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Fossils & Evolution "Diversity" 1
Fossil evidence for diversity trends
Key concepts
• Paleontologists attempt to document the history of biodiversity by tabulating fossil occurrences in large databases
• Diversity has increased during Phanerozoic time, but with significant short-term decreases caused by mass extinctions
• Rediversification following mass extinctions has resulted in the replacement of “evolutionary faunas”
• Rates of origination (and extinction) have decreased during Phanerozoic time
• ZFEL states that, absent selection or constraints, diversity and organismal complexity are expected to increase over time
Fossils & Evolution "Diversity" 2
Fossil evidence for diversity trends
Key terms
• Diversity (= “taxonomic richness” or “biodiversity”)
• “Pull of the Recent”
• Phanerozoic marine evolutionary faunas
• ZFEL
Fossils & Evolution "Diversity" 3
Fossil evidence for diversity trends
• For paleontologists, diversity means
taxonomic richness, or the number of taxa
present at a given time
• Diversity trends have been studied since at
least 1860
• Major questions:
– Has diversity steadily increased over time?
– Did Earth’s diversity increase and then level off at
some finite “carrying capacity”?
Fossils & Evolution "Diversity" 4
John Phillips
The first diversity curve
(Phillips 1860)
Fossils & Evolution "Diversity" 5
University of Chicago(“center of the universe”
for modern diversity studies)
Jack Sepkoski
David Raup
Michael Foote
Fossils & Evolution "Diversity" 6
Sepkoski’s diversity curve for marine animals
Fossils & Evolution "Diversity" 7
Sepkoski’s Phanerozoicmarine evolutionary
faunas
• “Cambrian fauna” was dominated by trilobites and other weakly calcified invertebrates
• “Paleozoic fauna” was dominated by sessile, filter-feeding invertebrates
• “Modern fauna” is dominated by active grazers and predators (invertebrates and vertebrates)
• Faunal replacementsoccurred in the aftermath of mass extinctions
Fossils & Evolution "Diversity" 8
Bambach’s diversity curve for marine animals
Fossils & Evolution "Diversity" 9
Vertebratediversity
num
ber
of
ord
ers
Fossils & Evolution "Diversity" 10
Vascular plant diversity(tracheophytes)
num
ber
of
spe
cie
s
(spore plants)
Fossils & Evolution "Diversity" 11
“Pull of the Recent”
• All diversity curves show taxonomic richness
reaching its peak at the present time
– Is today’s diversity really the peak?
• Yes, but values for today and the recent past
are inflated relative to the distant past
because the recent biota is much better
sampled than the older fossil record
Fossils & Evolution "Diversity" 12
An apparent conundrum:Phanerozoic decline in origination rates
perc
ent
ori
gin
ation
Fossils & Evolution "Diversity" 13
An apparent conundrum:Phanerozoic decline in origination rates
• How did diversity increase if origination rates decreased?
Time 1
Time 2
D2 = D1 + Orig2 – Extinct2
Diversity will increase if orig > extinctDiversity will decrease if extinct > orig
D2
D1
Fossils & Evolution "Diversity" 14
An apparent conundrum:Phanerozoic decline in origination rates
• Diversity is increasing, but the rate of
increase is slowing down
– Car analogy: Velocity will continue to increase
even when rate of acceleration decreases
• Origination rate is calculated on a per capita
(%) basis:
– If D = 100, then a 10% increase = 10 new taxa
– If D = 100,000, then a 1% increase = 1,000 new
taxa
Fossils & Evolution "Diversity" 15
Diversity, complexity and ZFEL
• In the broadest sense, there are threeseparate but related aspects to evolution:– Adaptation
– Diversity
– Organismal complexity
• Adaptation occurs as a consequence of natural selection
• Natural selection is not required for changes in diversity and organismal complexity
Fossils & Evolution "Diversity" 16
Diversity, complexity and ZFEL
• ZFEL = “Zero Force Evolutionary Law”
– Proposed by Daniel McShea & Robert
Brandon (2010)
• “In an evolutionary system in which
there is variation and heredity, in the
absence of selection or constraints,
diversity and complexity will increase
(on average).”
Fossils & Evolution "Diversity" 17
Daniel McShea(Biology)
Robert Brandon(Philosophy)
Fossils & Evolution "Diversity" 18
The law says that in the absence of selection and constraint, complexity – in
the sense of differentiation among parts – will tend to increase. Further, even
when forces and constraints are present, a tendency for complexity to
increase is always present. The rationale is simply that in the absence of
selection or constraint, the parts of an organism will tend spontaneously to
accumulate variation, and therefore to become more different from each
other. Thus, for example, in a multicellular organism, in the absence of
selection and constraint, the degree of differentiation among cells should
increase, leading eventually to an increase in the number of cell types. The
law applies at all hierarchical levels (molecules, organelles, cells, etc.). It
also applies above the level of the organism, to differences among
individuals in populations, and to differences among species and among
higher taxa. In other words, the ZFEL says that diversity also tends
spontaneously to increase.
(excerpted from McShea’s website)
ZFEL
Fossils & Evolution "Diversity" 19
Increasing complexity/diversity…
Fossils & Evolution "Diversity" 20
Traveling particle “diffusion”
Fossils & Evolution "Diversity" 21
Increasing complexity and diversity can be viewed as a “diffusion process”
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1 step
σ = 0.82
2 steps 3 steps 4 steps
5 steps 6 steps 7 steps 8 steps
σ = 2.4
Fossils & Evolution "Diversity" 22
time
std
. dev.
In a series of random steps (no selection
or constraints) the mean value never changes,
but variance about the mean increases indefinitely.This “diffusion process” is analogous to the ever-
increasing complexity/diversity predicted by ZFEL.