Study Questions:
1) What argument led Darwin to conclude that species change over time?
2) How were the Galapagos mockingbirds important to Darwin's development of evolutionary ideas?
3) Darwin spends several chapters in The Origin describing how humans have altered domesticated animals. Why were these observations so important to Darwin's argument?
4) Malthus suggested that organisms in a population will compete for resources, and that most will die. Darwin recognized that it was the variation in populations that was important, and he put Malthuses ideas together to with his understanding of variation to describe how populations would cahnge over time. This the called the theory of Natural Selection. Outline this theory as three premises, three conclusions, and a corollary. Note which elements were Malthus's, and which were Darwin's.
5) List Darwin's three dilemmas, and describe how he addressed them.
6) Outline Darwinian evolution, listing sources of variation and agents of change.
C. Observations - Evidence for Common Descent as Historical Fact
1. Geology
2. Paleontology
3. Anatomy
4. Biogeography
- Community convergence
- Island Faunas
Galapagos Mockingbirds
- Darwin thought he had one, very variable species. He sent specimens to John Gould, the premiere ornithologist in England at the time. On Darwin's return to England, he found out that Gould recognized
the mockingbird specimens as belonging to 4 separate species. This made Darwin consider that maybe the variation WITHIN a
species could be continuous with the variation BETWEEN species.... maybe varieties within a species could gradually become so different from one another that they would eventually become different species.
5. Argument For Evolution as a Historical Fact:
Premise 1: Species that are alive today are different from those that have lived previously.
Premise 2: Spontaneous Generation is refuted, so organisms only come from other organisms.
Conclusion 1: Thus, the organisms alive today must have come from those pre-existing, yet different, species.
Conclusion 2: There must have been change through time (evolution).
Correlated Patterns: The fossil record, vestigial organs, and homologies are all explicable and logical in this context, and inexplicable (even heritical) in some theological contexts (imperfection).
So, if species do change over time (evolve), the next question is "How?" How does this change occur?
a. Domesticated animals:
As a result of selective breeding, humans have taken certain species and modified them tremendously. So, from an ancestral population of wolves, we have created chihuahuas and St. Bernards. Now, there were never any Chihuahua sized wolves running around - we created this variability by breeding progressively smaller dogs with one another. Now, we have two groups, Chihuahuas and St. Bernards, that can't be bred together. So, we have created separate biological groups, that could be called different species. The KEY is that humans only allowed certain organisms to breed. So, only these traits were passed to the next generation. Darwin wondered if there was a mechanism that could do the SAME thing in nature (only allow certain organisms in a population to breed), and thereby explain the natural changes seen in the fossil record and implied by island faunas. (Just because man does it purposefully doesn't mean that nature can't due it WITHOUT purpose. We put water in the freezer to PURPOSEFULLY make ice cubes... but water freezes naturally, as well - without a purpose, but with the same CAUSE (low temperature).
b. In 1838, Darwin read a book published in 1798 by Thomas Malthus called Essay on Population. Malthus was a british aristocrat who was concerned that the british aristocracy might be overthrown like the French had been 10 years before (1788). He realized that all populations have the capacity to grow exponentially, but that resources are finite. As such, there will be a "struggle for existence" (Darwin's words). Malthus was worried that the british population would continue to grow, and that poverty and famine would lead to revolution.
Darwin read this and said "at last I have a theory by which to work"
- If there are limited resources of food, shelter, and mates, and if organisms in a population vary, then as a consequence of this variability, some will be more likely to gain the resources than others, and will thus be more likely to mate. So, the environment will 'select' which animals in a population will mate... and these traits that work to gain resources in this environment will be passed on.
2. Natural Selection: (Know this. Understand it. You WILL be asked to outline NS in this very form.)
P1: Populations over-reproduce (Malthus)
P2: resources are finite (Malthus)
C1: Eventually, a population will grow until it becomes limited by its resources. At that time, their will be a "struggle for existence" and most offspring produced will die. (Malthus)
P3: Individuals in a population vary, and some of this variation is heritable (Darwin - observations and animal/plant breeding) ** (aside - see below)
C2: Variations will not have the same probability of survival and reproduction in a particular environment; those well-suited to the environment will be more likely to survive and reproduce than others, passing on the genes for these adapted traits. There will be "Differential Reproductive Success" (Observations, breeding).
C3: Over time, adaptive traits will accumulate and the characteristics in a population will change. This is lineage evolution. (Like change in horse toes in a sequence of fossil species, or like the change in the chihuahua lineage from the ancestral wolves).
Corollary: Two sub-populations, separated in different environments,
would be selected for different traits and may subsequently lose the capacity
to interbreed. At this point, they are different biological species. This
is Speciation and Radiational Evolution. (like the production of different
Finches, mockingbirds, etc. on different islands in the galapagos, and
like the radiation of St. Bernards AND chihuahua's, which diverged from
one another over time).
** - aside: For 2000 years, Western Civilization had been dominated by the ideas of Plato - his ideas of unchanging eternal essences forming and unchanging harmonious creation dovetailed neatly with Judeo-Christian perpectives that would come to dominate Europe. In this world-view, the perfect essences exist on a different plane; our reality is only a shadowy reflection of that true reality. In terms of living things then, each individual is an imperfect reflection of the true 'essence' or 'archetype' of that species. All closed, three-sided polygons are triangles - they vary, but this variation is unimportant - it is their common attribute of TRIANGLE that is important as this is reflecting the 'essence' of their type. In this "typological" view, individual variation in a species is unimportant... and also, in this Platonic worldview, these essences are unchanging, static, and eternal... there is no way to change one thing into another. Darwin's appreciation for the biological IMPORTANCE of variation was a critical departure. All other natural theologians before him had viewed species as static and the variation as unimportant deviation from the perfect 'type' (think Linnaeus and all the classifiers that came before Darwin - they viewed each species as a separate 'box', and each individual organism had to fit in one box or another based on their similarity to a 'type' specimen). However, Darwin saw that variation was potentially critical, and was heritable. Darwin's "populational" thinking overthrew 2000 years of Platonic, typological thinking.
Chapter Six in "The Origin of Species" is entitled: Difficulties
on Theory
Darwin starts the chapter by writing:
"LONG before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory."
These dilemmas are:
1. How can we explain the existence of complex characteristics composed of mutually dependent parts?
- Paley's "watchmaker analogy" (1798) and the eye - the argument of design. Some elements of nature seem so complex, and composed of mutually dependent parts, that it does not seem possible that they could arise by sequential innovations. For instance, for Paley, half an eye won't work For modern Intelligent Design proponents, the cell is called "irreducibly complex" - suggesting that it can't come from any simpler precursor - everything has to be there. (Of course, for philosophical reasons, this is about as unscientific as it gets. The idea that something is simply "too complex" to understand thwarts curiosity and stifles TRUE scientific inquiry, which is fundamentally reductionistic - as we have discussed.)
- Darwin's Solution: comparative evidence Well, in molluscs, there are a variety of photoreceptive organs. They range from a sheet of receptive cells (a naked retina) to a cupped retina, to a lenseless eye, to a camera eye. If this sequecne exists today, it is possible that this sequence could have existed through time, as a sequential evolutionary series in eye evolution. Half an eye CAN work, if the half you evolve first is the retina.
2. Gradualism and Intermediate Fossils - If evolution proceeds by change from one generation to the next, we should see nearly continuous sequences of species in the fossil record. Where are all these intermediates?
- Darwin said that fossilization is rare... most organisms decompose; they are not fossilized
- also, Darwin suggested that we may find some once we look harder for fossils. But really, this was just an apology for the "incompleteness of the fossil record"
- 1861 - Archeopteryx lithographica unearthed from Bavaria. When previous fossils of this species had been unearthed, they were classified as reptiles because they had teeth, fingers, and a long bony tail. But, when this fossil (and 6 others) were pulled from the very fine sedimentary deposits of Bavaria, the impression of feathers were seen. So, here is an organisms with a very odd combination of traits - a lizard skeleton covered by feathers.
- IMPORTANT: Now, the existence of this odd animal is not the only thing that bears on evolution. Obviously, evolution does predict the existence of intermediates. But even more so, it also predicts WHEN in the fossil record this species should have lived. For instance, If Archeopteryx is a transitional species, it has to be after reptiles appear in the fossil record, and before true birds. It can't just be anywhere and be a transitional species between these groups. Well, it is just where evolutionary theory predicts it should be; after other reptiles and before all true birds. We will see lots of intermediates later in the course, when we look at post-Darwinian developments....
3. Source of Variability:
- All natural populations show variation among organisms. But how is the variability produced?
- first, if selection has been acting on a population for millenia, then the population should be "honed" to fine edge - all individuals should represent the same adaptive type, and thus not differ much from one another.
- second, if blending heredity works (as Darwin thought it did), then we should also get a homogenization of variation over time.... white paint plus black paint makes gray paint, but when you then mix grey paint together all you get is grey paint.
- Darwin proposed many hypothetical mechanisms that might
promote variation, including the "use and disuse" idea of Lamarck.
However, he conducted experiments that disproved these hypotheses, and
he died not appreciating the insights that had been made in Austria only
6 years later, in 1865.
F. A Summary of Darwinian Evolution:
1. Darwin's view of evolution can be summarized as follows:
Source of variation: UNKNOWN
but variation is observable, and exists
Agents of Evolutionary Change: (factors that cause change
in natural populations): Natural Selection
2. Where Now?
- Now we will explain what has happened in the last 150 years of biology. Evolutionary theory doesn't END with Darwin, it only BEGINS there. We will see how the theory has been tested and modified by subsequent scientists. In particular, we will address Darwin's dilemmas... and we will begin by describing the sources of variation and solving Darwin's third dilemma.....