Course Introduction and the Tree of Life

Biology 160: Diversity and the History of Life

Photo by Matt Ritter

“Each [organism] instructs; its form and behavior embodies general messages if only we can learn to read them. The language of this instruction is evolutionary theory. Exultation and explanation.” Stephen J. Gould

http://www.coopext.colostate.edu/TRA/PLANTS/index.html#http://www.colostate.edu/Depts/CoopExt/TRA/PLANTS/demogardens/html

Name that organism?

The Cell TheoryAre All Organisms Made of Cells?Where Do Cells Come From?

The Theory of Evolution by Natural SelectionWhat Is Evolution?What Is Natural Selection?

The Tree of LifeLinnaean Taxonomy– Taxonomic Levels– How Many Kingdoms Are There?

Using Molecules to Understand the Tree of Life– The rRNA Tree– The Tree of Life Is a Work in Progress

The Cell Theory

In the late 1660s, Robert Hooke and Anton van Leeuwenhoek were the first to observe cells.

Cells consist of a membrane that contains concentrated chemicals dissolved in water.

Figure 1-1

The first view of cells: Robert Hooke’s drawing from 1665 Anton van Leeuwenhoek was the first to view single-celled “animalcules” in pond water.The pore-like

compartmentsare cork cellsfrom oak bark A Paramecium

Are All Organisms Made of Cells?

Where Do Cells Come From?

The cell theory states that all organisms are made of cells and all cells come from preexisting cells.

Because all cells come from preexisting cells, single-celled organisms in a population are related to a single common ancestor, and all cells in a multicellular organism also descend from a single ancestral cell.

Be familiar with this seminal experiment by Pasteur

Figure 1-2-experiment

The Theory of Evolutionby Natural Selection

In 1858, Darwin and Wallace proposed that all species are related by descent from a common ancestor.

Species change over time and are related to one another. Natural selection explains how this occurs.

What is Natural Selection?

Two conditions must be met for natural selection to occur in a population:

(1) individuals in the population must differ from one another for some heritable trait(s); and (2) individuals with certain heritable traits must survive and/or reproduce better than individuals with other traits.

What is Natural Selection?

Result of Natural Selection

In natural selection, the population’s characteristics will change over time as the individuals with the favorable traits increase in frequency.

In artificial selection, individuals in a population are selected for mating based on particular traits. Repeating this process over generations results in a population with altered characteristics.

Figure 1-3

Wild Brassica oleracea: the parental generation

ARTIFICIAL SELECTION

Select individuals with the largest and most compact floweringstalks, and breed them.

Parental generation:

Of the offspring, selectthe individuals withthe largest and mostcompact floweringstalks, and breed them.

Broccoli: one descendantof wild Brassica oleracea

1.

Of the offspring, selectthe individuals withthe largest and mostcompact floweringstalks, and breed them.

After many generations, average flowering stalks are much larger and more compact.

These charts, calledhistograms, document howthe width of the floweringstalk changed throughtime in response to selection

Large,compactfloweringstalks

Spindlyfloweringstalks

Select theseindividuals fornext generation

Generation 2:2.

Generation 3:3.

Descendants:5.Repeat dozens of times.4.

• Began about 50,000 years ago

• 14,000 years ago - artificial selection

– Dogs with desired forms of traits were bred

• Modern breeds are the result

Domestication of Dogs

Conceptual Questions

• Does Evolution always result in species that get larger, more complex, or “better” in some sense?

• Do individuals change when natural selection occurs?

• Are individuals with high levels of fitness stronger or bigger or “more dominant?”

The Tree of Life

The cell theory and the theory of evolution by natural selection imply that all species are descended from a single common ancestor at the root of a family tree of all organisms—the tree of life.

Linnaean Taxonomy

• In Carl Linnaeus’ (1707-1778 ) taxonomic system for classifying organisms, each organism is given a unique two-part scientific name consisting of the genus and the species.

• Linnaeus’ system is hierarchical with nested taxa. The taxonomic levels from least to most specific are kingdom, phylum, class, order, family, genus, and species.

Figure 1-5

KINGDOM(Animalia)

PHYLUM(Chordata)

CLASS(Mammalia)

ORDER(Primates)

FAMILY(Hominidae)

GENUS(Homo)

SPECIES(Homosapiens)

How Many Kingdoms Are There?

• Carl Linnaeus proposed just two kingdoms, plants and animals. An alternative five-kingdom system based on phylogeny was proposed in the 1960s.

Phylogeny: The evolutionary development and history of a species or higher taxonomic grouping of organisms.

Figure 1-7

KINGDOM MONERA(includes allprokaryotes)

KINGDOM PROTISTA(includes severalgroups of unicellulareukaryotes)

KINGDOM PLANTAE

KINGDOM FUNGI

KINGDOM ANIMALIA

Using Molecules to Understand the Tree of Life

Carl Woese and colleagues studied small subunit rRNA, a molecule found in all organisms, as a means for understanding the evolutionary relationships among groups of organisms.

Figure 1-8

… to the nucleotide sequence found at thesame location in the rRNAmolecule of green algae.

Compare the rRNA nucleotidesequence observed in land plants…

Using Molecules to Understand the Tree of Life

Genetic Distances Among Organisms on Earth

Domains of Life

Carl Woese proposed a new taxonomic level called the domain. Each of the three domains (Bacteria, Archaea, and Eukarya) includes several related kingdoms.

The rRNA Phylogenetic Tree

The tree of life indicates three major groups of organisms: the eukaryotes (Eukarya) and two groups of prokaryotes—the Bacteria and the Archaea.

Figure 1-9

Plants, fungi, andanimals are smallbranch tips onthe tree of life

This node represents the commonancestor of archaea and eukaryotes

This node represents the commonancestor of all organisms alive today

KEY CONCEPTS

Biological science was founded with the development of:(1) the cell theory, which proposes that all organisms are

made of cells and that all cells come from preexisting cells, and

(2) the theory of evolution by natural selection, which maintains that the characteristics of species change through time—primarily because individuals with certain heritable traits produce more offspring than do individuals without those traits.

A phylogenetic tree is a graphical representation of the evolutionary relationships among species. Phylogenies can be estimated by analyzing similarities and differences in traits. Species that share many traits are closely related and are placed close to each other on the tree of life.

KEY CONCEPTS