Download ADAPTIVE RADIATION OF THE NEW ZEALAND GECKOS

ADAPTIVE RADIATION OF THE
NEW ZEALAND GECKOS
VERSION 2.0
Presented by Mike Kean
Waiopehu College, Levin
Copyright M.J. Kean
[email protected]
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Orders Of Reptiles
•
Squamata - snakes and lizards
•
Testudines - tortoises, turtles, and terrapins
•
Crocodilia - alligators and crocodiles
•
Sphenodontida - tuatara
Orders with Species Endemic to New Zealand
•
Squamata - lizards, comprising geckos and skinks
•
Sphenodontida - the two species of tuatara are the only
extant species of this order
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NZ has two families of native lizard
Scincidae (Skinks)
• moveable eyelids (can blink)
• no lamellae on digits
• skin shiny, smooth, and tight
Grand Skink
Gekkonidae (Geckos)
• fixed transparent eyelid
(how do they blink?)
• skin dull, rough, and loose
• lamellae on digits for grip
(see sticky fingers)
Forest Gecko
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Gecko Genera
Genus: Hoplodactylus
(the “brown geckos”)
• most are nocturnal
• most are terrestrial but climb
shrubs and small trees at
night
• are drably coloured in greys
or browns
• can lighten and darken their
skin tones.
• weakly vocal (chirrups and
squeals)
Genus: Naultinus
(the “green geckos)
• diurnal
• exclusively arboreal
• variously coloured in bright
greens, yellows,
browns
• cannot lighten or darken
their skin tones
• strongly vocal (loud “bark”)
Click for photos
Both genera are endemic to New Zealand
Click for photos
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Origin of the New Zealand Geckos
New Zealand’s first geckos walked here. Like
the tuatara they evolved in Gondwanaland while
the land that was to become New Zealand was
still attached. Separation began about 85 million
years ago (85mya)
Outline of NZ During Oligocene (in grey)
The Oligocene bottleneck (26mya) caused by rising
sea levels and subsiding landmass saw the extinction
of many of the species living in NZ at that time.
Surviving populations dropped to very low numbers
Possibly only one species of gecko survived this
period. This would have been the common ancestor
of all the modern NZ species.
DNA sequencing provides the evidence for
the above hypotheses. It gives us a form of
clock. By measuring the amount of variation
in the DNA of different species in NZ and
around the Pacific, scientists can estimate how
long ago they shared a common ancestor.
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Land of Opportunity
As the land rose again at the end of the Oligocene, conditions arose
which permitted many species to increase in numbers and diversify.
For the ancestral gecko these included:
• increased land area and new kinds of habitat
• the lack of other competitors (especially mammals) that might
have more successfully exploited the vacant ecological niches of
these habitats.
• the absence of mammalian predators.
• Large populations of insects and other arthropods due to the
complete absence of terrestrial or arboreal insectivorous
mammals.
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Steps to Speciation
• Oligocene bottleneck: Small population with reduced genetic diversity.
Some competitors and predators become extinct.
• Post Oligocene: NZ landmass re-emerges from the sea increasing land area
and providing new types of habitat. A population explosion follows, aided
by a plentiful supply of food, and reduced predation and competition. This
reduction in selection pressures allows genetic variation to increase
because most phenotypes are able to survive and reproduce.
• Migration into new and varied habitats to form separate populations where
selection pressures differ. Small initial population size also may result in
random genetic changes due to the founder effect and genetic drift.
•
Geographical barriers prevent gene flow between populations.
Continued next slide
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Continued from previous slide
•Competition steadily increases as the population grows so that natural
selection favours the best adapted geckos in each habitat. As selection
pressures are different in the different habitats each population evolves in
different ways.
•Eventually genetic, ecological, or behavioural isolating mechanisms
prevent gene flow should populations become sympatric. Such populations are
now considered to be separate species.
Click to view adaptive radiation diagram
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Three Species
Three species of gecko can be used to illustrate two of the
evolutionary trends seen in the New Zealand geckos.
Common Gecko
(H. maculatus)
Forest Gecko
(H. granulatus)
Wellington Green Gecko
(N. elegans punctatus)
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Evolutionary Trends
The differences in form and colour between these three species are the result
of two behavioural changes
1. Nocturnal to diurnal habit
2. Terrestrial to arboreal habit
What are the advantages of becoming
diurnal? Think about:
• competition for prey
• efficiency of hunting
• effect of increased metabolic rate due to
higher daytime temperatures – what
advantages are there in this for; catching
prey, body processes?
What are the risks of becoming diurnal? Think about:
• predators
• heat stress (a major problem for geckos) and dehydration
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What are the advantages of becoming arboreal?
Think about food sources. Geckos take moving prey –
mostly insects, but also eat small berries and lap up
nectar from flowers. Also think about predators.
What risks might there be in becoming arboreal? 11
Think this question over as you view the following slides
Genus: Hoplodactylus.
Common Gecko
We could refer to this species as the ancestral type, as it is similar to other
Pacific island geckos and probably most closely resembles the common ancestor.
This gecko is terrestrial, living mostly at ground level, although it will climb
shrubs and trees at night. It is nocturnal although it is sometimes seen basking in
the sun, always close to shelter.
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Forest Gecko
The forest gecko is currently
assigned to the genus
Hoplodactylus, but DNA
sequencing suggests it should be
placed somewhere between the
two genera.
The forest gecko exhibits
behaviour which appears to be
intermediate between the other
two species. While its activity is
primarily nocturnal, it is mainly
arboreal, staying in the trees
during in the summer months.
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Wellington Green Gecko
Diurnal geckos are rare elsewhere in
the World. Species of the genus
Naultinus are unique in their daytime
habits, brightly coloured camouflage,
and live birth. Their DNA shows the
greatest divergence from the
hypothetical common ancestor of the
Oligocene bottleneck.
The Wellington Green Gecko is a
confirmed tree dweller and is
superbly adapted for this lifestyle.
Living at the tips of the branches of
trees, its limbs, digits, tail, colours,
and patterns all combine to make this
master of the disappearing act a
successful species. While introduced
predators do take their toll,
Wellington suburbanites are still
occasionally charmed to find a green
gecko in the garden.
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NOCTURNAL TO DIURNAL
Common gecko
colour is drab, gray or brown. It is inconspicuous in the low light conditions
that the common gecko normally ventures out in.
escape is by running away and quickly disappearing into a crevice. In poor
light this is very effective. Dropping the tail is effective if seized. The tail
threshes around distracting the predator.
movements are quick and erratic making it difficult for a predator to follow
Wellington green gecko
colour is rich, mainly green with yellow spots which act as disruptive
camouflage. This is very effective in fine foliage near the tips of branches,
where these geckos wait for prey, or bask in the sun.
escape: green geckos if detected will try to bluff the predator, or they will
drop from the branch and, hopefully, out of sight. Because the tail is valuable
for climbing it is seldom dropped.
movements are slow and punctuated by pauses while the gecko is climbing
casually. This pattern of movement is difficult to spot.
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Forest gecko
Colour: Unlike the common gecko
the forest has rich colours – redbrowns, browns, yellows, black,
and white. The richer colours can
fade rapidly when the animal is
disturbed, or at night when it is
active, so most people only see a
predominately light brown to gray
animal.
Escape: Camouflage on bark
covered stems. Freezing. Running
away. Dropping tail.
Movement: Similar to the common
gecko – rapid movements to get out
of sight as quickly as possible.
Click for more information on colour
Click for examples of camouflage
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TERRESTRIAL TO ARBOREAL
What are the advantages of becoming arboreal?
• What kind of predators might be avoided?
• What additional foods might be available?
What are the possible costs of becoming arboreal?
• Staying in the trees means being visible during the day. What are the
implications of this?
• The common ancestor was almost certainly an egg layer and would
have buried eggs in sun-warmed sandy soils. How can an arboreal
gecko keep its eggs warm and safe from predators? (Give up? Wait
for slide 22 if you are stuck)
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TERRESTRIAL TO ARBOREAL
Compare the relative lengths of the legs and digits. What is the
advantage of a longer reach to the green gecko?
How do the shapes of the digits relate to movement on rocks,
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or twigs? For more detailed information see “Grip”
Tails
Compare the shapes of the tails. Which one
is most likely to be storing fat? Which
shape would be more use when moving
around in trees?
What term is used
to describe a tail
that can be used as
a fifth limb? How
might it be used to
help an arboreal
gecko capture its
prey?
Tails are covered more fully in
Grip
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Live Birth
Unlike almost all other geckos in the world, New Zealand geckos are ovoviviparous. This
means that the eggs remain in the mother’s body throughout development and live young are
born. This confers several advantages:
•
larger offspring can be born (increasing chances of survival).
•
in a cool climate the mother can keep the
eggs warm inside her by basking in the
sun.
•
eggs are protected from predators.
Given that New Zealand geckos are the southernmost in the world, and that all our gecko’s and all
but one of our skinks have live birth, which of the
above advantages must have triggered the change?
What major geological event might have provided
strong selection pressure for this adaptation?
Female Northland green gecko with week-old juvenile
Retaining the eggs in the mother’s body would be of particular advantage to an arboreal
species like the Wellington green. Why do you think this would be so?
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Geographical Barriers
Changing Sea Level
Since the Oligocene, tectonic uplift
and successive ice-ages have
resulted in large changes to sea
levels, first isolating, then reuniting,
island populations.
Glaciers
During ice-ages, glaciers form
effective barriers. Cold blooded
animals quickly slow down and
become immobile when placed on
ice.
Mountains
High mountain ranges present a
barrier to reptiles if the passes are
too cold for a gecko cross.
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Naultinus Species and Their Barriers
The Naultinus species (“greens”) are largely allopatric. In captivity
many species will interbreed and produce fertile offspring
Wellington Green Gecko and Nelson Green Gecko
Cook Strait forms a geographical barrier between them.
Rough Gecko (Nth Canterbury) and Jewel Gecko (Banks Peninsula)
It is likely that glaciers flowing from the Southern Alps originally
kept these two species apart. Wide stony riverbeds may have also
been a factor in more recent times.
Marlborough Green Gecko and Rough Gecko (North Canterbury)
There is some evidence that these two species once formed a cline.
Habitat destruction by humans has largely destroyed the intervening
habitat and most of the intermediate populations. The loss of habitat
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now prevents gene flow between the two species.
Hoplodactylus Species and their Barriers
Unlike the Naultinus (“green”) species, many of this genus occur
sympatrically. Also the Hoplodactylus species do not seem to
hybridise in captivity.
The Common Gecko and the Forest Gecko often occur in close
proximity to each other in the wild but each has its own preferred
habitat - “commons” on the ground and usually close to rocky
areas for daytime cover, while the “forests” remain in the trees.
This reduces the chances of boy meeting girl. The forest gecko’s
more diurnal behaviour may also reduce contact between the
species.
The giant Duvaucels Gecko looks as though it would regard any
potential mate of the other species as a dinner item (see photo), but
those breeders who have taken the risk report that it does not
happen. It can be found in close proximity to common geckos but
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may be that the physical size difference prevents cross mating.
Naultinus-Hoplodactylus Hybrids
Despite being classified in different genera, a forest gecko-jewel gecko cross has
occurred in captivity. It seems likely however that the hybrids are infertile. Also,
while they appear to be robust and strong, one factor may see them eliminated by
natural selection in the wild. Their colouration and patterning are quite different
from either parent species so they may be more conspicuous to predators.
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Forest
Hybrids
Jewel
Minis and Maxis
An Example of Parallel Evolution
Victoria University post-graduate Rod Hitchmough investigated populations
of common gecko for his thesis. He quickly noticed that in each of several
regions around the country there appeared to be two distinct populations
based on size. Rod quickly dubbed these “maxis” and “minis” and assumed
they must have been two separate species. However when Rod sequenced
DNA from maxis and minis in both Marlborough and Otago the results
surprised him. They showed that the maxis and minis in Otago were more
closely related to each other than to the maxis and minis from Marlborough.
It seems that in each of these two regions, a single ancestral population has
diverged along similar lines. Click here for tree diagram
The geckos in each region resemble each other, not because they inherited
their similarities from a common ancestor, but because they have been
exposed to similar environmental selection pressures.
This is an example of parallel evolution. Two closely related populations in
different regions have continued to evolve similar characteristics because they
occupy similar ecological niches in similar environments.
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For information about the New Zealand herp scene you may
like to visit the following website:
http://members.tripod.com/~hoodoodvb/herpweb/herpweb.html
END
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