|CHAPTER 22 Darwin and Evolution
22.1 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.
A) The pre-Darwinian world-view was different from the post-Darwinian.
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 worldview.
B) Darwin's ideas were part of a larger change in thought already underway among biologists; this concept would eventually be known as evolution.
2) Mid-Eighteenth-Century Contributions
A) Carolus Linnaeus and Taxonomy
1) Taxonomy is the science of classifying organisms; taxonomy had been a main concern of biology.
2) Carolus Linnaeus (1707-1778) was a Swedish taxonomist who developed a binomial system of nomenclature (two-part names for each species (Homo sapiens)
3) Like other taxonomists of his time, Linnaeus believed in the ideas of special creation and the fixity of species; each species had a place in the scala naturae, a sequential ladder of life.
B) Georges Louis Leclerc
1) Georges Louis Leclerc, known by his title, Count Buffon (1707-1788), was a French naturalist.
2) He wrote on the natural history of all known plants and animals, provided evidence of descent with modification.
3) His writings speculated on influences of the environment, migration, geographical isolation, and the struggle for existence.
4) Buffon's Law is considered the first principle of biogeography
C) Erasmus Darwin
1) Erasmus Darwin (1731-1802) was Charles Darwin's grandfather.
2) He was a physician and a naturalist whose writings on both botany and zoology contained many comments that suggested the possibility of common descent.
3) He based his conclusions on development, artificial selection, and anatomy
3) Late Eighteenth-/Early-Nineteenth Century Contributions
A) Cuvier and Catastrophism
1) George Cuvier (1769-1832), a French vertebrate zoologist, was the first to use comparative anatomy to develop a system of classifying animals.
2) 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.
3) To explain the fossil record, Cuvier proposed that a series of catastrophes and repopulations had occurred.
4) 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.
B) Lamarck's Acquired Characteristics
1) Lamarck (1744-1829) was the first to state that descent with modification occurs and that organisms become adapted to their environments.
2) Lamarck, an invertebrate zoologist, held ideas at odds with Cuvier's.
3) Lamarck mistakenly saw "a desire for perfection" as inherent in all living things.
4) 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.
5) Experiments fail to uphold Lamarck's inheritance of acquired characteristics
22.2 Darwin's Theory of Evolution
1) Darwin's Background
a) His nature was too sensitive to pursue medicine; he attended divinity school at Cambridge.
b) He attended biology and geology lectures and was tutored by the Reverend John Henslow.
c) Henslow arranged his five-year trip on the HMS Beagle; Darwin was an observant student of nature.
d) Darwin's father objected to him going on this trip
2) Geology and Fossils
a) Darwin took Lyell's book, Principles of Geology, on the voyage of the HMS Beagle.
b) In contrast to catastrophists, Hutton proposed that the earth was subject to slow but continuous geological processes that occur at a uniform rate, a theory called uniformitarianism.
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.
a) Biogeography is the study of the geographic distribution of life forms on earth.
b) Patagonian hares replaced rabbits in the South American grasslands.
c) The greater rhea found in the north was replaced by the lesser rhea in the south.
4) 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, each island had different variations
c) Each island had a variation of tortoise; long and short necked tortoises correlated with different vegetation.
5) Darwin's Finches
a) Finches on the Galápagos Islands resembled a mainland finch but there were more types.
b) Galápagos finch species varied by nesting site, beak size, and eating habits.
c) One unusual finch used a twig or thorn to pry out insects, a job normally done by (missing) woodpeckers
d) 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?
Questions to Ponder:
Did the animals on the islands descend from one mainland ancestor?
Did the island populations evolve independently?
Could present-day species have resulted from changes occurring in isolated populations?
6) Natural Selection and Adaptation
a) Natural selection was proposed by both Alfred Russel Wallace and Darwin
b) It is the driving mechanism of evolution caused by environmental selection of organisms most fit to reproduce, resulting in adaptation.
Wallace was not given credit for the theory because Darwin published first, however, there is a geographical area named for him called the "Wallace Line" which separates Australia and Asia.
c) There are three preconditions for natural selection.
1) The members of a population have random but heritable variations.
2) In a population, many more individuals are produced each generation than the environment can support.
3) Some individuals have adaptive characteristics that enable them to survive and reproduce better.
d) There are two consequences of natural selection.
1) An increasing proportion of individuals in succeeding generations will have the adaptive characteristics.
2) The result of natural selection is a population adapted to its local environment.
e) Natural selection can only utilize variations that are randomly provided; therefore there is no directedness or anticipation of future needs.
f) Extinction occurs when previous adaptations are no longer suitable to a changed environment.
7) Organisms Have Variations
a) Variations are essential to natural selection
b) Variations are random and heritable
c) The mechanism for variation was not known (genetics had not been discovered)
8) Organisms Struggle to Exist
a) Malthus proposed that human populations outgrow food supply and death and famine were inevitable.
b) Darwin applied this to all organisms; resources were not sufficient for all members to survive.
c) Therefore, there is a constant struggle for existence; only certain members survive and reproduce.
9) Organisms Differ in Fitness
a) Fitness is a measure of an organism's reproductive success
b) Fitness does not necessarily mean stronger
10) Artificial Selection
a) Early humans likely selected wolf variants; produced the varieties of domestic dogs.
b) Many crop plant varieties can be traced to a single ancestor.
c) Evolution by artificial or natural selection occurs when more fit organisms reproduce and leave more offspring
11) Organisms Become Adapted
a) An adaptation is a trait that helps an organism be more suited to its environment.
b) Unrelated organisms living in the same environment often display similar characteristics.
c) Because of differential reproduction, adaptive traits increase in each succeeding generation.
12) On the Origin of Species by Darwin
a) After the HMS Beagle returned to England in 1836, Darwin waited over 20 years to publish.
b) He used the time 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.
c) Darwin was forced to publish Origin of Species after reading a similar hypothesis by Alfred Russel Wallace.
Summarize the Main Points of Evolution by Natural Selection
Natural selection is the driving force of evolution
Natural selection acts upon individuals (death or survival) but changes POPULATIONS
In fact: The Theory of Evolution is more accurately called THE THEORY OF EVOLUTION BY NATURAL SELECTION
The Theory is not the result of ONE man's observations, though Darwin gets most of the credit, the theory was being established for many years, across many disciplines and still continues to be refined
Evolution in 4 Basic Steps
1. Heritable variation
2. More individuals are produced than can improve survival
3. Some individuals have traits (adaptations) that help them survive & reproduce
4. Survivors pass those traits to their offspring (increases the number that have the trait)
* Results in a population adapted to its environment
Drawbacks of Natural Selection
Can only utilize variations that are randomly provided; there is no directedness or anticipation of future needs
Extinctions occur when previous adaptations are no longer suitable to a changed environment
Macroevolution vs. Microevolution
These two ideas attempt to explain the difference between small changes (micro) versus large changes (macro). There is really no difference, other than macroevolution takes a very long time and results in profound changes in the species.
Microevolution refers to minor changes that can occur within a species in a relatively short period of time, like a change in coloration within a fish population.
There is a good tutorial on this topic at: http://evolution.berkeley.edu/evolibrary/article/evoscales_01
Imagine a Scenario of Evolution.....
1. Create a real or imagined organism
2. Describe 2-3 variations
3. Show how evolution would act on this population given a change in the environment (climate, predators, food change…)
4. Pay attention to which variations are beneficial, which are harmful.
5. Show how reproduction changes the overall population (with regard to these variations)
6. Be creative! You can map your organism through a few generations... You will present your scenario to the class