From Biol301

Contents 1 Darwin and his theory: 428*444 1.1 The Historical Context for Evolutionary Theory 1.2 The Darwinian Revolution 1.3 Other Evidence of Evolution Pervades Biology 1.4 What is Theoretical about the Darwinian view of life?

Darwin and his theory: 428*444

The Historical Context for Evolutionary Theory

• Darwin challenged the 6000 year old model and the creator model
• Plato believed in two worlds –the perfect ideal, and the imperfect world we perceive. If the animals were idea, why did they have to evolve?
• Plato and Aristotle influenced western civilizations the most and they didn’t believe in evolution
• Aristotle believed all things could fit on a ladder of increasing complexity (scala naturae) called a “scale of nature”.
• Natural theology dominated western countries with the idea of learning about god’s nature by studying nature. It held that animals were perfectly designed during the creation. These people focused on revealing the steps of the ladder.
• Linnaeus was a natural theologian. He developed the two name protocol for naming species. Linnaeus grouped animals for the first time, but not to show evidence of evolution, just to put like with like. A century later, Darwin used the groupings Linnaeus had come up with to show how the animals had evolved from one into several different but similar animals.
• Georges Cuvier developed paleontology (study of fossils). He found that fossils showed that species were less similar to modern species as the got older and that sometimes species disappear altogether. Instead of seeing this as supporting evolution, he suggested catastrophism: the idea that each boundary between strata corresponded in time to a catastrophe such as a flood or drought that killed some species.
• James Hutton suggested that the geography of earth could be explained by a gradual process by mechanisms still in effect (rivers, oceans, weather, etc.). This theory led a Scot named (see next point)
• Charles Lyell to incorporate “gradualism” into his theory stating that geological processes have not changed over time (same thing is happening today as back a million years).
• Darwin was influenced by this gradualism idea to come to two conclusions: if the world came to the condition that it is today slowly, than it cannot be 6000 years old. Second, slow continuous processes can add up to big changes –why not in biology, too? Darwin wasn’t the first to think of this, but it was only 2 generations before him –his grandfather’s age.
• Lamarck published his theory of evo in 1809 (the year Darwin was born). He had compared fossils to see multiple distinct lines of decent. Lamarck used two (common at the time) ideas to explain the lines of descent. The first was use/disuse –if you don’t use it, you lose it. If you use it lots, it becomes bigger and stronger. The second mechanism he used to explain the evo was “inheritance of acquired characteristics” by which offspring have characteristics that their parents obtained during their lives. (Remember that Mendel was working in 1860’s but didn’t get recognized until the early 1900’s.) Lamarck was ridiculed because everyone still believed that animals didn’t evolve. Lamarck is noted for his theory suggesting the great age of the earth, evolution as the cause of diversity for life, and that adaptation to the environment was the product of evolution.

The Darwinian Revolution

• Charles Darwin (1809*1882) went to the clergy after not liking med school. Most biologists were in the clergy at the time and thinking in terms of natural theology. He got his BA and was suggested as a conversation partner aboard the Beagle to sail around the world (22 years old). Darwin saw lots of animals and noted that the South American ones had a distinct South Americanism about them. And interestingly, the temperate animals of SA were more closely related to the extreme animals of SA than they were to the temperate animals of Europe. So when he got back he sat down and realized that he had evidence of small changes leading to different species. By 1840’s he had natural selection worked out but was sick, although visited by many of the great minds (including Lyell). Darwin is a pansy, doesn’t publish, Wallace asks him to review manuscript. Wallace’s paper gets presented, Darwin finishes his manuscript and gets credit b/c he has a more developed theory and had it before Wallace as is apparent from his logs. Within 10 years nearly everyone believes in natural selection as the mechanism for evo (mostly because of the torrent of evidence).
• Darwin’s work had two points: evolution is the reason for diversity and natural selection is the mechanism by which it works.
• Ernst Mayr summarizes how Natural Selection works with observations and inferences:

1. Since all animals have such fertility potential, if they all mated successfully their numbers would astronomically increase.
2. Populations end to remain stable in size.
3. Resources are limited.

INFERENCE: Production of more individuals than the environment can handle leads to competition where some will die.

1. There is diversity in each population.
2. Much of the diverse characteristics are heritable.

INFERENCE: Survival isn’t random but is affected by the characteristics of the animal. INFERENCE: The inequality of animals to survive and reproduce will change the characteristics of animals over time.

• Darwin recognized that overpopulating was a common capability for most species and what caused most competition; hence, the pressure on the best to survive.
• Darwin used “Artificial selection” to show how it could change a species. He used examples of domesticated animals and crops.
• a population is the smallest unit that can evolve
• natural selection takes place on the individual level, but evolution does not.
• evolution is only measured in relative proportions of traits in a population as time proceeds
• evolution can only amplify or diminish heritable traits.
• Natural selection is situational –what is good for bee here may not be good there.
• examples of natural selection: bugs and insecticides, HIV and 3TC (acts like a cytosine)

Other Evidence of Evolution Pervades Biology

• homology = similarity in characteristic because of common ancestry; hence the term homologous structures –structures that are similar because of common ancestry
• comparative biology supports the idea of remodeling as opposed to introducing new characteristics out of the blue. For example we can see that our hips and knees came from four legged creatures. Hence we have knee and hip problems; we’re catching up.
• vestigial organs: organs or traits that are unused but remain from ancestors. Example: some snacks have remnants of pelvises.
• embryological homologies: sometimes it is easier to see similarity between animals when you look at their embryos because parts have not specialized yet.
• molecular homologies often help to link species with no physical characteristics in common (i.e. plants and animals). I.e. DNA and RNA and the genetic code itself.
• homologies form a “layered pattern” that corresponds to the tree of life and where branches start.
• biogeography first suggested evolution to Darwin because he saw that species tended to be most closely related to species near to themselves.
• convergent evolution: when two species adapt to life in the same way but are not actually along the same lineage (example: sugar glider and flying squirrel, page 440)
• endemic = found no where else in the world
• Darwinian thinking suggests there would be fossils to connect older fossils to newer fossils; this is true; example = skull shape from reptiles to mammals.
• Darwinian thinking holds because “it is supported by independent types of evidence: evolutionary patters of homology that match patterns in space (biogeography) and time (the fossil record).”

What is Theoretical about the Darwinian view of life?

• Two errors in calling Darwin’s thinking “just a theory”.

1. It doesn’t separate evolution and natural selection as the mechanism for evolution.
2. It doesn’t give due respect to a scientist’s use of the word “theory.” Scientists mean a tested and not yet disproved hypothesis when they say theory. This theory in particular has stood for many years making it more than a trivial idea.