Called “one long
argument” for
evolution
Hundreds of
thousands of
observations
Detailed, testable
theory
No mechanism for HOW
variation is passed from
parent to offspring
Modern synthesis:
combination of Mendel’s
and Darwin’s ideas
Comparing fossils
presents a pattern of
gradual change
Darwin found gaps in
the fossil record
Scenario 1: Leaves towards the bottom of trees are disappearing. Some giraffes can reach leaves higher up, while others’ necks are not long enough?
Which giraffes will thrive? Which giraffes will die out?
What will future generations look like?
Scenario 2: A certain species of rabbits in a dense forest come in brown and white fur colors. Hawks, soaring overhead, often eat the rabbits.
Which rabbits will be easier to spot?
Which color trait will more likely be passed on to future generations?
Individuals that display an “extreme” trait are selected for. Other individuals are selected against.
Population shifts toward this trait, away from the opposite.
Scenario 3: Lizards
range in size from tiny
to very large. Small
lizards generally run
very slowly while
large lizards are
usually the easiest for
predators to see.
What size lizard
stands the best
chance of survival?
Scenario 4: Human babies of very low birth weight are often more prone to disease. Babies of very large weights are difficult for mothers to safely deliver.
What size babies stand the best chance of survival?
Individuals that display an average trait are selected for.
Individuals that display extreme traits are selected
against.
Population shifts towards the average trait, and those
with extreme forms die off.
Scenario 5: Limpets are marine animals that attach to
rocks. Their color varies from pure white to dark brown.
White-shelled that are on rocks covered with goose
barnacles (also white) are at an advantage. On bare,
dark-colored rocks, brown limpets are at an advantage.
What “advantage” is being referred to above?
If you were a dark limpet,
where would you live?
A white limpet?
What color would have the
hardest time surviving?
Individuals that display either extreme are selected for. “Average” individuals are selected against.
Population shifts towards away from the average trait, towards BOTH extremes.
Design three example situations (1 paragraph each) representing the three types of selection we covered. Include the following:
1. Description of the animals involved and their traits
2. How is nature selecting for/against certain animals
3. Which trait will be come more common/less common and why?
4. Name the type of selection.
1) A group of crabs live on
a sandy beach. The
crabs range in color from
very light tan to dark
brown.
2) Seagulls, flying
overhead, eat the crabs.
They can more easily
see the dark crabs, since
the lighter ones blend in
with the color of the
sand.
3) Because the seagulls
more easily eat the dark
crabs, the lighter ones will
more likely survive and
reproduce, passing that
trait onto their offspring.
4) This is called “directional
selection.
Change in the allele frequency of a population
over time
Does chance
play a role?
Is evolution
predictable?
Populations will
remain at equilibrium
(and therefore not
evolve) unless some
outside factor affects
them
Equilibrium: frequency
of alleles is
unchanging
1. Natural Selection
2. Sexual Selection (mate choice)
3. Mutations
4. Gene flow (migration)
5. Genetic Drift and population bottlenecks
Movement of
individuals
(and alleles)
from one
population to
another
What are
some
barriers?
Change in allele frequency caused by chance events
More common in small populations
Can be beneficial, harmful, or neutral
Examples:• Founder Effect
• Population Bottleneck
Convergent evolution – similar environmental
pressures produce similar organisms
Coevolution – two organisms adapt to coexist
and support one another
Environmental factors
allow the development
of similar traits in very
different organisms
Ex: bats, beetles, birds
Species that live near
one another evolve to
benefit one another
Ex. sharks and fish
Ex. Yucca moth and
Yucca plant
Adaptive Radiation –
one species,
subjected to different
environmental
pressures, can evolve
into multiple species
…formation of a new
species
Often, one species can
branch from another
Extinction: the death of all
members of a single
species
Gradualism – tiny changes
become large-scale changes
over a long period of time
Punctuated Equilibrium –
sudden environmental
changes lead to new species
developing “quickly”
Morphological Concept: uses similarities
(physical and physiological) between
organisms
Biological Concept: group of organisms
that can breed and have fertile offspring
Geographic Isolation (Allopatric speciation)
Reproductive Isolation (Sympatric
speciation)
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