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<div class=Section1>
<p class=MsoNormal><i><span style='font-family:"Arial","sans-serif"'>Nature </span></i><span
class=visually-hidden><b><span style='font-family:"Arial","sans-serif"'>volume</span></b></span><b><span
style='font-family:"Arial","sans-serif"'> 559</span></b><span
style='font-family:"Arial","sans-serif"'>, <span class=visually-hidden>pages</span>392–395<span
class=apple-converted-space> </span>(2018)<span
class=apple-converted-space> </span><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#article-info"><span
style='color:windowtext;text-decoration:none'>04 July 2018</span></a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<h1 style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:10.5pt;
margin-left:0in;font-size:3rem'><span style='font-family:"Arial","sans-serif";
font-weight:normal'>An inverse latitudinal gradient in speciation rate for
marine fishes<o:p></o:p></span></h1>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span
style='font-family:"Arial","sans-serif"'><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-1"><span
style='color:windowtext;text-decoration:none'>Daniel L. Rabosky</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-2"><span
style='color:windowtext;text-decoration:none'>Jonathan Chang</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-3"><span
style='color:windowtext;text-decoration:none'>Pascal O. Title</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-4"><span
style='color:windowtext;text-decoration:none'>Peter F. Cowman</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-5"><span
style='color:windowtext;text-decoration:none'>Lauren Sallan</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-6"><span
style='color:windowtext;text-decoration:none'>Matt Friedman</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-7"><span
style='color:windowtext;text-decoration:none'>Kristin Kaschner</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-8"><span
style='color:windowtext;text-decoration:none'>Cristina Garilao</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-9"><span
style='color:windowtext;text-decoration:none'>Thomas J. Near</span></a>,<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-10"><span
style='color:windowtext;text-decoration:none'>Marta Coll</span></a><span
class=apple-converted-space> </span>&<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#auth-11"><span
style='color:windowtext;text-decoration:none'>Michael E. Alfaro</span></a> <o:p></o:p></span></p>
<h2 style='mso-margin-top-alt:10.5pt;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;background:#EEEEEE'><span style='font-size:16.0pt;font-family:
"Arial","sans-serif";color:#222222;letter-spacing:.15pt'>Abstract<o:p></o:p></span></h2>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:22.45pt;background:white;word-wrap: break-word'><span
style='font-size:13.0pt;font-family:"Arial","sans-serif";color:#222222;
letter-spacing:.15pt'>Far more species of organisms are found in the tropics
than in temperate and polar regions, but the evolutionary and ecological causes
of this pattern remain controversial</span><sup><span style='font-size:9.5pt;
font-family:"Arial","sans-serif";color:#222222;letter-spacing:.15pt'><a
href="https://www.nature.com/articles/s41586-018-0273-1#ref-CR1"
title="Mittelbach, G. G. et al. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecol. Lett. 10, 315–331 (2007)."
id=ref-link-section-d927e655><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>1</span></a>,<a
href="https://www.nature.com/articles/s41586-018-0273-1#ref-CR2"
title="Schluter, D. & Pennell, M. W. Speciation gradients and the distribution of biodiversity. Nature 546, 48–55 (2017)."
id=ref-link-section-d927e658><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>2</span></a></span></sup><span style='font-size:13.0pt;
font-family:"Arial","sans-serif";color:#222222;letter-spacing:.15pt'>. Tropical
marine fish communities are much more diverse than cold-water fish communities
found at higher latitudes</span><sup><span style='font-size:9.5pt;font-family:
"Arial","sans-serif";color:#222222;letter-spacing:.15pt'><a
href="https://www.nature.com/articles/s41586-018-0273-1#ref-CR3"
title="Tittensor, D. P. et al. Global patterns and predictors of marine biodiversity across taxa. Nature 466, 1098–1101 (2010)."
id=ref-link-section-d927e662><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>3</span></a>,<a
href="https://www.nature.com/articles/s41586-018-0273-1#ref-CR4"
title="Stuart-Smith, R. D. et al. Integrating abundance and functional traits reveals new global hotspots of fish diversity. Nature 501, 539–542 (2013)."
id=ref-link-section-d927e665><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>4</span></a></span></sup><span style='font-size:13.0pt;
font-family:"Arial","sans-serif";color:#222222;letter-spacing:.15pt'>, and
several explanations for this latitudinal diversity gradient propose that warm
reef environments serve as evolutionary ‘hotspots’ for species
formation</span><sup><span style='font-size:9.5pt;font-family:"Arial","sans-serif";
color:#222222;letter-spacing:.15pt'><a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR5"
title="Kiessling, W., Simpson, C. & Foote, M. Reefs as cradles of evolution and sources of biodiversity in the Phanerozoic. Science 327, 196–198 (2010)."
id=ref-link-section-d927e669><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>5</span></a>,<a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR6"
title="Alfaro, M. E., Santini, F. & Brock, C. D. Do reefs drive diversification in marine teleosts? Evidence from the pufferfish and their allies (Order Tetraodontiformes). Evolution 61, 2104–2126 (2007)."
id="ref-link-section-d927e669_1"><span style='color:#006699;text-decoration:
none;vertical-align:baseline'>6</span></a>,<a
href="https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR7"
title="Cowman, P. F. & Bellwood, D. R. Coral reefs as drivers of cladogenesis: expanding coral reefs, cryptic extinction events, and the development of biodiversity hotspots. J. Evol. Biol. 24, 2543–2562 (2011)."
id="ref-link-section-d927e669_2"><span style='color:#006699;text-decoration:
none;vertical-align:baseline'>7</span></a>,<a
href="https://www.nature.com/articles/s41586-018-0273-1#ref-CR8"
title="Siqueira, A. C., Oliveira-Santos, L. G. R., Cowman, P. F. & Floeter, S. R. Evolutionary processes underlying latitudinal differences in reef fish biodiversity. Glob. Ecol. Biogeogr. 25, 1466–1476 (2016)."
id=ref-link-section-d927e672><span style='color:#006699;text-decoration:none;
vertical-align:baseline'>8</span></a></span></sup><span style='font-size:13.0pt;
font-family:"Arial","sans-serif";color:#222222;letter-spacing:.15pt'>. Here we
test the relationship between latitude, species richness and speciation rate
across marine fishes. We assembled a time-calibrated phylogeny of all
ray-finned fishes (31,526 tips, of which 11,638 had genetic data) and used this
framework to describe the spatial dynamics of speciation in the marine realm.
We show that the fastest rates of speciation occur in species-poor regions
outside the tropics, and that high-latitude fish lineages form new species at
much faster rates than their tropical counterparts. High rates of speciation
occur in geographical regions that are characterized by low surface
temperatures and high endemism. Our results reject a broad class of mechanisms
under which the tropics serve as an evolutionary cradle for marine fish diversity
and raise new questions about why the coldest oceans on Earth are present-day
hotspots of species formation.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'>/////////////////////////////<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal><span class=emphasis><i><span style='font-family:"Arial","sans-serif";
color:#222222'>Nature</span></i></span><span class=apple-converted-space><span
style='font-family:"Arial","sans-serif";color:#222222'> </span></span><strong><span
style='font-family:"Arial","sans-serif";color:#222222'>559</span></strong><span
style='font-family:"Arial","sans-serif"'>, 341-342 (2018)<o:p></o:p></span></p>
<p class=MsoNormal><i><span style='font-family:"Arial","sans-serif"'>doi:
10.1038/d41586-018-05575-2<o:p></o:p></span></i></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif";color:#1A1A1A;
text-transform:uppercase;letter-spacing:.7pt'>04 JULY 2018<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif";color:#1A1A1A;
text-transform:uppercase;letter-spacing:.7pt'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>New
species of marine fishes are found to emerge at a faster rate in high-latitude
oceans, which have lower densities of species, than in the species-rich
tropics. Are the tropics too crowded for new species to take hold?<o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:18.0pt;font-family:"Arial","sans-serif";
letter-spacing:-1.15pt'>Speciation far from the madding crowd</span></b><span
style='font-size:18.0pt;font-family:"Arial","sans-serif";letter-spacing:-1.15pt'><o:p></o:p></span></p>
<p class=MsoNormal><span class=block><span style='font-size:12.0pt;font-family:
"Arial","sans-serif"'><a href="javascript:;"><span style='color:windowtext'>Arne
O. Mooers</span></a></span></span><span class=apple-converted-space><span
style='font-size:12.0pt;font-family:"Arial","sans-serif"'> </span></span><span
class=block><span style='font-size:12.0pt;font-family:"Arial","sans-serif"'>&
<a href="javascript:;"><span style='color:windowtext'>Dan A. Greenberg</span></a></span></span><span
style='font-size:12.0pt;font-family:"Arial","sans-serif"'><o:p></o:p></span></p>
<p class=MsoNormal style='mso-margin-top-alt:auto;margin-right:7.5pt;
mso-margin-bottom-alt:auto;line-height:20.4pt;background:white'><span
style='font-family:"Arial","sans-serif"'>The tropics are, like many cities,
hot, busy and crowded. It was previously thought<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR1"><span
style='color:windowtext;vertical-align:baseline'>1</span></a></sup><span
class=apple-converted-space> </span>that these conditions in the tropics
generate a hotbed for the formation of new species (speciation). Species
diversity is remarkably high in the tropics and declines toward the poles.
However, newly developed tools to measure speciation rates, coupled with
ever-growing global data sets, have enabled the surprising finding that
terrestrial speciation rates for the past few million years are similar across
different latitudes<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR2"><span
style='color:windowtext;vertical-align:baseline'>2</span></a></sup><span
class=apple-converted-space> </span>or increase outside the tropics<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR3"><span
style='color:windowtext;vertical-align:baseline'>3</span></a></sup>.<span
class=apple-converted-space> </span><a
href="https://www.nature.com/articles/s41586-018-0273-1"><span
style='color:windowtext'>Writing in <i>Nature</i></span></a>, Rabosky<span
class=apple-converted-space> </span><i>et al</i>.<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR4"><span
style='color:windowtext;vertical-align:baseline'>4</span></a></sup><span
class=apple-converted-space> </span>document a speciation rate for marine
fishes at high latitudes that is twice the speciation rate in tropical seas.
This high speciation rate in cold, species-poor waters poses an interesting
conundrum for evolutionary biologists and ecologists.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>There are two potential drivers of
high speciation rates in the tropics. First, the elevated temperatures in the
region both speed up metabolism, increasing the number of mutations, and
decrease generation times. This is a potentially powerful combination,
producing more of the variation necessary for evolution and the possibility of
faster evolution. A second possible driver is ecological opportunity. The
energy-rich tropics offer abundant resources that can support many different
niches. And the tropics are so rich in species that the interactions of members
of a single species with its competitors, predators and parasites might differ
from place to place, leading to different adaptations and eventual divergence
into new niches<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR1"><span
style='color:windowtext;vertical-align:baseline'>1</span></a></sup>. Although
this narrative makes for a compelling theory, Rabosky and colleagues’
discovery suggests a different story, at least for marine fishes.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>The authors gathered genetic data for
11,638 species of marine and freshwater fish, along with information on inferred
evolutionary relationships based on taxonomic groupings for 19,888 additional
fish species for which genetic data were not available. Using these data, and
information from 139 dated fossil fishes, the authors generated a large set of
plausible phylogenetic trees detailing the evolutionary relationships between
all living marine fishes, and, crucially, estimates of when different lineages
diverged from one another. These dated trees enable speciation rates to be
inferred on the basis of the branching patterns of the tree. Species connected
by short branches, and with many close relatives, have high speciation rates,
whereas species that are separated by long branches and that have few close
relatives have low speciation rates.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>Most taxonomic groups are made up of
lineages with both low and high speciation rates. The marine fishes in the
authors’ large phylogenetic trees were no exception, with speciation
rates varying by more than 50-fold between lineages. The authors combined these
values with global maps of where these species live, revealing a clear
geographical structure to the speciation rates. Because small biases integrated
over large amounts of data could produce misleading inferences in these sorts
of studies<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR5"><span
style='color:windowtext;vertical-align:baseline'>5</span></a></sup>, the
authors considered several ways to estimate speciation rates and to map
species’ location. But whether the authors considered the patterns
looking species by species, place by place or ecoregion by ecoregion, there was
always a pattern of the average speciation rate increasing from the tropics
towards the poles.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>Rabosky and colleagues consider only a
few potential mechanisms that might affect the rate at which marine fishes
produce new species at high latitudes. At higher latitudes, marine fishes tend
to have longer generation times and slower metabolisms<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR6"><span
style='color:windowtext;vertical-align:baseline'>6</span></a></sup><span
class=apple-converted-space> </span>than have fishes in the tropics,
suggesting that such extended generation times and lower mutational input do
not limit speciation rate in these cold-adapted lineages. The authors also
tested and discounted the interesting possibility that high-latitude species
are the descendants of tropical lineages that exhibited adaptations for
cold-water living and also happened to have high speciation rates. This
negative result suggests that a high-latitude marine environment, rather than
the species that colonizes it, drives high speciation rates.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>The correlation of high speciation
rates with low diversity is consistent with the idea that there are unfilled
ecological opportunities near the poles. However, ecological opportunity is
something that is inferred rather than witnessed<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR1"><span
style='color:windowtext;vertical-align:baseline'>1</span></a></sup>. The shape
of a phylogenetic tree can indicate slowing speciation as species numbers rise
— a pattern that is consistent with diminishing ecological opportunity<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR7"><span
style='color:windowtext;vertical-align:baseline'>7</span></a></sup>. If closely
related species in such clades occupy different niches, as might be the case
for the high-latitude Antarctic icefishes<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR8"><span
style='color:windowtext;vertical-align:baseline'>8</span></a></sup><span
class=apple-converted-space> </span>(Fig. 1), this would be consistent
with ecological opportunity having a key role in driving their diversification.
Such analyses are needed to determine whether high-latitude groups have reached
the ecological limits of their ecosystems or whether high-latitude fish
diversity might be expected to continue increasing.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>Continued diversification at higher
latitudes might seem reasonable, given that Earth’s cooling over the past
30 million years or so<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR9"><span
style='color:windowtext;vertical-align:baseline'>9</span></a></sup><span
class=apple-converted-space> </span>has given rise to the present,
relatively young temperate and polar realms. However, the rate of speciation at
high latitudes reported by Rabosky (roughly 0.2 new species per species per
million years) is high. If this rate had been sustained over the whole of the
past 30 million years, high latitudes would have tropical levels of species
diversity by now. Given that the accumulation of diversity depends on both
speciation and extinction rates, one explanation that reconciles a high
speciation rate and low current diversity is if both speciation and extinction
are elevated outside the tropics<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR10"><span
style='color:windowtext;vertical-align:baseline'>10</span></a></sup>. This
could result in a pattern in which an increase in the number of species is
limited by high extinction rates, and poleward realms would have few, but
relatively young, species. Measuring extinction rates is almost as difficult as
trying to assess ecological opportunity, but new approaches that combine
information on extinct species represented in the fossil record with
information from their living relatives<sup><a
href="https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&spMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=MTQ0MDUyODUxMQS2#ref-CR11"><span
style='color:windowtext;vertical-align:baseline'>11</span></a></sup><span
class=apple-converted-space> </span>might offer a way to investigate
whether extinction rates are greater at higher latitudes.<o:p></o:p></span></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:21.0pt;
margin-left:0in;line-height:20.4pt;background:white'><span style='font-size:
11.0pt;font-family:"Arial","sans-serif"'>The view of high-latitude oceans as
‘sleepy’ backwaters remote from the exciting evolutionary bustle of
the tropics will need to change if it turns out that both speciation and
extinction of marine fishes occur at a faster pace beyond the tropics. Such a
pattern would imply that living cheek by jowl, or rather gill by jaw, in the
tropics is a condition that is more constraining than productive, such that the
real biodiversity action is taking place where there is less, rather than more,
biodiversity. Far from the madding crowd, as it were.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
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