15.1 History of Evolutionary Thought
1. In 1831, Charles Darwin, a 22-year-old naturalist, accepted a position aboard the ship HMS Beagle that began a voyage around the world; it provided Darwin with many observations.
2. The pre-Darwinian worldview was different from the post-Darwinian.
a. Pre-Darwinian worldview was determined by intractable theological beliefs.
i. The earth is young.
ii. Each species was specially created and did not change over time.
iii. Variations are imperfections varying from a perfectly-adapted creation.
iv. Observations are to substantiate the prevailing worldview.
b. Darwin, however, lived during a time of great change in scientific and social realms.
c. Darwin’s ideas were part of a larger change in thought already underway among biologists; this concept would eventually be known as evolution.
d. Evolution proposed that species arise, change, and become extinct due to natural forces.
A. Mid-Eighteenth-Century Contributions
1. Taxonomy is the science of classifying organisms.
a. Biologists in the mid-eighteenth century used comparative anatomy to classify organisms into groups.
2. Carolus Linnaeus and Taxonomy
a. Carolus Linnaeus (1707–1778) was a Swedish taxonomist.
i. Linnaeus developed a binomial system of nomenclature (two-part names for each species [e.g., Homo sapiens]).
ii. He developed a system of classification for all known plants.
iii. 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.
b. Linnaeus thought that classification should describe the fixed features of species and reveal God’s divine plan.
c. His ideas reflected the ideas of Plato and Aristotle: the ideal form can be deduced, and organisms can be arranged in order of increasing complexity.
d. His later work with hybridization suggested species might change with time.
3. Georges Louis Leclerc
a. Georges Louis Leclerc, known by his title, Count Buffon (1707–1788), 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.
d. His writings speculated on influences of the environment, migration, geographical isolation, and the struggle for existence.
e. Buffon vacillated on whether he believed in evolutionary descent and he professed to believe in special creation and the fixity of species.
4. 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:
i. Changes undergone by animals during development,
ii. Artificial selection by humans, and
iii. The presence of vestigial structures (structures or organs that are believed to have been functional in an ancestor but are reduced and nonfunctional in a descendant).
d. Erasmus Darwin offered no mechanism by which evolutionary descent might occur.
B. Late Eighteenth/Early-Nineteenth Century Influences
1. George Cuvier (1769–1832), a French vertebrate zoologist, used comparative anatomy to develop a system of classifying animals.
a. 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.
b. To explain the fossil record, Cuvier proposed that a whole series of catastrophes (extinctions) and repopulations from other regions had occurred.
c. Cuvier was also a staunch advocate of special creation and fixity of species; this presented him with a problem when geological evidence of a particular region showed a succession of life forms in the Earth’s strata.
d. Catastrophism is the term applied to Cuvier’s explanation of fossil history: the belief that catastrophic extinctions occurred, after which repopulation of surviving species occurred, giving an appearance of change through time.
2. Jean-Baptiste de Lamarck (1744–1829) was the first to state that descent with modification occurs and that organisms become adapted to their environments.
a. Lamarck, an invertebrate zoologist, held ideas at odds with Cuvier’s.
b. Lamarck mistakenly saw “a desire for perfection” as inherent in all living things.
c. Inheritance of acquired characteristics was Lamarck’s belief that organisms become adapted to their environment during their lifetime and pass these adaptations to their offspring.
d. Experiments have failed to uphold Lamarck’s hypothesis of inheritance of acquired characteristics; the molecular mechanism of inheritance shows phenotypic changes do not result in genetic changes that can be passed on to the next generation.
3. James Hutton (1726-1797) concluded that extreme geological changes can be accounted for by slow, natural processes.
4. In his book Principles of Geology, Charles Lyell (1797-1875) presented arguments to support a theory of geological change proposed by James Hutton.
a. In contrast to catastrophists, Lyell proposed that Earth was subject to slow but continuous geological processes (e.g., erosion and uplifting) that occur at a uniform rate, a theory called uniformitarianism.
5. Thomas Malthus (1766-1834), an economist, published An Essay on the Principle of Population (1798).
a. He proposed that the size of human population was limited only by the quantity of resources available.
b. Famine, war, and epidemics are a result of overpopulation and limited resources.
c. Darwin used Malthus’s principle to formulate his idea of natural selection.
15.2 Darwin’s Theory of Evolution
1. Darwin’s nature was too sensitive to pursue medicine; he attended divinity school at Cambridge.
2. He attended biology and geology lectures and was tutored by the Reverend John Henslow.
3. Henslow arranged his five-year trip on the HMS Beagle; Darwin was an observant student of nature.
A. Observations of Change over Time
1. During his travels, Darwin observed that:
a. The Argentina coast had raised beaches; he witnessed earthquakes raising the Earth several feet.
b. Marine shells occurred far inland and at great heights in the Andes.
c. Fossils of huge sloths and armadillo-like animals suggested modern forms were descended from extinct forms with change over time; therefore species were not fixed.
2. Darwin accepted the possibility that the Earth is very old and there would have been enough time for descent with modification to occur.
B. Biogeographical Observations
1. Biogeography is the study of the geographic distribution of life forms on Earth.
2. Patagonian hares replaced rabbits in the South American grasslands.
3. The greater rhea found in the north was replaced by the lesser rhea in the south.
4. Comparison of the animals of South America and the Galápagos Islands caused Darwin to conclude that adaptation to the environment can cause diversification, including origin of new species.
5. The Galápagos Islands
a. These volcanic islands off the South American coast had fewer types of organisms.
b. Island species varied from the mainland species, and from island to island.
c. Each island had a variation of tortoise; long and short-necked tortoises correlated with different vegetation.
d. Darwin’s Finches
i. Finches on the Galápagos Islands resembled a mainland finch but there were more types.
ii. Galápagos finch species varied by nesting site, beak size, and eating habits.
iii. One unusual finch used a twig or thorn to pry out insects, a job normally done by (missing) woodpeckers (Darwin never witnessed this finch behavior).
iv. The variation in finches posed questions to Darwin: did they descend from one mainland ancestor or did islands allow isolated populations to evolve independently, and could present-day species have resulted from changes occurring in each isolated population?
C. Natural Selection and Adaptation
1. Darwin decided that adaptations develop over time; he sought a mechanism by which adaptations might arise.
2. Natural selection was proposed by both Alfred Russel Wallace (1823-1913) 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. Natural selection is a process based on these observations:
a. The members of a population have random but heritable variations.
i. In contrast to the previous worldview where imperfections were to be ignored, variations were essential in natural selection.
ii. Darwin suspected, but did not have today’s evidence, that the occurrence of variation is completely random.
iii. New variations are as likely to be harmful as helpful.
iv. Variations that make adaptation possible are those that are passed on from generation to generation.
v. Darwin could not state the cause of variations because genetics was not yet established.
vi. Natural selection only operates on variations that are already available in a population’s gene pool.
b. Organisms compete for available resources.
i. Darwin applied Malthus’s treatise to all organisms; resources were not sufficient for all members to survive.
c. Some individuals have adaptive characteristics that enable them to survive and reproduce better.
i. Organisms whose traits enable them to reproduce to a greater degree have a greater fitness.
ii. Fitness is a measure of an organism’s reproductive success.
iii. Black western diamondback rattlesnakes are more likely to survive on lava flows; lighter-colored rattlesnakes are more likely to survive on desert soil.
d. Organisms become adapted to conditions as the environment changes
i. An adaptation is a trait that helps an organism be more suited to its environment.
ii. Unrelated organisms living in the same environment often display similar characteristics.
iii. Because of differential reproduction, adaptive traits increase in each succeeding generation.
E. We Can Observe Selection at Work
1. Darwin noted that humans carry out artificial selection.
a. Early humans likely selected wolf variants; consequently, desirable traits increase in frequency in subsequent generations and produced the varieties of domestic dogs.
b. Many crop plant varieties can be traced to a single ancestor.
2. The Galapagos finches have beaks adapted to the food they eat, with different species of finches on each island.
a. Peter and Rosemary Grant of Princeton University are documenting natural selection on the Galapagos Islands.
3. Biotechnology advances allow documentation of phenotype evolution at the gene level.
F. The Tree of Life: 150 Years of Support for the Theory of Evolution by Natural Selection (Nature of Science reading)
1. Darwin challenged the idea that species were “fixed” and did not change.
2. His hypothesis of evolution by natural selection explained how nature shapes variation in a population.
3. However, Darwin could not answer the question of how diversity arises in the first place.
4. It was Gregor Mendel’s work in 1900 that provided the explanation of how new variation in populations could arise and be sustained.
5. Over the last 150 years there has been so much scientific support for Darwin’s ideas that his hypothesis is now accepted as the theory of evolution by natural selection.
6. The Tree of Life project’s aim is to determine how all life on Earth is related and descended from a common ancestor.
15.3 Evidence for Evolution
A. Fossil Evidence
1. Fossils are the remains and traces of past life or any other direct evidence of past life.
2. Fossils include skeletons, shells, seeds, insects trapped in amber, and imprints of leaves.
3. Transitional fossils represent the intermediate evolutionary forms of life in transition from one type to another, or a common ancestor of these types.
a. Archeopteryx is an intermediate between reptiles and birds.
b. Ambulocetus natans is a whale with legs.
B. Biogeographical Evidence
1. Biogeography studies the distribution of plants and animals worldwide.
2. Distribution of organisms is explained by related forms evolving in one locale and spreading to other accessible areas.
a. Darwin observed South America had no rabbits; he concluded rabbits originated elsewhere.
b. Biogeography explains the abundance of finch species on the Galápagos Islands lacking on the 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 joined; Australia separated before placental mammals arose, so only marsupials diversified in Australia.
C. Anatomical Evidence
1. Organisms have anatomical similarities when they are closely related because of common descent.
a. Homologous structures in different organisms are inherited from a common ancestor.
b. Analogous structures are inherited from unique ancestors and have come to resemble each other because they serve a similar function.
c. Vertebrate forelimbs contain the same sets of bones organized in similar ways, despite their dissimilar functions.
2. Vestigial structures are remains of a structure that was functional in some ancestors 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 tailbone but no tail.
c. Presence of vestigial structures is explained by the common descent hypothesis; these are traces of an organism’s evolutionary history.
3. Embryological development reveals a unity of plan.
a. During development, all vertebrates have a post-anal tail and paired pharyngeal pouches.
i. In fishes and amphibian larvae, the pouches become gills.
ii. In humans, the first pair of pouches becomes a cavity of middle ear and auditory tube; the second pair becomes tonsils, while the third and fourth pairs become thymus and parathyroid glands.
iii. The above features are explained if fishes are ancestral to other vertebrate groups.
D. Biochemical Evidence
1. All living organisms use the same basic biochemical molecules, e.g., 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 the 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. This is substantiated by the analysis of the degree of similarity in amino acids for cytochrome c among organisms.
5. These similarities can be explained by descent from a common ancestor.
6. Life’s vast diversity has come about by only a slight difference in the same genes.
E. Evidence from Developmental Biology
1. The diversity of life has come about by the same set of regulatory genes that control the activity of other genes involved in development.
a. Hox, or homeobox, genes orchestrate the development of the body plan in all animals.
b. All animals share a Hox gene common ancestor, but the number and type differ from species to species.
F. Criticisms of Evolution
1. The following are some criticisms to evolution along with a scientific explanation supporting evolution.
a. Evolution is a theory about how life originated.
i. The origin of life is not the focus of evolutionary biology.
ii. Evolutionary biology studies life following the origin of life.
b. There are no transitional fossils.
i. There are specific conditions that must be met in order for an organism to be fossilized.
ii. Transitional fossils have been found of whale and fish ancestors.
c. Evolution proposes life changed as a result of random events; clearly traits are too complex to have originated ‘by chance.’
i. Natural selection shapes variation and is not random.
ii. Complexity is a result of millions of years of modifications to pre-existing traits.
d. Evolution is not observable or testable, thus it is not ‘science.’
i. Genetic research is used to observe and test variations.
ii. Evolutionary biology gathers evidence from multiple sources in order to observe and test the theory of evolution.