PATTERNS OF EVOLUTION

PATTERNS OF EVOLUTION

1. phylogenies enable us to confidently infer history of changes in characteristics of organisms, even without fossils

   • ONLY way of inferring history of evolution of features that leave little or no fossil trace eg DNA sequences, biochemical pathways & behaviours
   • phylogenetic & systematic studies enable us to describe past changes in genes, genomes, biochemical & physiological features, development & morphology, life histories & behaviour, as well as changes in geographic distribution, habitat associations & ecological interactions among different species
     • from inferences of such changes in many kinds of organisms possible to find common themes or patterns of evolution

2. general patterns of change most important phenomena for evolutionary biology

   • enables explanation of eg which clades genome size increased & which decreased, rather than just knowing genome of one species is bigger than another

3. early in development of all animals, embryo becomes hollow ball of cells - blastula

   • all phyla (except sponges) one side of ball folds inwards leaving external opening (blastopore) to form gastrula
     • in time, infolding becomes gut & in protosomes blastopore becomes mouth
     • in duteurostomes, mouth forms separately & blastopore becomes anus
   • chordates probably monophyletic group - united by dorsal nerve cord underlain (in early development) by flexible rod (notochord)

4. ANAGENESIS

   • evolutionary change in evolving lineage
   • grades - taxa based on anagenetic changes deemed important enough to separate descendent from ancestral stocks
     • eg traditional reptiles - birds excluded because evolution of many profound changes in structure & physiology associated with powered flight

5. CLADOGENESIS

   • splitting of lineage into 2 or more descendant lineages

6. features of organisms almost always evolve from pre-existing features of ancestors

   • eg wings of birds etc are modified forelimbs (not arising from shoulders)

7. character may be homologous among species, eg toes, but given character state may not be, eg number of toes

8. character or character state defined as homologous in 2 species if derived from their common ancestor

   • common criteria for hypothesising homology are correspondence of position relative to other parts of body & correspondence of structure
     • shape & function NOTuseful)
   • embryological studies often important for hypothesising homology
     • eg hindlimbs of birds & crocodiles more evident in embryo than adult because many birds bones fuse during later development
   • most important criterion for judging if character homologous among species is if its distribution on phylogenetic tree indicates continuity of inheritance from their common ancestor