[OANNES Foro] An inverse latitudinal gradient in speciation rate for marine fishes

Jue Sep 6 09:16:14 PDT 2018

Nature volume 559, pages392-395 (2018) 

<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#article-info> 04 July 2018

An inverse latitudinal gradient in speciation rate for marine fishes

<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-1> Daniel L. Rabosky,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-2> Jonathan Chang,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-3> Pascal O. Title,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-4> Peter F. Cowman,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-5> Lauren Sallan,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-6> Matt Friedman,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-7> Kristin Kaschner,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-8> Cristina Garilao,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-9> Thomas J. Near,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-10> Marta Coll &
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#auth-11> Michael E. Alfaro 

Abstract

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
<https://www.nature.com/articles/s41586-018-0273-1#ref-CR1> 1,
<https://www.nature.com/articles/s41586-018-0273-1#ref-CR2> 2. Tropical
marine fish communities are much more diverse than cold-water fish
communities found at higher latitudes
<https://www.nature.com/articles/s41586-018-0273-1#ref-CR3> 3,
<https://www.nature.com/articles/s41586-018-0273-1#ref-CR4> 4, and several
explanations for this latitudinal diversity gradient propose that warm reef
environments serve as evolutionary 'hotspots' for species formation
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#ref-CR5> 5,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#ref-CR6> 6,
<https://www.nature.com/articles/s41586-018-0273-1?WT.ec_id=NATURE-201807&sp
MailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=M
TQ0MDUyODUxMQS2#ref-CR7> 7,
<https://www.nature.com/articles/s41586-018-0273-1#ref-CR8> 8. 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.

/////////////////////////////

Nature 559, 341-342 (2018)

doi: 10.1038/d41586-018-05575-2

04 JULY 2018

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?

Speciation far from the madding crowd

 <javascript:;> Arne O. Mooers &  <javascript:;> Dan A. Greenberg

The tropics are, like many cities, hot, busy and crowded. It was previously
thought
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR1> 1 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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR2> 2 or increase outside the tropics
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR3> 3.
<https://www.nature.com/articles/s41586-018-0273-1> Writing in Nature,
Rabosky et al.
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR4> 4 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.

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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR1> 1. Although this narrative makes for a compelling
theory, Rabosky and colleagues' discovery suggests a different story, at
least for marine fishes.

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.

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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR5> 5, 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.

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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR6> 6 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.

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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR1> 1. The shape of a phylogenetic tree can indicate
slowing speciation as species numbers rise - a pattern that is consistent
with diminishing ecological opportunity
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR7> 7. If closely related species in such clades
occupy different niches, as might be the case for the high-latitude
Antarctic icefishes
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR8> 8 (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.

Continued diversification at higher latitudes might seem reasonable, given
that Earth's cooling over the past 30 million years or so
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR9> 9 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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR10> 10. 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
<https://www.nature.com/articles/d41586-018-05575-2?WT.ec_id=NATURE-201807&s
pMailingID=56937555&spUserID=MjA1NTE2ODQxMAS2&spJobID=1440528511&spReportId=
MTQ0MDUyODUxMQS2#ref-CR11> 11 might offer a way to investigate whether
extinction rates are greater at higher latitudes.

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.

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