An asteroid strike, dinosaur extinction, and a bench-top fossil dig.

I teach a weekly hands-on laboratory as part of my evolution course each Spring at Saint Michael's College in Vermont. Fossils are essential components of the evolutionary biologist's toolbox and I wanted an engaging way to look at fossils that would generate quantitative data for student lab reports.  I'm lucky to to have access to the college's modest fossil collection.  The collection includes some fascinating examples but does not lend itself to a particularly quantitative laboratory exercise.

Show-and-tell with fossils has it's place but I had a more comparative laboratory in mind.  I wanted students to experience the sense of discovery of a fossil dig, generate quantitative data, and have a comparative question to answer using real data.  Microfossils turned out to be the answer!

Where ever marine sediments accumulate, the hard parts of microscopic organisms also accumulate.  These microfossils tend to be drawn from diverse and unrelated groups but are studied as a single category because they are all small, and are extracted from mixed sediments using the same techniques.

There certainly are abundant marine sediments right under our campus in the Champlain Valley, but they are from a limited number of time periods.  My interest in working in sediments of diverse ages brought me to one of my favorite sources of specimens: Ebay!  A fossil dealer in England offered a collection of pre-sifted fossiliferous sediments in labeled vials.  The samples come from sediments spanning a more than 200 million years of deposition.

I purchased vials that came from before and after the KT boundary that marks the time in Earth's history when the dinosaurs disappeared in a geological blink of the eye.  While various hypotheses regarding dinosaur extinction have been tested, an asteroid strike near the Yucatan Peninsula in Mexico is supported by the overwhelming force of evidence.

Dinosaurs capture the imagination and the headlines, but most other groups of organisms on the planet suffered severe declines at the time the non-avian dinosaurs vanished.  Conveniently, microfossils are drawn from several groups of organisms and so we can reasonably expect a reduction in the number of species found in microfossil samples following the KT boundary.

My students picked through the small vials of sand to isolate and count microfossils using techniques developed by Charlie Drewes. They then counted the number of distinct types of fossil in each sample and measured a diversity index.  After repeating this procedure for each vial, they ran statistical analyses to determine whether there was, as predicted, a fall in the number of species and diversity detected following the KT boundary.

I'm happy to say that the new laboratory was a success and will become part of the regularly scheduled labs in the Evolution course.

Image source: http://commons.wikimedia.org/wiki/File:Mikrofossils_hg.jpg

Artificial selection: simulating rapid evolution on a spreadsheet.

I was rereading Richard Dawkins' The Blind Watchmaker recently and became interested in one of the analogies he used to illustrate natural selection and artificial selection.  The analogy he presented was that of a monkey poking at keys on a typewriter.The chances of the monkey producing a particular phrase are vanishingly small and the time needed to repeat the process until the correct phrase was randomly typed would exceed the lifetime of the monkey and indeed several generations of monkeys.

I have access to neither a monkey nor a typewriter so I settled upon MS Excel instead.  I instructed Excel to randomly plop letters in each of 8 cells to see if I could match an 8-letter word: "CharlesD".  Not surprisingly, I never found a match despite running several thousand iterations of my experiment.  It illustrates beautifully the idea that random evolution could never work.  Expecting several random mutations to spontaneously generate an organism with traits that fitted it for a particular set of environmental conditions is similarly unlikely.

But evolution is far from a random process.  Certainly mutations can be random, but the survival and reproduction of the resulting mutants is certainly non random. If several mutants are about average, better, or worse at catching prey and therefore surviving and reproducing, then those better suited to predation will survive, reproduce, and pass those superior mutant genes in greater numbers to the next generation.  This is the process of natural selection as outlined by Darwin and Wallace in the 19th century.

Artificial selection is rather like natural selection on steroids.  A plant or animal breeder can artificially select some desirable trait over several generations.  Each time a particular plant or animal better matches that trait, it will be selected as breeding stock.  This brought me to a second simulation. Again MS Excel plopped random letters in cells in a spreadsheet row, but checked the letters against the target phrase "CharlesD".  Any letters that matched the target phrase were placed in the next row and new random letters or 'mutations' were placed in other the remaining slots.  Every time a letter matched one from the target phrase, it was retained and passed onto the next row or 'generation'.

I found that in this simulation the phrase evolves to match the target phrase in a short number of repetitions; often fewer than 100 generations.  One such simulation is presented below.  The letters plopped into the phrase are just as random as the mutant letters in the first simulation I mentioned above.  The crucial difference is that the target letters are retained in successive generations as would be the case if a cattle breeder selected for higher butterfat content of milk over generations.

Try it using your own phrase in this simple MS Excel sheet.

An example that converged on the target in 30 generations:

	Target phrase:	C	H	A	R	L	E	S	D
generation
1		H	D	A	V	R	X	P	U
2		T	K	A	L	G	L	U	N
3		G	I	A	P	P	X	B	P
4		W	S	A	W	S	U	O	J
5		I	W	A	R	X	V	O	C
6		Q	R	A	R	K	V	O	Z
7		P	W	A	R	G	X	M	L
8		C	J	A	R	U	I	G	P
9		C	P	A	R	A	E	T	A
10		C	W	A	R	I	E	M	F
11		C	H	A	R	Y	E	T	Y
12		C	H	A	R	I	E	F	I
13		C	H	A	R	Q	E	L	B
14		C	H	A	R	Q	E	D	E
15		C	H	A	R	E	E	L	W
16		C	H	A	R	J	E	Y	J
17		C	H	A	R	X	E	S	D
18		C	H	A	R	Z	E	S	D
19		C	H	A	R	Z	E	S	D
20		C	H	A	R	I	E	S	D
21		C	H	A	R	E	E	S	D
22		C	H	A	R	J	E	S	D
23		C	H	A	R	H	E	S	D
24		C	H	A	R	O	E	S	D
25		C	H	A	R	Z	E	S	D
26		C	H	A	R	M	E	S	D
27		C	H	A	R	M	E	S	D
28		C	H	A	R	P	E	S	D
29		C	H	A	R	U	E	S	D
30		C	H	A	R	L	E	S	D