Rise of animals 4 Flashcards
What is macroevolution?
This is evolution above the species level and is illustrated by the large-scale morphological changes which can be seen in the fossil record
What is microevolution?
This is small-scale evolutionary changes such as changes in gene frequency within populations over generations
What do the differences in micro and macro evolution mean for our ability to study them?
As microevolution takes place at the scale of populations in the time frame of animal generations it can be studied in the lab or field
Macro evolution in contrast deals with timeframes of tens of thousands of years and therefore is much more difficult for laboratory based study
What is evo-devo?
This is short for evolutionary developmental biology which is a recently developed field which examines the connection between the developmental processes that produce body structure in each generation and the evolutionary processes that produce new anatomical features and novel animal body plans
How can profound changes in morphology be produced?
Changing the relative rates at which different parts of the body develop
Changing the timing of developmental events which affect the sequence in which different parts of the body appear
Homeosis or an alteration in the spatial patterning of body parts this is controlled by the homeotic or hox genes
What is the homeobox?
This is a sequence found in all homeotic genes in drosophila with the best studied class of these genes being the hox genes which are typically found clustered next to each other along animal chromosomes Identical or similar genes have been found in every bilaterian animal to date, they also exist in cnidrians and ctenophores but only one sequence has been found in sponges suggesting these genes evolved early and had a high selective pressure to be conserved
How do the hox genes reflect the phylogenetic tree?
The pattern of similarity in these genes matches the molecular bilaterian tree with each of the major clades having their own distinct hox cluster
What si the function of hox genes?
These genes regulate development by controlling the transcription order of other developmental genes
In bilaterians the hox cluster specifies the developmental fate of individual regions within the body with the genes usually being expressed in the order that they are found in the cluster
This makes these genes crucial for the timing and spatial location of the pattern forming on the embryo
What were the first bilaterian animals likely to be?
These were probably small, bilaterally symmetrical, wormlike creatures with a coelom, heart-like structure, tube like gut and anterior tentacular apendages they also probably had a ciliated larval phase
What are the two important periods of bilaterian evolutionthat preceded the Cambrian?
The evolution of the bilaterian stem lineage leading to urbilateria and the diversification of the three major bilaterian clades
What do the major differences between bilateria and cnidarian body plans suggest?
That the evolution of bilaterian characters must have required many innovations incuding triploblasty, bilateral symmetry, a central nerve cord, a through gut, various primitive organs and the hox system necessary to control the development of these features
Can bilateral symmetry be seen outside the bilaterians?
Yes as although the ctenophores and cnidarians are often said to have radial symmetry many of them contain bilateral symmetry for example hydrozoans have radial symmetry but anthozoans have bilateral symmetry with the primary body axis of a sea anemone beign from the foot to the mouth with the secondary body axis crossing the pharynx at right angles to the main axis and have hox expression which mimics that of bilaterians
This makes it likely that bilateral symmetry actually evolved before bilaterians and cnidarians diverged
What are the problems with the theory that bilaterians clades diverged as larvae evolving their adult ontogenic states independently?
Indirect-developing larvae seem to be derived in invertebrates with direct development the basal condition while the opposite would be the case if this theory were true
Body cavities, blood vascular systems, complex muscular and skeletal systems are all features required in larger animals not tiny ones where cilia are effective for locomotion and diffusion is sufficient for excretion and respiration creating a minimum size for complex animals
Even tiny animals should have left a fossil record as in the Doushantuo phosphorite embryos
Body patterning in the three major bilaterian clades is controlled by a very similar set of hox genes and this is unlikely to be the case if these clades diverged as larvae without adult body forms
What are the problems for the theory that ancestral bilaterians were all soft bodied to explain why there is a lack of fossils?
Even soft bodied animals leave trace fossils (even diploblasts) no burrows whch indicate presence of coelom have been found which are older than 535 MYA while the Urbilateria would be coleomated, perhaps segmented and probably inhabited the marine benthos suggesting they should have left excellent fossils
How did gould see the problem of the discrepancy between the Cambrian explosion fossil record and the molecular tree of life?
He said the molecular biologists are confused in their thinking that their deep Precambrian splitting times contradict the reality of the Cambrian explosion saying that the Cambrian explosion marks the geologically abrupt origin of the major bilaterian body plans but is not a claim as to when the groups diverged
He concluded that bilaterian diversity represents extensive modification of a basic developmental pattern that was inherited from a common ancestor and that all body plans that could be built with this basic developmental pattern evolved very rapidly during what we call the Cambrian explosion
