insect evolution
Bio153: lecture 17
deuterostomes and the rise of
chordates
•
•
• invertebrate terrestrial invasion
• transition from invertebrate ⇒ vertebrate
•
•
insect diversification
•
showed “bursts” of diversification
from wingless insects….
1. winged insects
2. folded wing insects
3. metamorphosis
4. hardened 1st pair of wings
most successful group ever
more spp. of beetles than of all
vertebrates!
Hexapoda: 6 legs
no legs on abdominal segments
1) evolution of wings
- not “limbs” (bird, bat)
possibly:
- accessory of respiratory system?
- fanning increased gas exchange
- during Jurassic (O2 levels ↑ / CO2↓):
dragon flies: 75 cm wingspans!
• an adaptation, but also an encumbrance!
2) folded wings
- streamlined
- crawl through vegetation, burrow…
3) metamorphosis
- larva, pupa, adult
- ↓ competition between adult and
offspring – exploit more environments
1
• beetles: appear ~ 230 mya
• quickly rose to 40% of all insects
• why so successful?
4) hardened 1st pair of wings
- protection from
mechanical damage
predators
parasites
echinoderms (starfish, sea urchins etc.)
• abundant in Paleozoic: crinoids
• strictly marine – reduced in Permian
extinction
deuterostomes
•
•
•
•
echinoderms, hemichordates & chordates
blastopore = anus
coelomate, triploblastic
radial cleavage (cell fate determined late)
• unique to echinoderms : pentamerous
symmetry (5-sided)
• radial symmetry is a derived trait
(Echinoderms are in Bilateria)
• larvae are bilaterally symmetrical
2
chordate characters:
1. notochord
• stiff rod derived from embryonic gut
• mesodermal
without notochord
2. pharyngeal gill slits
• used in filter feeding in
early chordates
• slits persist in aquatic
vertebrates
• become parts of jaw,
inner ear in terrestrial
vertebrates
with notochord
3. dorsal hollow nerve cord
• derived from ectoderm
• “flipped” from annelid, arthropod
arrangement
hemichordates (closely related to
chordates)
•
•
acorn worms
only 1 chordate character
(pharyngeal gill slits)
4. post-anal tail
• extension of notochord, musculature
• important in locomotion
chordates
1. urochordates (tunicates)
2. cephalochordates (lancelets)
3. vertebrates
all chordates
show the 4
chordate
characters at
some point
in life cycle
3
1.
•
•
•
urochordates (sea squirts, tunicates)
all 4 chordate characters as larvae
lost in adult – sessile lifestyle
heterochrony: descendent retains larval
form
2. cephalochordates
(e.g. amphioxus): all 4 characters
•
•
chordate origins still obscure
~ 540 mya (Cambrian)
steps in vertebrate evolution:
1. pre-vertebrate – suspension feeder
(similar to amphioxus)
2. agnathan – no jaws; muscular pump to
move food current
3. gnathostome – vertebrate with jaws
probable vertebrate phylogeny:
agnathans
lamprey
cartilaginous
fish
ray-finned
fish
lobe-finned
fish
lungfish
tetrapods
• agnathans: e.g. hagfish
• no bones, no jaws; cartilaginous skeleton
• only vertebrate with partially open circulatory
system
• several hearts to help move blood (cardinal
hearts + caudal heart + branchial heart)
jaws arose from
gill arches:
1. both are bars of
tissue that hinge
and bend forward
2. both derived from
neural crest
3. attached muscles
have same
embryonic
development
4
ray-finned fish:
• comprise half of all
verts
• ~20,000 spp.
• swim bladders
• gas exchange
through gills
• some gulp air
• cartilaginous
fish:
e.g. sharks, rays
• lack swim
bladders
• heterocercal tail:
gives lift
• complex sense
organs
lungfish:
3 species (tropical)
lobe-finned fish:
• one of 2 extant
lineages with jointed
limbs
• loss of swim bladder
• flourished in
Devonian; thought
extinct until 1938
- bi-lobed sac attached to floor
of pharynx
- highly vascularized
- gulps air
-similar to fish that gave
rise to amphibians
the terrestrial invasion (part 3)
• late Devonian (~ 360 mya)
•Panderichthys (extinct) – type of lobefinned fish
• shallow ponds are stagnant – little O2
(lots of decaying algae)
• gills not effective
• Panderichthys:
- bi-lobed sac in pharynx
probably could swallow air (like lungfish)
- lobed fins to push on substrate
Ichthyostega:
• dorsoventrally flattened
• dorsally placed eyes
suggest adaptation to
life in shallow ponds
Devonian/
Carboniferous
Greenland
5
Ichthyostega:
1) powerful limbs and feet
2) vertebra are interlocking – prevents
twisting of backbone
3) strong rib cage – prevents crushing of
internal organs
April 6th, 2006
why live on land?
• limbs evolved
from fins
• Hox genes in
fish expressed in
tetrapod limbs
1) more O2
2) less competition
3) less predation (esp. in
reproduction
New adaptations:
• neck!
– shoulder free; arms and head have greater
movement
• new sensory organs
– lower jaw senses vibration
6