leading to benzene (C6H6) is barrierless and
therefore efficient at low temperature (8).
In both hot and cold gas-phase chemistry,
benzene derivatives—such as C6H4 and C6H5—
that can lead to further growth toward larger
aromatic species are involved. Observations
of benzene-type species are therefore crucial
for constraining these chemical models.
McGuire et al. were able to detect benzonitrile in a cold molecular cloud of the Taurus
region thanks to an elegant spectral-stacking
procedure (9) that increases the chance of
detecting molecules with aromatic rings.
Among the species that they searched for,
only benzonitrile was identified as a promising candidate. The authors confirmed its
detection after identification of individual
rotational lines, including their hyperfine
structure, through detailed spectroscopic
work in the laboratory. The presence of a CN
side group leads to a substantial dipole moment and thus facilitates detection of benzonitrile. Calculated benzonitrile abundances
from a chemical model that includes different gas-phase reactions at low temperature
are lower than those observed by a factor
of four. The authors suggest that alternative
mechanisms involving cosmic-ray radiation-induced chemistry at the surface of grains
produce the missing benzonitrile. The mismatch
between observations and
models shows that, despite
the low observed abundance
of benzonitrile, its detection
remains important in constraining chemical models.
Is there any relation between the detection of the
first aromatic ring in dark clouds and the
presence of PAHs, the carriers of the mid-IR aromatic emission bands, in the external
UV-irradiated regions of the clouds? In addition to the gas-phase chemical reactions
mentioned above, these PAHs and related
species, such as C60, could be produced
through UV processing of dust grains (10).
Other scenarios have also been proposed.
For instance, large hydrocarbons, including
PAHs, could be formed by chemical processing on the surface of silicon carbide grains, a
mechanism that could be efficient in the envelopes of carbon-rich red giant stars (11, 12).
It remains unclear how many of the PAHs
and their precursors are synthesized in the
dense and hot envelopes of evolved stars
and how many arise from photo- or radiative chemistry in interstellar environments.
The detection by McGuire et al. of a ben-
zene derivative in a cold molecular cloud
indicates that it can form even at very low
temperatures and without UV radiation. The
authors did not detect larger species with
two or three cycles, but the species they se-
lected have lower dipole moments compared
to benzonitrile, which reduces the chance for
their detection unless they have an anoma-
lously large abundance.
Among the 200 molecules detected in
space, many are organic species. Studying
their composition and chemical networks is
key for understanding molecular complexity in protoplanetary disks surrounding
young stars (13). The search for complex
molecules has mainly been performed in
the millimeter and submillimeter domains.
The work of McGuire and collaborators (3,
14) shows the potential of centimeter-wave
instruments for chemical complexity studies. This opens avenues for research at the
upcoming Square Kilometer Array, which
will operate in this spectral range.
Knowledge of astrochemical networks
also helps in understanding the nucleation
and growth of interstellar dust (including
PAHs) and its role in star and planet formation. However, the detection of benzonitrile in the Taurus region is not sufficient to
conclude on the possibility to form PAHs in
cold molecular clouds. It also remains to be
shown whether the detection of benzonitrile
indicates that PAHs could contain nitrogen
(15). More insights into the chemistry of
PAHs and related species
are expected from combining data from radio and infrared waves with the James
Webb Space Telescope, due
to launch in 2019. In addition to observations, guidelines from laboratory astrophysics studies are key to
progress in this area. These
include spectroscopic and kinetic studies
but also experimental simulations in reactors in order to provide scenarios that can
explain the building of molecular complexity in cosmic conditions. j
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“Among the 200
in space, many are
Darwin’s finches prove
a mechanism for the rapid
formation of new species
By Catherine E. Wagner
Darwin’s finches, a group of 18 species endemic to the Galápagos archipel- ago, are a classic example of adap- tive radiation—the process whereby a single ancestral species multiplies in umber to produce divergent species,
often in rapid succession (1). These birds
are evolutionary biologists’ most celebrated
example of natural selection in action. On
page 224 of this issue, Lamichhaney et al. (2)
have succeeded in observing a process even
more elusive than natural selection—the
formation of a new species (speciation). Because speciation typically takes place on time
scales that are too long for direct human observation, before now it was only in organisms with very fast generation times, such
as viruses and bacteria, that scientists had
directly observed this process [for example,
(3)]. Lamichhaney et al. show through direct
observation and DNA sequencing that new
species can form very rapidly: within three
generations. The key, in this case, is hybridization between different species.
Lamichhaney et al. report that in 1981, a
male large cactus finch (Geospiza conirostris) arrived on the island of Daphne Major
in the Galápagos. It had come from Española, another island more than 100 km
away in the archipelago. Rosemary and
Peter Grant and their collaborators were
there to notice it, band it, and watch what
it did. Although there were no other individuals of this species on the island,
Lamichhaney et al. observed that the bird
succeeded in finding a mate—a medium
ground finch, G. fortis. This pair produced
offspring, and, with only one exception,
the hybrid lineage has bred only within
the lineage, exclusively finding mates that
are descended from the original pair, for
more than 30 years. Because they have a
larger body size than other finch species on
Daphne Major, they are known as the Big
Birds (see the figure).
Botany Department and Biodiversity Institute, University of
Wyoming, 1000 East University Avenue, Laramie, WY 82071,
USA. Email: firstname.lastname@example.org
12 JANUARY 2018 • VOL 359 ISSUE 6372 157