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Lecture 18

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Western University
Biology 1001A
Tom Haffie

Biology Lecture 18 Macroevolution When we say how closely related or how distantly related are two species what we really mean is how far back in evolutionary time did they last share a common ancestor Patterns in macroevolution o Most of what we know of macroevolution comes from the fossil record How old is that fossil o Law of Superposition when dealing with sedimentary rock formation rocks deposited in layers newer rocks get deposited on top of older layers o Therefore fossils embedded in the top layer of a particular rock formation are going to be more recent and fossils in the bottom layers are going to be relatively more ancient o However this tells us nothing about the absolute ages of the different rock layersHow old is that fossil o The half lives are a period of time required for half of the remaining parent atoms to decay or decompose to the daughter atoms or isotopes o Therefore when rocks are newly formed they are 100parent isotopes and after one half life has elapsed we will be left with a 11 ratio of parents to daughter isotopes and after another halflife we will be left with 25 parent and 75 daughter isotopes o This allows us to count how many half lives have elapsed and multiply by the length of the half live which gives us an estimat
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