Transcript Microevolution - Cloudfront.net

Chapter 23-Microevolution
Population genetics
• Population:
a localized group of individuals
belonging to the same species
• Species:
a group of populations whose
individuals have the potential to
interbreed and produce fertile offspring
• Gene pool:
the total aggregate of genes in a
population at any one time
• Population genetics:
the study of genetic changes in
populations
• Modern synthesis/neo-Darwinism
• “Individuals are selected, but populations
evolve.”
Hardy-Weinberg Principle
• States: frequencies of alleles & genotypes in a population’s
gene pool remains constant from generation to generation.
• Model proposed in 1908
• Represents an ideal situation
• Seldom occurs in nature
• Mathematical model is used to compare populations
• Allows biologists to calculate allele frequencies in a
population
• Serves as a model for the genetic structure of a
non-evolving population (equilibrium)
Represents “genetic equilibrium”
If the allele frequencies deviate from the predicted values of HW then
the population is said to be evolving.
Hardy-Weinberg Theorem
5 conditions for Equilibrium
-Very large population size
- No migration
- No net mutations
- Random mating
- No natural selection
**when all these are met then
a population is not
evolving
Hardy-Weinberg Equation
• p=frequency of one allele (A);
p+q=1.0
•
•
•
•
•
q=frequency of the other allele (a)
(p=1-q & q=1-p)
P2=frequency of AA genotype
2pq=frequency of Aa
q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
**Operates like a deck of cards: No matter
how many times you shuffle the deck the deck
itself remains the same.
Solving & Analyzing HW Principle
•
Problem: If you had 90 individuals that possessed the recessive
condition in a population of 1000 individuals, determine the
frequency of dominant and recessive alleles present in the
population as well as the genotypic and phenotypic frequencies.
(1) Always start with the # of homozygous recessive alleles
- aa = 90 and q2 = 90/1000 which is 0.09
- a = square root of 0.09 which is 0.3
- A = (1 – 0.3) which is 0.7
- AA = (0.7) 2 which is 0.49
- Aa = ???
**Remember that p2 + 2pq + q2 = 1
(AA)
(Aa)
(aa)
Ch. 23 Microevolution (Video)
1. How are Mosquito populations affected by genetic and
environmental conditions
2. What affect did the insecticide have on the insects?
3. Which forms of isolation are presented in the last
segment?
4. What are some examples of species that are endemic to
Catalina Island? How is inbreeding prevented for these
species?
***Write the title for each segment and FIVE statements
for each segment.
Microevolution
Two situations increase the impact of Genetic drift:
**Bottleneck & Founder Effect
Microevolution
• Involves small or minor changes in the allele frequencies
within a population
• Five processes have been identified that change allele
frequencies: (see pg. 458)
–
–
–
–
–
Nonrandom mating
(sexual selection)
Gene flow
(migration between populations)
Genetic drift
(bottleneck effect & Founder effect)
Mutations
(unpredictable change in DNA)
Natural selection
(differential reproduction)
**certain alleles are favored over others in nature
Microevolution
A change in the gene pool of a population over a
succession of generations
Genetic drift: changes in the gene pool of a small
population due to chance
-Usually reduces genetic variability (losses of alleles)
-Caused by Bottleneck Effect & Founder Effect
Microevolution
• The Bottleneck Effect: type
of genetic drift resulting from a
reduction in population (natural
disaster) such that the surviving
population is no longer
genetically representative of the
original population
• (Fires, Floods, etc.)
• Caused by Chance Events
• Humans can have an impact
Bottleneck Effect
• Sudden change in the environment causing
a shift in allele frequencies.
• Present Population does not reflect the
original population
• Certain alleles become over represented
Ex. Elephant Seal Hunting (1890’s) were
limited to just 20 individuals.
**Genetic Variation is reduced
Microevolution
• Founder Effect:
2nd cause of genetic drift
attributable to
• Colonization by a
limited number of
individuals breaking off
and becoming isolated
from the parent
population.
Ex. Retinitis pigmentosa
occurrence in
Tritan da Cunha
Microevolution
Gene Flow:
genetic exchange due to the
migration of fertile
individuals or gametes
between populations
(reduces differences
between populations)
Microevolution
Mutations:
A Change in the DNA
- source of new alleles
- genetic variation
- “raw materials of natural selection
-unpredictable in nature
-Doesn’t determine the direction of
evolution
-causes small changes in allele
frequencies
Approx. Mutation rate: One in every
100,000 genes per generation
Microevolution
Nonrandom mating: sexual selection
Mates are chosen according to desired characteristics
(Phenotypic traits)
Microevolution
• Natural Selection:
– Adapts a population to its environment
– Accumulates and maintains FAVORABLE
genotypes in a population
– differential success in reproduction
-only form of microevolution that adapts a
population to its environment
**recognizing Friends in a crowd.
Natural selection
• Fitness: refers to the contribution an individual
makes to the gene pool of the next generation
3 types of Selection:
• A. Directional
• B. Diversifying
• C. Stabilizing
Microevolution
• Small genetic changes in a
population
• Change in frequency of a single
allele due to
selection
Three Types of Selection
Three modes of Selection
• Stabilizing Selection:
-well adapted to the environment
-observed in many plants
-selection eliminates extreme phenotypes
-intermediate form is favored
• Directional Selection:
-one phenotype extreme is favored
-bell shaped curve is shifted (genetic drift)
-Examples: Darwin’s Finches & Peppered moth
• Disruptive Selection:
-causes divergence; splitting apart of the extreme phenotypes
-extreme traits are favored
-intermediate traits become elimanated
Natural Selection in a Population
• Selects only favorable phenotypic traits
• Unfavorable alleles are eliminated
• Can maintain genetic diversity
-heterozygous advantage (sickle cell anemia) Pg. 399
-frequency-dependent selection: rarer phenotypes are
maintained, most common phenotypes eliminated and
decrease in number. (Observed in cichlids)
• Neutral Variations: offers no selective advantage or
disadvantage Examples ???
• Geographical variations and Clines (Clinal variation)
**Observed in the common yarrow wildflower in the
Sierra Nevada Mtns. (Pg. 401)
Population Variation
• Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
population
• Geographical
variation: differences in
genetic structure between
populations (cline)
Preserving Variations in a Population
Prevention of natural selection’s
reduction of variation
Diploidy
2nd set of chromosomes hides
variation in the heterozygote
Balanced Polymorphism
- heterozygote advantage (hybrid
vigor; i.e., malaria/sickle-cell
anemia);
- frequency dependent selection
(survival & reproduction of any 1
morph declines if it becomes too
common; i.e., parasite/host)
Sexual selection
• Sexual dimorphism:
secondary sex
characteristic distinction
• Sexual selection:
selection towards
secondary sex
characteristics that leads
to sexual dimorphism
Balanced Polymorphism
Two or more alleles persist in a population
over many generations.
Preserved by:
•
Heterozygote advantage
•
Frequency-dependent selection
Ch. 23 Microevolution (Video )
1. How are Mosquito populations affected by genetic and
environmental conditions
2. What affect did the insecticide have on the insects?
3. Which forms of isolation are presented in the last
segment?
4. What are some examples of species that are endemic to
Catalina Island? How is inbreeding prevented for these
species?
***Write the title for each segment and FIVE statements
for each segment.