Download File

DAY 1 –
NATURAL SELECTION
& EVIDENCE
Evolution
Unit
■ Chapters 22, 23, 24, 25
and 26
What is Evolution?
■ Change in the genetic makeup of a population over time.
■ Fitness – those with favorable variations for survival and reproduction.
– Populations can evolve, not individuals.
■ Diverse gene pool good for long-term survival of a species. Genetic
variations are important!
■ How do genetic variations occur?
Where does Variation come from?
■ Mutation
– random changes to DNA
■ errors in mitosis & meiosis
■ environmental damage
■ Sexual reproduction
– mixing of alleles
■ genetic recombination
– new arrangements of alleles in every offspring
■ new combinations = new phenotypes
Genetic variation in a population
Essence of Darwin’s ideas
■ Natural selection
– heritable variation exists in populations
– competition
■
for food, mates, nesting sites, escape predators
– differential survival
■
successful traits = adaptations
– differential reproduction
■
adaptations become more
common in population
Lamarckian vs. Darwinian view
■ LaMarck
– in reaching higher
vegetation giraffes
stretch their necks & transmits the acquired
longer neck to offspring
■ Darwin
– giraffes born with longer necks survive better &
leave more offspring who inherit their long
necks

Natural Selection
■ Major mechanism of evolution
■ Environment is always changing
■ Acts upon the phenotype of the population
■ Based on Darwin’s idea that resources are limited and that there is
competition for those resources.
■ Adaptation = a genetic variation favored by natural selection.
■ When allele frequencies shift, speciation occurs
– Thus, the frequency change is NOT RANDOM
Effects of Selection
■ Changes in the average trait of a population
DIRECTIONAL
SELECTION
giraffe neck
horse size
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
human birth weight
rock pocket mice
Natural selection
in action
Resistance…
NOT immunity!
MRSA
Heterozygote Advantage
■ Keeps the recessive allele
in the population
■ Ex: Sickle Cell Anemia
– aa – dies of sickle cell
anemia
– Aa – some side affects BUT
resistant to malaria!
– AA – no disease present
BUT prone to malaria
Hidden variations can be exposed through selection!
Terminal
bud
Lateral
buds
Cabbage
Artificial selection
Brussels
sprouts
Leaves
Flower cluster
Kale
Cauliflower
Stem
Flower
and
stems
Broccoli
Wild mustard
Kohlrabi
IN ADDITION TO NATURAL
SELECTION,
EVOLUTIONARY CHANGE IS
ALSO DRIVEN BY RANDOM
PROCESSES…
Genetic Drift
■ Chance events changing frequency of traits in a population
– not adaptation to environmental conditions
■
not selection
– founder effect
■
■
small group splinters off & starts a new colony
it’s random who joins the group
– bottleneck
■
■
a disaster reduces population to
small number & then population
recovers & expands again but
from a limited gene pool
who survives disaster may be random
Ex: Cheetahs

All cheetahs share a small number of alleles
 less than 1% diversity

2 bottlenecks
 10,000 years ago

Ice Age
 last 100 years

poaching & loss of habitat
Conservation issues
Peregrine Falcon
■ Bottlenecking is an important concept in
conservation biology of endangered species
– loss of alleles from gene pool
– reduces variation
– reduces adaptability
Breeding programs must
consciously outcross
Golden Lion
Tamarin
Human Impact on variation
■ How do we affect variation in other populations?
– Artificial selection/Inbreeding
■
Animal breeds
– Loss of genetic diversity
■
Insecticide usage
– Overuse of antibiotics
■
resistant bacterial strains
Evidence Supporting Evolution
2006 Fossil Discovery of Early Tetrapod
“Tiktaalik”
“missing link” from sea to land animals
Evidence for Evolution
• Paleontology – fossils show change in a species over
time
• Biogeography – Similar species are found in similar
ecosystems around the world
• Morphology – Comparing structures
– Homologous structures – body parts with similar
structure but possible different function. Shows
common ancestry
– Analogous structures – similar structure develops in
organisms that share a common ecosystem but not a
common ancestry
• Biochemical or Molecular –
Similarities in gene sequences, proteins, DNA
Fossils
■ Preserved remains of living things
■ Paleontology is the study of the fossil record
■ Most organisms do not leave a fossil after death
– Explains the “missing links”
■ Sedimentation Fossils
– As the organism decomposes the spaces will be filled with the minerals from
the silt
The Archaeopteryx Fossil
Reptilian Features
Forelimb has three
functional fingers
with grasping
claws.
Lacks the
reductions and
fusions present in
other birds.
Breastbone is small
and lacks a keel.
True teeth set in
sockets in the jaws.
The hind-limb girdle
is typical of
dinosaurs, although
modified.
Long, bony tail.
Avian Features
Vertebrae are
almost flatfaced.
Impressions of
feathers attached
to the forelimb.
Belly ribs.
Incomplete fusion
of the lower leg
bones.
Impressions of
feathers attached
to the tail.
LEFT: Archaeopteryx lithographica
Found in 1877 near Blumenberg, Germany
How old is that fossil?
■ Relative Dating
– Age of fossils based according to their location in
strata
■ Absolute Dating
– Age of fossils determined by analyzing the content of
radioactive isotopes found in the fossil.
 Half-life: The length of time required for half of the
radioactive elements to change into another stable
element.
 Unaffected by temperature, light, pressure, etc.
 All radioactive isotopes have a dependable half life. Ex: C14
decays into N14
Relative Dating
Absolute Dating
How radioactive
“naturally
occurring”
elements get inside
an organism:
A.K.A –
Radiometric dating
Homologous Structures
Anatomical evidence
Analogous structures
Convergent
Don’t
be fooled
by evolution
their looks!
Those
tails
Does fins
this &
mean
& sleek
they bodies
have a are
recent
common
ancestor?
analogous
structures!
Solving a similar problem with a similar solution
Molecular Homology
Human
Macaque
Dog Bird
Frog
Lamprey
The sequence in
DNA
proteins
Why &compare
is a &molecular
DNA
proteins
record
of evolutionary
across
species?
relationships.
Comparative hemoglobin structure
8
0
32
45
67
125
10 20 30 40 50 60 70 80 90 100 110 120
Number of amino acid differences between
hemoglobin (146 aa) of vertebrate species and that of humans
Vestigial organs
Why would whales
have pelvis & leg bones
if they were always
sea creatures?
These are
remnants of
structures that were
functional in
ancestral species
Evolution evidence at the cellular level
■ Domains: Archaea, Bacteria and Eukarya
– Elements conserved through all: DNA, RNA and many metabolic pathways.
■ Eukaryotes – core features:
– Cytoskeleton
– Nucleus
– Membrane-bound organelles
– Linear chromosomes
– Endomembrane system