Change over time (LOTS of TIME!)
Biological evolution, simply put, is
descent with modification
The central idea of biological evolution is that all life on
Earth shares a common ancestor, just as you and your
cousins share a common grandmother.
Through the process of descent with modification, the
common ancestor of life on Earth gave rise to the fantastic
diversity that we see documented in the fossil record and
around us today. Evolution means that we're all distant
cousins: humans and oak trees, hummingbirds and whales.
Modifications diversity to fit an available niche (lifestyle)
The history of life:
looking at the patterns
The central ideas of
evolution are that life
has a history
Life has changed over
The family tree
The process of
evolution produces a
This produces a
branching pattern of
Understanding a phylogeny is a lot like reading a family tree.
The root of the tree represents the ancestral lineage, and the
tips of the branches represent the descendents of that
a) As you move from the root to the tips,
you are moving forward in time.
b) When a lineage splits (speciation),
it is represented as branching.
c) Phylogenies trace patterns of shared
ancestry between lineages.
Each lineage has a part of its history
that is unique to it alone and parts
that are shared with other lineages.
Similarly, each lineage has ancestors
that are unique to that lineage and
ancestors that are shared with other
lineages — common ancestors.
Trees, not ladders
Aristotle's vision of a Great Chain of Being,
above. We now know that this idea is incorrect.
Similarly, it's easy to misinterpret phylogenies
as implying that some organisms are more
"advanced" than others; however, phylogenies
don't imply this at all.
In the phylogeny above it is equally valid to list
events from right to left or left to right.
Ferns are no more “advanced” than moss
Misconceptions about humans
Humans did not
Humans and chimpanzees are
evolutionary cousins and share
a recent common ancestor that
was neither chimpanzee nor
Humans are not "higher" or
"more evolved" than other
Since our lineages split,
humans and chimpanzees
have each evolved traits
unique to our own lineages.
Using shared derived characters
Our goal is to find
evidence that will help
us group organisms
into less and less
Homologies and analogies
Since a phylogenetic tree is a hypothesis about
evolutionary relationships, we want to use characters
that are reliable indicators of common ancestry to
build that tree.
Homologous (common inheritance)
a) Homology = An example is the presence of four limbs.
b) Analogy –same function,
Bat and bird wings are
examples of analogies
Not all characters are
homologies. For example,
birds and bats both have
wings, while mice and
crocodiles do not.
Does that mean that birds and
bats are more closely related
to one another than to mice
No. When we examine bird
wings and bat wings closely,
we see that there are some
hint at different ancestors
Bat wings consist of flaps of
skin stretched between the
bones of the fingers and arm.
Bird wings consist of feathers
extending all along the arm.
These structural dissimilarities
suggest that bird wings and
bat wings were not inherited
from a common ancestor with
This idea is illustrated by the
phylogeny, which is based on
a large number of other
TIME ( Millions of Years)
Adding time to the tree
If you wanted to squeeze the
3.8 billion years of the history of
life on Earth into a single
minute, you would have to wait
about 50 seconds for
multicellular life to evolve,
another four seconds for
vertebrates to invade the land,
and another four seconds for
flowers to evolve — and only in
the last 0.002 seconds would
"modern" humans arise.
The following phylogeny
represents vertebrate evolution
— just a small blade on the tree
of life. The lengths of the
branches have been adjusted
to show when lineages split and
How we know what happened when?
Life began 3.8 billion years ago, and insects
diversified 290 million years ago, but the
human and chimpanzee lineages diverged
only five million years ago.
a) This long span of time allows for the
slow changes of evolution to
dating relies on
half-life decay of
elements to allow
scientists to date
rocks and materials
ii. Stratigraphy provides a sequence of events from
which relative dates can be extrapolated.
iii Molecular clocks
Mutation rates can
give us the years
since two species
Descent with modification
We've defined evolution as descent with
modification from a common ancestor, but
exactly what has been modified? Evolution
only occurs when there is a change in
gene frequency within a population over
These genetic differences are heritable
and can be passed on to the next
generation — which is what really matters
in evolution: long term change.
Increasing the amount of a certain gene (trait) over
time due to an advantage gained from that trait.
Imagine that green beetles are easier for birds to spot (and
hence, eat). Brown beetles are a little more likely to survive
to produce offspring. They pass their genes for brown
coloration on to their offspring. So in the next generation,
brown beetles are more common than in the previous
More brown genes are present, just by chance!
Individuals moving introduce a different gene
into a population
Some individuals from a population of brown
beetles might have joined a population of green
beetles. That would make the genes for brown
beetles more frequent in the green beetle
A DNA mistake introduces a new allele of a gene
into a population
A mutation could cause parents with genes for
bright green coloration to have offspring with a
gene for brown coloration. That would make the
genes for brown beetles more frequent in the
Changes in how an organism grows introduce a
trait giving some advantage.
Developmental changes may help explain, for
example, how some hoofed mammals evolved into
ocean-dwellers, how water plants invaded the land,
and how small, armored invertebrates evolved wings.
contain clues about
its history that
biologists can use to
Human selections can
modify a population of
A striking example is
that of the population
of dark moths in the
19th century in
England, which rose
and fell in parallel to
These changes can
often be observed and
Choice of Mate
Individuals must be
chosen to mate for
genes to be passed
No kids, no
One of my favorite portions of Charles Darwin’s
Origin of Species are his opening chapters detailing
the very successful human efforts at selection.
Mimicry of leaves by
insects is an
This example is a
katydid from Costa
nicotine, a potent
poison, to kill insect
and animal pests.
nutrients and water.
Echolocation in bats
is an adaptation for
Is evolution progressive?
This is not an easy question to answer.
From a plant's perspective, the best measure of
progress might be photosynthetic ability;
From a spider's it might be the efficiency of a
venom delivery system.
Evolution = ?
Descent with modification
Mechanisms of evolution
i. Genetic Drift
ii. Natural Selection
v. Developmental Changes
Why does evolution matter?