Form, function & fitness—
J. Brad
Players:
Dragonflies
are mean!
Protagonist
Lie, wait,
strike!
Antagonist
2. Concepts
Identifying selection on a trait is difficult because it may be correlated with
other traits. Plus, the correlated traits also may be targets of selection.
Lande & Arnold (1983) proposed we use multiple regression to disentangle
the complexity of correlated traits under selection. In the same year, Arnold
also pointed out that primary traits are not typically direct targets of selection.
Rather, primary traits such as morphology merely enable aspects of performance.
And performance determines survival and reproduction. To admit this complexity,
Arnold suggested path analysis as illustrated in the left panel of the figure below.
Expanding that model (right panel) can even help uncover aspects of performance
due to unmeasured traits (Garland & Losos 1994; Johnson et al. 2008).
z1
z2
fA
w
z1
z2
fB?
fA
fC?
D. Brent
1
Burt ,
Thomas J.
2
DeWitt
1Dept.
Selection and performance gradients
for tadpoles exposed to predatory naiads
(Evolution 62:1243-1251)
1,2
Johnson ,
Biology, Stephen F. Austin State University
2Dept. Wildlife & Fisheries Sciences, Texas A&M University
4. Results—Our major result is summarized by the path diagram:
1. Natural history
Two hypotheses suggest how
tadpole morphology mediates
survival with predators:
1) morphology confers
increased swimming speed:
0.01
Significant
standardized
coefficients are
in black.
0.13
0.01
0.01
(e.g Dayton
et al. 2005)
Burst
swim
speed
0.55
2) morphology induces
predators to misdirect strikes
(Van Buskirk et al. 2003)
0.04
Survival
(chance to
become a
0.11 leopard frog!)
0.13
0.38
5. Conclusions
Tadpole survival with dragonfly nymphs was increased
by deep tailfins and large size. Based on work by uberawesome ecologists (Van Buskirk et al. 2003), we
know deep tails draw predator strikes to nonvital
tailfin tissue and hence away from the trunk.
Perhaps surprisingly, fast swimming did not
seem to confer defense against nymphs.
Now frogs eat usI
w
http://www.dragonflies.org
3. Methods
We collected tadpoles from the wild, measured their morphology (per figure), startled them three times while filming with
high-speed video equipment, and then exposed tadpoles in groups of 10 to predatory Anax larvae in wading pools.
Morphology was summarized as the three major principle components of shape (i.e. relative warps). Survival (fitness) was
scored dichotomously and a path analysis was conducted using the expanded Arnold paradigm as depicted above.
Surviving tadpoles were also examined for tail damage.
Acknowldegements
Thanks RB Langerhans, A Pease, B Shipley, D Pfennig
and two referees for manuscript review, and to
C Kaufman for empirical assistance. Funded by NSF
(DEB0344488 to TJD) and Dept. Biology, SFA (JBJ)
References
Arnold SJ (1983) Morphology, performance and fitness. Am. Zool. 23:347–361.
Dayton GH, Saenz D, Baum KA, Langerhans RB, DeWitt TJ (2005) Body shape, burst speed and predator
susceptibility of larval anurans. Oikos 111:582-591
Garland T Jr, Losos JB (1994) Ecological morphology of locomotor performance in squamate reptiles.
In PC Wainwright PC and SM Reilly, eds. Ecological Morphology. U. Chicago Press.
Johnson JB, Bert DB, DeWitt TJ (2008) Form, function, fitness—pathways to survival. Evolution 62:1243-1251.
Lande R, Arnold SJ (1983) The measurement of selection on correlated characters.
Evolution 37:1210–1226.
Van Buskirk J, Anderwald P, Lüpold S, Reinhardt L, Schuler H (2003) The lure effect, tadpole tail shape
and the target of dragonfly strikes. J.Herpetol. 37:420–424.