Tree thinking
Tree Thinking” Reading and Notes
Pgs 17–21
We sort sea shells into mammals, birds, reptiles, amphibians, and fish.
For a long time, scientists have looked for systems for naming and grouping the diversity of life.
In the mid-1700s, a Swedish scientist named Carolus Linnaeus (1707–1778) organized living things into groups.
Characteristics – A trait that helps identify an organism.
Then he grouped them based on their similarities and differences.
About 100 years later, Charles Darwin (1809–1882) suggested organizing life in a tree pattern.
The branching tree he sketched suggests that all life on Earth is related.
Related – Belonging to the same group or family connected by common ancestry.
Each branching point on Darwin’s tree represents a common ancestor that divided into descendants.
Common Ancestor – An organism from the past that is related to all the organisms in the group.
Descendant – An organism related to an organism that lived earlier.
The root of the tree is the ancestor of all the species on the tree of life.
Species – A group of organisms that can interbreed or pass on its genes to following generations.
Today, scientists still use Darwin’s “tree thinking” to model evolution. They make hypotheses about the relationships among species using a tree diagram called a cladogram.
Cladogram – An evolutionary tree diagram based on shared characteristics.
A cladogram maps patterns of relatedness based on patterns of shared inherited characteristics called traits.
Inherited Characteristics – A genetic trait that an ancestor passes on to its descendants.
Traits – The specific way a feature is expressed in an individual organism; for example, blue and green eyes are traits for eye color.
To be useful in making a cladogram, a trait must have been inherited by two or more of the species on the tree.
It must have appeared for the first time in those species’ most recent common ancestor.
Most Recent Common Ancestor – The first organism from the past that is related to all the organisms in the group.
Extinct species can be included in the cladogram.
A cladogram shows the order in which traits appeared.
It does not represent a straight line from ancestor to descendant.
Finally, a cladogram shows that one trait evolved in a group of organisms before or after another trait.
Evolve – To change.
A cladogram is only as accurate as the information used to construct it.
Scientists may use physical characteristics, such as the arrangement of bones in a limb.
Or they might use the kind of tissues or reproductive structures of a plant.
Using only physical characteristics requires careful study.
DNA (Deoxyribonucleic acid) – A molecule that contains an organism’s genetic information.
A cladogram is a hypothesis about the relationships among groups of organisms.
It represents our best model of evolutionary relationships.
It changes as new fossils or genetic information is discovered.
What cladograms reveal is that all living things are a part of one huge bushy tree.
They share a 3.8 billion-year-old common aquatic ancestor.
As we gather more evidence and develop new methods for examining traits, our view of the tree of life becomes clearer.