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Phenetics vs. Cladistics

Phenetics vs. Cladistics

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Phenetics vs. Cladistics

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  1. Phenetics vs. Cladistics

  2. Phenetics • The study of relationships among a group of organisms on the basis of the degree of similarity between them, be that similarity molecular, phenotypic, or anatomical. • A tree-like network expressing phenetic relationships is called a phenogram or a phylogentic tree

  3. Phenetics • Considers as many characteristics as possible to classify organisms into groups based on overall similarity • Does not necessarily attempt to reconstruct evolutionary relationships, just to produce groups that can be named

  4. Cladistics • The study of the pathways of evolution. • Cladists are interested in the branching sequence. • A ladder-like network that expresses such ancestor-descendant relationships is called a cladogram.

  5. Cladistics • Considers only a restricted set of characteristics of the organism that is being classified • Organisms are assigned to a group because they share unique derived characteristics not found in other organisms • Seeks to determine the order in which evolutionary lines diverged or branched

  6. vertebrates fish amphibia reptiles mammals birds Cladistics vs. Phenetics • Linnaean classification is essentially phenetic, that is, it groups organisms with similar characteristics together into a hierarchial series of levels or grades

  7. Cladistics vs. Phenetics • Modern tendancy is to order by descent (cladistic order). The previous group is not a cladistic arrangement since amphibia and reptiles are both descendents of fish and mammals and birds are both descendents of reptiles.

  8. Evolution: change in gene frequency from generation to generation Phylogeny: evolutionary history of the development of a taxon Taxon: name designating an organism or group of organisms. (A taxon is assigned a rank and can be placed at a particular level in a systematic hierarchy reflecting evolutionary relationships. )

  9. Hierarchy: ranking or ordering Cladistics: system of arranging taxa by analysis of derived characteristics Derived characters: unique characteristics which evolved from a common ancestor

  10. Amniote Egg • Having an amnion (innermost of the extraembryonic membranes) forming a fluid filled sac around the embryo • Contains nutrients for developing embryo • Found in reptiles, birds and mammals

  11. Clade: a taxon or group consisting of a single species and all of its descendents Cladogram: diagram of evolutionary relationships

  12. Phylogenetic Trees and Cladograms • Phylogenetic patterns generated from branching processes may be represented in at least two different ways: phylogenetic trees and cladograms. • We can think of both cladograms and trees as phylogenetic maps. Both express hypotheses about a phylogeny — a pattern of evolution.

  13. Phylogentic trees

  14. Cladograms • Cladograms are reconstructed by comparing the distribution of characters among species. • Characters are inherited attributes of organisms; • Morphological • Genetic • Developmental • Behavioral • Physiological • Biochemical • and so forth. • Outer covering, number of legs, or gene sequence, for example, are characters of organisms that vary. The variation can be characterized by counting or measuring or describing it.

  15. Cladograms

  16. Cladogram based on amino acid sequences

  17. What this cladogram suggests is that the lineage leading to vertebrates has been dominated by the evolution of characters that make an animal stronger, faster, and smarter.

  18. Variation in same cladogram • These are the same cladogram, just drawn a bit differently. In this cladogram, for example, a bird and crocodile are sister taxa, relative to a snake or lizard, or even more distantly related vertebrates. That information about branching pattern is useful in itself, but branching pattern alone does not really tell us much about the group or the sorts of evolutionary changes that occurred in it. Plus, one has to assume that there is some good reason why this cladogram is drawn, and not some other alternative cladogram, for example, one that puts birds and mammals together. After all, both birds and mammals are endothermic (i.e., "warm blooded"), and crocodiles, lizards, and snakes are not.

  19. Branching diagram that illustrates both branching patterns and time; branch lengths have meaning in the sense that longer branches imply longer periods of time. General hypothesis of evolutionary relationships Branching diagrams that illustrate patterns of phylogenetic relationships in a nested hierarchy. (Time is included in cladograms only in a relative sense, in the internested structure of the cladogram itself.) Shows specific evolutionary connections of derived characters Phylogenetic tree vs. cladogram