Unit 6: Evolution
• The process of change over time
• Charles Darwin developed the theory of how
living things evolve
Darwin's Theory of Evolution
• Based on his findings on
the Galapagos Islands
and elsewhere, Darwin
said that all living things
are descended (with
modifications) from a
common ancestor.
• “Descent With
Modification”
Descent with Modification
• The Earth is about 4.5 billion years old
• The first living cells (similar to bacteria) are
believed to have appeared on Earth about 3.5
billion years ago
• Darwin theorized that all living things on Earth
are descendants of an ancient organism from
the past.
• This ancient organism is the “common
ancestor” for all living things on Earth.
Descent with Modification (cont.)
• Small changes every
generation for billions of
years have led to different
organisms branching off
into new species, and
forming the “Tree of Life”.
• We now know that these
small changes occur in the
DNA as mutations.
How does Decent with Modification occur?
• Darwin proposed that evolution occurs by
Natural Selection
• Natural Selection- the process by which
organisms with variations most suited to their
environment survive and leave more offspring.
How Natural Selection Works
A. “The Struggle For Existence”
- If more individuals are born than can survive,
there must be competition for food, living
space, etc.
- Darwin wanted to know which individuals
would come out on top
How Natural Selection Works
B. “Variation and Adaptation”
- Some variations in individuals are better
suited for a certain environment
- Adaptation- any heritable characteristic that
increases an organism’s ability to survive in a
certain environment
- Well adapted individuals survive and
reproduce
Yeti
Crab
How Natural Selection Works
C. “Survival of the Fittest”
- Fitness- how well an organism can survive
and reproduce in its environment
- If an organism is well adapted to its
environment and successfully reproduces it
has “high fitness”
- Over time, the traits of the well adapted
individuals become the more common traits
of the population
Snowy egret nest
Example of Natural Selection
1. Grasshopper lays eggs
3.Predator eats the
grasshoppers that are
easy to see
2. Yellow and green
grasshoppers hatch
4.Green grasshoppers are
left to reproduce
After many
generations,
almost all
grasshoppers
will be
__________.
green
Thus, nature has
“selected for”
green
grasshoppers!
Patterns of Evolution
1. Divergent Evolution• the accumulation of differences between
organisms of a species which can lead to the
formation of new species
• Involves speciation through adaptive radiation
Speciation is how divergent
evolution occurs
The Galapagos finches
underwent speciation
- A species is a
population or group of
populations whose
members can interbreed
to produce fertile
offspring
- Speciation- the
formation of a new
species
How does Speciation Occur?
• When two populations can no
longer interbreed, the gene
pool will split
• This is called reproductive
isolation
• Reproductive isolation can
lead to the formation of two
separate species or divergent
evolution
Adaptive Radiation leads to
speciation
• The process by which a single species or small
group of species evolve over a relatively short
period of time into several different forms that
live in different ways
Reasons why Adaptive Radiation Occurs
A. Species migrate to a new environment
B. Extinction clears an environment of many
inhabitants
- Example: after the dinosaurs became extinct,
mammals were able to reproduce and survive
more, spread out, and diversify.
C. A species evolves a new feature that allows it
to live in a new environment
- Example: Galapagos finches developed different
beaks to allow them to eat different types of
food on the different islands.
2. Convergent Evolution
• When organisms in
different locations, but
similar environments
evolve in a similar way.
• Natural selection may
cause different body
structures to have similar
functions.
• Example: rheas (South
America) and ostriches
(Africa) are both birds
that lost their ability to fly
RheaSouth
America
OstrichAfrica
3. Coevolution
• The process by which two species evolve over
time in response to changes in each other.
• Often species that coevolve can not survive
without each other.
• Example: flowers and pollinators (organisms
that cause the pollen (sperm) to reach the egg
of a flower)
Example of Coevolution
• The Madagascar Star Orchid and
Hawkmoth
– The orchid has a long tube with nectar
(food) at the bottom
– The moth has a long tongue to reach
the nectar
– When the moth eats, it rubs against
the pollen of the flower, and brings it
to other flowers
– Over generations, as the moth’s
tongues get longer, the orchid’s tube
gets longer so that the moth will
always rub against the pollen.
Flowers and Pollinators
• Pollination
• Beauty of Pollination