mechanisms be similarly flexible depend-
ing on environmental circumstances?
At a more proximate level, it would be
worth investigating the cue used by the fe-
male toads to determine what constitutes a
shallow enough pond to trigger the attrac-
tion to heterospecific males. The females
spend up to several hours swimming and
submerged in the ponds before they select
a mate, but how the water level is assessed
and how that evaluation translates into a
preference for a particular mating-call fre-
quency remain unknown. Females of many
species, from butterflies to birds, choose
egg-laying or nesting sites ( 9 ), but such
habitat selection is not generally linked to
mate choice, as it is in the toads.
The toads might also illustrate another
phenomenon gaining attention: evolution-
ary rescue. In this case, an adaptive change
allows persistence of a population that
would otherwise face extinction ( 10 ). The
classic case of evolutionary rescue entails
a large population that, when subjected to
a rapid ecological change, evolves its way
out of the new pressure because genetic
variation enables the population to adapt.
Here, the behavioral plasticity of female S.
bombifrons, rather than their genetic vari-
ability, is key to their survival. The females
exert a preference for particular males,
but only when it serves as an adaptive re-
sponse to impending death of the tadpoles
in shallow ponds. Whether such behav-
ioral responses account for other instances
of evolutionary rescue remains to be seen.
The new work underscores the complex
interaction of genes, environment, and
behavior in evolution. Although the im-
portance of the first two are widely acknowl-
edged, behavior is sometimes dismissed as
ephemeral and less likely to contribute to
evolutionary changes. The interspecific hy-
bridization that simultaneously rescues S.
bombifrons in dry environments and exerts
selective pressure on the mating calls of S.
multiplicata occurs only because of a finely
tuned sensory response to both ecological
and behavioral signals. jREFERENCES AND NOTES- C. Chen, K. S. Pfennig, Science 367 , 1377 (2020).
- M. J. Ryan, A. S. Rand, Evolution 47 , 647 (1993).
- T. C. Mendelson, K. L. Shaw, Trends Ecol. Evol. 27 , 421
(2012). - E. Mayr, Systematics and the Origin of Species from the
Viewpoint of a Zoologist (Columbia Univ. Press, 1942). - S. A. Taylor, E. L. Larson, Nat. Ecol. Evol. 3 , 170 (2019).
- M. Zuk, L. W. Simmons, Sexual Selection: A Very Short
Introduction (Oxford Univ. Press, 2018). - D. T. Gwynne, D. C. F. Rentz, Aust. J. Entomol. 22 , 79
(1983). - M. A. Peterson et al., Evolution 59 , 2639 (2005).
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10.1126/science.abb2398
A S. bombifrons female (left) will mate with an
S. multiplicata male (right) to increase their offspring’s
fitness under harsh environmental conditions.depth in the United States and Mexico (see
the photo ). In shallow ponds, but not deep
ones, female Plains spadefoot toads, Spea
bombifrons, prefer to mate with a male
of another species, the Mexican spadefoot
toad, S. multiplicata. The resulting hybrid
tadpoles develop faster than nonhybrids,
which means they stand a greater chance
of becoming adults before harsh conditions
cause the ponds to dry up. But the S. bom-
bifrons females do not mate with just any
heterospecific; they prefer males with par-
ticularly attractive mating calls.
Female S. bombifrons that follow these
rules yield offspring with greater fitness
than if they mated with male S. bombi-
frons. This means that females exert sexual
selection pressure on males of a different
species, a phenomenon that has not previ-
ously been reported.
Two of the many intriguing aspects of
this finding are the demonstration that
hybridization can be adaptive and that fe-
males drive the process. It has long been
known that animals do not always mate
with the “correct” species and sometimes
even mate with inanimate objects. Austra-
lian jewel beetles, for example, famously
attempt to copulate with beer bottles, but
the actors in such cases are virtually al-
ways males ( 7 ). The sex difference in such
so-called mistakes is usually attributed to
the higher cost of misidentification for fe-
males, who generally make the larger in-
vestment in offspring ( 8 ). Mating with the
“wrong” species could, therefore, mean loss
of reproductive success for an entire breed-
ing season. Conversely, a male that mates
with a heterospecific species or an object
risks only the time and effort expended in
the mating. It might even be beneficial to
have a broad filter for acceptable mates,
because missing a mating opportunity is
likely more costly than wasting sperm.
Although the bottle-infatuated beetles do
experience reduced survival, perhaps sci-
entists have been too hasty in dismissing
all male-initiated hybridization attempts
as mistakes.
The study by Chen and Pfennig suggests
many avenues for future research, starting
with the question of just how widespread
sexual selection across species might be.
Opportunities could be afforded in the
many groups of animals that occur in spe-
cies “flocks” or “swarms,” including cichlid
fishes, Hawaiian crickets, and drosophilid
flies. Plants are another unexplored ave-
nue for deciphering the cross-species mat-
ing phenomenon. Could incompatibilitySCIENCE
CHEMICAL ENGINEERINGTaking on all
of the biomass
for conversion
A catalytic process
converts hardwood into
phenol, propylene,
and other valuable products
By Tao ZhangT
he old saying that “you can make
everything from biomass except
money” sums up the huge chal-
lenges of converting lignocellulose
into fuels and chemicals in a cost-
competitive manner. Most efforts
separate its more readily processed cellu-
lose and hemicellulose components from
lignin. However, upgrading the methoxyl-
ated phenylpropanoid lignin biopolymer (a
process called valorization) has become the
bottleneck ( 1 ). On page 1385 of this issue,
Liao et al. ( 2 ) address this key issue in a
more holistic way. Rather than separate
the components, they converted woody
biomass into phenol and propylene, two
bulk chemicals widely used in the poly-
mer industry, and coproduced valuable
phenolic oligomers and a carbohydrate
pulp amenable to bioethanol production.
The total conversion efficiency is impres-
sive, up to 78% by weight (wt %) based
on the initial mass input. Moreover, both
technoeconomic analysis and life-cycle as-
sessment reveal that such an integrated
process can be profitable and sustainable.
Intensive research efforts have been in-
vested toward valorization of lignin ( 3 ), but
most studies have focused on the conver-
sion of lignin-related model compounds;
few succeeded with real lignocellulose. In
contrast to those chemistry-centered stud-
ies, Liao et al. is more engineering-guided.
In particular, the efficient valorization of
lignin involves reductive catalytic frac-
tionation (RCF), extraction of phenolic
monomers, and catalytic funneling (see the
figure). On a lignin basis, this process pro-
vided phenol in 20 wt % yield with copro-
duction of 9 wt % propylene. Most state-State Key Laboratory of Catalysis, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian
116023, China. Email: [email protected]20 MARCH 2020 • VOL 367 ISSUE 6484 1305