Evolution, 4th Edition

218 CHAPTER 9

occasionally, among sympatric species in many groups of plants and animals [51],

and genes are sometimes incorporated into the gene pool of one species from

another, a process called introgression (or introgressive hybridization). Some such

genes may enhance adaptation [1]. For instance, Heliconius butterflies are distaste-

ful to predators and have warning coloration: predators do not attack butterflies

with this pattern after one or two experiences in which they learn to associate the

coloration with distastefulness. Alleles that determine part of the color pattern of

the wings of certain Heliconius species have spread among even distantly related

species (FIGURE 9.5).

Biological species are seldom distinguished in practice by directly testing their

propensity to interbreed or their ability to produce fertile offspring. Indeed, this

is usually not necessary. Morphological and other phenotypic characters are the

usual evidence used for diagnosing sympatric species (FIGURE 9.6), because they

can serve as markers that indicate reduced gene flow—that is, reproductive isola-

tion—among sympatric populations. If a sample of sympatric organisms falls into

two discrete clusters that differ in multiple characters, it is likely to represent two

species. In modern studies, genetic markers are often used to reveal the existence

of two or more sympatric species. A polymorphic locus that shows few heterozy-

gotes, and so departs strongly from Hardy-Weinberg equilibrium, is a signal that

there are likely to be more than one species. (BOX 9A provides an example.)

FIGURE 9.5 Advantageous alleles have

spread by introgression between distantly re-

lated species of Heliconius butterflies in South

America. The phylogeny is based on many

genes. The DNA sequence of two genes that

control color pattern shows that H. timareta

ssp. nov. acquired the “postman” pattern in

the hindwing from H. melpomene amaryl-

lis, and that H. elevatus acquired the “rayed”

hindwing pattern from H. melpomene aga-

lope/malleti. (From [35]; large wing images

courtesy of J. Mallet.)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_09.05.ai Date 01-23-2017

H. numata

H. ethilla

H. hecale

Heliconius hindwing patterns

Rayed Postman

H. pardalinus sergestus

H. elevatus

H. pardalinus butleri

H. timareta orencia

H. timareta ssp. nov.

H. heurippa

H. cydno

H. melpomene rosina

H. melpomene melpomene

H. melpomene amaryllis

H. melpomene aglaope/malleti

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