Download 14.3: Natural Selection is the Mechanism of Evolution

Natural Selection is the
Mechanism of Evolution
Objectives:
1. Summarize Darwin’s
theory of natural selection.
2. Compare and contrast
artificial selection w/
natural selection.
3. Relate pesticides
resistance in insects to
natural selection.
Vocab.
Population
Variation
Artificial Selection
Evolution occurs in
POPULATIONS, NOT
INDIVIDUALS
Population:
Group of individuals of the
same species living in the
same area at the same time.
Evolutionary changes occur as a
result of a CHANGE IN
THE FREQUENCY OF
ALLELES IN A
POPULATION.
NATURAL SELECTION
selects favorable alleles!
DARWIN’S Natural Selection is
based on 2 observations!
1.
All species tend to produce EXCESSIVE # of
OFFSPRING; MORE than NATURE CAN SUPPORT
/Resources. <Malthus>
RESULT = STRUGGLE FOR SURVIVAL; Small %
survive to reproduce.
2.
There is VARIATION (traits) among indviduals in a
population.
Variation is due to different alleles (genotypes)
producing different phenotypes
Natural Selection & Evolution
Individuals w/ traits (variations) favored by
their environment will survive &
reproduce more offspring. Over many
generations, the favorable traits will be
more common.
Adaptations = traits favored by environment.
Artificial Selection Encourages
Darwin
Artificial Selection: selective breeding of
domesticated plants and animals to produce
offspring w/ traits desired by humans.


Like Natural Selection … it can change the gene
pool and change a species.
Unlike Natural Selection … PEOPLE select
traits (not nature)
Natural Selection & Pesticide
Resistance in Insects
Some insects have natural
resistance (mutation in
genes).
Resistant insects survive
and pass on resistance to
offspring.
Over generations, resistant
insects b/cm more common.
Summary
Process of Natural Selection
(S.N.A.S.D.)
• Struggle for Existence: Organisms generally have more
offspring than resources can support
• Natural Variation: Even within a species there will be distinct
differences between individual organisms
• Adaptation: Some variations provide individuals with an
advantage that increases their chances of survival.
• Survival of the Fittest: Those with the adaptations are more
likely to survive and reproduce
• Descent with Modification: Genes for the beneficial
adaptations are passed on and the trait becomes increasingly more
common in each new generation of the species
Microevolution & Changes in
the Gene Pool
Objectives:
1. Explain the significance
of gene pools in
understanding evolution.
2. Tell how the genetic drift,
gene flow, mutation, and
natural selection
contribute to changes in
the gene pool.
3. Explain what is meant by
fitness.
Vocab.
Gene pool
Hardy-Weinberg equilibrium
Genetic drift
Gene flow
Fitness
Darwin didn’t know about
genes… we do.


Natural Selection acts on variations
(alleles) in the gene pool.
Result = Evolution
Gene Pool
All of the alleles in all of the
individuals that make up a
population; Healthy gene pools
have variation!
Evolution occurs when there is any
change in the gene pool.
Change in Gene Pool = Change in allele
frequencies
Microevolution = change in frequency of alleles fr.
generation to generation.
Genetic Drift
Change in gene pool
due to RANDOM
CHANCE. (instead
of Natural Selection)
Microevolution: What can cause a
population to evolve?
Five Fingers of Evolution
1. Pinky: Chance events SHRINK popluation.(bottleneck
effect or Founders); New population is established from small
gene pool that may or may not represent allele freq. in original
population.
2. Ring: Mating/Marriage is not random (Mates are picked
based on certain characteristics); some alleles are favored.
3. Middle: Mutations occur and add new alleles.
4. Pointer: Movement (migration of individuals) into or out of
an area affects allele freq.
5. Thumb: Natural selection – nature selects traits that are
favorable = adaptations (like opposable thumbs)
Change in Gene Pool: Chance events SHRINK gene pool.
Pinky: Population shrinks (bottleneck effect or Founders); New
population is established from small gene pool that may or may not
represent allele freq. in original population.
Bottleneck Effect
Disaster drastically reduces population (& gene pool)
Few individuals repopulate.
New population lacks genetic variation.
Pinky: Chance SHRINKS population; New
population is established from small gene pool that
may or may not represent allele freq. in original
population.
Founder’s Effect
Change in Gene Pool
Ring: Mating/Marriage is not random (Mates are
picked based on certain characteristics); some alleles
are favored.
Change in Gene Pool
Middle: Mutations occur and add new alleles
Sickle cell trait, lactose tolerance, etc.
Change in Gene
Pool
Thumb: Natural
selection – nature
selects traits that
are favorable =
adaptations (like
opposable
thumbs)
Change in Gene Pool
Pointer: Movement (migration of individuals)
into or out of an area affects allele freq (Gene
Flow)
Gene Flow: Exchange of genes b/w populations.
 Reduces genetic differences b/w populations.
Contributing to the Gene Pool =
FITNESS
Fitness = Contribution an individual
makes to the gene pool compared to
other individuals.
You don’t have to be the “Fittest” … you just have to
survive & contribute genes to next generation
Genetic Equilibrium
= allele frequencies are not changing from one
generation to the next
• Hardy-Weinberg theory: a population is in genetic
equilibrium if the following conditions are met
simultaneously:
–
–
–
–
–
Large population size
Random mating
No mutations
No migration
No natural selection
Hardy Weinberg Equilibrium Tutorial
• http://www.phschool.com/science/biology_place/labbench/lab8/hardwein.html
Nat. Selection & Sickle Cell Disease
African populations have high incidence of
sickle cell.
 Carriers of sickle cell have resistance to
malaria.
 Malaria = major cause of infant death in
Africa.
 Carriers (heterozygous) have
advantageous adaptation.
 Higher incidence of sickle allele remains
in African population
Natural Selection:
Selected for Sickle
cell allele b/c it
provides an
advantage
(Resistance to
Malaria!).
Natural Selection & Antibiotic
Resistance
 Some bacteria have natural
resistance to antibiotics
(mutation, share by
conjugation)
 Some survive treatment --> pass
resistance genes on -> alleles
for resistance b/cm more
common.

NOT COMPLETING COURSE OF
ANTIBIOTICS CAN INCREASE
RESISTANCE.