Evolution, 4th Edition

38 CHAPTER 2

Sometimes the information about DNA

sequences or other characteristics simply is

insufficient to resolve the relationships among

taxa. (Often the tree will then be shown with

a polytomy, a node from which three or more

lineages emerge.) That often is the case if suc-

cessive speciation events happened so rapidly

that there was not enough time for many

mutations to become fixed in between succes-

sive branching points.

Variations on the Phylogenetic Theme

Branches of a phylogenetic tree sometimes rejoin

The results of phylogenetic studies are often consistent with the assumption that the

various lineages that arise from common ancestors remain separate and diverge from

each other—that the tree consists only of bifurcations. But branches sometimes rejoin,

in whole or in part, so that relationships among organisms may form a network rather

than just a branching tree. For example, some species have evolved from hybrid

crosses between two different ancestors, a pattern that is especially common in plants

(FIGURE 2.11). In these cases of hybrid speciation, various phenotypic features and

DNA markers throughout the genome reveal two ancestral sources.

More commonly, analysis of one or a few genes suggests a radically differ-

ent phylogeny than most other genes. For example, aphids are obviously insects,

based on both their morphology and almost all DNA sequences. A few species

of aphids, unlike almost all other animals, can synthesize carotenoid pigments.

A phylogenetic analysis of the genes that enable this biosynthesis placed these

aphids among the fungi—clear evidence that they acquired these genes from

a fungus (FIGURE 2.12). In contrast to “vertical” inheritance of genes by off-

spring from parents, such nonreproductive passage of genes among organisms is

horizontal gene transfer (HGT; see Chapter 4). The genome of most eukaryotes,

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H. niveus

H. neglectus

H. petiolaris

H. annuus

H. argophyllus

H. anomalus (sand dune)

H. deserticola (desert oor)

H. paradoxus (salt marsh)

FIGURE 2.11 Hybrid origin of some diploid

species of sunflowers. The phylogeny,

based on sequences of chloroplast DNA

and nuclear ribosomal DNA, shows that Heli-

anthus anomalus, H. paradoxus, and H. de-

serticola have arisen from hybrids between

H. annuus and H. petiolaris. (After [11].)

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Note: Aphid a roughly cropped screencapter FPO.

Xanthophyllomyces dendrorhous

Rhodosporidium sp.

Phycomyces blakesleeanus

Blakeslea trispora

Mucor circinelloides

Acyrthosiphon pisum

Acyrthosiphon pisum

Acyrthosiphon pisum

Myzus persicae

Ustilago maydis

Pyrenophora tritici-repentis

Phaeosphaeria nodorum

Neurospora crassa

Podospora anserina

Aspergillus oryzae

Nectria haematococca

Gibberella fujikuroi

Fungus genes

Fungus genes

Aphid genes

FIGURE 2.12 Genes that encode the

enzymes that synthesize carotenoid com-

pounds are found in one group of aphids,

but not in any other animal that has been

studied. This phylogeny of copies of a gene

found in aphids and of the homologous

gene in fungi shows that the ancestor of

these aphids acquired the gene from a fun-

gus. The photo shows pea aphids (Acyrtho-

siphon pisum) that have this gene. (After [27];

photo courtesy of Nancy Moran, University

of Texas at Austin.)

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