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Ch 23 - Evolution in Populations
Transcript of Ch 23 - Evolution in Populations
– determines a trait (ex. eye color)
– A variant of a gene (ex. brown eyes vs. blue eyes)
All sexually reproducing organisms have
for any trait.
– An allele that will show a trait, regardless of the other other allele (ex. brown eyes)
– An allele that will only show a trait if both alleles are recessive (ex. blue eyes)
- a localized group of interbreeding individuals
- the collection of alleles in the population
remember difference between alleles & genes!
= change in allele frequencies in a population
p + q = 1
p + 2pq + q = 1
assume 2 alleles = B, b
frequency of dominant allele (B) =
frequency of recessive allele (b) =
frequencies must add to 1 (100%), so:
p + q = 1
Individuals are selected
Differential survival, differential reproductive success
The genetic makeup of a population changes over time
Fitness increases: Favorable traits (greater fitness) become more common
Natural Selection acts on Variation
Variation is the raw material for natural selection.
There have to be differences within population. Some individuals must be more fit than others
Where does variation come from?
: random changes to DNA (Why?)
: mixing of genes ("
"). New arrangements in every offspring
Offspring inherit traits from parents
5 Sources of Evolution
Describes a Hypothetical, non-evolving population that
preserves allele frequencies.
Serves as a model for comparison (
Natural populations are never in H-W equilibrium.
Useful model to measure how forces are acting on a population.
Mutation creates variation
new mutations are constantly appearing
Mutation changes DNA sequence, changes amino acid sequence, changes protein structure & function, changes traits, changes fitness (maybe)
Movement of individuals & alleles in & out of populations.
seed & pollen distribution by wind & insects
migration of animals
reduces differences between populations
Gene flow in human populations is increasing today thanks to modern travel technology
Effect of chance events; founder effect, bottlenecks
Loss of alleles from gene pool: reduces variation, reduces adaptability
Differential survival & reproduction due to changing environmental conditions
Combinations of alleles that provide “fitness” increase in the population
Adaptive evolutionary change
An Application of H-W principle: Sickle cell anemia
Due to a mutation in a gene coding for
(oxygen-carrying blood protein)
recessive allele =
- makes a defective protein
dominant allele =
- makes a normal protein
A recessive disease: individuals must be
to have sickle cell anemia
low oxygen levels causes RBC to sickle
breakdown of RBC
clogging small blood vessels
damage to organs
often lethal in childhood
Sickle cell frequency:
High frequency of heterozygotes ("
sickle cell trait
") in African population
1 in 5 in Central Africans =
Unusual for allele with severe detrimental effects in homozygotes
1 in 100 = Hs/Hs
usually die before reproductive age
In tropical Africa, malaria is common.
): confers resistance to malaria.
Homozygous dominant (
): die/reduced reproduction from malaria.
Homozygous recessive (
): die/reduced reproduction from sickle cell anemia.
Heterozygote carriers survive & reproduce: Hs allele becomes common in population
p + q = 1
p + 2pq + q = 1
frequency of homozygous dominant: p x p =
frequency of homozygous recessive: q x q =
frequency of heterozygotes: (p x q) + (q x p) =
frequencies of all individuals must add to 1 (100%), so:
p + 2pq + q = 1
Make Sure You Can
Solving HW Problems
1. Write both equations.
2. Identify any given information.
3. Don't screw up squaring frequencies!
Remember that the square root of a decimal is a LARGER number than that decimal.
ex: What is the square root of 81? What is the square root of 0.81?
4. First: figure out q!
5. Work your way around the problem until all terms are solved for.
6. Practice makes perfect!
A. If 64% of the individuals in a population exhibit the recessive appearance, what % of the gene pool is dominant (assume HW equilibrium)?
B. A population contains individuals, 16% of whom show the recessive trait What % of the population is pure dominant? What % of the gene pool is recessive? What percent of the population is hybrid (assume HW equilibrium)?
C. How can you identify if a population is in HW equilibrium? Refer to the HW equations in your answer.
aka "Enter Math"
How is variation generated and maintained in a population?
How do we know evolution is happening in populations?
What aspects of a population contribute to evolution?
How can evolution be qualitatively and quanititatively measured?
How does measuring evolution help us understand how populations are evolving?
Explain how variation is produced and maintained in a population.
Define all new terms used in this presentation in your own words and give descriptive examples.
Explain how each source of evolution in a population affects variation and selection.
Use the HW theorem with facility (be able to move through all terms of both equations).
Apply the HW theorem to actual populations.
In the absence of all other factors the segregation and recombination of alleles during meiosis and fertilization will not change the genetic make-up of a population
1. No Mutations
2. Random Mating
3. No Gene Flow
4. No Natural Selection
5. No Genetic Drift = large populations
The following 5 conditions must be met
All Alleles in the population = the Gene Pool
What it is:
Change in the frequency of traits in a population due to gender preferences
change. May seem
Acts in all sexually reproducing populations.
"The traits that get you mates!"
You don't REALLY need to survive after mating...
Why does this happen?
Sexually selected traits serve as fitness markers for mating.
You know what they say about crabs with big claws...
What it is:
Change in the frequency of traits in a population due to chance events.
A new population is started by a small group of individuals.
Somes rare traits may be at high frequency (or "
" at 100%); others may be missing (or "
Common in island populations (both physical and reproductive--Why?)
Polydactly, one a sign of Ellis-van Crevald syndrome. Common among the Amish in Pennsylvania
Diagram of the Founder Effect
When a large population is drastically reduced by a non-selective disaster: famine, natural disaster, loss of habitat.
Loss of variation by chance event narrows the gene pool
Cheetah's have been through 2 bottleneks in recent history:
Recent ice age (~10 kya)
Living cheetahs are as genetically similar to each other as identical twins.
What problems does this cause?
Data showing the effect of habitat destruction on the Illinois population of the Greater prarie chicken.
Diagram of a "Bottleneck"
Effects of Evolution
How does evolution work?
Traits must be inherited!
...at least to some degree
Modes of selection:
How selection affects a population.
Evolution is a population level phenomenon.
It emerges from the selection of individuals by the environment.
Against the mean, towards both extremes
Towards the mean, against extremes
Towards one extreme
Traits can be physiological OR behavioral
Reproductive success is ALL that matters.
The different morphologies of Nemoria arizonaria catterpillars is due entirely to chemicals in their diet, NOT genetic differences
Oak leaf diet
Oak Flower diet
Mantis mating behavior results in the death of the male during copulation