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Lecture 14

BIOC50H3 Lecture 14: Lecture Fourteen

Biological Sciences
Course Code
Jason Weir

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Lecture Fourteen
The Evolution of Biodiversity
- Why do some geographic regions like the tropics have more species than others, such as
the arctic?
- Why are there more species in some groups than others? For example, rodents versus
primates or angiosperms vs ferns?
- Why has the diversity of species changed over evolutionary time?
- Does diversity steadily increase, or does it have a limit?
Three approaches to studying biodiversity:
- Ecological
o Ecologists focus on factors that operate over short time scales to influence
diversity within local habitats or regions
- Evolutionary
o On the scale of million of years, extinction, adaption, speciation, climate change
and geological change create the potential for entirely different assemblages of
species to evolve
- Integrated Approaches:
o Ultimately speciation is what causes biodiversity to arise
o Extinction causes biodiversity to decline
o Ecology often has a large effect on both of these processes
Several problems exist in estimating biodiversity lists from the fossil record/ estimate patterns
over time?
- Knowledge of the fossil is incomplete therefore we have biased sampling; this
provides a coarse time scale
- Our sampling of the fossil record is not even at all time periods; therefore, we have better
understanding of the levels of biodiversity at some time periods than others
- Geological strata generally become difficult to locate the further back in time we go due
to erosion
o Paleontologists have statistical methods to correct for sampling effort in different
strata to correct for these issues
- The best sampling of species diversity is of living species at the present
o Pull of the Recent: Because the more recently a taxon arose, the more likely it is
to still be extant
Bias will occur if we include both fossil and living species as we have
more information of the living species
Diversity will seem to increase as we approach the present
o Many congeneric species are not distinguishable in fossil form

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o To fix this: Biases can be reduced by counting only fossil occurrences of each
taxon and excluding it for the time intervals between its last occurrence and the
- Fossils of very rare species are known only from a single fossil/ strata even though they
almost certainly lived longer than that
o These are termed singleton; don’t provide good diversity over time
o Singletons can result in bias as it creates false impression that these taxas are
originating and going extinct very rapidly
o These are generally left out of biodiversity analyses
Taxonomic Diversity Through Time (Look at the patterns of diversity through time)
- By excluding singletons, you can get rid of a proportion of the species, but it doesn’t
change the overall pattern of species
- Extant genera cause pull of the Recent; creates an overestimation of diversity to what we
got from earlier time periods
o There is an upturn of biodiversity
o Better to take out the extant genera
Different taxonomic groups and general trends over time
- Insects, nonmarine tetrapod and vascular land plants have an increase in biodiversity ever
since they came onto land
o They accumulated biodiversity over time; usually occurs at the K/T mass
- If there a general increase in all living groups over time? Has biodiversity globally
increased over time:
o Graph corrected for artifacts and biases (based on 44,000 samples)
o Results: Small increase in time but it’s a huge increase in time when the graph
was not corrected
o Implication: Thus, it is not possible to know if increases in biodiversity levels
through time were just an artifact
- Observations of this:
o There is a general decline in diversity in the Devonian instead of a Paleozoic
o A sharp increase in diversity during the Permian followed by a mass extinction at
the end of the Permian
o A less steep increase in biodiversity from the Jurassic to present
- Rates of Origination and Extinction
o The origination is the rate at which new lineages arrive over time
o Extinction rate: where taxa goes extinct
o The increase in diversity during the Mesozoic and Cenozoic tells us that on
average, the rate of origination has been higher than the rate of extinction
o Rate of origination of genera was highest in Cambrian and Ordovician, and then
later during the early Triassic following the P/T mass extinction
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