Please enable JavaScript.
Coggle requires JavaScript to display documents.
THE ORIGINS OF HIGHER TAXA - Coggle Diagram
THE ORIGINS OF HIGHER TAXA
what is a higher taxon
various definitions of what constitutes a
species
at least some of these assume that species are real
higher taxa require some subjective assessment of "difference" or
disparity
higher taxon = a monophyletic group of organisms that is sufficiently distinctive and morphologically discontinuous from other
contemporaneous
taxa
lots of small steps or a few big ones
morphological distinctness of higher taxa could arise through a succession of ordinary speciations and the extinction of intermediate species
or processes that cause macroevolutionary change might be different from those that cause microevolution
what use are transitional forms
development of eye - all stages would be enormously useful for the organisms who had them
not necessarily as sophisticated as a mammalian eye but still useful
adaptive landscapes
different combinations of genes and characters have different implications for fitness
selection usually pushed populations to the top
copies are useful to have as a failsafe
they are mostly dead weight but might become useful without being detrimental in the first place
whole genome duplication events = more rapid rates of evolution/speciation
developmental biology
all heritable evolutionary change involves a modification of the developmental programme
does the evolution of a higher taxon differ from normal evolution?
morphological distinctness of higher taxa could arise through a succession of ordinary speciations and the extinction of intermediate species
on the other hand the processes that cause macroevolutionary change might be different from those that cause microevolution
what might increase the rate of evolution?
key innovations or first filling of the barrel
invasion of a new adaptive zone, for which many new adaptations are possible
vacated niches following extinction
heterochrony (also maintains functional integration)
change form markedly during lifestyle
sexual reproduction is at a different point
can result from mutations in control genes
paedomorphosis
organism achieves sexual maturity and adulthood at a juvenile stage in overall development
humans have undergone pedomorphosis
we look similar to the juveniles of other great apes
peramorphosis
sexual maturity is delayed, while overall development continues beyond the point of adulthood in the ancestor
heterochrony can also have the effect of shuffling juvenile and adult characters within the same adult body
draws on the developmental repertoire of the organism to generate new morphologies
there's alternation of generations between land plants
tunicate - sedentary filter feeders
chordates that are very weekly cephalised
in development go through a stage much like a tadpole - free living larva that swims to distribute
metamorphoses to become a filter feeder
modern vertebrates evolve from pedomorphic ascidian tadpole
changes in master control genes (especially homeobox)
hox & other regulatory genes
control positional information in developing animals
homologous versions of these genes are known from virtually all major animal taxa
particular control of segmentation patterns (its not just the groups with obvious external signs of segmentation that require this body patterning control)
relatively simple patterns of mutations in these genes could have given rise to new body plans
homology between hox genes
copying of hox genes has been retained
whole genome is duplicated multiple times in higher taxa vertebrates
polyploidy
can change rate of speciation
genome duplications
some evidence that the origin of vertebrates was marked by the duplication of the entire genome
a second duplication may have occurred at the origin of jawed fishes
duplication frees up more copies
you can afford to have more mutations
allopolyploidy
polyploidy from outside - if you cross two species with different numbers of chromosomes meiotic division cannot occur
if you can duplicate an entire genome you can carry on reproducing
can't reproduce with original complement of chromosomes
driven trends
strong selection to go in one direction
directionality
leaving aside the possibility of an instantaneous macro mutational event, the origin of a higher taxon implies the evolution of a series of forms intermediate between the ancestor and the new taxon
implies a trend - a period during which organisms evolved in an
approimately
constant morphological direction for long enough to accumulate the necessary differences
does not imply constancy of rate or direction
explanations for long term trends
chance
there's no reason to suppose there were any special processes at work in generating it
any path through a phylogenetic tree if extracted and considered in isolation can look like a trend
orthogenesis
basically nonsense
internal forces drive evolutionary change in a particular direction
also aristogenesis
and nomogenesis
there may be mechanical constraints on the evolution of form, such that even random drift can only yield change in one direction
eg. there may be lower size limit for a viable metazoan. it may be possible only to get bigger
small mammals all look v similar
medium to large mammals have fewer biomechanics constraints and constraints of miniaturisation
very large mammals need massive skeletons and are limited in design by the mechanical properties of bone, muscle, the need to pump blood etc.
natural selection
standard Neo-darwinian explanation -
amelioration
dynamic situation: adaptive tracking
of an environment changing slowly and extensively enough through time
how rapid can the change be before natural selection cannot keep pace
this explanation for the origins of higher taxa is usually accepted by default
evidence is impossible to acquire because it isn't preserved in the fossil or stratigraphic record
species selection
difference concept from natural selection (operates on organisms)
suppose some species tend to split into new species more often than others
these species produces more variants on which natural selection can act
more variants = more chances of getting it right
if the daughter species inherit the propensity to speciate more themselves, species selection becomes a possibility
what sort of character is both a property of a whole species (rather than individuals) and can affect the probability of speciation
population size and structure
in order for species selection to create a continuous morphological trend, there must be some morphological characteristic of the organism causing the different population characteristics
how is morphological integration maintained
cop out explanation
there are no intermediated - big macro mutational jumps
ancestor and descendant are both viable. intermediates are not
must go in a single large jump. biologically unlikely
correlated progression
small amount of variation in a single character possible
selection can act on this to a limited extent but any further change ruins its integration with other characters
after this there may be a selective advantage for change in other characters
only when these have caught up is more change in the first character again possible
developmental feedback
involves feedback between developing cells and tissues and between those tissues and the environment
flight in insects
primitive insects are known from Devonian rocks
flying insects may not have evolved until the late carboniferous (where many groups are found)
in all extant insect groups except mayflies only the last moult stage the adult has wings. drastic metamorphosis
wings must be as light and strong as possible
in living taxa done by withdrawing as much liv tissue as possible
leaves a light mass of dead tissue that cannot be repaired
once damaged that it
consequences: tremendous flying efficiency, short adult lifespan
consequences for the evolution of eusociality (cheap throwaway flying workers eg. bees
some Hymenoptera remove their wings
wings from gills
large plate like gills derived from the three thoracic segments and smaller plates from the abdominal segemnts
functions
flaps covering spiracles (nymphs can leave the water)
ventilation
movement of the flaps would increase the rate of gas exchange
aids to locomotion
eventually becoming the main locomotory system
wings from epipodites
most aquatic arthropods (crustaceans, xiphosurans) have biramous limbs
kukalová-peck: theory that wings derive from exopods
averof and Cohen: genes with wing-specific functions in insects (
pdm
and
apterous
)
expressed in epipodites of crustaceans
flight in insects
young adult insects emerged to feed on plant juices or other insects
used flapping movements for balance and to break falls
as movement in air became more important for feeding, mating, escape from predators and dispersal, flaps became larger and stronger
became more difficult to have a normal growth pattern because of difficulty in molting wings
preadaptations for basking
some living butterflies and moths use their wings as an essential part of their preflight warmup procedure (also shivering)
transluscent wings can also act much like a greenhouse
wings predated for light structure and large size well before they were used for flight
doesn't explain the development of strong muscles
preadaptations for surface skipping
possibly used large wings as sails initially
surface skipping is important in courtship displays
hox genes
high level regulators affecting the position and form of whole segments
mutations in these genes generate large morphological changes but without losing integration between tissues and organs within segments