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nultiple vertebrate

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)L.

The

Pattern and Process

of Speciation

Margaret

B. Ptacek- dnd

Shala

J.

Hank"ison

Tl-re essential

bit of evolutionary theor\.which

is concerned rvjth

the ori-

gin and

nirture of spet:ies remains

r.rtterly rnyster',.rut.

*. Batcso.

(192r)

Speciation, the evolutionary

process by which

new and distinct lineages arise

and mirintain their

independent traiectories,

has been a perplexing

phenome-

non ever since Dirrwin

(18-59) first proposed

that all living organisms

have

diver:sified frorn shared

ancestors. There are several

reasons why unclerstand-

ing how speciation oceurs

is chellcnging. First.

most specirrion tvcnts

hrvc

occurred in the past, leaving

only the end products,

species, :rs a signattrre

of

their

occurrence. Species,

l.r<tth exrant and extinct tornts,

[re,rr the cher,rctcris-

tics

rlssociated with their divergence

from conlrlon

ancestors' but in addi-

tion, they

also carry unique

phenotypic and

genotypic changes that have

occurrecl

since speciation. Disentangling

the characteristics

associatecl rvith

the

speciation process frclm those

thzlt have evolved

since divergellce can be

particularly dif6cult.

Second,

new species arise through

r'r variety of evolutionary

mechrrnisms

(e.g.,

natural

selecticln, sexual selection,

genetic drift,

anci mutatioll), and

therefore,

finding a unifying definition

that describes and

cle fines all sf-'ecies is

an impossible

challenge. Thus much

clebate has occurred al'rout

the nature of

species characteristics

and how scientists

can distinguish species

for taxo-

nomic, systelratic,

and conservatiorr

purposes.

Finaily, speciation

is a series of

processes, with :r beginning stage

of initial

divergence, a

middle stage wherein species-specific

characteristics

are refined

bv vari<,rus forces of

evolutiotr, and an end

point at which ir new

species be-

comes a completely

separaCe evolutionary

lineage on its 1;wn trajectqry

of

evolutionary change

with the potential for

extinction or further diversifica-

rion into new

lileages (Figure 1). Knorving the

locirtion of ur species alor-rg

this

continuum oi speciation

has proved to be

a perplexing

;lroblem

ftlr evolu-

tionary biologists who

wish to understand

how new species :rrise.

Despite

this hurdle, the study of

speciation is one of

the most importarrt

ancl excit-

ing

areas

of el,olutionrrr,v biologv.

A resursence of

interest in speci:rtion

by