Function vs effect
Otte, D. 1974. Effects and functions in the evolution of signalling systems
Annual Review of Ecology and Systematics 5: 385Re communication systems: we need to
“distinguish between evolved functions
and incidentaI effects” (Otte 1974)
Function defined here as: “the special
action of any part of a living organism
that evolved because such action
fostered survival or reproduction”
Effect: a by-product of a characteristic.
Structural adaptation, another
defn: structural features with a
history of genetic selection to a
specified end of increased fitness.
Structural effect would then be
one with no history of selection
to a specified fitness goal*.
*teleology
Library ann.
Red of the deepwater sponge, red of
the male cardinal, red of maple leaves
in the fall: only the cardinal’s red is a
structure associated with adaptive
consequence.
Welcome Wildlife
Fall Colours
an
adaptation?
Anthocyanins can give flowers red and purple
colour, a structural adaptation for attracting
pollinators (e.g., hummingbirds). As a plausible
hypothesis, red (anthocyanin) coverings of fruits
could be selected to attract animals whose feeding
on this food gift leads to dispersal of plant seeds.
In photosynthetic tissues (such as leaves and
sometimes stems), anthocyanins have been shown
to act as a "sunscreen", protecting cells from highlight damage by absorbing blue-green and
ultraviolet light, thereby protecting the tissues
from photoinhibition, or high-light stress. (Wikki).
The anthocyanins of sugar maple leaves are a
structural adaptation: they have a history of
selection to promote fitness of trees by protecting
from too much light.
This red colour of maples in the fall has probably not been selected
as a signal or cue (e.g., conveying information to leaf-feeding insects)
The redness is a beautiful effect, a by-product of having anthocyanins
revealed by the dying away of green chlorophyll.
G.K. Morris
What are the boundaries of a structural adaptation? What are its limits? Think about
evolutionary timelines: generation after generation comprising a history of
adaptation. Adaptations group together into a historical theme: e.g., flight.
Multiple structural adaptations of the
historical theme of flight: structural features
with a history of genetic selection toward the
specified end of increased flight fitness: from
bones, to feathers, to an aerofoil wing shape,
a nontidal respiratory system, air sacs:
multiple structural adaptations contributing
toether to better flight fitness.
What were once adaptations become
vestigial, like the human tailbone, or occur
as a partial reading out of genes during
development that has no cost but never
comes to function and is not subject to
selection in males: e.g., breasts (nipples) of
male humans.
Matt Woods
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Arthropods waltz about in chitin, cuticular exoskeleton. Like a human
wearing a suit of armor, the surface needs be segmented and jointed – and
presumably parts moving against parts – rather noisy. One part comes near
another and makes contact – one wing bangs into another as they are flexed
and put away after landing. To start with this sound of wings being put away
is incidental. At first the sound produced has no history of selection and so
by these definitions would be properly regarded as an effect – an incidental
effect.
The singing of a cricket to attract a mate comes about via an evolutionary
process the ethologists called ritualization. Sound effects (intentional pun)
of putting the wings away could be selected for because they helped bring
the attention of a female nearby in the dark. Sound became an adaptation
enhanced by special wing structures furthering the sound-maker’s fitness.
Stridulation (the production of sound through frictional contact by
exoskeleton) is another historical theme of selection.
Alexander, R.D. 1957. Sound production and associated behavior in
insects. Ohio Journal of Science 57: 101- “The number and variety of
insects which produce sounds with specialized apparatus undoubtedly
exceed those of all other living organisms combined…”
Tettigonia viridissima (a katydid): the arrows indicate the level of the tentorium
within the head, just above the articulations, anterior and posterior, of the
mandibles. During development tentorial arms like other apodemes, arise as
inflections of ectoderm; the inflected embryo tissue acts to produce chitin,
fusing internally into the body of the tentorium. One has a paradox of
exoskeleton that functions internally.
Orthoptera all
have a
tentorium: the
Order includes
katydids,
crickets,
grasshoppers.
Locust tentorium from below: a truss anchoring the
Ventral corners of the head for mandible support
Condyle for mandible
Anterior arm
Posterior arm
Sara Jane Gutierrez
dissection
Locust tentorium from above
posterior
anterior
Region into
which the
abductor and
adductor
apodemes
project
A diagram of forces: the tentorium is what an engineer would
call a truss.
The insect head can be modelled as a six-sided box, one with no bottom and no rear.
Lacking two sides leaves the two rear corners movable under stress. The mandibles,
mounted beneath the cheeks and rotated together toward the midline to crush
vegetation, needs a firm base. Crushing in the absence of the tentorium would let the
two ‘free’ box corners move apart and so reduce the squeezing force.
Function of the tentorium: a truss
that keeps the head stable as a
base for mandible rotation.
Variation in tentorium structure among taxa
Evolution of the
beetle gular sclerite
Gula
Prognathous – forward directed – mouthparts are typical of
the Order Coleoptera and so is the sclerite called a gula (a
sclerite is a region of the insect’s exoskeletal surface, set off
by membrane, sutures or sulci). Early ancestors of beetles,
had hypognathous mouthparts, but evolved this prognathous
orientation, accessing food (or prey) in front rather than at its
feet. The region where the posterior arms meet the cranium
was involved in this head angle change and internally drew
the posterior arms (red) out into a very lengthened structure
helping to offset forward-originating forces..
Force moment
When “a force has a line
of action lying to one side
of an axis of rotation...
we call the shortest, or
perpendicular, distance
between the force’s line
of action and the axis,
the ‘moment arm’ or the
‘lever arm’ of the force”
(Vogel 2nd). A moment of
force is the product of
the force magnitude and
this lever-arm distance to
the line of action.
“Levers are practical applications of
...moments” Vogel
•
•
•
•
•
Moments involve rotation.
“The effectiveness of a force is the product of
the magnitude of the force and the
perpendicular distance from the line of action of
the force to the axis of rotation.” This is its
moment. Making d large increases the moment
of force that the mandible can apply.
Compromise is necessary.
Axis of rotation of the mandible is an imaginary
line joining the two articulation points situated
on the lower margin of the cranium.
The mandible turning about this axis, completes
an arc of a circle, which can, e.g., crush a seed
against the opposing mandible.
The mandible, suspended beneath the insect’s
head, is anchored at two articulations ; joining
these articulations with an imaginary straight
line gives an axis of rotation or fulcrum. The
mandible rotates about this axis, completing
the arc of a circle. Forces at the mandible faces
have magnitude and direction: vectors.
Vogel 2nd edition, See
Appendix 2 Motion and
Direction, p. 547
Imagine it as it isn’t: imagine the
adductor apodeme insertion at any
other locus around the base of the
mandible -- the moment of force
exerted would be smaller.
Skeletons both exoskeletons and endoskeletons, move forces about.
They translocate them, they leverage them.
Three classes of lever are named on the basis of sequencing* effort, load,
fulcrum.
• FIRST
• SECOND
• THIRD
EFFORT
FULCRUM
FULCRUM
FULCRUM
LOAD
EFFORT
LOAD
EFFORT
LOAD
• For more background on levers see Vogel 2nd ed. Chapter 24, p. 473.
• *sequencing horizontally
Class 1 lever: insect wing
Scallop adductor
works with a 2nd
class lever
Centre of gravity
taken as place
where load is
applied
Abductin at the hinge is not a
muscle but a rubbery material,
antagonist of the adductor
muscle; it acts elastically,
storing energy in its distortion
when the adductor contracts,
to return it at a later time.
Abductin as inner and outer
ligament: one acts as a 2nd
class other as 1st .
The load of the shell is
taken as acting through
the centroid of the
bivalve, this being
closer to the hinge than
the muscle; the effort
of the adductor muscle
‘lifts’ this load.
Leverage involves a
force causing body-part
rotation: the force from
shortening adductor
muscles pulls on the
adductor apodeme
inserted on the
mandible base; this
rotates the whole
structure through a
short arc toward the
midline (fat blue arrow)..
[Force can be represented as a vector,
showing magnitude and direction.]
To decide where the load should
be considered to act, you need
the point of balance, i.e., the
centroid or centre of gravity.
“Levers are practical applications of ...moments” Vogel
Force moment
When “a force has a line
of action lying to one side
of an axis of rotation...
we call the shortest, or
perpendicular, distance
between the force’s line
of action and the axis,
the ‘moment arm’ or the
‘lever arm’ of the force”
(Vogel 2nd). A moment of
force is the product of
the force magnitude and
this lever-arm distance to
the line of action.
Force-advantage levers vs distance- (or) speedadvantage levers
• Vogel advocates using the term force advantage instead of mechanical
advantage, because mechanical advantage can be misleading: a muscle
often actually works at a leverage ‘disadvantage’.
• Force advantage is the ratio by which the applied force is multiplied
(amplified) by the lever.
• Where force is more important than speed in the life of an animal
evolution will want a force-advantage lever, one where the effort arm is
longer than the load arm, so maximizing the moment of the effort – the
force-in or effort moment.
• Distance advantage is force advantage’s reciprocal: the ratio of the
distance the load moves to the distance moved by the effort. [“Distance
advantage must correspond to ‘speed advantage’ – if an action takes a
given time, then going farther means going faster”.]
• When speed and distance are more important than force you will want a
longer load arm than effort arm giving a relatively greater moment to the
load.
Redrawn Fig. 24.1 of Vogel
Force advantage: ratio by which
the effort is multiplied by the
lever; moment arm of effort
divided by moment arm of the
load.
Distance advantage: ratio of the
distance moved by the load relative to
that moved by the effort.
Speed advantage: ratio of the
speed at which the load moves
relative to that of the effort.
Force and distance/speed
advantage are inversely related:
good force advantage goes with a
relatively poor distance/speed
advantage; good distance/speed
advantage with a relatively poor
force advantage.
Both up and down insect-wing movements are a (first class) distance-increasing lever – a lever
with good speed advantage, and a relatively poor force advantage; there is a very short force
arm, the moment arm of the effort is divided by the much larger moment arm of the load – the
centre of gravity of the wing being much farther from the fulcrum.
“A muscle... is relatively
good at producing force
and relatively bad at getting
shorter. ...any engine that gets
only 20 % shorter will have to
operate with a substantial
distance advantage to move a
long limb [wing] through an
angle that may approach 180
degrees. For that good
distance advantage it will
necessarily suffer a poor force
advantage because the
product of the two must be
unity...” (Vogel 2nd , p. 475)