Transcript Temporal Radiations

Species Radiations
Readings:
Grant & Grant (2002) Science paper
Species radiations
The opposite of extinction is radiation – speciation
diversifying a taxon. One example was introduced at
the beginning of the previous slide set: the radiation of
Darwin’s finches on the Galapagos.
The Galapagos islands are a group of 16 major islands
plus an equal number of smaller islets located on the
equator about 1000 km west of Ecuador. The
archipelago was formed following volcanic eruptions of
the seafloor ca. 3 - 5 million years ago. The
islands, because of their remoteness from mainland
sources of plants and animals, have a relatively
impoverished biota.
Galapagos Islands: Most islands are not occupied; Darwin
visited the islands in 1830; Cocos Island is 600km NE
26 species of land birds occurred naturally on the
Islands before human introductions; 13 of these are
finches. The islands also support 4 mockingbird, 2
flycatcher, 2 owl, 1 hawk, 1 dove, 1 cuckoo, 1 warbler
and 1 martin species (Pianka 1983)
The island finches belong to a distinct subfamily of
finches, endemic to the Galapagos and Cocos Island
(Costa Rica). Cocos Island is several hundred km
north of the Galapagos and about the same distance
from the mainland. It supports only 1 finch species, the
Cocos Island finch Pinaroloxias inornata. The Cocos
finch is a generalist feeder.
On the Galapagos Islands, adaptive radiation has resulted from
geographic isolation and reduced gene flow among islands.
Three distinct genera (Geospiza, Camarhynchus and
Certhidea) occur on the islands.
Members of these genera differ in where they forage, how they
forage, and what they eat.
Genus Geospiza includes 6 ground-foraging species with broad
beaks that crush different species and/or sizes of seeds, or use
flowers of Opuntia cactus.
Darwin's finches (classical view)
Cocos finch
A more modern view... (Sato et al .1999) PNAS
based upon
mtDNA sequences
A more modern view... (Sato et al .1999) PNAS
based upon
mtDNA sequences
•Warbler finch may be the ancestral
species, not the Cocos finch, which
groups with the tree finches.
• Vegetarian finch most closely related
to warbler finch
•tree and ground finches remain
separate
Finch diversity by island
The 6 species of Camarhynchus finches
forage in trees, have narrower beaks,
and eat either vegetation (1 species) or
different sizes of insects. C. pallidus
uses a stick or cactus spine to probe for
insects.
Another genus has one species, Certhidea olivacea, called
a 'warbler-finch'.
Beak lengths and depths vary widely from island to island,
and in some cases, provide evidence for competitive
displacement
On islands (Abington, Bindloe, James, Jervis, Albemarle,
Indefatigable, Charles, Chatham) where Geospiza
fuliginosa and G. fortis occur sympatrically, they tend to
have widely divergent beak depths.
Only one of these species is on each of Daphne and
Crossman. The beak morphologies are very similar
when the species occur in isolation.
Before it can be confirmed that character displacement
has occurred, 4 conditions must be met:
1) the change in mean character state in areas of overlap
should not be predictable from variation within areas of
overlap/isolation
2) sampling should occur at more than one set of
locations to eliminate local variation effects
3) Heritability of the feature must be high if genetic
variation is thought to underlie variation in the feature,
and if it is to be passed to subsequent generations
4) Evidence must be presented to demonstrate that
competition occurs and that the measured feature has
relevance to competition among groups
Conditions 1 and 3 have been demonstrated: The variation
observed in G. fortis in isolation on Daphne greatly exceeds
that observed on other islands. Heritability of the trait for
beak morphology [which can range from 0 (no relation
between parental and offspring morphology) to 1 (absolute
relationship)] has been estimated as 0.82.
There is good tentative evidence for criterion 4. Peter and
Rosemary Grant recently demonstrated that co-occurring
finches diverged in beak morphology after a drought which
greatly reduced seed abundance, relative to their respective
beak shapes prior to the drought.
There is, however, no way to deal with criterion 2 since there
is only one Daphne and one Crossman. Satisfying this
requirement would necessitate observing the same pattern
on other islands like Daphne and Crossman.
G. fortis on Daphne Major (Grant & Grant 1993, Proc. R. Soc. Lon. B)
rainfall on Daphne Major
seed size (white) and hardness (black)
small seeds = solid large
seeds = open
relative abundance of small seeds increased dramatically (right) following
El Nino in 1983 (left).
fortis (top), scandens (bottom)
Strong selection for fortis, which
feeds on small seeds, over
scandens, which feeds on rare
cactus seeds, following drought
after 1983
G. fortis on Daphne Major (Boag & Grant 1981, Science)
PC 1 (morphology)
popn size
seed
abundance
seed
size
• 1500 birds banded and studied between 1975-78
• regular rainfall (large finch populations) until 1977, when only
24mm rain fell. fortis did not breed and 85% decline in
population (A); seed abundance declined (B), big males
survived best (C), corresponding with decline of small seeds (D)
The distributions of individual species also suggest
competition has occurred among the finches:
Two closely related taxa, G. fortis and G. conirostris, have
completely different distribution patterns and never cooccur.
G. difficilis is found on very few islands located at different
ends of the archipelago, suggesting that it has had
opportunity to colonize islands in between. Grant and
Schluter (1984) attributed its rarity and absence on
additional islands to competition from other finch species.
Geospiza bill morphology.
Both geographically
restricted species
(conirostris, difficilis) have
bill depth similar to
widespread species (fortis,
fulignosa)
G. difficilis is found on 4 widely separated islands and
extinct on 4 others. It co-occurs with G. fuliginosa on
large central islands (presumably with large habitat area
supporting large populations), but does not co-occur on
smaller central or small outlying islands. Grant and
Schluter used statistical comparisons to suggest that
these species co-occur much less than one would
expect by chance alone.
Grant & Grant (2002) Science paper
• studies the same populations of G. fortis (medium ground
finch) and G. scandens (cactus finch) on Daphne Major
for 30 years
• survival of marked and measured individuals measured
each year
• 6 traits studied were reduced by Principal Components
Analysis, which break down as body size, beak size and
beak shape
If species were not changing,
each measure should have
stayed within its original 95%
confidence interval (horiz.
lines). Clearly this is not
happening. scandens
converged on fortis' morphology
1. body & beak sizes selected
more than beak shape
2. species differed in net selection
direction on size traits (fortis in
both directions with equal
frequency, scandens selection
repeatedly favoured large body
size and never small beak size)
3. unidirectional selection (up to 3
yrs) occurred in both species
4. selection events in the species
were not synchronized except
in late '70s during a drought
5. each of these studied traits was
highly heritable, so evolution
followed for fortis (4 body size
3 beak size, 1 beak shape) and for scandens (2 body size, 5 beak size)
Predicted and
observed
evolutionary
responses in
beak size and
shape are
determined by
the product of
selection
differentials and
heritabilities
Proportion of hybrids and
backcrosses are higher in
scandens than in fortis (see
b), thus new variation
introduced to the population
should follow the same pattern
Mean (a) and variance (b) of
beak shape changes more in
scandens than fortis due to
presence of hybrids and
backcrosses (white bars).
Result due to higher mate
competition in fortis (1:1 sex
ratio) after a drought had
killed many scandens females
(male bias afterwards).
The Galapagos finches experienced adaptive radiation, a
process whereby the species diversified to exploit a wide
variety of available niches.
As in the following cases of the Hawaiian honeycreepers and
cichlid fishes of the African Great Lakes, closely related taxa
co-exist by exploiting different habitats.
If individuals with similar niches practiced assortative mating,
in which they preferentially mate with individuals sharing
similar traits or habits, then sympatric speciation is possible.
Aging the Hawaiian Islands
Fleisher et al. (1998) Mol. Ecol.
Hawaiian honeycreepers
These birds experienced much greater adaptive radiation
than Darwin's finches, though, sadly, many of the taxa
have been driven extinct due to introduction of diseases,
other passerine birds and mammals, and destruction of
habitat.
On Laysan Island introduced rabbits and a windstorm
destroyed vegetation resulting in the extinction of the
Laysan honeycreeper.
Bird pox virus and avian malaria (introduced to the
Hawaiian islands by mosquitoes on ships in 1826) have
caused sharp declines in Drepanidid species, including
the honeycreeper.
wide diversity, some now extinct (E)
beak shape
highly variable
and highly
adapted to
feeding mode
Origin of the Honeycreepers
• Johnson et al. (1989) used starch gel electrophoresis to
study 8 genera (9 species) of honeycreepers
• they are a monophyletic group (only one ancestor)
• the ancestral species colonized the Hawaiian archipelago
7-8 million yr ago
• this agrees with the emergence of Nihoa (now largely
submerged) but predated the island of Kauai (5 MYBP)
• Oreomystis and Paroreomyza are the oldest and most
diverse lineages
• youngest lineages are the nectar feeders and thick billed
finch types
Views of honeycreeper phylogeny based on mtDNA
(top) and allozymes (20, variable) (bottom)
true creepers
Time difference between island
formation dictates genetic distance
between species
Fleisher et al. (1998) Mol. Ecol.
Time difference between island formation
dictates genetic distance between species
Honeycreepers
Hawaiian Drosophila
sequence divergence rate
0.016 per Million/yr
0.019 per Million/yr
• Currently, honeycreepers occur primarily at altitudes
above 600 m on the main islands and on several
smaller remote islands in the NW part of the
archipelago.
• Mosquitoes, by contrast, occur primarily below 600 m,
and overlap very little with the honeycreepers.
• At the lower altitudinal end of the honeycreeper
species ranges, between 2 and 7% of individuals have
avian malaria.
• Rats have also played a large role in species
extinctions.
• in total, 28 of 53 known species are extinct (34 known from
historical records, 19 from fossil records)
• 18 of the remaining species are endangered
• 9 extinctions have occurred since introduction of malaria to
the islands. Of these, 6 occurred on Lana'i, Moloka'i and
O'ahu. These islands have been radically modified by
humans, thus habitat destruction appears to have played a
large role.
• Behaviour may have affected 2 large, nectar-feeding
Drepanidid species driven extinct and 2 smaller ones
extirpated ('akoekoe and 'i'iwi) on some islands.
• The rare 'akiapola'au, a specialized insectivore, has
become endangered because it lives primarily in large koa
trees; koa trees have been widely harvested for furniture
lumber.
• Another endangered species, the palila, a granivore,
exploits seeds of one tree, the mamane. This tree has
been adversely affected by introduced goats and sheep,
thereby endangering the bird.
• Honeycreepers are endangered because of their extreme
specialization (habitat or food), which itself is a result of
dramatic adaptive radiation. This problem may be
compounded by introduced diseases, mammals, and
exotic birds.
What's the Honeycreeper's Future?
Benning et al. (2002) PNAS
• habitat loss began with the Polynesian colonists (9001000 yrs ago), who cleared low elevation and seasonally
dry forest
• European colonists brought new agricultural technology,
domesticated cattle
• hunting, beginning with Polynesians, and introduction of
dogs and rats that preyed on nesting birds. These
predators were followed by mongoose, cats and 2 more
rat species
• introduction of mosquitoes and of avian pox, and more
recently avian malaria, had the greatest consequences
Temp. <13°C is critical to prevent malaria infection. In Hanawi Forest
(Maui), a 2°C increase would cut this zone area by half (665 to 285Ha).
low mosquito risk zone, Hanawi, Maui
low mosquito risk zone, Hakakau Refuge, Hawaii
(low risk areas declines from 3120 to 130 HA)
Alakai Swamp, Kauai. No area currently below 13°C,
area of possible high risk moves up 300m. Must focus
on disease prevention in remaining honeycreeper
populations.
Hawaiian Drosophila
More than 900 endemic species, each typically restricted
to one island.
Speciation of these flies is also speculated to have
occurred as a result of adaptive radiation. The
speciation mechanism is not entirely clear, though two
main models are:
1) Mayr's founder effect model
2) Brian Charlesworth’s model of adaptive divergence
under new selective pressures
• molecular data suggest separation of Hawaiian lineage
from mainland lineage between 10-32 MYBP
Hawaiian
Drosophila
Mayr’s model: a few new colonists to an island represent
only a small fraction of the total Drosophila gene pool.
Genetic drift then results in dramatic 'peak shifts' in the
genome of colonizing species relative to the parental stock
(so long as gene exchange is precluded).
This genetic change in the population would occur very
rapidly and would be driven by local selective pressures in
a relatively homogeneous environment.
Genetic variation is far more pronounced if you start with small as
opposed to large populations
Hampton Carson modified this model slightly.
Certain loci form strong epistatic relationships (synergistic
effects caused by multiple loci) and a 'closed variability
system'. For example, in fruit flies, genes controlling mate
recognition and behaviour may form such a closed
variability system' in which females will recognize only
specific behaviours or morphologies when preparing to
mate.
These blocks of genes may become destabilized during
founding events. Recombinants that previously had low
fitness may now thrive, bringing the new population to a
new 'adaptive peak'.
Templeton modified the model differently, suggesting that
change in allelic frequencies for a few key genes (owing to
genetic drift) could precipitate changes in modifier loci in
the new environment, leading to a new co-adapted state of
the character.
One evidence: Kaneshiro has determined that some
species differ only by one mutation. This mutation modified
the behavioural repertoire of males during mating
rituals. Derived species females would recognize the ritual
and mate with the mutated individual, whereas ancestral
females (the parental species) would not. The species were
behaviourally (sexually) isolated from one another.
Charlesworth’s model: founding may not be the impetus
for speciation; it may occur as a result of adaptive
divergence from the parental stock under a new regime of
selective pressures.
Newly colonized islands  new environmental conditions
 new selection pressures  divergence from parental
stock
Problem for both hypotheses: immunological tests
with Drosophila suggested that the flies were far older than
the islands. If this were so, how could all of these endemic
species occur on the islands? Where did they come from?
Colonization events (direction and number)
for Hawaiian Drosophila