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Major Evolutionary Patterns Aims • Discuss major evolutionary patterns • The common occurrence of certain features or sets of features • Evolutionary implications • Investigation with cladistics Forms of evolution • There are a number of types of evolutionary patterns and trends that repeatedly occur and can be seen in both extant taxa and the fossil record. • These do not generally have ‘strict’ definitions and are given titles as a way of labeling them for convenience. • Like many things in biology, these form more of a continuum than discreet states. Forms of evolution • Divergence: evolution away from a common form. • Convergence: evolution of unrelated taxa towards a common form. • Parallelism: evolution of similar forms in similar ways by related taxa. • Iterative evolution: repeated evolution of a structure of form (i.e. over time, multiple convergences). Divergence What is convergence? • Convergent evolution is where two (or more) species (or clades) who do not have a recent shared common ancestry independently evolve very similar structures to ‘solve’ a similar problem. • It is generally ecologically or functionally driven. • Before cladistics, convergent structures were often thought to be homologous and thus led to erroneous conclusions about systematic relationships. Detecting Convergence • In a cladistic context convergence can be easy to detect. Two taxa with superficially similar structures that are not closely related can be assumed to have gained the feature convergently. • Outside of cladistics, convergence may be detected by how the structure has been formed. • For example to elongate the snout phytosaurs elongated the premaxilla, and crocodilians the maxilla. Crocs & Phytosaurs • Although both are archosaurs and therefore relatively closely related, they do not share a recent common ancestor with elongate jaws. • The feature of elongate jaws has evolved convergently. Derived theropods Spinosaurs Basal theropods Pterosaurs Crocodilians Aetosaurs Phytosaurs Famous examples of convergence Gliding Fast bipedal herbivores Leg loss in aquatic and fossorial herptiles Multiple effects • Convergence can lead to a whole suite of characters changing in unison, even across the entire anatomy of the animal. • In the case of the formicivores, digging requires adaptations to the claws, hand shape, ulna and humerus, ribs, and vertebrae, while the elongate snout is usually accompanied by a reduction in the teeth (number, size and complexity) and the development of a long tongue. Formicivores Marsupial Convergence • Marsupials (both extant and extinct) provide an interesting case of multiple convergences. • With their isolation in Australasia, multiple lineages of marsupials have evolved to mimic eutherian mammals in other parts of the world to fill certain ecological niches. • Some cases are unique (kangaroos as grazers) and others are only loosely similar (e.g. the numbat) while others show striking similarities (e.g. marsupial moles). Eutherians vs Marsupials Parallelism • This is the evolution of parallel forms from homologous structures in animals with a shared ancestry. • The structures do not get much more similar (convergence) or more dissimilar (divergence) but evolve in similar trajectories. • These traits have evolved independently, but their origin and development is facilitated by their underlying similar developmental pathways making them relatively easy to acquire, and to evolve multiple times. Spines in mammals Parallelism in Archosaurs Iterative evolution • This is the repeated evolution of a structure or form by a lineage in response to a recurring evolutionary pressure. • It essentially represents multiple lineages over time acquiring similar (or even identical) characteristics. Quadrupedality • While the earliest dinosaurs were bipeds, at least 4 separate lineages (3 of the ornithischians) adopted facultative (blue) or obligate (red) quadrupedaility as an adaptation towards larger size. Quadrupedal Ornithischians Sabers Changes • Obviously, these terms are dependent on how the changes are perceived or the evolutionary relationships between the taxa in question. • These terms are not mutually exclusive – one could consider the archosaurian filaments to be iterative (repeated modification of scales into filaments at different times) or parallelism (related taxa evolving filaments). Differing phylogenies / information • If pterosaurs are derived archosaurs then their body covering would likely be considered parallelism with those of dinosaurs. If they are more basal this would probably be termed convergence. Dinosaurs Dinosauromorphs Pterosaurs ? Crocodiles Euparkeria Pterosaurs ? • If basal theropods, sauropodomorphs or dinosauromoprhs are ever discovered with body fibers then these features might be shown to be truly homologous and not convergent, or a result of parallelism. Derived theropods Basal theropods Sauropodomorphs Ornithischians Dinosauromoprhs Pterosaurs Vestigiality • Vestigiality is the massive reduction and even loss of non-functioning systems. • Since these are not being used they are under neutral selection, or if they are a hindrance may be actively selected against. • Determining if systems (be it biochemical, anatomcial, behavioural etc.) are truly nonfunctional can be very hard. • Classic examples of vestigiality (such as the legs of whales) have later been found to have a function. Alverezsaurs Vestigial digits • Derived alvarezsauroids have a single hypertropied manual digit (II) and two much reduced fingers. • However the recently discovered Linhenykus suggests that these other digits are vestigial and under neutral selection. Atavism • Atavisims are reversions to ‘ancestral’ conditions. The reemergence of characteristics through developmental anomalies but which are coded for in the genome of the species. In conclusion • These labels are primarily one of convenience – there is no strict definition or rule as to what is considered parallelism or iterative evolution. • However, they are important and can be used to describe complex patterns simply. • Of course they also demonstrate the complexity of evolution and the way systems and organisms evolve in response to ecological challenges. Further reading • http://www.zo.utexas.edu/courses/THOC/ Convergence.html • http://www.nature.com/scitable/topicpage/ atavism-embryology-development-andevolution-843