UNIT IV: MODERN EVOLUTION
Overview:
Our understanding of genetics has certainly had an extraordinary effect on our understanding of evolution; we know genes are particulate (not fluids), and we understand how genetic and phenotypic variation is produced genetically. We know where new genes come from - they typically come from pre-existing genes that were then altered by a substitution mutation. So, we have solved Darwin's dilemma about the source of variation. But what ELSE have we learned in the 150 years since Darwin and Mendel that bears on Evolutionary Theory?
I. Post-Darwinian Facts
A. Physics - Radioactivity
1. The great age of the Earth:
Darwin had guessed that the earth had to be at least 300 million years old to explain the evolution of life through the stately process of natural selection. W. Thompson ("Lord Kelvin") wa a physicist who demonstrated that the Earth could be no more than 24 million years old. He did this by calculating how long it would take for a molten object with the Earth's mass to cool to the current temperature of the Earth. However, he made his predictions before Becquerel and the Curies discovered radioactivity - a process that releases heat and keeps the Earth warmer than it "should be" (based on Thompson's inference).
2 Marie Curie - discovered radium and studied radiactivity (1898) - Two nobel prizes in chemistry.
3. Radioactive dating-based on constant rate of ‘decay’ from one isotope to another
a. Principle:
- measure amt of parent and daughter isotopes = total initial parental
- with the measureable1/2 life, determine time needed to decay this fraction
- K40-Ar40 suppose 1/2 of total is Ar40 = 1.3by
(Now, the creationists say "be real"! How can we measure something that is this slow?
- Well, 40 grams of Potassium (K) contains:
6.0 x 1023 atoms (Avogadro's number, remember that little chemistry tid-bit?).
So, For 1/2 of them to change, that would be:
3.0 x 1023 atoms in 1.3 billion years (1.3 x 109)
So, divide 3.0 x 1023 by 1.3 x 109 = 2.3 X 1014 atoms/year.
Then, divide 2.3 x 1014 by 365 (3.65 x 102) days per year = 0.62 x 1012 per day ( shift decimal = 6.2 x 1011)
Then, divide 6.2 x 1011 by 24*60*60 = 86,400 seconds/day: (= 8.64 x 104) = 0.7 x 107 atoms/second
0.7 x 107 = 7 x 106 = 7 million atoms changing from Potassium to Argon every second!!!
This radiation is detectible and measureable...
It is actually a very easily measured rate. And you can come back and measure it again tomorrow, next week, next year, or in 50 years...always the same... There are LOTS of atoms, so even if the percent changing is very small, or the rate is very slow, there is still a sizeable, constant stream of atoms decaying at any one time.
b. C14 - decays to C12 with a half-life of 5730 years
c. Longer periods - use different elements with longer half-lives
- K40 decays to Ar40 at a rate of 50%/1.3 b years
- U238 - Pb208 = 4.5 by
- Rb87 - St87 = 47 by
- U238 - Thorium 230 = 80,000
- U235 - Protactinium231 = 34,000
d. Fission tracking
- U238 is found in zircon crystals. When it decays, the nuclear particles shoot of through the crystal and disturb the lattice - leaving a track. Count up the tracks with an electron microscope.
e. Results and Corroborations
- oldest rocks on earth = 3.8 by (solidification of earth’s crust)
- moon rocks, meteorites = 4.5 by (origins of solar system)
4.. corroborations:
- our ability to understand many atomic properties in physics (nuclear power) is dependent on constant decay rates. If they weren’t constant, our predictions wouldn’t fit the observations
-C14, U235 tests - can be tested against known historical events (Wood in Tombs, Pompeii)
- multiple, independent tests on same sample - give same date
- astronomers have postulated that, because of tidal friction and the mass of the oceans and the earth, the duration of an earth’s rotation (day) has slowed at a rate of 2 seconds every 100,000 years. (So days were SHORTER in the past and have SLOWED to current 24 period) This means a year in the past would have MORE days than it does now. Corals lay down a layer of calcium carbonate 'exoskeleton' every day. There are seasonal changes in the thickness of the layers, so years can be distinguished, too. Astrophysicists predict that, based on tidal friction, a year 380 million years ago would have contained 400 days.
So, if corals are dated by radioactive means at 380 million years, then they should have 400 layers (days)/year. Prediction - test - data: They do. So, we can use basic physics to predict the "age" of a coral. And when we compare that "age" to the "age" determined by radioactive decay, the ages are the same. So, either they are both true, or they are both wrong in the same way. It is unlikely that we have such basic physics wrong. Very unlikely. So, the Earth is, indeed, very very old. Radioactive decay is constant; if it wasn't, or if it hadn't been in the past, none of these comparisons would work. period. But they DO work, and so it is irrational to conclude that the Earth is young, or that radioactive dating doesn't work or is somehow "dubious". Again, our ability to harness the power of decay in reactors is powerful testimony to the degree of confidence we have in our knowledge and understanding of the decay process. And thus, we have great confidence in the great age of the Earth.
B. Geology/Paleontology
1. Plate Tectonics
1912 - Alfred Wegener - continental drift ( also called plate tectonics): He observed the similarity in the shapes of South America and Africa and hypothesized that they might have once been joined together. This implied that the continents moved explained. Although this could explain organisms with"disjunct distributions" (such as the Southern Beech and marsupials that live in South Amercia and Australia); the idea was ridiculed for years.
- An alternative model to explain disjunct distributions was the idea of "land bridges". These were hypothetical links between the continents that would have been exposed during the ice ages (when sea levels were lower).
- In the 1960's and 1970's, sonar proved that there were NO land bridges under the ocean, and also showed mid-ocean ridges where volcanic magma was coming to the surface. Measurements subsequently revealed that the sea floor was indeed expanding from these ridges. Continenetal movement and plate tectonic was tested and supported.
2. Mass Extinctions
- Paleontologists had long recognized that the fauna of the Earth occassionally changed abruptly:
500 mya - First appearance of large numbers of hard-bodied organisms - the "Cambrian Explosion"
240 mya - First great mass extinction - loss of the trilobites, etc. Paleontologists called the period from the Cambrian to this extinction in the Permian Period the PALEOZOIC ERA.
65 mya - Extinction of the Dinosaurs. This distinguishes the Cretaceous Period from the Tertiary Period ("K"-T boundary; K instead of C, because C is used for 'C'ambrian). From 240-65mya = MESOZOIC ERA.
65 mya to present - the CENOZOIC ERA.
- So, paleontologists were aware of these abrupt changes. In the '70's and 80's, Walter Alvarez found a layer of iridium in rock 65 mya around the world. Iridium is rare on Earth, but abundant in metoerites. He hypothesized that the earth as hit by a meteor, resulting in an ecological winter that killed off the dinosaurs. Alvarez tested his hypothesis of metoeric impact and found the huge hole in the Gulf of Mexico, which dates to 65 mya. This was strong support for the impact theory, and demonstrated that organisms could go extinct for reasons other that competitive inferiority in a slow, gradual selectionist race.
Permian and K-T extinctions associated with major climatological change; ice ages on a smaller temporal scale.
3. Transitional Fossils
'Transitional' fossils are interesting evolutionarily for two reasons:
First, they contain a complement of traits that makes them hard to pigeonhole
into one group of organisms or another...
in other words, they have a combination of traits from two separate
groups.(an intermediate morphology).
But this is not all. I mean, there are alot of crazy organisms out there. The existence of weird organisms does not prove evolution. But, not ONLY does evolution predict that such weird intermediates should EXIST, it predicts WHEN those intermediates should exist.
Let's apply these ideas with some real fossils....
a. 1861 - Archeopteryx lithographica
Has fingers, teeth, and a bony tail..... all reptilian traits
Has feathers............................ a bird trait
So, it had a combination of traits from two major groups. No birds
today have teeth or fingers, and no reptiles have feathers.
So, it IS intermediate in morphology. Well, that is certainly interesting.
BUT, evolution PREDICTS something else about this organism. IF
it was a biological link between reptiles and birds, then it would
have to have lived after other reptiles (who were its ancestors) and BEFORE
all true birds (who might be its descendants. Evolution predicts
that it can't be just "any old place" in the fossil record - if it is biologically
descended from pre-existing reptiles but ancestral to birds, THEN it must
be in to time interval marked XXXX:
ancient reptiles--------------------------------- modern reptiles
XXX
------------ modern birds
|
First True Birds
It is. Hypothesis tested by evidence from the physical world.Evolution is a testable, supported THEORY.
b. Ichthyostega
Has feet - an adaptation for terrestrial life (like amphibians)
Has an operculum covering the gills,
cartilaginous struts in its tail fin (like Fish)
So, it has an intermediate morphology. Well, that is certainly interesting,
but Evolutionary theory demands MORE. Where is it?
If it is descended from fish, but ancestral to amphibains, it has to occur
in the interval marked XXXX:
ancient fish------------------------------------- modern fish
XXX
----------------- modern amphibians
|
First True Amphibians
It does. Hypothesis tested by evidence from the physical world. It doesn't "have to" be here, except if evolution is true. If it was just a wacky animal, unrelated to either fish or amphibians, it could be anywhere. But it is not. It is just where evolutionary theory predicts it should be.
- has a mammalian skeletal system, and specialized teeth (like mammals)
- form a nice sequence of intermediates from the three-bones
jaw of reptiles to the one-boned jaw of mammals.
- covered in scales (like reptiles)
So, it has an intermediate morphology between reptiles and mammals. Well, that is certainly interesting, but Evolutionary theory demands MORE. Where is it? If it is descended from reptiles, but ancestral to mammals, it has to occur in the interval marked XXXX:
ancient reptiles------------------------------------- modern reptiles
XXX
--------------------- modern mammals
|
First True Mammals
It does. Hypothesis tested by evidence from the physical world. It doesn't
"have to" be here, except if evolution is true. If it was just a
wacky animal, unrelated to either reptiles or mammals, it could be anywhere.
But it is not. It is just where evolutionary theory predicts it should
be.
d. Hominids
There are lots of hominid fossils
I'll focus on just one of note - Australopithecus afarensis
- head with 1/4 cranial volume as humans, equal to chimps
- bipedal; knee joint at angle like humans, pelvis bowl-shaped
- jaws more ape-like - a snout, but a rounded "arcade" of teeth yet still with large canines.
So, it has an intermediate morphology between apes and humans. Well, that is certainly interesting, but Evolutionary theory demands MORE. Where is it? If it is descended from apes, but ancestral to humans, it has to
occur in the interval marked XXXX:
ancient apes------------------------------------- modern apes
XXX
--------------------- modern humans
|
First members of the genus Homo
It does. Hypothesis tested by evidence from the physical world. It doesn't "have to" be here, except if evolution is true. If it was just a wacky animal, unrelated to either apes or humans, it could be anywhere. But it is not. It is just where evolutionary theory predicts it should be.
Yes, this is redundant. But it is redundant for a reason. There aren't just one or two fossils that 'conform' to the expectations of evolutionary theory. There are 1000's of intermediates that provide tests and confirmation of evolutionary theory. Creationists like to say that "evolutionism" is as much of a religion as "creationism". Uh, no. Common ancestry was testable, falsifiable hypotheses about the physical world. It has been confirmed by evidence from the physcial world . It is not religion, it is science. Creationists also claim "where are the intermediates?" Well, you've seen a few now. Darwin predicted their existence. Although only Archeopteryx was discovered in his lifetime, we now have intermediates linking all major groups of vertebrates. This was just a sample. So, next time you read that "intermediates don't exist", you will recognize that this is a falsehood - a lie. And it might lead you to question why certain people keep repeating this, when they know it is not true...
And, it is important to realize that evolution by common descent is NOT the only "naturalistic" hypothesis of species origins. There are others, and all these hypotheses are testable. The hypotheses are:
A. Single separate origin, no change - predicts similar fossils throughout strata and no intermediates between groups.
B. Multiple separate origin, no change - intial fauna goes extinct - a new fauna is produced - different fossils in different strata; no intermediates
C. Single separate origin, but each lineage changes over time - differnt fossils, intermediates, no "linking" common ancestors.... This was actually Lamarcks idea... he accepted the fossil evidence that species changed over time, but he also believed in a perfect initial divine creation. He reasoned that the perfect harmony of the whole system could persist even if species changed over time, if LOTS of species were changing. Sort of like a soap bubble - you can see that the oils on the surface of the bubble are swirling on its surface, changing all the time, yet still maintaining the total 'perfection' of an intact bubble.
When we test hypotheses of origins with fossils, hypotheses of "no change", "multiple origins", and "Lamarckian transformation" are falsified by the fossil data:
1) Different organisms are preserved at different times (falsifying A, the "no change" hypothesis),
2) There are intermediates (falsifying B, the "multiple origins" hypothesis)
3) There are linking intermediates (falsifying C, the "Lamarckian transformation" hypothesis).
4) The fossil records confirms the Theory of Evolution by Common Descent from shared ancestors (hypothesis D).
Study Questions:
1. What was Thompson's estimate of the Earth's age? How did he reach this conclusion, and why was he wrong?
2. If the half life of K->Ar decay is 1.3 by, and we have a rock with a 3:1 ratio of Ar to K, how old is it?
3. Describe an independent test of radioactive dating see notes above.
4. What are disjunct faunas and how does plate tectonics solve that "problem"?
5. What are the two interesting things about intermediates, with respect to evolutionary predictions?
8. What traits make Archeopteryx an intermediate? Ichthyostega? Therapsids? Australopithecines?
9. What data falsifies other models of origins and supports common descent?