Rise of Animals 6 Flashcards
What is one of the most striking features of current biodiversity?
It is dominated by arthropods with estimates of the number of species being from 10-100 million with half of these being insects and half of those being beetles
What are the explanations for the high diversity seen in insects?
Small size and rapid geographic dispersal with adaptability of the arthropod body plan with its ability to mix and match segments and appendages has been suggested as a partial answer though it does not explain why particular groups are so much more speciose
Another explanation has been that there was coevolution with angiosperms or flowering plants these are the most diverse group of plants and many of these species are fertilized by insects in a mutually beneficial arrangement so the two groups drive each others diversity
There is also another theory where modern insect diversity results not from high rates of caldogenesis but rather from low rates of extinction which were established in the Mesozoic
What is the evidence behind the theory that insects are so diverse due to angiosperm diversity?
If this were true an examination of the fossil record would show a corresponding increase in the diversity of the angiosperms and the insects
A molecular clock study paces the divergence between insects and crsutaceans at 434-421 MYA which coincides with the earliest fossil of vascular plants
Insects are actually found in the fossil record about 390 MYA but are uncommon as fossils for the first 60MY of their history and huge radiations were observed about 325 MYA with peaks 290MYA and 260 MYA
Beetles and flies started radiating in the Mesozoic and continued this to the Cenzoic
When we plot the number of insect families against time shows that the expansion of angiosperms had no influence on the diversification of insect families with the diversification before angiosperms evovled
What is the Carboniferous period?
This is a period where there is thought to have been high atmospheric oxygen with much of the land being covered by dense forests of giant tree-ferns, horse tails, club mosses and lycopods up to 40m high Insects at this time grew to huge sizes such as dragonflies and mayflies with wingspans over 70-45cm long this was likely possible due to high atmospheric O2 elvels
This period takes its name from the fact that most of the worlds coal deposits formed during this region
What was found in the two studies that attempted to relate the Cambrian explosion to current patterns of diversity?
The first study compared 25 Cambrian arthropod taxa with representatives of 21 groups of extant arthropods by analysing 134 morphological characters with disparity being estimated by using a principal component analysis of the morphological data to plot the 46 living and fossil taxa in so-called morphospace this showed that the extant arthropods occupied a volume equal to 95% of the range occupied by the Cambrian forms concluding that arthropod disparity was no greater in the Cambrian than it is now and that phylogenetically that the Cambrian animals examined all fell within the crown group of arthropods
The second study used a combination of character based morphospace analysis and phylogenetic analysis to examine Cambrian and extant Priapulids these were a major component of the burgess shale fauna while most species today are restricted to marginal habitats this analysis showed us that the Cambrian and extant Priapulids occupy adjacent not overlapping regions of morphospace and that extant priapulids are more disparate than Cambrian taxa this phyla explored a wide variety of body plans and continued to evolve new body plans after the Cambrian while some of the Cambrian plans went on to become extinct
What does direct fossil evidence show about the deuterstome lineage?
That all major lineage had appeared by 520 MYA this creates a 50 MY gap in the fossil record if the divergence truly happened at 575 MYA as predicted by molecular clock studies
What are the two recent clock studies that incorporate rate differences to reach divergence dates that correspond well with the fossil record?
These studies place Urbilateria at 620-570 MYA and the common ancestor of the protostomes at about 580-550 MYA this causes the Cambrian explosion to reflect the diversification of the bilaterian phyla
Though others argue with the methodologies used in these studies saying that the divergence was actually far more ancient
Are the Cambrian animals likely to represent lineages that no longer exist on earth or represent branches off the lineages that lead to groups of extant groups of animals?
Detailed study of Cambrian organisms has shown that most fall out as a stem or crown group members of extant phyla. Odontogriphus is a stem group, shell-less mollusc with a radula and is actually a relict from Precambrian faunas. Wiwaxia is also a mollusc
Some animals do remain as problematica but it is likely that the phylogenetic relationships of many of these forms will be resolved by the discovery of new fossils
Is modern day disparity less than that seen in the Cambrian?
Studies on arthropods and priapulids show that disparity is similar or greater than in the Cambrian. This raises the question as arthropods appear to have undergone as much morphological evolution during the Cambrian as they have in the following in 500 MY, this suggests only a limited number of changes are possible in the arthropod body plan and that all or most of these plans were explored in the Cambrian
Was the Cambrian explosion the result of unique evolutionary processes that have not occurred again in the history of life on earth or did it involve the same conventional evolutionary processes?
The answer to this question may lie somewhere in the middle where perhaps the uniqueness of the developmental homologies shared by bilaterians suggests the establishment of these developmental systems constrained subsequent body plan rearrangements so there is a limit to the range of body plans that can be built with the regulatory gene complement inherited from the Urbilateria, so all the radical body plan transitions which have occurred since the Cambrian have simply been a reshuffling of the deck
What do the three different levels of hierarchy in developmental gene regulatory networks in bilaterians?
These correspond to each of the three taxonomic levels the highest network level controls the development of phylum specifc body plans and perhaps general super-phylum and bilaterian body plans as well
These networks must have evolved during the initial diversification of the bilateria between 600 MYA and 520 MYA and retained their basic pattern ever since
These networks are critical for the normal embryonic development of bilaterian animals making them resistant to evolutionary change as any slight alteration is likely to cause catastrophic disruption to development
