Download lecture 18 - adaptive radiation - Cal State LA

How do _____ evolve?
microevolution
populations
allele
frequencies
- genetic drift
- natural selection
- migration
How do _____ evolve?
microevolution
populations
species
over time, populations
can diverge & produce
two or more daughter
species from one
ancestral species
allele
frequencies
- genetic drift
- natural selection
- migration
each species must be
reproductively isolated
reproductive
isolation
adaptation
each species becomes
adapted to its niche by
natural selection
How do _____ evolve?
microevolution
populations
allele
frequencies
- genetic drift
- natural selection
- migration
macroevolution
species
reproductive
isolation
adaptation
lineages
(clades of species)
diversification
why do some groups
have more species
than related groups?
one common
ancestor
clade of 3 surviving (modern)
species
1 surviving species
2 daughter lineages,
of equal age
Evolutionary success = number of living species
 Why does one lineage diversify into many more species
than its less-successful sister lineage?
2 living species
of Bosellia
- flat sea slugs
- eat one algal
genus
- tropical only
134 species
in sister clade
Plakobranchidae
- sides rolled up
- eat >20 algal
genera
- tropics to poles
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 freshwater or terrestrial habitat
- got into a new lake, or onto a new island
- 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
Adaptive radiation
When an ancestor colonizes a new habitat, its offspring may
undergo an adaptive radiation
- descendents diversify (get different) and occupy all the
available ecological niches
- each lineage adapts, becomes phenotypically differentiated
by natural selection (= ecological speciation in action)
May follow colonization of islands, newly-formed lakes
- could provide opportunities for sympatric speciation
Often follows mass extinction events that remove previously
dominant competitors
Adaptive radiation via natural selection
Example: cichlid fishes in volcanic crater lakes in Africa
- monophyletic assemblage of fishes arose in sympatry by
descent from common ancestor that colonized new lake
- each species maintains its differences by assortative mating:
depends on ecological differences and mating preferences
Evidence for sympatric speciation in cichlids
All fishes from Lake Mbo
are each other’s closest
relatives
All fishes from Lake
Bermin also group
together
Suggests they arose in
sympatry, following an
initial colonization event
Adaptive radiation 1: Disruptive selection
Lake Victoria was completely dry 12,000 years ago
- now contains 500 species of cichlid fish
An adaptive radiation happened after an ancestral fish got
washed into the empty lake when a river flooded
Most species pairs have different feeding behaviors, due to the
adaptive evolution of their jaws
- by occupying different ecological niches, species avoid
competing with each other (necessary for co-existence)
Disruptive selection on feeding specializations can
drive adaptive radiation, by promoting species divergence
Disruptive selection on feeding: Cichlid jaws
(1) Diversification: each species feeds something different,
due to adaptive evolution of their jaws
front jaws catching food
back jaws processing food
Adaptive radiation 2: Sexual selection
Within a “feeding type,” there are often several sister species
(each other’s closest relatives) that differ only in color
All weird predators group
together (sponge-eaters)
All plankton-feeding
species group together
All normal predatory
species group together
Adaptive radiation 2: Sexual selection
Females with eye pigment alleles that see blue better prefer
bluer males; females that see red better like redder males
Sexual selection: female preference for color keeps different
species from hybridizing
reproductive isolation
End up with a blue, a yellow, and a red sponge eater;
.. a blue, a yellow, and red clam eater... etc
males can be
red, yellow, or
blue with
different
markings
Adaptive radiation 2: Sexual selection
sexual selection thus splits one group in two
Mate choice is determined by coloration
- strong assortative mating quickly leads to isolation of
different color morphs
- different species can interbreed without loss of fertility, but
normally they are pre-zygotically isolated by mate choice
Disruptive selection on feeding fueled diversification of sister
species; sexual selection provided reproductive isolation
one-two punch that drove the most explosive speciation
in the history of vertebrates
Adaptive radiation 3: end of cichlid diversity?
Recently, pollution has clouded Lake Victoria so badly, fish are
unable to see color differences
- species barriers are collapsing, as different species start
hybridizing with each other
Human activities that cloud the water are thus destroying cichlid
biodiversity
- unravels the mechanism of reproductive isolation by relaxing
sexual selection
- loss of evolutionary novelty, due to human disruption of
the environment
2) Specialization: why are there so many beetles?
~25% of described living species are beetles (flying insects)
- more than 135,000 species of beetles feed only on
angiosperms (flowering plants)
other beetles are:
- fungus-eaters
- predators
- aquatic
evolution.berkeley.edu
2) Specialization: why are there so many beetles?
2 factors may favor diversification in herbivorous beetles
2-A) Co-speciation
- when one plant speciates (evolves into 2 new species), its
pollinators and the herbivores that eat it may also speciate
2-B) Ecological specialization + host-shifting
- specialization = eat one species of host plant
(or animal, if you are a parasite)
- many related species can co-occur without competing,
which allows greater diversity of species in an area
- speciation can occur by host-shifting (as in Rhagoletis flies)
2-A) Plant-insect coevolution
When a plant speciates, so may its pollinators and specialized
herbivores
pollinator
(butterfly)
herbivore
(beetle)
2-A) Plant-insect coevolution
When a plant speciates, so may its pollinators and specialized
herbivores
Rain
forest
species
Desert
species
Pollinators and herbivores may also form new species when their
host plant speciates
coevolution promotes speciation
2-B) Specialization and “inordinate fondness”
Each time a lineage of beetles started to feed on angiosperms
(flowering plants), it quickly evolved into many more species
than did its sister lineage that did not eat flowers
- its rate of speciation increased, suggesting the association
with flowering plants in turn promoted beetle biodiversity
Farrell (1998)
proposed that
beetle diversity
resulted from their
associations with
flowering plants:
as plants diversified, so
did their beetle pests
Farrell 1998
Hunt et al. (2007) argued the radiation of beetles was due to:
A) specialization on different plant parts in some groups
(roots, flowers, fruit, leaves)
14,000
 herbivory
 predation
35,000
 aquatic
 eat fungus
Hunt et al. (2007) argued the radiation of beetles was due to:
A) specialization on different plant parts
B) frequent ecological shifts among major feeding strategies
23,000
14,000
 herbivory
 predation
48,000
35,000
 aquatic
 eat fungus
35,000
Hunt et al. (2007) argued the radiation of beetles was due to:
A) specialization on different plant parts
B) frequent ecological shifts among major feeding strategies
C) partly just because beetle lineages rarely go extinct
23,000
14,000
 herbivory
 predation
48,000
35,000
 aquatic
 eat fungus
35,000
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 freshwater or terrestrial habitat
- got into a new lake, or onto a new island
- 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
3) key innovation: evolution of a trait that allows exploitation
of new niches, or greater competitive ability
For instance, one group of fish diversified in the Antarctic
after evolving anti-freeze glycoproteins, allowing them to
survive water temperatures below freezing
9 species, non-Antarctic
(no anti-freeze)
123 species, Antarctic
- anti-freeze glycoproteins
- within Antarctic, species also
diversified into benthic and
pelagic forms, like lake fish
‘Big 5’ mass extinction events
During 5 mass extinctions, 50-90% of species disappeared
over a period of one million years
- the ‘big 5’ eliminated 20-60% of families of plants + animals
(whole families, or kinds of organisms)
end Permian extinction: 90% of marine
species gone
end Cretaceous K-T extinction,
65 Mya ago: bye-bye dinosaurs
Triassic-Jurassic boundary, 215 Mya
Mass extinctions vs. background extinctions
Despite their immediate impact, the Big 5 mass extinctions only
account for 4% of total extinctions over the last 500 million yrs
- 96% of species suffer background extinctions
- they just die out, or differentiate into new species
Episodic mass extinctions are important because they clear the
way for new adaptive radiations
(1) what causes them?
(2) why do some species survive them?
Causes of mass extinction: Deep Impact
Many forms of evidence support asteroid impact theory of K-T
mass extinction, possibly others as well
(1) iridium layer in rocks at the K-T boundary
- rare on earth, common in meteors
(2) microtektites also found in rocks at K-T boundary
- little glass particles formed when minerals melt at impact
- cool while flying through the air
(3) huge crater found off Mexican coast, 180 Km diameter,
dating to K-T boundary
(4) extraterrestrial origin suggested for noble gases trapped in
“buckey balls”, carbon spheres found at extinction boundaries
Causes of mass extinction: Deep Impact
K-T Impact likely had numerous environmental consequences
(1) injected SO2 and water into atmosphere, producing acid rain
(2) global cooling as dust blocked sunlight
(3) huge wildfires
(4) massive earthquake and tidal wave, supported by geological
evidence
(5) massive die-off in ocean phytoplankton
(photosynthetic plankton) disrupted marine food chains
Survivor’s guide to mass extinction
Studies on marine snails (good fossil record) indicate that the
lineages which survived mass extinctions had member species
scattered in many different biomes, or environmentally
different regions of the world
- surviving lineages had some species in the deep sea,
some in the tropics, some in cold water, etc
In other words, more biogeographically diverse lineages were
less likely to be wiped out by asteroid impact
- hedges against the total wipeout of any one niche or region
following a deep impact
# of families
Plant Evolution following Mass Extinctions
Gymnosperms
Seedless
dominate
plants
Angiosperms dominate
First
First
gymnosperms angiosperms
Lineages are often around, but not very successful, until a mass
extinction event wipes out the dominant competitors
 clears the way for adaptive radiation
Mackenzie 2003
What makes a lineage an evolutionary “winner”...?
1) specialization to exploit different niches
(co-existence without competition)
2) something that promotes rapid speciation:
- sexual signaling
- strong host association
- tendency to get allopatrically isolated (dispersal)
- fast-evolving gamete recognition proteins
3) key innovation (trait) allowing exploitation of new niches
or greater competitive ability
4) in the long run, being biogeographically widespread –
more likely to survive mass extinctions
Why only 2 Bosellia
but 134 plakos?
- flat sea slugs
- eat one algal
genus
- tropical only
134 species in clade
Plakobranchidae...
- have sides of body rolled up, which
protects stored chloroplasts from sun
(key innovation?)
- each species feeds on just one of >20
kinds of algae (specialized)
- species live and mate on their host
- colonized cold water habitats