Divergent and Convergent Pathways
Once a new species forms, its evolutionary pathway may diverge from the original species. Disruptive selection continues long after speciation has occurred resulting in divergent evolution.
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Divergent evolution: occurs when two or more species evolve increasingly different traits, resulting from differing selective pressures or genetic drift.
Species with significantly different morphological and behavioural traits may arise. (ex. Various modifications to vertebrate limbs and their uses).
Convergent Evolution on the other hand occurs when two or more species become increasingly similar in phenotype in response to similar selective pressures.
Ex. Sharks and Dolphins- both developed streamlined bodies well suited for their carnivorous behaviour.
Sometimes divergent evolution occurs in rapid succession, or simultaneously, amoung a number of populations. This process is known as adaptive radiation.
Adaptive radiation results in one species giving rise to three or more species.
Phylogenetic Relationships: classification method
Phylogeny: the theoretical evolutionary history of a species or group.
In these systems closely related organisms are classified together with all organisms that share a common ancestor.
These related organisms are placed in the same monophyletic group called a clade
– ends up not necessarily reflecting the degree of relatedness between a members of a clade
Phylogeny and Cladistics
From assumption that all species have evolved from common ancestor, all species must have evolved through a series of events in which ancestral species gave rise to new species, each giving rise to a new linage.
This can be illustrated in a phylogenetic tree (or cladogram).
– Branches represent theoretical sequence of events.
Reconstruction of phylogenetic relationships of evolutionary histories is based on a careful evaluation of a wide range of evidence, including the fossil record, comparative anatomy, biochemistry, and genetics.
How do you make a tree?
Cladistics: a phylogenetic system of classification used to infer and construct cladograms based on shared derived traits or synapomorphies
Synapomorphies: shared traits that evolved only by members of the group and that have been inherited from a single common ancestor in which the features first evolved.
EX. Most vertebrates have paired appendages: fish have fins, while amphibians reptiles, birds and mammals are tetrapods. Fins are considered more ancient- therefore fish with fins didn’t evolve from tetrapods- tetrapods evolved from fish with fins. Therefore A is an ancestor with fins, while B would be a tetrapod.
Ancestral features cannot be used to determine the relationship of species or groups; for example monkeys and salamanders have tails- while chimps do not.
Cladograms provide information about relative sequence of events (although if they both end at the same time it doesn’t mean they were around at the same time) but which ancestor is more ancient.