Ask Mendel's Mother

Family Trees

"My son has asked you to construct family trees, called 'phylogenies', of the strange "Three part-ite" creatures he found in church floor tiles.

"These tiles were cut from slabs of rock dug out of the ground and contain the fossilized bodies of long extinct animals that crawled around on the bottom of the sea over 300 to 500 million years ago.

"Because the slabs of rock were cut out in sections, one after another, you can look at each tile or slab, and see a cross section of want kind of creatures were alive at that time, and how many of them there were. You can then look at the next slab in the sequence and see what has changed during that time period. This is very valuable data.

"By comparing all the tiles or slabs in sequence you will be able to see trends in the lineages of these creatures. If you have forgotten what a 'trend' or a 'lineage' is, then go back to the story of my son (Chapter the Second) and re-read the hyperlinked sections on these topics. They will be titled "Patterns of Evolution".

"You must do four things in order to get the patterns, or phylogeny, of the creatures within each set of tiles:

you must first determine the age of the rock you are looking at. To do that you must calculate how deep it was in the strata of rocks containing these fossils, and then convert that depth to an age in "millions of years" by using the 'rate of sedimentation' which tells you how quickly this type of rock is formed. Hummm .... can you work that one out for yourself?

then you must look at the examples of "Threepartites" in each slab or tile. At this point it might be a good idea to give each species a scientific binomial name using the system devised by Linnaeus. If you have forgotten how, re-read my son's story (Chapter the First, 'Interruption', 'classification system' and 'classification of species').

now you should make some notes about what you see. How big is each creature? What does its body look like? Compare the examples from the different tiles in the sequence. Has anything changed since last time, if so, what?

you must also look at the distribution and numbers of each type of creature at each age in time. Are they increasing in numbers, if so, how fast? Are they decreasing in numbers? Are they moving around or staying put? What is going on?
Now you are ready to make a diagram. (It may be a good time to re-read the section on Phylogeny, or Patterns of Evolution, in Chapter the Second!).

"In this phylogeny, species "A" started the lineage millions or years ago (you have to calculate how long ago!). The width of the blue lines in this lineage indicates how many individuals were alive at any one moment (called the "abundance"). The wider the line, the more of species A was found in the rocks.

"Then a 'speciation even' or branching occurred and species "C" was formed. It started to evolve in its own way and became more and more abundant (see the line widening).

"But species A was in trouble. It gradually changed, perhaps enough to be considered a new species (B), but its numbers got less and less until it became extinct.

"You can draw your phylogenies this way, if you want.

"This is a different way of presenting a phylogeny. In this type of diagram only the major species are represented (not their abundances) and all we see is when the branching occurred.

"This type of diagram has other uses and can be very helpful sorting out the relationship between species.

"Each member of each team should complete at least one of the phylogenies from the sets of tiles A to E.

"Draw out the phylogeny for your set and collect all the sets together. Look at them closely. What is going on?

"Write a short (two paragraph) description of the patterns or trends you seen in the phylogeny you have constructed.

Put all the diagrams and descriptions into your team portfolio and hand it to the instructor for grading.

"Good luck!"

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