Download How Populations Evolve

How Populations Evolve
Before Darwin – 2 main ideas
1. Species are fixed – not changing
2. Earth relatively young - <10,000 yrs
These ideas began to change as people studied organisms,
fossils, geology, and human population growth
Organisms – Greater diversity than previously known
Fossils – Suggest old Earth, Fossils similar to existing
organisms
Geology – Gradual and observable geologic events
(weathering/erosion, earthquakes) could
explain current physical features.
Darwin
 Dec. 1831 HMS Beagle – 5 year voyage
Darwin’s Job:
Study geology, plants and animals during voyage
Send samples home to England
 Data/observation convinced him species changed as they
adapt to their environments
o Not the individual, but the population
 Darwin documented regional differences
o Fossils of South America more similar to South
American species
 Islands were of particular interest
o Island organisms similar to nearest mainland
 Made geologic observations
o Experienced earthquake and resulting uplift of
ocean floor
o Collected shells on mountain tops
Back in England
 Combined observations from voyage with problems from
human population growth
o Organisms produce more offspring than
environment can support
 This leads to a struggle for survival
2 Main Points
1. Descent with Modification – organisms alive today are
descendants of ancestral species.
o Different habitats result in an accumulation of
different traits (modification)
2. Natural Selection - Those organisms with inherited
characteristics best suited for that environment survive
better and pass on those traits/genes to their offspring.
o Adaptation – Inherited characteristics that improve
an organism’s ability to survive and
reproduce in a specific environment.
 Characteristics must already be present in the
population (variation).
Giraffe Evolution
How is artificial selection different from natural selection?
Humans act as the selective pressure – select for traits they
want in the next generation.
 Wild mustard:
o broccoli, cauliflower, cabbage, Brussels sprouts and
kale
 Dog breeds
?
Using genetic tests,
scientists now know
that dogs and wolves
share a common
ancestor, not a direct
lineage.
This common ancestor
was a prehistoric wolf
that lived in Europe or
Asia between 9,000 and
34,000 years ago.
Evidence for Evolution
1. Fossil Record
 What types of parts generally fossilize?
o Hard parts: teeth, bones, shells
o Rarely soft tissue and markings
How do fossils form?
A. Petrification - Organism dies and minerals from
groundwater invade the organic tissue and turn the
organism to stone.
B. An organism dies and is covered by sediments.
Organic tissue decays and leaves an empty mold
Minerals from water fill mold and solidify
C. Other fossils can be formed in resin amber, ice or
acid bogs.
a. At times organic
tissue can be preserved
Rock Strata and the Fossil Record
Sedimentary rocks form from the compaction of sand, mud
and other rocks over time.
 New sedimentary rock layers are formed on top of old
layers
o The rock layers are called strata
Young strata found above older strata*
Fossils found in lower layers are older than fossils found in
higher layers – Relative age
Radiometric dating uses radioactive isotopes can be used
to determine the actual age of rock or fossil.
Oldest fossils
are
prokaryotic
cells from
3.5 billion
years ago.
Fossil Record
Evolution of the Horse
Whale Evolution
2. Molecular Biology
Heritable information is stored in DNA
 Siblings are more closely relate (genetically) to each
other than two unrelated members of the same species
 The more similar the DNA sequences of two species
the more closely related.
o Greater differences means less closely related
If you can’t use DNA, what else could be used?
Proteins – Why?
DNA  RNA  Protein
3. Biogeography – organisms are more similar to those
living in close proximity than to organisms living further
away.
 Galapagos animals and South America
 Australian marsupials, Plant distribution
Galapagos Finches
Mockingbirds of
the Galapagos
Islands.
4. Homologous Structures / Comparative Anatomy
Homologous structures – structures shared by organisms
because of a more recent common ancestor.
 Function can be different
Mammal forelimb - all contain the same bones, but limbs
used for different functions
Vestigial structures – structures with no clear or
current function but served a purpose in an ancestor.
o Whale hind pelvis
o Snakes with pelvis
o Flightless birds/insects with wings
o Blind cave fish with eyes
o Human tailbone
o Human goose bumps
Analogous Structures
Don’t show close evolutionary relationships
5. Embryology
Embryology is the study of how multicellular organisms
develop from an embryo towards an adult.
 Study the rate of bone/structure development
 Observe the pattern of development
All vertebrates have gill pouches
 In fish – become gills
 In terrestrial animals – skull bones
Organisms more closely related share more similar stages
of development
Artist rendering of concept
Natural Selection
http://peppermoths.weebly.com/
What could account for the change in beak depth over
time?
Precipitation  food availability  beak depth
Natural selection “edits” traits already in the population
Natural selection is specific to time and location
Natural selection can change populations quickly
Natural Selection - Those organisms with inherited
characteristics best suited for that environment survive
better and pass on those traits/genes to their offspring.
 Adaptation – Inherited characteristics that improve an organism’s ability
to survive and reproduce in a specific environment.
 Characteristics must already be present in the population (variation).
Populations and Evolution
What makes a population?
 Organisms of the same species
o Species – organisms that can reproduce together
and produce fertile offspring
 Living in the same place at the same time
Why is this important?
Must be able to sexually reproduce – pass on “favorable”
genes to future generations
Population genetics – study how populations change
genetically over time.
Gene pool – the total collection of genes in a population at
any one time.
 The gene pool consists of all the alleles in all the
individuals making up a population
Microevolution – when the relative frequencies of alleles
in a population change over generations