Fossils & Evolution Chapter 4 1

Ch. 4—Key concepts

• Systematics is the study of the kinds (diversity) of organisms and of the evolutionary relationships among them.

• A goal of systematics is to classify organisms “naturally,” so that the various groups are evolutionarily meaningful.

• Cladistics is a scientifically testable method for inferring evolutionary relationships on the basis of shared, derived characters.

Fossils & Evolution Chapter 4 2

Ch. 4—Key terms

• Taxonomy / systematics

• Cladistics / cladogram

• Principle of Parsimony

• Cladistic classification– Monophyletic grouping– Paraphyletic grouping– Polyphyletic grouping

Fossils & Evolution Chapter 4 3

Classification

• Classification is necessary because there are more species than any person can keep track of…

• Purpose of classification is communication– “Birds” is a convenient and readily understood

category for all feathered vertebrates

Fossils & Evolution Chapter 4 4

Taxonomic categories

• Taxonomy = the science of classification• Linnean taxonomic hierarchy (nested structure of

groups within groups):

KingdomPhylum

ClassOrder

FamilyGenus

species

Fossils & Evolution Chapter 4 5

Classification and Evolution

• Biologic classification should reflect evolutionary relationships– Species with a common ancestor should be

grouped into a single genus– Closely related genera should be grouped into a

single family

Fossils & Evolution Chapter 4 6

Systematics

• If classification is to reflect phylogeny, how do we assess genealogic relationships among species?

• Systematic approaches– Evolutionary taxonomy– Cladistics

Fossils & Evolution Chapter 4 7

Evolutionary taxonomy

• “Intuitive” taxonomy based on the views of “authorities”

• Genealogic relationships inferred on the basis of– Morphology– Stratigraphy– Biogeography

• Pitfall: often not scientifically testable

Fossils & Evolution Chapter 4 8

Cladistics

• Cladistics is a method of inferring genealogy and classifying organisms on the basis of shared, derived traits

• In a group of related species, two species are judged to be most closely related if they share a “derived” trait that is not shared by other species in the group

Fossils & Evolution Chapter 4 9

Cladistics

• All living things are related (like cousins)

• Any two taxa share a common ancestor

ASSUMPTIONS

memy

cousins

grandpagrandma

mysister

myparents

Fossils & Evolution Chapter 4 10

Cladistics

Branch tips are taxabeing investigated

Node representslatest common ancestor

(possessing featuresshared by taxa at

branch tips)

Fossils & Evolution Chapter 4 11

Cladistics

When more than two taxa are being considered,there are multiple possible branching arrangements

Two taxa are considered to be closely related if they share derived traits that are not present in the third taxon

Fossils & Evolution Chapter 4 12

Trees, trees, trees…….

number of taxabeing investigated

23456789

10

number ofpossible trees

14

26236

2,75239,208660,032

12,818,912282,137,824

Fossils & Evolution Chapter 4 13

Cladistics procedure:e.g., coelomates

coelom notochordmammary

glandstetrapod

coral 0 0 0 0brachiopod 1 0 0 0fish 1 1 0 0human 1 1 1 1turtle 1 1 0 1

0 = absent1 = present

Fossils & Evolution Chapter 4 14

Test two hypotheses

coelom

turtl

efis

hhu

man

brac

hiop

od

cora

ltu

rtle

fish

hum

an

brac

hiop

od

cora

l

coelom

notochord notochord

tetrapod

mammaryglands

mammaryglands

tetrapod

Tree length = 5 steps Tree length = 4 steps

Fossils & Evolution Chapter 4 15

Cladistics procedure (cont.)

• Given multiple hypotheses, the favored one is the one that requires the fewest steps– Simplest explanation is most parsimonious

Fossils & Evolution Chapter 4 16

Cladistics and classification

• “Clade” = an ancestor and all of its descendants– A clade is a monophyletic group

• Clades should form the basis for a classification that reflects genealogic relationships

A B C D

rectangles surroundmonophyletic groups

Fossils & Evolution Chapter 4 17

Cladistics and classification

A B C D A B C D E F

Paraphyletic group—doesn’t containall descendants of a given ancestor

Polyphyletic group—containsdescendants from more than oneimmediate ancestor

Fossils & Evolution Chapter 4 18

mam

mal

s

turtl

es

Liza

rds

& sn

akes

croc

s

dino

saur

s

bird

s

ClassMammalia

ClassReptilia

ClassAves

Cladistic analysisof amniotes

Fossils & Evolution Chapter 4 19

mam

mal

s

turtl

es

Liza

rds

& sn

akes

croc

s

dino

saur

s

bird

s

Amniota

Reptilia

Sauria

Archosauria

Dinosauria

Fossils & Evolution Chapter 4 20

Old

Wor

ldM

onke

ys

gibb

ons

Ora

ngut

an

Gor

illa

Chi

mpa

nzee

hum

an

FamilyPongidae

FamilyHominidae

Cladistic analysisof Old World apes

Fossils & Evolution Chapter 4 21

Old

Wor

ldM

onke

ys

gibb

ons

Ora

ngut

an

Gor

illa

Chi

mpa

nzee

hum

an

Superfamily Hominoidea

Family Hominidae

Unnamed clade

Tribe Homininae

Fossils & Evolution Chapter 4 22

Cladistics • Now the most widely used method for

classification and phylogeny reconstruction• But, beware of “derived” traits that have

originated independently in multiple lineages– e.g., eyes in arthropods, mollusks and

vertebrates

• Biogeographic and stratigraphic information cannot be ignored

Fossils & Evolution Chapter 4 23

Molecular systematics

• Classification and phylogeny reconstruction on the basis of genetic and other biochemical information (rapid advances beginning in 1980’s)

• Example:– Identify sequence of base pairs in the same

gene segment among taxa being analyzed

Fossils & Evolution Chapter 4 24

Structure of DNA

Doublehelix

Strands bondedby base pairs

A = adenineT = thymineG = guanineC = cytocene

Fossils & Evolution Chapter 4 25

Data Matrix

Fossils & Evolution Chapter 4 26

Cladistic analysis of base pair sequences

SPEC

IES1

SPEC

IES2

SPEC

IES5

SPEC

IES3

SPEC

IES4

SP

EC

IES

1

SP

EC

IES

2

SP

EC

IES

5

SP

EC

IES

3

SP

EC

IES

4

4

6 9

10