limblessness and venom evolution Flashcards
what are squamates
the largest groups of reptiles aka snakes and lizards
outline limblessness in squamates
this is a continuum in this clade as there are many examples of highly reduced limbs in lizrads and even examples of remanants of ancentral limns which are maintained for other functions
e.g.
slow worms = reduced and very small limbs whcih are non functional as limbs
Burmese python = spikes/claws connected to remanent of pelvic boned
outline partial limb loss
even when limb loss does occur it doesnt mean all the limbs
= can just loose front or back legs but usually remaining ones are highly reduced
why does limb loss occur in some species
evolved as an adaptations to one of two habitat types
1) fossorial/burrowing
2) grass-swimming in thick grassland
= both these habitats pose a challenge for limbs which interfere with movement
what are the four basic types of movement shown by limbless squamates
1) rectillinear = aka rib walking and used by heavier spp, ribs connected to ventral scales and are moved back and foward in a walking movement pulling along in straight line
2) serpentine = involves S-shaped movements where the body is alternatively pushing left and right off of substrate
3) concertina = involves anchoring one half of the body at a time then stretching out and anchoring the front half before pulling the back half forward before repeating
4) sindwinding = used mostly in surfaces with low friction such as sand, invloves lifting part of the body up, pushind down againts the substrate, throwing it forward and the putting it down in a new location
outline the evolution along side other evolutionary changes which led to the snake like body form
1) ELONGATION occurs via addition of verterbrate controlled by HOX genes
2) limbs get shorter and smaller= as they decrease in size the number of digits also reduces, usually lost in a 5-> 3-> 1->0 pattern
the evolutionary drivers of limblessness (burrowing vs grass-swimming habitats) result in two body plans whcih relate to the ecology they are what are these ecomorphs and describe what they look like
Burrowing ecomorph = short tail compared to length of the body
surface dwelling ecomorph= long tail comprising a substantial proportion of total body length
what does ecomorphology suggest about the ancestroal snake
= short tail compared to body => burrowing ecomorph
suggest lifestyle was likley responsible for the evolutionof limblessness in that group
how can the presence of lots of shared derived traits such as seen in squamates e.g. enlongated, loss of digits etc cause problems with working out phylogeny
when creating phylogenetic trees people look at derived states shared through desendants as it provided evidence that taxa sharing them have a more common ancestor than other taxa
HOWEVER
if lots of traits evlove together it can mislead inference by suggesting lots of support e.g. this has elongated body AND loss of digits
this fails to ignore that if one of those traits didnt happen the others wouldnt either e.g. reduced limb size causes loss of digits = linked not apart
outline how the conflict between morphological data and molecular data has been somewhat resoloved in explaining phylogeny
combining two traits together in one analysis looking for patterns in which traits evolve together and weighting the support to give phylogeny
= when looking at it like this traits associated with limblessness were seen to evolve together
what is venom
Broadly speaking, venom is a toxic substance which is produced by one animal and transferred to another via a wound, though there are borderline cases for which details of the definition matter.
what does venom do
they are a mixture of individual molecles (toxins) which have a range of effects on the envenomated animals
three major types of functional activities of venom
1) Cytotoxicity = damages cells and as a consequence tissues
2)haemotoxicity= interfers with ciruclatory system including blood coagulation
3)Neurotoxicity = interfers with nerve transmission and causes paralysis or other neurological symptoms
what is venom used for
1) predation = incapacitation of prey
2) defence
3)competition with conspecific e.g. slow loris (only venomous mammal)
4) reproduction
what are thought to be the main drivers of snake venom evolution
1) diet
2) defence
outline some evidence suggesting diet as a driver for venom evolution
Barlow et al 2009 = looked at vipers with different diets, either mostly mice or mostly scorpian, and then the amount of a certain toxin in their venom, those feeding moslty on scorpians had greater toxicity than those feeding on mice suggesting that venom evolved for diet preference
Pawlak et al 2006 = looked at individual toxin within venom of mangrove snakes, denmotoxin (3 finger toxin) and found that the toxin had little effect on verterbrate bit was 100x more toxic in birds = what the magrove snake moslty predates on
