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MACROEVOLUTION - DIVERSITY THROUGH TIME 2 - Coggle Diagram
MACROEVOLUTION - DIVERSITY THROUGH TIME 2
LOGISTIC GROWTH (Sepkoski)
new families arise from old ones
ASSUMPTIONS
asssumption 1
rate of origination > rate of extinction
assumption 2
at any given time, there is a constant probability of one family giving rise to another
assumptions 1 & 2 together imply exponential growth of the number of families through time
assumption 3
the earth cannot sustain an unlimited number of families
there is a
carrying capacity
K = carrying capacity
in N is small, the 1-N/K will be nearly 1, and the reduction in growth per unity time will be small
as N approaches K, then 1-N/K will tend to 0
any rN term multiplied by 0 will also be 0
as the absolute number of families increases it becomes more difficult for new ones to arise or old ones to go extinct
assumption 1, 2 & 3 together imply sigmoidal growth of the number of families through time up to the carrying capacity K
all other things being equal, the more families present at any given time interval, the more are likely to be present in the next interval
at any given interval of time the probability of a new family arising is contingent of how many families you have
start with exponential growth
let the number of families present at a given time interval = N
rate of change of N with time =
let the intrinsic rate of familial origination per unit time (t) per family = r
Sepkoski (1981), Sepkoski & Miller (1985)
theory that the three evolutionary faunas (Cambrian, Palaeozoic,, and Modern) can each be described by a simple logistic equation
each successive fauna competed with and largely replaced the one before it
increasingly advanced adaptations of the member species
Kitchell & Carr (1985)
disagreed with Sepkoski's assumption that there is some theoretical equilibrium level (K)
simulated the actual pattern of changing marine diversity by assuming only that
occasional perturbations to the environment cause markedly elevated levels of extinction
occasional evolutionary innovations led to the expansion of particular groups
1&2 together prevent the simulated diversity from ever reaching any equilibrium
how has the marine biosphere changed?
1. more geographical locations for different species to live in
plate tectonics
more geographical locations
one landmass
less coastline
fewer distinct landmasses or oceans
little latitudinal variation
multiple landmasses
new groups tend to originate in high energy environments
more coastline
more distinct oceans and landmasses
more latitudinal variation
the effects of continental distribution
latitudinal diversity gradient
tropics
very little seasonal resource variation
species adapt to very fine but stable niches
greater diversity, but species are more vulnerable to environmental change
2 higher latitudes
greater abundance of resource in the summer with little in the winter
species have broader niches, greater adaptability but lower diversity
large continental landmasses
severe climates given their latitudes
arid interiors
extreme variations in temperature
2, summer monsoon
very hot during summer
low pressure drags air in from oceans
drags nutrient poor surface sea water towards the coasts
resources poor
winter monsoon
colder continental interior
high pressure generates winds offshore
drags up nutrients from deeper water
resources briefly rich
monsoons and supercontinents
when continents merge, monsoon variation in resource supply, as well as climatic variation more generally, become more marked
large supercontinents favour species able to obtain resources in a greater diversity of ways
broader niches
lower diversity overall
global diversity tracks movement of continents
faunal interchange
we are concerned about invasive species in conservation
consider the effects of a whole continent of invasive species arriving on the shore
diversity tends to decline when continents group
ocean ridge spreading
can be a significant supplier of nutrients to the seafloor
resources increase with continental dispersal
2. increase in the number of ways of making a living (Guilds)
guild = group of often phylogenetically inrelated taxa that all 'earn their living' in a similar manner
the number of guilds or mode of life has increased since the Palaeozoic
most of this evolution concerns the origins of new families and genera (single orders can have representatives in many different guilds)
it doesn't involve the origins of fundamentally new body plans which are usually reflected by recognising new classes and phyla
eg. suspension, herbivore, carnivore
baleen whale, manatee, blue shark
3. increase in the vertical extent of habitat use (tiering)
PELAGIC
: within the water column
EPIFAUNAL
: on the sediment/rock surface
most palaeozoic species low level epifaunal
lived on or close to surface of sediment
in the Mesozoic fauna a mixture of epifaunal and infaunal species
mobile, attached low, attached erect, reclining
suspension, deposit, herbivore, carnivore
INFAUNAL
: buried or burrowing in the sediment/rock
in cenozoic greater numbers of infaunal species and burrowing to a greater depth
shallow passive, shallow active, deep passive, deep active
suspension, deposit, carnivore
tiering in early palaeozoic
signor and vermeil (1994)
increasingly complex relationship between substrate community and plankton
many new niches and more species
early to mid cambrian
few benthic suspension feeders
those that existed were either passive or fed on tiny prey
late Cambrian and ordovician
explosion of filter feeders
many active suspension feeders able to create their own water currents
cephalopods, graptolites and other planktotrophic larvae became increasingly abundant
4. escalation: between predators and prey result in more ways of killing and of avoinding being killed (arms races)
vermeij (1987)
arms race
New adaptations in predators countered by adaptations in prey, and so forth
Certainly there are more ways of killing or avoiding being killed now than in the Cambrian
limits to how far this can escalate
kitchen (1990)
organisms make trade-offs in the way they use their resources
any change in one species affects the selective forces acting on the second and
vice versa
the system is dynamic, and organisms define aspects of each others environments
escalation may be a factor, but the interrelationships are extremely complex and in a constant state of flux
characteristics of Sepkoski's faunans
cambrian
continents goruped
food supplies variable
"mud grubbers"
deposit bottom feeders
palaeozoic fauna
ordovician onwards
continents more dispersed
more reliable food supply, hence more plankton
filter feeders (mostly sedentary) and greater energy flux
niche subdivision
FF fared badly at P/Tr
modern fauna
continents more widely spaced from P/Tr to present day
Palaeozoic FF ought to have fared well, but replaced by more motel FF of the modern fauna
Palaeozoic counterparts are mostly stationary
bulldozing hypothesis (Thayer 1983)
Predation hypothesis (Stanley and Vermeij 1997)
natural selection = better adaptations = lower extinction?
van valen (1973,1985)
studies the survivorship of a large range of taxa (foraminiferans, brachiopods, molluscs, actinopterygians, mammals)
from species to orders
half life
constant probability of extinction
some intervals of time witness more extinctions then others, but these are no more likely to hit old taxa than young taxa (analogy with the radioactive decay of isotopes)
if the environment (biotic or physical) is constantly deteriorating as far as the organisms are concerned, there will always be an
adaptive lag
complete adaptation is never achieved
all organisms are constantly 'running to catch up'
only the biotic environment shows constant change. the physical environment changes but episodically.
THE RED QUEEN AND THE LAW OF CONSTANT EXTINCTION
red queen
many ecologists
evolution driven by changes in
biotic
environment
macroevolution is simply scaled-up microevolution
big pattern can be produced by
biotic
interactions
stationary model
many palaeontologists
stenseth & maynard smith (1984)
extinction does not occur unless there is a change in the
physical
environment
patterns caused by changes in the physical environment
organisms generally rather poorly adapted
natural selection is a sporadic force
constant environment vs periodically disturbed environment
to test should look at fossil examples
hard to find an example spanning a few million years with no environmental change
kitchell et al 1989
- predator prey relationship
modelled mathematically
whatever the starting point, tended to a static position
bivalves reproduce early or thicker shells
RQ implies constant running in
one
direction
this is often not possible - limited by genetic variation, developmental and design factors
conclusions
almost impossible to distinguish biotic and abiotic factors in order to assess their relative importance
test is inconclusive. if the environment is periodically disturbed (and it almost always is), then it's impossible to distinguish between RQ stepping (continuous) and stationary model stepping (staggered)
if the environment is constantly changing then even the stationary model predicts a sloping graph (same as RQ)
BIOTIC ENVIRONMENT DRIVES EVOLUTION
NO!
