Biology and the Tree of Life
Biological science was founded with the development of :
(1) the cell theory
(2) the theory of evolution by natural selection.
Aphylogenetic tree is a graphical representation of the evolutionary relationships among species.
Phylogenies can be established by analyzing similarities and differences in traits.
Biologists ask questions, generate hypotheses to answer them, and design experiments that test the
predictions made by competing hypotheses.
The Tree of life
The cell theory and the theory of evolution by natural selection imply that all species come from
preexisting species and that all species, past and present, trace their ancestry back to a single common
Speciation is a divergence process in which natural selection has caused populations of one species to
diverge to form new species.
The tree of life is a family tree of organisms that describes the genealogical relationships among
species with a single ancestral species at its base.
Taxonomy is the effort to name and classify organisms.
In Linnaeus’taxonomic system for classifying organisms, each organism is given a unique two-part
scientific name consisting of the genus and the species.
1. Agenus is made up of a closely related group of species.
2. Aspecies is made up of individuals that regularly breed together or have characteristics that
are distinct from those of other species.
Linnaeus’system is hierarchical with nested taxa. The taxonomic levels from least to most specific are
species (Homo sapiens)
How Many Kingdoms Are There? Linnaeus proposed just two kingdoms—plants and animals. However, not all organisms fall neatly into
these categories. An alternative five-kingdom system was proposed in the 1960s.
The Origin of the Phylogenic Tree of Life
Carl Woese and colleagues studied small subunit rRNA, as a means for understanding the
evolutionary relationships among organisms.
1. rRNAsequences should be very similar in closely related organisms but less similar in
organisms that are less closely related.
2. Aphylogenetic tree reflects these relationships among species. Branches that are close to one
another represent species that are closely related; branches that are farther apart represent
species that are more distantly related.
Changes to the Tree of Life
The tree of life indicates three major groups of organisms:
and two groups of prokaryotes
• Bacteria andArchaea.
Therefore, Woese created a new taxonomic level called the domain, consisting of Bacteria,Archaea,
The location of certain branches on the tree is hotly debated and the shape of the tree will continue to
change as databases expand.
Why Do Giraffes Have Long Necks?
Biologists test ideas about the way the natural world works by testing the predictions made by
alternative hypotheses using carefully designed experiments.
Simmons and Scheepers tested the food competition hypothesis that giraffes evolved long necks by
natural selection because those with long necks reach food unavailable to others mammals.
The Food Competition Hypothesis
• Neck length is variable among giraffes • Neck length in giraffes is heritable
• Giraffes feed high in trees
One of the three predictions of the food competition hypothesis—giraffes feed high in trees—does not
Thus, there may be better alternative hypotheses to explain neck length in giraffes.
An alternative hypothesis is that giraffe evolved long necks because longer-necked males win more
fights than shorter-necked giraffes, and can then father more offspring.
Data support this hypothesis.
Eukaryotes have nucleus.
Amule does not have same number of chromosomes because parents do not share the same number of