Transcript Natural Selection - Willimon-PHS

Evolution and Natural Selection
Natural Selection in Populations
Population – group of organisms of the same species living
together in a given region
Natural Selection – process whereby organisms with
favorable variations survive and produce more offspring
than less well-adapted organisms
•
•
Proposed by Darwin
Key mechanism of evolution
Scientists who came before Darwin
Lamarck’s theory of evolution
• Inheritance of acquired characteristics – the
characteristics an organism develops over time can be
passed on to offspring
– Mouse gets its tail cut off, offspring should have no tails
– Giraffe stretches its neck out to reach tall leaves, offspring should have
longer necks
• Incorrect, not supported by scientific evidence
• Stimulated thought about evolution
Natural Selection in Populations
No individual is capable of evolving
• Natural selection produces changes in populations, not
individual organisms
• Organism may be born with a favorable mutation
– Example: dark-fur for pocket mouse living on lava rock
• Acquired traits are not passed on to offspring
Natural Selection in Moth Population
• Random mutations - color variations in moth population
– Allows the moths to blend into tree trunks
– Harder for birds to locate “camouflaged”
moths
• Natural selection occurs
– Moths with mutation survive and reproduce
– Favorable coloring is passed on to offspring
Image by Gilles San Martin
(Own work) [CC-BY-SA-3.0]
Natural Selection
Natural selection leads to development of
• Adaptations
• Genetic diversity
What are Adaptations?
Adaptation – trait that helps
an organism to survive and/or
reproduce in its unique
environment
• Natural selection favors
variations of traits that increase
organism’s ability to survive and
reproduce
Images by L. Shyamal
(Own work) [CC-BY-SA-2.5]
Development of Adaptations
Variations in
traits occur
naturally in a
population
Individuals
with favorable
variations
produce more
offspring
• Example:
– Darwin’s Finches
– Rock Pocket mouse
Frequency of
favorable
variation
increases in
population
Variation
becomes an
adaptation of
the population
over time
What is Genetic Diversity?
• Genetic diversity –
differences in genetic
characteristics among
organisms within the same
species or among different
species
• Natural selection can
increase or decrease genetic
diversity
Image by Graphodatsky et al.
[CC-BY-2.0]
Monday in Biology
• Rock Pocket Mouse QUIZ
– You will have 15 minutes to complete the quiz over the Rock Pocket mouse
– Two grades One lab and one quiz grade (both major grades)
• Take notes over Evolution
• Vimeo Video: Selection, Gene Flow, Genetic Drift
– Handouts and questions
Types of Natural
Selection and
other
mechanisms of
evolution
Genetic Diversity Within a Species
1. Directional
2. Diversifying
3. Stabilizing
Frequency
• Three major types of natural selection can affect the
diversity within a species
Frequency
Frequency
Frequency
Phenotypes
Phenotypes
Phenotypes
Phenotypes
Directional
Diversifying
Stabilizing
Directional Selection
• Directional selection – a single variation of a trait that
was not previously favored is now favored in a species
Frequency
– Result of migration or environmental changes
Frequency
Phenotypes
Phenotypes
• May increase or
decrease diversity
within a species
• Diversifying selection –
multiple variations of a trait
are favored in a single
species
Frequency
Diversifying Selection (Disruptive)
Phenotypes
Frequency
– Increases diversity
Phenotypes
– Decreases diversity
Phenotypes
Frequency
• Stabilizing selection – a
single variation of a trait
is favored in a species
Frequency
Stabilizing Selection
Phenotypes
Mechanisms of Evolution
1.
2.
3.
4.
Natural Selection “survival of the fittest”
Mutations create new alleles
Migration moves alleles into and out of a population
Genetic drift happens when random events change the
allele frequencies of small populations
5. Sexual Selection non-random mating
• Giraffe video
• https://vimeo.com/19797501
Tuesday/Thursday
Species
• A group of organisms
that can interbreed in
nature and produce
fertile offspring
Speciation
How Do New Species Form?
• Must be isolated from
other populations for a
long period of time.
• Different selection
pressures cause different
traits to be selected for or
against.
Geographic Isolation
• Species are separated from each other by a physical barrier
Geographic Isolation
• The Kaibab and Albert squirrels are separated by the Grand Canyon
Behavioral Isolation
• Species cannot interbreed because they have different
mating rituals or behavior
Temporal Isolation
• Species mate at different times of the day or the year
Adaptive Radiation
• A single species
evolves into many
species
• Occurs as new
opportunities for
survival arise
Co-evolution
• When two species evolve together in response to
changes in each other
• Example: Bees and Flowers
Evidences of Evolution
Homologous Structures
• Similar body structures that are adapted in different
ways for different organisms
• Provide evidence that related organisms descended,
with modifications, from a common ancestor
Vestigial Structures
• The structures of
organisms that remain
from a common
ancestor, but may no
longer affect its ability
to survive and
reproduce
• Example: Appendix,
Tail Bone
Analogous Structures
• Unrelated organisms
in similar
environments develop
similar adaptations.
• Example: Flight of
Insects vs. Birds vs.
Bats
Fossil Record
• The fossilized remains of organisms found in the layers of
rock and soil
• Shows that the diversity of organisms has changed over
time
Fossil Record
• https://vimeo.com/19796909
Embryology
• Organisms with a
common ancestor share
similar stages of
development.
DNA Analysis
• All organisms use the
same four bases for their
DNA (genetic code).
• The more similar two
sequences of DNA are, the
more recent their
common ancestor, or the
more closely related the
organisms are.
DNA Analysis
• Modern
biologists compare
DNA sequences to
determine which
organisms are
most-closely
related to each
other