Classification

Hierarchical classificationPantherapardus

Panthera

Felidae

Carnivora

Mammalia

Chordata

Animalia

EukaryaDomain

Kingdom

Phylum

Class

Order

Family

Genus

Species

 The connection between classification and phylogeny

Panthera pardus

(leopard)

Mephitis mephitis

(striped skunk)

Lutra lutra (European

otter)

Canis familiaris

(domestic dog)

Canislupus (wolf)

Panthera Mephitis Lutra Canis

Felidae Mustelidae Canidae

Carnivora

Ord

er

Fa

mil

yG

en

us

Sp

ec

ies

Monophyletic, paraphyletic, and polyphyletic groupings

(b) Paraphyletic. Grouping 2 does not meet the cladistic criterion: It is

paraphyletic, which means that it consists of an ancestor (A in this case) and some, but not all, of that ancestor’s descendants. (Grouping 2 includes the descendants I, J, and K, but excludes B–H, which also descended from A.)

(c) Polyphyletic. Grouping 3 also fails the cladistic test. It is polyphyletic, which

means that it lacks the common ancestor of (A) the species in the group. Further-

more, a valid taxon that includes the extant species G, H, J, and K would

necessarily also contain D and E, which are also descended from A.

D E

C

G H

F

J K

I

D E

C

G H

F

J K

I

D E

C

G H

F

J K

I

B

A

B

A

B

A

Grouping 2 Grouping 3Grouping 1

(a) Monophyletic. In this tree, grouping 1, consisting of the seven species B– H, is a

monophyletic group, or clade. A mono-phyletic group is made up of an

ancestral species (species B in this case) and all of its descendant species. Only

monophyletic groups qualify as legitimate taxa derived from cladistics.

Not all Similarities Represent Common

Ancestry• Homologous structures indicate shared

common ancestry• Homologous structures are therefore

evidence of divergent evolution• Analogous structures are similar in

function but not in evolutionary history• Analogous structures are evidence of

convergent evolution• It is not always easy to sort

homologous from analogous structures

How would you compare the fins in these 2 organisms?

In what way are these organisms displaying examples of convergent evolution?

• How do we differentiate between homologous and analogous structures?

• Compare embryonic development of the structures in question

• Look for structures that are complex.• The more complex the structures are the

more likely that they are homologous.•

Compare macromolecules along with anatomical features.

Phenetics• Phenetics =

– compares many anatomical characteristics to (overall phenotype) to construct phylogeny

– Does not sort homologous from analogous structures.

• phylogenetic trees

Cladistics

• Cladistics = – sorts homologous from analogous

structures– sorts primitive and shared derived

characteristics

Shared Derives Characteristics

• Need to differentiate between shared primitive characters and shared derived characters

All similar characters

Analogies

Homologies

Primitive (ancestral)

Derived (unique to

a clade)

Outgroups

• Distinguishes between shared primitive and shared derived characteristics

• Closely related to ingroup

Building a Cladogram

What is the shared primitive characteristic?

Does not mean that turtles evolved more recently than salamander

Notochord

Parsimony and the analogy-versus-homology pitfall

Lizard

Four-chamberedheart

Bird Mammal

Lizard

Four-chamberedheart

Bird Mammal

Four-chamberedheart

(a) Mammal-bird clade

(b) Lizard-bird clade

Parsimony does not always work, nature does not always take the simplest course

Modern Systematics

 Aligning segments of DNA1 C C A T C A G A G T C C

1 C C A T C A G A G T C C

2 C C A T C A G A G T C C

2 C C A T C A G A G T C C

G T A

Deletion

Insertion

1 C C A T C A A G T C C

2 C C A T G T A C A G A G T C C

1 C C A T C A A G T C C

2 C C A T G T A C A G A G T C C

1 Ancestral homologous DNA segments are

identical as species 1 and species 2 begin to

diverge from their common ancestor.

2 Deletion and insertion mutations shift what had been matching

sequences in the two species.

3 Homologous regions (yellow) do not all align

because of these mutations.

4 Homologous regions realign after a computer program adds gaps in

sequence 1.

Bird Phylogenetic Tree

• And sometimes the simplest explanation is not the best…

Classical Taxonomy

Hominidae Pongidae

Cladistic Taxonomy

PongidaeHominidae

Trees with different likelihoods

Tree 2 assumes different rates of

DNA changes

Molecular Clocks• Proteins and mitochondrial genomes

have constant rate of change over time• Use these rates to determine relative

evolutionary relatedness.

Review Questions

• 1. The temperature at which hybrid DNA melts is indicative of the degree of homology between the DNA sequences. The more extensive the pairing, the higher the temperature required to separate the strands. You are trying to determine the phylogenetic relationships among species A, B, and C. You mix single-stranded DNA from all three species (in test groups of two) and measure the temperatures at which the hybrid DNA melts (separates). You find that hybrid BC has the highest melting temperature, AC the next highest, and AB the lowest.

• (cont.) From these data you conclude that *A. species A and B are most closely related,

whereas B and C are least closely related.B. B and C must be the same species, and A is

more closely related to C than to B.C. species B and C must have diverged most

recently, and A is more closely related to C than to B.

D. A hybridizes most easily with B, and they must have a more recent common ancestor than do A and C.

E. these tests are inconclusive and you had better go back and check the fossil record.

• 2. According to this dichotomous phylogenetic tree created using cladistic analysis, C and D are most closely related because they

A. do not share a common ancestor with O, A, or B.

B. are monophyletic.• evolved from a

common ancestor a long time ago.

• have the most shared derived characters in common.

• have the greatest number of anatomical similarities as shown by statistical analysis.

• 3. A biologist discovers two new species of organisms, one in Africa and one in South America. The organisms resemble one another closely. Which type of evidence would probably be least useful in determining whether these organisms are closely related or are the products of convergent evolution? A. the history and timing of continental drift B. a comparison of DNA from the two speciesC. the fossil record of the two speciesD. analysis of the behavior of the two speciesE. comparative embryology

• Use Figure 25.1 to answer the following questions.

4. A common ancestor for species C and E could be at position number

• 1.• 2.• 3.• 4.• 5.

• E. The two extant species that are most closely related to each other are1. A and B.2. B and D.3. C and B.4. D and E.5. E and A.

• 5. If this evolutionary tree is an accurate depiction of relatedness, then which of the following should be correct?

• The entire tree depicts anagenesis.

• If all species depicted here make up a taxon, this taxon is monophyletic.

• The last common ancestor of species B and C occurred more recently than the last common ancestor of species D and E.

• Species A is the ancestor of both species B and C.

• The species present at position number three is ancestral to three extant species.

1. B only 2. A and C• C and D• B, C, and D• B, C, and E

• A researcher compared the nucleotide sequences of a homologous gene from five different species of mammals. The sequence homology between each species' version of the gene and the human gene are presented as a percentage of similarity.

Species Percentage

Chimpanzee 99.7

Orangutan 98.6

Baboon 97.2

Rhesus Monkey 96.9

Rabbit 93.7

• 6. What conclusion can be validly drawn from these data? A. Humans and other primates evolved from

rabbits.B. All organisms have similar DNA.C. Among the organisms listed, humans

shared a common ancestor most recently with chimpanzees.

D. Humans evolved from chimpanzees.E. Both A and D are correct.

• 7. Which statement represents the best explanation for the observation that the nuclear DNA of wolves and domestic dogs has a very high degree of homology?

A. Dogs and wolves have very similar morphologies.

B. Dogs and wolves belong to the same genus.

C. Dogs and wolves are both members of the family Canidae.

D. Dogs and wolves shared a common ancestor relatively recently.

E. Convergent evolution has occurred.