Evolution is not goal oriented Evolution is like “tinkering”—it is a process in which new forms arise by the slight modification of existing forms

Evolutionary Novelties Most novel biological structures evolve in many stages from previously existing structures For example, complex eyes have evolved from simple photosensitive cells, independently, many times Natural selection can only improve a structure in the context of its current utility

Fig (a) Patch of pigmented cells Optic nerve Pigmented layer (retina) Pigmented cells (photoreceptors) Fluid-filled cavity Epithelium (c) Pinhole camera-type eye Optic nerve Cornea Retina Lens (e) Complex camera-type eye (d) Eye with primitive lens Optic nerve Cornea Cellular mass (lens) (b) Eyecup Pigmented cells Nerve fibers

Investigating the Tree of Life How do biologists trace the evolutionary history of a species, or group of related species? Phylogenetics – phylon = tribe – genesis = origin

Investigating the Tree of Life Biologists draw on the fossil record, which provides information about ancient organisms Figure 25.1

The trunk and branches represent the past and the leaves represent the species alive at present. Investigating the Tree of Life

Currently, biologists use morphological, biochemical, and molecular comparisons to infer evolutionary relationships between organisms, both past and present

The Fossil Record Though sedimentary fossils are the most common, Paleontologists study a wide variety of fossils Figure 25.4a–g (a) Dinosaur bones being excavated from sandstone (g) Tusks of a 23,000-year-old mammoth, frozen whole in Siberian ice (e) Boy standing in a 150-million-year-old dinosaur track in Colorado (d) Casts of ammonites, about 375 million years old (f) Insects preserved whole in amber (b) Petrified tree in Arizona, about 190 million years old (c) Leaf fossil, about 40 million years old

The Fossil Record The fossil record is based on the sequence in which fossils have accumulated in such strata Fossils can reveal ancestral characteristics that may have been lost over time Figure Rivers carry sediment to the ocean. Sedimentary rock layers containing fossils form on the ocean floor. 2 Over time, new strata are deposited, containing fossils from each time period. 3 As sea levels change and the seafloor is pushed upward, sedimentary rocks are exposed. Erosion reveals strata and fossils. Younger stratum with more recent fossils Older stratum with older fossils

Morphological and Molecular Homologies In addition to fossil organisms – Phylogenetic history can be inferred from certain morphological and molecular similarities among living organisms In general, organisms that share very similar morphologies or similar DNA sequences – Are likely to be more closely related than organisms with vastly different structures or sequences of DNA

Sorting out Homology from Analogy A potential problem in constructing a phylogeny of organisms – Is similarity due to convergent evolution (analogy), rather than to shared ancestry (homology) vs.

Sorting out Homology from Analogy Convergent evolution occurs when similar environmental pressures and natural selection – Produce similar (analogous) adaptations in organisms from different evolutionary lineages Figure 25.5 Australian mole North American mole

Sometimes, morphological differences are much greater than their underlying molecular differences (Hawaiian silversword plants) Sorting out Homology from Analogy

Evaluating Molecular Homologies Systematists analyze comparable DNA segments from different organisms using computer programs and mathematical tools

Binomial Nomenclature – Classifying Organisms Binomial nomenclature (“two names”) – Is the two-part format of the scientific name of an organism (developed by Carolus Linnaeus) The binomial name of an organism or scientific epithet – Is latinized (Homo sapiens) – Is the genus and species

Hierarchical Classification Linnaeus also introduced a system for grouping species in increasingly broad categories Figure 25.8 Acronym: KPCOFGS King Philip came over for good spaghetti

Linking Classification and Phylogeny Systematists show evolutionary relationships in branching phylogenetic trees

Each branch point represents the divergence of two species. Top of tree is now, further down is evolutionary past Leopard Domestic cat Common ancestor

“Deeper” branch points represent progressively greater amounts of divergence between the organisms at top Leopard Domestic cat Common ancestor Wolf Extant species at ends of branches currently exist Extinct, ancestral species are below the branch ends

Evolutionary Taxonomy There is one unique way to classify organisms that reflects the reality of the tree of life. This results in perfect-nesting (no overlap- see next slide). Ex. Classification of mammals: 1= rodents 2= cetaceans 3= primates a= monkeys b= apes

Perfect Nesting

An Evolving System of Classification Taxonomists try to classify organisms so that the groups they form contain close relatives descended from a common ancestor. Taxonomists attempt to transcend time to determine evolutionary relationships. This is a difficult task, and as a result the classification scheme is under constant revision as our understanding of the tree of life increases.

Perfect Nesting of Taxa This occurs if taxa are constructed to be monophyletic and not paraphyletic or polyphyletic.

Monophyletic (“Single Tribe”) Taxon A taxon that includes an ancestral species and all of its descendent species (Clade).

Paraphyletic (“Beside the Tribe”) Taxon A taxon whose members have all descended from a common ancestral species, but does not include all of the descendents of the common ancestor.

Polyphyletic (“Many Tribes”) Taxon A taxon in which species are descendents of more than one ancestral species.

Phyletic Relationships

Cladistics

A shared ancestral character is a character that originated in an ancestor of the taxon A shared derived character is an evolutionary novelty unique to a particular clade A character can be both ancestral and derived- it depends upon the context!

Cladistics

Salamander TAXA Turtle Leopard Tuna Lamprey Lancelet (outgroup) Hair Amniotic (shelled) egg Four walking legs Hinged jaws Vertebral column (backbone) Leopard Hair Amniotic egg Four walking legs Hinged jaws Vertebral column Turtle Salamander Tuna Lamprey Lancelet (outgroup) (a)Character table. A 0 indicates that a character is absent; a 1 indicates that a character is present. (b)Cladogram. Analyzing the distribution of these derived characters can provide insight into vertebrate phylogeny. CHARACTERS

Fig b Hair Hinged jaws Vertebral column Four walking legs Amniotic egg (b) Phylogenetic tree Salamander Leopard Turtle Lamprey Tuna Lancelet (outgroup)

Among phylogenetic hypotheses – The most parsimonious tree is the one that requires the fewest evolutionary events to have occurred (in the form of shared derived characters) – “Occam’s Razor”- “shave away unnecessary complications;” that is, use the simplest explanation that is consistent with evidence first

Fig Species I Three phylogenetic hypotheses: Species II Species III I II III I I II III

Fig Species I Site Species II Species III I II III I I II III Ancestral sequence 1/C C C C C T T T T T TA AA A G G I I I II III 3/A 2/T 4/C I I I II III 7 events 6 events

Phylogenetic Trees as Hypotheses The best hypotheses for phylogenetic trees – Are those that fit the most data: morphological, molecular, and fossil – Not ALWAYS the most parsimonious!

Sometimes there’s good evidence that the best hypothesis is not the most parsimonious one!