ch. 7—evolution and the fossil recordfaculty.chas.uni.edu/~groves/ehch07lecture.pdfbody size to...

Earth History, Ch. 7 1

• Evolution (popular definition) = descent

with modification

• Evolution (technical definition) = change in

gene frequencies or gene combinations in a

series of populations, brought about by

natural selection

Ch. 7—Evolution and the fossil

record


• The basic tenets of evolution

– In natural populations, more offspring are produced than can be sustained in the environment

– Mutations and genetic recombinations are the sources of variability among individuals

– Natural selection results in the differential survival of variants

– Variation is heritable: therefore, the more successful variants preferentially will pass on their genes to following generations, and through time the overall composition of the gene pool will shift

Evolution and the fossil record


Extinction

• Extinction may result from one or more of these causes, if taken to extremes

– Predation

– Disease

– Competition from other species

– Change in physical environment or ecosystem

• Rates of extinction vary by type of organism, for example

– “lifespan” of mammal species is 1 to 2 million years (extinction rate of 50–100% every million years)

– “lifespan” of bivalve species is ~10 million years (extinction rate of 10% every million years)


Extinction

• Mass extinctions = intervals during which unusually large numbers of taxa suddenly become extinct (e.g., 40% or more genera)

• Causes of mass extinctions may be extraterrestrial or a combination of earthly factors

• Five major mass extinctions in Phanerozoic history


Origination

• Evolutionary radiation = episode of rapid evolutionary expansion (production of large numbers of new taxa)

• Radiations usually occur shortly after the origin of a new major taxon

• Radiations may follow extinctions—as ecologic “replacements”

• Radiations may follow an adaptive breakthrough

– The appearance of some key feature that allows ecologic and morphologic diversification


Origination

• Example: radiation of hexacorals in

Triassic Period

– Radiation quickly followed the extinction of

Paleozoic rugose and tabulate corals (vacant

niche)

– Radiation was facilitated by adaptive

breakthrough: the ability to quickly secrete

large skeletons using relatively little CaCO3


Triassic hexacorals


Adaptive radiation of hexacorals in middle Triassic time(in the aftermath of the end-Permian mass extinction)


Extinction & origination

• Organisms that have high rates of origination may

also have high rates of extinction, making them well

suited as guide fossils—ammonoid cephalopods


• Convergence and iterative evolution

– Marsupial/mammal and foram examples

• Evolutionary trends

– Cope’s rule

– Evolution of the whales

– Evolution of the horses

• Phyletic gradualism vs. punctuated equilibria

Patterns of Evolution


Evolutionary convergence

• Convergence = the evolution of similar

form in two or more distinct biologic groups

– Example: similarity between marsupials and

placental mammals

• Unrelated or distantly related groups

typically converge on particular forms that

have high adaptive value


Convergence

Although marsupials and placentals mammals have a common ancestor, there has been no genetic communication between the two groups sincethe breakup of Pangaea, during the Mesozoic Era.


Iterative evolution

• Iterative evolution = the repeated evolution of a

particular form from the same ancestor, but at

different times

• Ancestral form is usually a stable (morphologically

conservative) and long-ranging taxon

• Similar descendant forms arise periodically by

chance? [Or, genetic regulation may allow variation

in only one direction?]


Iterative evolution

A

B

C

tim

e

morphology

A second descendant, C,

which may be nearly identical

to B, originates from A long

after the extinction of B.

Stable ancestral taxon A

gives rise to slightly more

complex descendant B.


Iterative Cenozoic planktonic foram radiations

Quaternary

Pliocene

Oligocene

Miocene

Eocene

Paleocene

glo

big

erin

oid

glo

big

erin

oid

has

tiger

ine

has

tiger

ine

orb

uli

ne

orb

uli

ne

pull

enia

tine

glo

big

erin

e turb

oro

tali

dtu

rboro

tali

d

glo

boro

tali

dglo

boro

tali

d

conic

alco

nic

al

Pa

leo

gen

eN

eog

ene


Evolutionary trends

• Cope’s rule = the general tendency for

body size to increase during the evolution of

a particular group of organisms. Examples:

– Whales probably originated from even-toed

ungulates ~50 mybp

• Progressive modification of appengages

• Progressive adaptation to marine environment

• Progressive increase in size


Evolutionary trends (cont.)ti

me


Whale

ancestor

(note hoof-like

nails and position

of nostrils)


Whale

ankle bones

A = newly discovered bone

B = ankle of extinct, carnivorous

ungulate

C = ankle of even-toed ungulate

(like a hippo)

AB C


Evolutionary trends (cont.)

• Horses

– Originated ~ 55 mybp

– Earliest forms were dog-size, with four toes on

front feet and three toes on rear feet, and small

molars

– Modern horse is large, with a single hoofed toe

on each foot, and complex molars for grinding

grasses


Modes of evolution

• Phyletic transition

– Gradual, stepwise evolution

• Punctuated equilibria

– Long periods of stability “punctuated” by short

bursts of evolutionary change


Types of evolutionary changeti

me

Gradual, or phyletic transition

in response to directional

environmental change

Speciation, as a result of

geographic isolation and

then reproductive isolation

gene pool gene pool

modif

icat

ion o

f a

spec

ies

thro

ugh t

ime

splitting of one

species into two


Phyletic

transition

Gradual evolution of the

Jurassic oyster, Gryphaea.

Note increase in size as

well as progressive flattening

of the lower valve.


Punctuated

equilibrium

Evolution of bowfin fishes.

Note long period of stability

(Cretaceous–late Eocene),

then rapid speciation followed

by another long period of

stability (late Eocene–present).

ch. 7—evolution and the fossil recordfaculty.chas.uni.edu/~groves/ehch07lecture.pdfbody size to...

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