How should we organize the diversity of animal life?

Save this PDF as:
 WORD  PNG  TXT  JPG

Size: px
Start display at page:

Download "How should we organize the diversity of animal life?"

Transcription

1 How should we organize the diversity of animal life? The difference between Taxonomy Linneaus, and Cladistics Darwin What are phylogenies? How do we read them? How do we estimate them?

2 Classification (Taxonomy) Natural system Often do it without understanding how

3 Based upon shared, homologous traits Animals that share the most traits are most closely related. What is a trait? Morphologic physical appearance Developmental sequence physical appearance during embryonic development; common cell fates, timing of cell migrations & gene activity Genetic DNA sequence similarity; chromosome number

4 Based upon shared, homologous traits Animals that share the most traits are most closely related. What is homologous? A trait that is shared, by two or more taxa, because their COMMON ANCESTOR evolved the trait. The opposite is analogous. Shared by two or more taxa, because each one independently evolved the trait.

5 Origin; Figure 1 What does this figure explain? Diversity Unity 99% of all species that ever lived are extinct

6 Charles Darwin and Fig. 1 Species diverge; they do so gradually Articulated ideas of shared ancestry, divergence of lineages (cladogenesis) and change along branches (anagenesis) Only figure in OS: a dendrogram

7 Problems with Linnean system It does NOT represent evolutionary history Hierarchical categories do not imply degree of evolutionary change or divergence. Are Rabbits as different from Rodents as they are from Elephants? When and how should we split groups into separate categories? - splitting vs. lumping How different is different enough to warrant separate groupings? Ex: Dogs & wolves

8 Cladogram ~ Phylogeny Phylogeny: Interpretation of the evolutionary history and relationships between a group of organisms Phylogenetic tree: Hypothesis of ancestor-descendent relationships among populations, species, or larger groups* These are based on shared traits among organisms

9 Why construct phylogenies? 1. To understand evolutionary history 2. System of classifying organisms & organizing diversity Most biologists agree: most efficient way to classify organisms is based on phylogenetic relatedness

10 All species coalesce to a common Recall our lab: Just as all genes coalesce to a single ancestral gene, all individuals coalesce to a single ancestor. Like tracing a family tree ancestor:

11

12

13

14

15

16

17

18 Tips Branches Nodes Root Most Recent Common Ancestor (MRCA) 1 0

19 Phylogenetic Tree (Cladogram) of Vertebrates Name another tip Name another node

20 Who is the MRCA of Mammals and Crocodiles? Monophyletic groupings

21 Nodes name monophyletic groups: An ancestor and ALL of its descendents Monophyletic groupings

22 Which group is more closely related to Mammalia, Lepidosauria or Aves? Reading relationships

23 Topology of phylogenies 5 of these are identical (have the same topology) Which one is not the same?*

24 Paraphyletic groupings

25 What data should we use to group organisms into clades? Traits (characters): distinguishable variations of an organism Morphologic Developmental Genetic Quantifiable Independent of each other ACGTTGAA ACCTTGTA Homologous: shared by two groups because they inherited it from a common ancestor who evolved the trait

26 Similar vs. homologous Are the bones of the upper limb homologous structures? Are the wings of bats, birds and pterosaurs homologous structures?

27 Pitfalls When Similarity Common Ancestry! 2 taxa may share a derived character state OR may share an ancestral character state OR may share a character state due to convergence to same phenotype (MRCA did not have the character); analagous traits or homoplasies Bird and bat wings Dermopterans, Flying squirrels, Sugar gliders Only if first case is true will you ALWAYS infer correct phylogenetic relationship.

28 Who is the MRCA of Crocodylia & Testudines?

29 Name the monophyletic group that includes Aves, Crocodylia & Testudines

30 Willi Hennig created Cladistics Grouping scheme based ONLY on phylogenetic relationships Not on degree of similarity Uses ONLY shared derived characters to infer evolutionary relationships Based trees upon total weight of ALL synapomorphies

31 Synapomorphies define monophyletic groups Unite groups with shared, (only among each other) derived (from same ancestor ) characters

32 Method of inferring a tree Choose the most parsimonious one Parsimony: Methodological reductionism; Explanation which requires the fewest undocumented assumptions is probably correct. Occam s Razor: The easiest explanation is probably the correct one. William of Occam: 14th century Franciscan monk In cladistics/phylogeny: The tree which requires us to postulate the least evolutionary events (fewest changes and homoplasies) is probably correct

33 Parsimony Find all possible tree topologies; calculate total number of changes required to produce each topology. Topology with fewest changes = the most parsimonious tree* A B C D C A B D Taxa: A, B, C, D Characters:

34 Ideal character Character states are discrete # of states are knowable (across taxa) Mutation frequencies (rates of change) and direction of change are estimable DNA! ONLY 4 states; A, C, G, T No polarity Measurable mutation frequencies

35 Placement of Cetaceans Morphology of astragalus unites Artiodactyls Most parsimonious placement = one gain of pulley-shaped astragalus

36 Placement of Cetaceans Mounting DNA evidence AND fossil whale morphology suggests that Hippos share ancestors with Cetaceans more recently than they do with other Artiodactyls (e.g. deer)

37 Placement of Cetaceans Appearance of SINE s (mutations) throughout the genome suggests that Cetaceans & Hippos are close relatives Rare; Selectively neutral; Once established, lost rarely & randomly