fossil evidence for diversity trends key...

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



Key terms

• Diversity (= “taxonomic richness” or “biodiversity”)

• “Pull of the Recent”

• Phanerozoic marine evolutionary faunas

• ZFEL



• 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”?


John Phillips

The first diversity curve

(Phillips 1860)


University of Chicago(“center of the universe”

for modern diversity studies)

Jack Sepkoski

David Raup

Michael Foote


Sepkoski’s diversity curve for marine animals


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


Bambach’s diversity curve for marine animals


Vertebratediversity

num

ber

of

ord

ers


Vascular plant diversity(tracheophytes)

num

ber

of

spe

cie

s

(spore plants)


“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


An apparent conundrum:Phanerozoic decline in origination rates

perc

ent

ori

gin

ation



• 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



• 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


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


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).”


Daniel McShea(Biology)

Robert Brandon(Philosophy)


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


Increasing complexity/diversity…


Traveling particle “diffusion”


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


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.

fossil evidence for diversity trends key...

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