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Evolutionary success is unevenly distributedEcological success is measured as population growth rate - how fast does one population grow, compared to others?
Microevolutionary success is measured as fitness – how many of your offspring survive to reproductive age?
Macroevolutionary success equals clade biodiversity, or number of surviving species in a clade
- typical animal phyla have ~5,000 species (=spp.)
- lineages below the phylum level with >5,000 spp. are thus unusually successful
Identifying characteristics that make one lineage especially successful is a major goal of evolutionary biology
populations species lineages(clades of species)
microevolution macroevolution
- genetic drift- natural selection- migration
allelefrequencies reproductive
isolation
How do _____ evolve?
adaptation
diversification
why do some groupshave more species
than related groups?
one commonancestor
2 daughter lineages,of equal age
clade of 3 extant species (surviving today)
1 surviving species
Evolutionary success = number of living species
Why does one lineage diversify into many more species than its less-successful sister lineage?
Evolutionary success is unevenly distributed
Major goal of macroevolutionary studies: explain why some groups are more species-rich than others
3 spp.
60 spp.
Can we identify the traits that explain why biodiversity is unevenly distributed among sister clades?
what led this group to out-radiate its sister group by 20 to 1?
- change in habitat, feeding method, traits involved in competition or reproduction...?
Evolutionary success is unevenly distributed
Major goal of macroevolutionary studies: explain why some groups are more species-rich than others
**Winners**Woo-hoo!
beetles: 350,000 spp.named (probably >1 million)
Pulmonata: land / freshwater snails + slugs~60,000 spp. (including marine members)
vertebrates: ~45,000 spp.
colonizing dry land led to explosive radiations in many groups
Evolutionary success is unevenly distributed
other lineages can hover at low species numbers despite being ecologically abundant and important
- may survive unchanged for hundreds of millions of years and be very well adapted to their niche, yet never diversify
Losers – the “200 club”
cephalopods: pinnacle of invertebrate vision & intelligence
sharks + rays: top marine predators
Some lineages undergo adaptive radiations, filling all available ecological niches and diversifying into many species
1) opportunity: ancestor colonized an empty habitat with many unoccupied niches...
- went from marine into terrestrial + freshwater habitats
- got onto an empty continent, early
- survived mass extinction of dominant competitors
2) specialization: when related species exploit different ecological niches (i.e., food or host), many related species can co-exist in one place without competing
3) key innovation: evolution of a trait that allows exploitation of new niches, or greater competitive ability
Some lineages undergo adaptive radiations, filling all available ecological niches and diversifying into many species
4) evolved a trait that promotes rapid speciation:- sexual signaling or mating system- strong host or habitat association- tendency to get allopatrically isolated (dispersal)- fast-evolving gamete recognition proteins
5) being biogeographically widespread – meaning, some member species are distributed across different regions and biomes across the globe - lineage is more likely to survive local wipe-outs, and global mass extinction events
Evolutionary success is unevenly distributed
shift in rate ofdiversification(speciation - extinction)
- thus, either of two things can lead to a lineage diversifying more:
1) increase in speciation rate ()
2) decrease in extinction rate ()
Rabosky 2014
Diversification rate of a lineage (r) is the net difference between speciation (new spp. born) and extinction (existing spp. vanish)
r =
Evolutionary success is unevenly distributed
shift in rate ofdiversification(speciation - extinction)
- thus, either of two things can lead to a lineage diversifying more:
1) increase in speciation rate ()
2) decrease in extinction rate ()
Rabosky 2014
Diversification rate of a lineage (r) is the net difference between speciation (new spp. born) and extinction (existing spp. vanish)
r =
Evolutionary success is unevenly distributed
key innovationevolves, sets off burst of diversification
1) key innovation may lead to an adaptive radiation into many new ecological niches
problem: typically a one-time event, not naturally replicated
Rabosky 2014
Diversification rate of a lineage is the net difference between speciation (new spp. born) and extinction (existing spp. vanish)
2 living species of Bosellia
- flat sea slugs
- eat one algal genus
- tropical only
134 speciesin sister clade Plakobranchidae
- parapodia: sides rolled up
- eat >20 algal genera
- tropics to poles
Why only 2 Bosellia but 134 plakos?
- flat sea slugs
- eat one algal genus
- tropical only
134 species in clade Plakobranchidae...
- parapodia (sides of body rolled up) may protect stored chloroplasts from too much sun (possible key innovation?)
- each species feeds on just one of >20 kinds of algae (specialized)
- species live and mate on their host (host choice may promote speciation)
Candidate key innovation: antifreeze proteins
9 species, non-Antarctic (no anti-freeze)
123 species, Antarctic - anti-freeze glycoproteins
One group of fish diversified in the Antarctic after evolving anti-freeze glycoproteins, allowing them to survive water temperatures below freezing
- within Antarctic, species also diversified into benthic (bottom) and pelagic (open water) forms
- again, however, only happened once so hard to test hypothesis
Identifying trait-dependent diversification
Easier to test hypotheses if diversification rate is character state-dependent, and character state changes often ancestral state
derived state 3x higher rate of diversification
repeated, independent shifts between states naturally replicated experiment
Comparative methods can identify such traitsRabosky
& McCune 2010
Identifying trait-dependent diversification
Easier to test hypotheses if diversification rate is character state-dependent, and character state changes often derived state 3x higher rate of diversification
Traits that cause greater diversification result in species selection
- form of selection acting on trait(s) shared by all members of a species, or that are a species property (e.g., range)
- unrelated to fitness within species
Rabosky& McCune 2010
Identifying trait-dependent diversification
= speciation rate
From a model-fitting perspective, the question is:
Does a model with two different speciation rates (one for state “blue”, one for “red”) fit the data better than a default model with the same speciation rate for both states?
Goldberg et al. 2008, Science
Species selection in plants
Selfing
Non-selfing
Flowering plants repeatedly evolved self-compatible pollen, allowing self- fertilization, from self- incompatible pollen (cannot self-fertilize)
Species selection in plants
In non-selfing plants, estimated speciation rate is higher than extinction rate – thus, lineages diversify (r > 0)
- however, some non-selfers are always gradually evolving into self-fertilizers by character change..
non-selfing
selfing
diversificationrate (r)
Goldberg et al. 2008, Science
Species selection in plantsIn selfing plants, rates of both speciation and extinction increase... however, extinction increased more than speciation
- selfing plants have decreased diversification rates (r < 0)
- this explains why non-selfing plants persist, even though some keep turning into selfers: the remaining non-selfers outcompete the species that undergo character change and become selfers
non-selfing
selfing
diversificationrate (r)
Marine larval type and dispersalmarine invertebrates produce microscopic larvae that swim for
short periods (0 - 5 days) or long periods (>30 days)
Planktotrophy
long-distance dispersal
lecithotrophy
short-distance dispersal
Consequences of long-distance dispersal
planktotrophy lecithotrophy
populationconnectivity
gene flow
local adaptation
speciation rate
extinction risk
planktotrophic populations remain connected over evolutionary timescales
Evolutionary consequences of larval type
planktotrophy lecithotrophy
populationconnectivity
gene flow
local adaptation
speciation rate
extinction risk
ancestrallecithotroph
Evolutionary consequences of larval type
planktotrophy lecithotrophy
demographicconnectivity
gene flow
local adaptation
speciation rate
extinction risk
populationsdiverge...
Evolutionary consequences of larval type
planktotrophy lecithotrophy
demographicconnectivity
gene flow
local adaptation
speciation rate
extinction risk
theory and genetic data suggest lecithotrophic populations will split and diverge into new species...
Evolutionary consequences of larval type
planktotrophy lecithotrophy
demographicconnectivity
gene flow
local adaptation
speciation rate
extinction risk
theory and pop-gen data suggest lecithotrophic populations will split and divergence into new species...
Evolutionary consequences of larval type
planktotrophy lecithotrophy
demographicconnectivity
gene flow
local adaptation
speciation rate
extinction risk
...but may also go extinct more often
Evolutionary consequences of larval type
For 40 years, paleontological studies of snail fossils have inferred larval type from the shape of the larval shell, at the tip of adult shell
lecithotrophic shape
Shuto 1974
Six studies, cited >1,400 times, concluded lecithotrophs diversify more than planktotrophs, so benefit from species selection
Shuto 1974, Hansen 1978, 1980, 1982, Jablonski & Lutz 1983, Jablonski 1986
- that’s 1/12th the number of citations of the discovery of PCR!
Paleontological Perspectives
short-distance long-distance
Hansen 1978, Science
65 million years ago
each vertical line is a species,showing where it 1st appearedin the fossil record, and when itdisappeared (went extinct)
Paleontological Perspectives
1. short-distance dispersers speciate more often, but survive for short periods
2. long-distance dispersers survive for longer, but speciate less
lecithotrophic plankto.
Hansen 1978, Science
However, these studies never calculated diversification rate:
r = speciation - extinction short-distance may increase speciation and extinction rates, but the net difference between the two is what matters
Paleontological Perspectives
long-distance (n = 50)
short-distance (n = 50)
duration (m. y.)
%
%
Jablonski (1982, 1986) confirmed for several groups of snails that lecithotrophs have higher rates of both speciation and extinction
inferred that species selection favors lecithotrophs, because:
i) they speciate faster
ii) they accumulate in fossil record over time
Has been cited >450 times, and become a textbook example of species selection
Paleontological ProblemsStudies also did not address the fact that short-distance migration arises in two ways: 1) when a short-distance ancestor speciates, or 2) when a long-distance species undergoes character change
short-distance dispersal evolves once, triggers rapid diversification
‘species-selection’hypothesis
short-distance evolves 4 times from different long-distance ancestors; short-distance species don’t diversify
‘character-change’ hypothesis –accumulation w/out diversification
Paleontological Problemsi) studies did not factor in rates of character change
ii) paleontological studies never calculated diversification rate
short-distance may increase both speciation and extinction rates, but it is the net difference between the two that matters
speciation rate ()
extinction rate ()
diversification rate (r), the rate at which a lineage accumulates species (the measure of evolutionary success)
r = net gain in species over time
Paleontological Problemsi) studies did not factor in rates of character change
ii) paleontological studies never calculated diversification rate
short-distance may increase both speciation and extinction rates, but it is the net difference between the two that matters
extinction rate ()
speciation rate ()
r = 2
r = 1
both and go up, yet r decreases
long-distance short-distance
Paleontological Problemslong-distance (n = 50)
short-distance (n = 50)
duration (m. y.)
%
%
speciation rate () = 0.23extinction rate () = 0.17
diversification rate: (r) = = 0.06
Jablonski 1986
speciation rate () = 0.43extinction rate () = 0.34
diversification rate: (r) = = 0.09
1) minimal difference (if any...)
2) assumes all “appearances” of short- distance dispersers reflect speciation, but some must result from character change (long turns into short)
Using sea slugs to study macroevolution
Objective: identify traits that promote diversification, using herbivorous slugs in clade Sacoglossa as a model
Oxynoacea - 6 genera, 74 spp.
Plakobranchoidea- 4 genera, 137 spp.
(103 in Elysia)
Limapontiodea - 18 genera, 152 spp.
shelled
cerata-bearing
photosynthetic
Ancestral devel. modeinferred usingevolutionary quantitative genetics modelprobability that an ancestor had a
given type of larval dispersal
short-distance
long-distance
Limapontioidea
- more lecithotrophs in Plakobranchoidea, but only two pairs of lecithotrophic sister species
Plakobranchoidea
species-selection hypothesis predicts (a) clades of short-distance dispersers, which (b) should contain more species
NOT the case!
short-distance
long-distance
1. Testing for shifts in
diversification rate
Software ‘Medusa’ used to model diversification across 32 genus-level clades, using total # of known spp.
Medusa identifies shifts in the overall rate of diversification, not taking into considerating character state
two branches where rate of diversification accelerated:
1) after loss of shell
2) after photosynthesis evolved
Alfaro et al. 2008
1
2
We then modeled rates of speciation, extinction, and change between long- and short-distance larvae
Tested whether data better fit a model in which rates depended on larval dispersal, or if ignoring larval type fit the data just as well
Considered the three superfamilies of Sacoglossa as distinct, since they diversify at different background rates
Speciation rate depends on larval type df ln(L) AIC χ2 P a) unrestricted BiSSE (1), (1), q (1) 9 -68.73 155.46 n/a n/a (1), (2), q (1) 12 -66.06 156.12 5.33 0.149 (2), (1), q (1) 12 -61.90 147.79 13.67 0.003 (2), (2), q (1) 15 -61.20 152.40 15.06 0.020 b) restricted BiSSE (1), (1), q (1) 9 -70.33 158.66 n/a n/a (1), (2), q (1) 12 -66.87 157.75 6.91 0.075 (2), (1), q (1) 12 -63.78 151.55 13.10 0.004 (2), (2), q (1) 15 -63.29 156.58 14.08 0.029
best-fitmodel
- model which allowed speciation rate to vary with larval type was highly preferred over model which ignored larval type
- letting extinction rate covary with larval type did not improve fit
= speciation rate
= extinction rate
q = rate of character changeMaddison et al. 2007, FitzJohn 2012
Species selection favors planktotrophy
diversification rate (speciation – extinction) was always higher for long-distance (rP) than short-distance (rL) dispersers
most short-distance dispersers arose recently by character change, when a long-distance species evolved into a short- distance species
rP rL q1
Oxynoacea 3.2 1.8 4.3
Limapontioidea 10.4 <0 1.1
Plakobranchoidea 26.1 10.1 9.8
long- short-distance distance
Are sacoglossans just weird, though?
“This is not a group that appears to have speciation rates driven by lecithotrophy: lecithotrophy is the much rarer state in this group. Presumably this is not the case for many other clades.”
“You are characterizing patterns in a single somewhat odd clade of mollusks, with relatively poor fossilization.”
- anonymous reviewer comments about this work
Are sacoglossans just weird, though?
Heterobranchia #P #L % P Anaspidea 17 2 89.5 Cephalaspidea 47 13 78.3 Notaspidea 7 3 70.0 Nudibranchia 171 60 74.0 Sacoglossa 108 35 75.5
Caenogastropoda Calyptraeidae 39 39 50.0 Conidae 56 35 61.5 Fasciolariidae 9 25 26.5 Littorininae 139 13 91.4 Muricidae 36 46 43.9 Volutidae 0 9 0.0
% of known species with planktotrophic development
outliers are some clades in Neogastropoda that have few surviving planktotrophs
...but guess who paleontological studies focused on?
Are sacoglossans just weird, though?
“This is not a group that appears to have speciation rates driven by lecithotrophy: lecithotrophy is the much rarer state in this group. Presumably this is not the case for many other clades.”
“You are characterizing patterns in a single somewhat odd clade of mollusks, with relatively poor fossilization.”
As a function of changes per branch, larval type changed about as often in Sacoglossa (0.067) as in cone snails (0.067), and less often than in slipper shells (0.176)
Thus, Sacoglossa is typical in its % of planktotrophs, and in its rate of developmental evolution
Short-term solutions to a long-term problemSpecies selection favors long-distance dispersal in Sacoglossa, and perhaps (probably?) in most invertebrate groups
Loss of dispersive larvae is..
i) favored at ecological timescales, so change is frequent
ii) a dead-end at macro-evolutionary timescales
Most short-distance dispersers are the Walking Dead:
short-lived lineages that go extinct before they can diversify into daughter species
Short-term solutions to a long-term problemSpecies selection favors:
1) self-incompatible pollen in plants
2) long-distance larval dispersal in molluscs in both cases, the derived state (selfing in plants, short-distance larvae in sea slugs) evolves frequently, but increases extinction more than speciation, so dooms that lineage
thus, what’s favored by selection in the short-term or within a species may not be an evolutionarily “winning strategy” in the long term
>1,400 citations support a hypothesis that our results indicate is wrong. Don’t believe everything you read!