On the wings of checkerspots : a model system for population biology

408 Pages·2004·37.574 MB·English

by  Paul R. Ehrlich, Ilkka Hanski

ON THE WINGS OF CHECKERSPOTS
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ON THE WINGS OF CHECKERSPOTS
A Model System for Population Biology
Edited by
Paul R. Ehrlich
Ilkka Hanski
OXFORD
UNIVERSITY PRESS
2004
OXFORD
UNIVERSITY PRESS
Oxford New York
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Library of Congress Cataloging-in-Publication Data
On the wings of checkerspots : a model system for population biology / edited by Paul R.
Ehrlich, Ilkka Hanski.
p. cm.
Includes bibliographical references and index.
ISBN 0-19-515827-X
1. Nymphalidae. 2. Population biology. I. Ehrlich, Paul R. II. Hanski, Ilkka.
QL561.N9O5 2004
577.8'8—dc21 2003051740
9 8 7 6 5 4 3 21
Printed in the United States of America
on acid-free paper
I/
To Anne and Eeva
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Preface
Bay Checkerspots In the course of my training with Charles
Michener, Robert Sokal, and Joseph Camin, I devel-
I clearly remember the first time I saw a Bay checker- oped a strong interest in understanding evolutionary
spot butterfly. It was not in a sunlit patch of ser- processes in natural populations. Joe Camin and I
pentine grassland overlooking San Francisco Bay, had been fortunate enough to elucidate one of the
but in my bedroom in suburban New Jersey in 1948. few examples documenting natural selection in the
There, as a 16-year-old devoted butterfly collector, field, a task made possible by an unusual combina-
I opened the first box of butterflies I had ever re- tion of strong selection balanced by migration in a
ceived in exchange from another collector. Bill population of water snakes (Camin and Ehrlich
Hammer in California had mailed me a generous 1958, Ehrlich and Camin I960).1 My early experi-
lot of specimens, which he traded to me for some ence with that project greatly shaped my career.
rather common eastern butterflies. In that pile of Other investigations of selection in nature also have
specimens in neatly folded paper triangles was a been focused on unusual situations. One classic ex-
series of Euphydryas editha bayensis. Little did I ample is the reaction of moth populations to in-
know then that I was to spend most of my life in creased industrial pollution, first studied in the Mid-
California and devote much of it to the study of that lands of Great Britain and later in Europe and North
very butterfly. America (Kettlewell 1955, 1973, Owen 1997,
About a decade later I arrived at Stanford Univer- Majerus 1998, Brakesfield and Liebert 2000). An-
sity, thrilled to have been hired as an assistant profes- other was strong selection on land snails detectable
sor of biology to teach evolution and entomology,
replacing the eminent morphologist and student of
scale insects, Gordon Floyd Ferris. Jobs were hard to
1. Water snakes (Nerodea sipedon) on the islands of Lake Erie,
get, and I considered myself especially lucky to land
unlike those in the swamps of most of North America, tend to have
one where the opportunities to do field work stretched reduced banding. This is a result of selection by visual predators
through much of the year. I was tired of always wait- for bandlessness against the uniform gray limestone rocks of the
islands. This selection pressure is balanced by individuals migrat-
ing until May or June for interesting butterflies to start
ing from banded lakeshore swamp populations. It is these oppos-
flying. The Bay area and Stanford seemed like para-
ing forces that allowed selection to be detected by comparing the
dise on earth to one fresh from graduate school and amount of banding in juvenile individuals with that in adults. At
postdoctoral training in Kansas and Chicago. Indeed, birth there were many banded individuals, but relatively few of them
survived to adulthood. More recent work on the N. sipedon system
almost four decades later I still feel fortunate to live
has confirmed and expanded our early observations (King 1993,
in the land of milk and honey. King and Lawson 1995).
vii
VIM Preface
because thrushes that prey on them break their shells Fortunately, just after my arrival at Stanford, I
on traditional rocks, permitting the color patterns of decided to go to the 1959 meeting of the Lepidop-
the snails being eaten to be compared with those in terists' Society in Santa Barbara and drove down
the entire population (Sheppard (1951, 1952, Cain with two local butterfly collectors, who have since
and Sheppard 1954).2 All those studies showed that passed away: Bill Tilden and Elton Sette. I discussed
predation could be a potent factor in determining the my plans with them, and Elton suggested that I
color patterns of both moths and snails by favoring study the Bay checkerspots on Jasper Ridge, a partly
genotypes whose phenotypes were better camou- undeveloped area of Stanford campus used for graz-
flaged against the backgrounds on which they lived. ing, recreation, and occasionally for teaching and
I was (and still am) concerned with whether these and research by the Department of Biological Sciences.
the handful of other studies of natural selection were Elton said that there was a population in the grass-
a biased sample of selection pressures in nature. land on the ridge and said he would be glad to give
Other sampling problems have hindered understand- me samples of specimens that he had taken on the
ing the dynamics of natural populations. People have ridge at different times going back to 1936.
been mostly concerned with changes in the size of The die was cast. In the fall of 1959, I mapped
large populations. That is because of our interest in the Jasper Ridge area where Elton said the butter-
pests and in organisms that we want to harvest. With flies flew and divided it into arbitrary areas, from
trivial exceptions, these have large populations. But A to H (figure 1.2). In the spring of 1960 I began
most natural populations are small, and if we want to visit the area with the intention of starting a
to understand how the world works, understanding mark-release-recapture study. I was accompanied
the dynamics of small populations is important. In- by a graduate student, Susan Davidson, now Susan
deed, it is essential in a world entering an unprece- Thomas, wife of my late Stanford colleague, bota-
dented era of extinctions (see, e.g., Myers 1979, nist John Thomas. I saw no butterflies on the first
Ehrlich and Ehrlich 1981, Ehrlich and Wilson 1991, two visits, but on March 31 at 10 AM I saw and
Wilson 1988, 1992, Heywood 1995). Central ques- caught my first live Bay checkerspot. My ancient
tions of conservation biology deal with predicting the mimeographed data sheet says it was a male, taken
fates of small populations and finding ways to in- in area G, in excellent condition. It was marked with
crease their chances of persistence. a coded number one, a felt-pen smear on the un-
When I arrived at Stanford in the fall of 1959,1 derside of the apex of the right forewing, and re-
was determined to make at least a small start at cor- leased. Then Susan and I moved off in search of
recting the sampling bias of studies of natural popu- number two, little realizing the numbers marked on
lations. I decided to put a portion of my research Jasper Ridge would eventually reach many thou-
effort into teasing apart the population dynamics, sands in a research program that would span more
general ecology, and population genetics of some than four decades and would be central in a long
populations that were not doing anything spectacu- battle to prevent Jasper Ridge from being converted
lar—just apparently going along with no vast out- into a housing development (figures 1-3).
breaks of numbers or dramatic changes in the pro- So the circle was closed. The butterflies that I
portions of different kinds of individuals because fondly remember arriving dead in little triangular
of strong selection. My long-term interest in butter- envelopes, smelling faintly of moth balls, were now
flies (my dissertation had been on their morphol- living entities that would help me understand more
ogy and higher classification) and the pioneering about how the world worked. I have never regretted
work of E. B. Ford (e.g., Ford and Ford 1930) on the decision to work with Euphydryas, and one of
their ecological genetics made choosing a butterfly the greatest pleasures of doing so has been the num-
population, especially a checkerspot population, as ber of graduate and postdoctoral students and col-
a model system a natural decision. leagues that I have gotten to know as friends as we
struggled together to solve the mysteries of the bi-
2. Detecting selection coefficients in the vicinity of .01 (one ology of their populations. Much of the Euphydryas
genotype reproducing itself 99% as well as another) is virtually
work reported here was done by those colleagues
impossible in the field. One of the triumphs of theoretical popula-
tion genetics has been to show that such selection pressures would (as the citations will show). Several of them have
have been sufficient, given the enormity of the time available, to authored chapters in this volume.
account for evolution from coacervate droplets to people. Thus
We are an inbred lot. Mike Singer, now a profes-
we do not know whether the high levels of selection that have been
observed are due simply to our ability to observe them or whether sor at the University of Texas, is one of the most
they are indeed typical of natural situations. knowledgeable Euphydryas biologists. He and his
Preface ix
Figure 1. Susan Davidson Thomas and Duncan
Porter marking Euphydryas editha at Jasper
Ridge, spring 1960. Photo by Paul R. Ehrlich.
students are unsurpassed as investigators of Euphy- from runaway development to devote much time to
dryas oviposition behavior and the evolution of host his first love, Euphydryas) and me on the most in-
plant choice, and he is married to another Euphydryas tensive study ever done of a Jasper Ridge £. editha
biologist, Camille Parmesan, known for her impor- population. After getting his doctorate at Harvard
tant work on the response of checkerspots and other doing research on fishes, Alan returned to Stanford
butterflies to global climate change. Carol Boggs, di- to work more on Euphydryas and to become the
rector of Stanford's Center for Conservation Biology university's official conservation biologist. Dennis
(CCB), has worked on various problems in the repro- Murphy, both as a graduate student and as a former
ductive biology of Euphydryas and has broad expe- director of the CCB, mentored Stu Weiss. Stu has
rience with other butterflies. She is one of my academic devoted decades of his life to understanding the
grandchildren, having received her Ph.D. with another dynamics of Bay area E. editha populations, both
distinguished Euphydryas biologist at Texas, my for- as a researcher at the CCB, during the course of his
mer student Larry Gilbert (who deserted Euphydryas doctoral work, and as a biological consultant. John
for Heliconius). Carol is also the wife of my Stanford McLaughlin got his Stanford doctorate in ecologi-
colleague Ward Watt, a leading butterfly evolution- cal theory with Joan Roughgarden, returned to the
ist who has been continually helpful to me as I have CCB as a postdoc to work on Euphydryas, and now
struggled to understand the Euphydryas system. collaborates with us as a faculty member at West-
Alan Launer, when he was an undergraduate, ern Washington University. The youngest member
worked with my student Dennis Murphy (now too of our Euphydryas team is Jessica Hellmann, who
busy saving the biota of the western United States got her doctorate in our group working on E.
Figure 2. Using a felt-tipped marker to number
(1-2-4-7 system; see Ehrlich and Davidson 1960)
a Euphydryas editha at Jasper Ridge in spring
1960. With these markers it was best to support
the wing with a piece of cardboard, making it
necessary to have two people in a marking team.
Photo by Paul R. Ehrlich.