• Consider an individual organism, a ground beetle named Eric.

o Eric’s species reproduces sexually, so a parent passes on ½ of his or her genes to an offspring. Therefore, an offspring inherits ½ of its genes from each parent.

What does an evolutionary tree represent?

o Eric’s family lives under a log with other beetles. Offspring hatch every summer and live for about one year. So each year there is a new generation of beetles.

o Eric’s log is on a beach where there are other beetles of his species living under other logs.

Driftwood Beach State Recreation Site, Oregon

o If we zoom out of the previous pedigree to include many more individuals—say, from under all the logs on the same beach—and more generations, and if we simplify by showing just the lines representing genetic descent, we would get the diagram on the next slide.

o Eric’s population living on his beach is but one of many on many beaches that make up his species. Beetles sometimes travel from one population to another and start new families.

o Thus, each species moves through time as a bundle of genetic connections, represented by a branch of an evolutionary (aka phylogenetic) tree.

o Occasionally, a species might be split into two reproductively isolated populations, and each would become its own isolated bundle of genetic connections.

o Something like this must have happened a long time ago when Eric’s species, Bembidion zephyrum, evolved from the ancestor it shares with its closest relative, Bembion levettei.

o If we went further back in the evolutionary history of Eric’s genetic lineage, we would see a sequence of ground beetle lineages splitting in two and giving rise to new species, some of which would have gone extinct. o An evolutionary lineage is a sequence of species each of which is

considered to have evolved from its predecessor.

o This process has resulted in an ever growing evolutionary tree of species lineages, with the species living today sitting at the tips of actively growing branches.

What’s a phylogeny?

• An evolutionary tree represents a phylogeny, which is a testable hypothesis about the evolutionary relationships among a set of organisms.

o However, the terms "evolutionary tree" and "phylogeny" are often used interchangeably.

Darwin's first drawing of an evolutionary tree from his “First Notebook on Transmutation of Species”

(1837)

Important Terminology:

• At the tips of an evolutionary tree are the leaves, which represent the living descendants of a single ancestor that is at the root of the tree.

• A taxon (pl. taxa) is any named group of organisms – a population, species, or group of species. Living taxa are the leaves of the tree.

• A node, or branch point, represents both the common ancestor and the speciation event that produced its descendants.

o Although the taxa at the tips of the branches are sometimes groups of species (such as reptiles, mammals, birds, and so on), a node still represents a speciation event because even a group of species originates from a single ancestral species.

• Branches connect nodes to nodes or nodes to leaves; thus, branches are evolutionary lineages.

• Each taxon ("A", "B", and "C") has a part of its history that is unique to it alone and parts that are shared with other taxa.

How to Read Evolutionary Trees

• Similarly, each taxon has ancestors that are unique to it alone and ancestors that are shared with other taxa – common ancestors.

• For any given branch point on a tree, which lineage goes to the right and which goes to the left is purely arbitrary.

• In fact, a particular phylogeny can be presented in very different ways without altering what it depicts.

o For example, these three trees display the same evolutionary relationships despite being different in style (↓).

Note: You are not responsible for this phylogeny.

• When comparing taxa at more than two tips, the most closely related pair are the two that have the most recent common ancestor.

o Question: In this evolutionary tree (↓), is the rose more closely related to the fern or to the moss?

• When comparing taxa at more than two tips, the most closely related pair are the two that have the most recent common ancestor.

o Question: In this evolutionary tree (↓), is the rose more closely related to the fern or to the moss?

o Answer: The nodes in the lineage of the rose from the root to the tip are numbered 1, 2, and 3. Node 2 represents the most recent common ancestor of the rose and fern, while node 1 is the most recent common ancestor of the rose and moss. Since node 2 is more recent than node 1, the rose and fern are most closely related.

• Unless time is marked on a tree, the chronology of nodes can be determined only for nodes that are on the same lineage.

o According to this tree (→), y happened before z, but the tree does not specify when x occurred relative to y and z.

• Just because you tend to read a tree along the tips from left to right, do not interpret them as a series of ancestors and descendants.

o No living taxon is an ancestor or descendant of any other living taxon.

• Similarly, do not infer increasing level of "advancement" as you read along the tips from left to right.

o The taxa at the tips of the tree are all equally "advanced" or "evolved".

REMEMBER:

• The points described in the last couple of slides cause the most problems when it comes to human evolution. The evolutionary tree depicting the phylogeny of living apes looks like this (→):

o Humans did not evolve from chimpanzees. We are evolutionary cousins and share a most recent common ancestor that was neither chimp nor human.

o Humans are neither "higher" nor "more evolved" than other modern organisms.

Clades

• A clade is an ancestor and all of its descendents (living and extinct).

o In other words, a clade includes all and only the descendents of a particular ancestor.

Any part of a tree you can remove with just ONE cut is a clade.

• Clades descended from more recent common ancestors are included within those descended from more distant ones – they form a nested hierarchy.

Clades-within-clades are color-coded in the evolutionary trees above.

Shared Derived Characters

• A shared derived character is a homology all members of a clade, and only members of that clade, possess.

o Shared derived characters distinguish one clade from another.

o For example, amphibians, turtles, lizards, snakes, crocodiles, birds, and mammals all have, or their ancestors had, four limbs. Four limbs is a shared derived character inherited from a common ancestor that helps set apart this particular clade of vertebrates, called the tetrapods.

Vestigial pelvic and thigh bones of python.

• On an evolutionary tree, a shared derived character appears at or just before the common ancestor of the clade that it helps define.

Biological Classification

• One of the many uses for evolutionary trees is to classify (i.e., group) organisms. The modern evolutionary classification system names only taxa that are clades.

o Clades-within-clades become groups-within-groups.

o The evolutionary classification system combines data from many sources, including the fossil record, comparative anatomy, and comparison of DNA sequences among organisms.

o The older Linnaean classification system – which "ranks" species into kingdoms, phyla, classes, orders, families, and genera – is not based on evolution. Created long before scientists understood that organisms evolved, it classifies organisms based on physical similarities and differences.

Darwin and the Evolutionary System

"We can understand why a classification founded on any single character or organ…is almost sure to prove unsatisfactory. Classifications may, of course, be based on any character whatever, as on size, colour, or the element inhabited; but naturalists have long felt a profound conviction that there is a natural system. This system, it is now generally admitted, must be, as far as possible, genealogical in arrangement, – that is, the co-descendants of the same form must be kept together in one group, apart from the co-descendants of any other form; but if the parent-forms are related, so will be their descendants, and the two groups together will form a larger group…For this object numerous points of resemblance are of much more importance than the amount of similarity or dissimilarity in a few points."

– Charles Darwin, The Descent of Man (London: John Murray, 1871). Volume 1, First edition, p 188.

• Under a system of evolutionary classification, we could name any clade on this tree. For example, the Testudines, Squamata, Archosauria, and Crocodylomorpha all form clades.

• However, the reptiles do not form a clade, as shown in this tree.

o That means that either "reptile" has to be abandoned as a taxon, or we have to start thinking of birds as reptiles (that is, avian dinosaurs).

• Another cool thing about evolutionary classification is that it means that dinosaurs are not entirely extinct.

o Birds are, in fact, dinosaurs (part of the clade Dinosauria). It’s pretty neat to think that you could learn something about T. rex by studying birds!

"Recently, fossils of early birds and their most immediate predecessors have been collected at an unprecedented rate from Mesozoic-aged rocks worldwide. This wealth of new fossils has settled the century-old controversy of the origin of birds. Today, we can safely declare that birds evolved from a group of dinosaurs known as maniraptoran theropods—generally small meat-eating dinosaurs that include Velociraptor of Jurassic Park fame.

"Evidence that birds evolved from the carnivorous predators that ruled the Mesozoic ecosystems is plentiful and it comes from disparate lines of evidence."

— Dinosaur Institute of the Natural History Museum of Los Angeles County

http://www.nhm.org/site/research-collections/dinosaur-institute/dinosaurs/birds-late-evolution-dinosaurs

Great Transitions: The Origins of Birds https://www.youtube.com/watch?v=z4nuWLd2ivc&feature=youtu.be

Scientific Names

• The scientific name of an individual organism, whether living or extinct, consists of two Latinized words: the genus and the species to which it belongs.

o The first word is capitalized and both words are italicized or underlined. o A genus (pl. genera) is a group of closely related species, such as Homo.