Biology Lecture 18.docx

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30 Apr 2012
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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 layers
How 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 100 % parent isotopes, and after one
half life has elapsed we will be left with a 1:1 ratio of parents to daughter isotopes and
after another half-life, 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 estimate on the absolute age of that rock
Fossil record is incomplete and biased:
o In order to form and preserve fossils, many conditions have to be met
o The vast majority of species that have lived have never completely formed fossils
o Not everything is equally likely to fossilize (biased)
Temporal biases: you are more likely to find a new fossil then an ancient fossil
Taxonomic biases: some organisms are more likely to fossilize than others
because of the body parts or structures they have (Example: animals with hard
shells, teeth, bones are able to fossilize well)
Habitat biases: some habitats just have better conditions for fossil formation
(Example: marine conditions are great for fossil formations)
New species can "appear" via anagenesis or cladogenesis:
o Cladogenesis: new species appear because of speciation events (at some point lineages
become reproductively isolated from one another and leads to the formation of two
separate lineages from a branching event)
o Anagenesis: changes without branching or speciation (no branching event, it simply
indicates morphological change within one lineage)
Tempo of evolution:
o Gradualism: lineages are changing and evolving all the time, whether or not that change
is associated with a branching event, speciation event (Darwin)
o Punctuated Equilibrium: all morphological change is associated with speciation events
(rapid evolution) and is followed by periods of stasis (absence of evolutionary change)
o In the fossil record it is more common to see the relatively more sudden appearance of
new forms of new species (coincides with punctuated equilibrium)
o Gradualists would explained this by stating the problems with the fossil record
o Some groups evolve suddenly, and some groups evolve gradually
Evolutionary trends:
o Cope's Rule: body size tends to increase over evolutionary time
Possible explanations for Cope’s Rule:
o Individual-level selection (fitness advantages)
o Species-level selection
o Passive “floor effect”: first living things were small-bodied, nowhere to go but up
Exceptions to Cope’s Rule:
o Animals with exoskeletons typically do not change body size because the exoskeleton
sets a limit on the body size of these living things (Example: insects)
o Does not always occur, it is just a general trend