. Homologous structures in different organisms are inherited from a common ancestor.
b. Vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions. (Fig. 18.15) [transp. 113]
2. Vestigial structures are remains of a structure that was functional in some ancestor but is no longer functional in the organism in question.
a. Most birds have well-developed wings; some bird species have reduced wings and do not fly.
b. Humans have a tail bone but no tail.
c. Presence of vestigial structures is explained by the common descent hypothesis.
3. Embryological development reveals a unity of plan. (Fig. 18.16)
a. During development, all vertebrates have a notochord and paired pharyngeal pouches.
1) In fishes and amphibian larvae, the pouches become gills.
2) In humans, first pair of pouches becomes a cavity of middle ear and auditory tube; second pair becomes tonsils, while third and fourth pairs become thymus and parathyroid glands.
3) Points 1) and 2) are explained if fishes are ancestral to other vertebrate groups.
E. Biochemical Differences
1. Almost all living organisms use the same basic biochemical molecules, including DNA, ATP, and many identical or nearly identical enzymes.
2. Organisms utilize the same DNA triplet code and the same 20 amino acids in their proteins.
3. Many organisms share same introns and types of repeats, which is remarkable since there is no obvious functional reason why these components need to be so similar.
4. These similarities can be explained by descent from a common ancestor.
5. This is substantiated by analysis of degree of similarity in amino acids for cytochrome c among organisms. (Fig. 18.17) [transp. 114]
F. Because it is supported by so many lines of evidence, evolution is no longer considered a hypothesis.
1. Evolution is one of the great unifying theories of biology.
2. In science, theory is reserved for those conceptual schemes that are supported by a large number of observations or a large amount of experimental evidence and have not been found lacking.