is the core theme of biology
Evolution is the one unifying biological
- Life evolves. Species change
over time and their history can be described as a branching tree of life.
- Species that are very similar
share a common ancestor at a recent branch point on the phylogenetic tree.
- Less closely related organisms
share a more ancient common ancestor.
- All life is connected and can
be traced back to primeval prokaryotes that existed more than three billion
In 1859, Charles Darwin published
On the Origin of Species in which he made two major points:
1. Species change, and contemporary
species arose from a succession of ancestors through a process of "descent with
2. A mechanism of evolutionary
change is natural selection.
Darwin synthesized the concept of
natural selection based upon the following observations:
- Individuals in a population
of nay species vary in many inheritable traits.
- Populations have the potential
to produce more offspring than will survive or than the environment can
- Individuals with traits best
suited to the environment leave a larger number of offspring, which increases
the proportion of inheritable variations in the next generation. This differential
reproductive success is what Darwin called natural selection.
Organisms' adaptations to their
environments are the products of natural selection.
- Natural selection does not
create adaptations; it merely increases the frequency of inherited variants
that arise by chance.
- Adaptations are the result
of the editing process of natural selection. When exposed to specific environmental
pressures, certain inheritable variations favor the reproductive success
of some individuals over others.
Darwin proposed that cumulative
changes in a population over long time spans could produce a new species from
an ancestral one.
Descent with modification accounts
for both the unity and diversity of life:
- Similarities between two species
may be a reflection of their descent from a common ancestor.
- Differences between species
may be the result of natural selection modifying the ancestral equipment
in different environmental contexts.