We will use an attribute table to make a phylogenetic tree based on 3 lines of evidence • Observations of habitat and eating habits • Observations of skeletons • Observations of gene sequences • After each observation we will modify our tree
Skeletal evidence • Skeletons provide strong evidence that all vertebrates share a common ancestry • Skeletal evidence comes from species that are now living and from fossils of species that have become extinct
Several fossil discoveries show how amphibians descended from fish • These fossils are literally half fish, half amphibian • Fossils that show transitions between species are called “transitional fossils” Ichthyostega
Fossil record also clearly shows the reptile to mammal transition • Examples of features that are part reptilian and part mammalian: • Jaw joint • Tooth • Ribs on neck vertebrae Lycaenops -- a carnivorous therapsid
Vestigial bones also provide more evidence of common ancestry among vertebrates • Pelvic girdle in some snakes, tailbone in humans • Remnants of structures with important functions in ancestors but no longer used
Homologous structures in mammal skeletons demonstrates common ancestry • Features, like the bones of mammals, are said to be homologous, because they share a common structural pattern • Conclusion: all mammals are derived from a common ancestor
Molecular biology evidence • A common genetic code for all living things is evidence that all are related • Comparison of DNA among living organisms has strengthened and clarified our understanding of evolutionary relationships
% Genes from other organisms that also occur in H. sapiens: deep genetic homologies • Mouse - 86%* Fruit fly - 44% • Nematode worm - 25% Yeast - 30% • Amoeba - 22% Mustard (plant) - 19% • E. coli (bacterium) - 9%*Of those genes now identified in mice, 86% of them also occur humans
Because it was present in a common ancestor billions of years ago! Hb Common ancestry of organisms explains many puzzles such as the distribution of the Hb gene Hb Hb • Puzzle posed earlier: The hemoglobin gene is widely distributed throughout living organisms. Why? Hb Hb Hb Hb Hb
Hemoglobin (again) - how molecular biology is used to estimate dates of common ancestry • All vertebrates have genes that make hemoglobin • Like many other genes, hemoglobin genes mutates at a fairly constant rate, even if they are in different animal groups • Rate of change can be used to estimate how long ago groups or organisms diverged from one another! * Changes per 100 codons
Using molecular biology evidence to draw phylogenetic trees • Evolutionary relationships are reflected in the similarity of DNA and proteins among species • The closer the match between sequences, the more recent the common ancestor
Closely related species have similar DNA (and proteins). Similarity reflects ancestry.
Two related species start with similar DNA, but mutations occur, making their DNA different • Assume species A & B just arose from the same common ancestor • There DNA is the same (or nearly so) • Each of their proteins are the same • With time mutations make their DNA (and proteins) different • Computers can align regions of DNA that did not change Computer alignment:
Now draw a tree similar to this example based on DNA sequence data
About the hippo-whale relationship • DNA data suggested hippos as whale’s closest land relative but there was no fossil evidence to support this theory • Recent discovery of 47 million year old fossils from a proto-whale provided fossil evidence -- hippo’s and whales are closely related • Key fossil evidence -- the hippo has a distinctive ankle bone and so does the proto-whale!
Recent discoveries of transitional fossils show that whale ancestors did have legs Ambulocetus (the walking whale) Carl Dennis Buell