evolution and classification

Genetic Connections Between Organisms

• Evidence from morphological, biochemical, and gene sequence data suggests that all organisms on Earth are genetically related.

• But how do you get from the genetic relationships between parents and their offspring to the more distant genetic relationships between species that are as different as daisies and elephants?

• Consider an individual organism, a 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.

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

• Eric’s log is on a beach where there are other populations of beetles under other logs. Occasionally, individuals move between populations and mate, so Eric is actually part of a group of populations (see next slide).

Driftwood Beach State Recreation Site, Oregon

• The genetic connections among organisms within a species form a tangled web. However, these genetic connections do not extend much between different species, because organisms generally look for mates within their own species.

• Thus, each species descends through time as a bundle of genetic connections isolated from other such bundles formed by other species.

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

• Something like this must have happened a long time ago when Eric’s species, Bembidion zephyrum, evolved from the ancestor it shares with it closest relative, Bembion levettei (see next slide).

• 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 A genetic lineage is a continuous line of descent from a particular ancestor to a particular descendant.

• This process has resulted in an ever growing evolutionary tree of species lineages, with the species living today sitting like leaves at the tips of actively growing branches (see next slide).

What is 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:

• The leaves at the tips of an evolutionary tree represent the modern 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 most recent common ancestor and the event, or speciation, that produced its two descendants.

o The taxa at the leaves of the branches are sometimes groups of species (such as reptiles, mammals, birds, and so on), instead of species, but a node still represents a speciation event, because groups of species originated as species.

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

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

How to Read Evolutionary Trees

• Unless time is marked on the tree, the chronology of nodes can be determined only for nodes that are on the same lineage. According to this tree (→), y happened before z, but the tree does not specify when x occurred relative to y and z.

• 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 (↓).

• Thus, just because we tend to read trees along the tips from left to right, there is no correlation with a sequence of evolution.

• Likewise, there is no correlation with level of “advancement” as we read along the tips from left to right.

REMEMBER:

• The points described in the last couple of slides cause the most problems when it comes to human evolution. The phylogeny of living species most closely related to us 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 not “higher” or “more evolved” than other modern organisms. No living organism is an ancestor or descendant of any other.

• When comparing taxa at more than two tips, the most closely related pair are the two that have the most recent common ancestor – not necessarily the two that are next to each other in the evolutionary tree.

o Question: In this phylogeny (↓), 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 – not necessarily the two that are next to each other in the evolutionary tree.

o Question: In this phylogeny (↓), 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.

Clades

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

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

• 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.

• Homologies are used to group organisms into clades; a homology shared by all members of a clade and only by members of that clade is called a derived character.

o For example, amphibians, turtles, lizards, snakes, crocodiles, birds, and mammals all have, or their ancestor had, four limbs containing the same bones; no other animals have them. Thus, four bony limbs is a derived character that helps set apart this particular clade (tetrapods) of vertebrates.

Some snakes actually have a vestigial pelvis

and femur.

• Derived characters appear on an evolutionary tree at or just before the last common ancestor of the clade.

Biological Classification

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

o Clades-within-clades become groups-within-groups. o The phylogenetic 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 Phylogenetic 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 phylogenetic 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. That means that either “reptile” is not a valid phylogenetic grouping or we have to start thinking of birds as reptiles.

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

Scientific Names

• All known organisms, both living and extinct, have a scientific name, consisting of two Latinized words: the genus and the species. 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.