Population Evolution - Marblehead High School

Evolution of Populations
AP Biology 2014 - 2015
Campbell Biology in Focus: Chapter 21
I. Microevolution: change in the allele frequencies of a population over generations
A. Darwin did not know how organisms passed traits to offspring
B. 1866 - Mendel published his paper on genetics
C. Mendelian genetics supports Darwin’s theory → Evolution is based on genetic variation
II. Source of variation
A. Point mutations: changes in one base (eg. sickle cell)
B. Chromosomal mutations: delete, duplicate, disrupt, rearrange → usually harmful
C. Sexual recombination: contributes to most of genetic variation in a population
i. Crossing Over (Meiosis – Prophase I)
ii. Independent Assortment of Chromosomes (during meiosis)
iii. Random Fertilization (sperm + egg)
III. Population genetics: study of how populations change genetically over time
A. Population: group of individuals that live in the same area and interbreed,
producing fertile offspring
B. Gene pool: all of the alleles for all genes in all the members of the population
C. Diploid species: 2 alleles for a gene (homozygous/heterozygous)
D. Fixed allele: all members of a population only have 1 allele for a particular trait
The more fixed alleles a population has, the LOWER the species’ diversity
IV. Hardy-Weinberg Theorem
The allele and genotype frequencies of a population will remain constant from generation to
generation…UNLESS they are acted upon by forces other than Mendelian segregation and
recombination of alleles
Equilibrium = allele and genotype frequencies remain constant
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A. Conditions for Hardy-Weinberg Equilibrium
1. No mutations.
2. Random mating.
3. No natural selection.
4. Extremely large population size.
5. No gene flow.
If at least one of these conditions is NOT met, then the population is EVOLVING!
B. Allele Frequencies:
Gene with 2 alleles : p, q
p = frequency of dominant allele (A)
p+q=1
q = frequency of recessive allele (a)
C. Genotypic frequencies:
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V. Strategies for solving H-W problems
A. If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of
A and a alleles.
B. If you know phenotypes, then use “aa” to find q2, and then q. (p = 1 - q)
C. If a population IS evolving, then the allele frequencies, and therefore the frequencies of
all genotypes, will change over time. If there is no change in the allele and/or genotype
frequencies, then the population is NOT evolving.
VI. Practice Problem
The scarlet tiger moth has the following genotypes. Calculate the allele and genotype
frequencies (%) for a population of 1612 moths.
AA = 1469
Aa = 138
aa = 5
Allele Frequencies:
A=
a=
Genotypic Frequencies:
AA =
Aa =
aa =
VII. Causes of Evolution - see the 5 conditions for Hardy-Weinberg
A. Minor Causes of Evolution:
#1 - Mutations: rare, very small changes in allele frequencies
#2 - Nonrandom mating: affect genotypes, but not allele frequencies
B. Major Causes of Evolution: natural selection, genetic drift, gene flow (#3-5)
C. Natural Selection
Individuals with variations better suited to environment pass more
alleles to next generation
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D. Genetic Drift: small populations have greater chance of fluctuations in allele frequencies
from one generation to another
i. Founder effect: a few individuals isolated from larger population
Certain alleles under/over represented
ii. Bottleneck effect: sudden change in environment drastically reduces
population size
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E. Gene Flow
i. Movement of fertile individuals between populations
ii. Gain/lose alleles
iii. Reduce genetic differences between populations
VIII. Adaptive Evolution and Natural Selection
A. Fitness: contribution that an individual makes to the gene pool of the next generation
B. Natural selection can alter frequency distribution of heritable traits in 3 ways:
1. Directional selection
2. Disruptive (diversifying) selection
3. Stabilizing selection
Directional Selection:
eg. larger black bears
survive extreme cold
better than small ones
Disruptive Selection:
Stabilizing Selection:
eg. small beaks for small eg. narrow range of
seeds; large beaks for
human birth weight
large seeds
C. Sexual Selection
i. Form of natural selection – certain individuals
more likely to obtain mates
ii. Sexual dimorphism: difference between
2 sexes (size, color, ornamentation, behavior)
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iii. Intrasexual – selection within same sex
(eg. M compete with other M)
iv. Intersexual – mate choice
(eg. F choose showy M)
D. Preserving Genetic Variation
i. Diploidy: hide recessive alleles that are less favorable
ii. Heterozygote advantage: greater fitness than homozygotes
eg. Sickle cell disease
E. Natural Selection cannot fashion perfect organisms
1. Selection can act only on existing variations.
2. Evolution is limited by historical constraints.
3. Adaptations are often compromises.
4. Chance, natural selection, and the environment interact.
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SAMPLE PROBLEMS
Define the following examples as directional, disruptive, or stabilizing selection:
1. Tiger cubs usually weigh 2-3 lbs. at birth
2. Butterflies in 2 different colors each represent a species distasteful to birds
3. Brightly colored birds mate more frequently than drab birds of same species
4. Fossil evidence of horse size increasing over time
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