Microevolution Part 1
Evolution of Populations
Chapter 18
Some Vocabulary

Microevolution – Evolutionary change below
the species level


Example: change in a population’s allele
frequencies over generations
Macroevolution – Evolutionary change above
the species level including origin of a new
group of organisms or a shift in the broad
pattern of evolutionary change over a long
period of time

Examples: appearance of new organism features,
mass extinction and recovery of diversity
Populations Evolve

Natural selection acts on individuals

differential survival


differential reproductive success


“survival of the fittest”
who bears more offspring
Populations evolve
genetic makeup of population changes
over time
 favorable traits (greater fitness) become
more common

In Summary:

Individuals DON’T Evolve
Individuals survive or don’t
 Individuals reproduce or don’t

Individuals are SELECTED
 Populations EVOLVE!

Fitness

Survival & Reproductive
Success

individuals with one
phenotype leave more
surviving offspring
Body size & egg laying in water striders
Variation & Natural Selection

Variation is the raw material for natural
selection

there have to be differences within population

some individuals must be more fit than others
Nonconstancy of Species

Nonconstancy refers to
variation within a
species


This is part of Darwin’s
theory
There is variation within
a species


Example: dog and cat
breeds
All are capable of
interbreeding but they
look different.
Where Does Variation Come From?
Mutation

random changes to DNA


Wet year
Beak depth

Dry year
Dry year
errors in mitosis & meiosis
environmental damage
1977
Dry year
1980
1982
1984

Sexual Reproduction

mixing of alleles

recombination of alleles



new arrangements in every offspring
new combinations = new phenotypes
spreads variation

offspring inherit traits from parent
Beak depth of
offspring (mm)
11
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
5 Agents of Evolutionary Change
Mutation
Genetic Drift
Gene Flow
Non-random mating
Selection
Mutation

Change from one allele to another
Mutation changes the DNA
sequence
 It may alter the resulting amino
acid sequence which could effect
the corresponding protein/enzyme


Alters proportion of alleles in population


Generally low mutation rates in a population
Ultimate source of genetic variation making evolution
possible

Despite what the word “mutation” sounds like, it is not
necessarily a bad thing!
Chromosomal Mutations

Deletion

Inversion

Translocation
Chromosome Mutations

Nondisjunction
DNA Mutations

Point Mutations



Substitution
Addition
Deletion
DNA Mutations

Additions & Deletions may lead to a reading
Frame Shift
Gene Flow

Movement of individuals &
alleles in & out of populations
seed & pollen distribution by
wind & insect
 migration of animals

sub-populations may have
different allele frequencies
 causes genetic mixing
across regions
 reduce differences
between populations

Non-Random Mating

Example - Sexual selection
Genetic Drift

Effect of chance events

founder effect


small group splinters off & starts a new colony
bottleneck

some factor (disaster) reduces population to
small number & then population recovers &
expands again
Natural Selection
Differential survival &
reproduction due to changing
environmental conditions

climate change
 food source availability
 predators, parasites, diseases
 toxins


combinations of alleles
that provide “fitness”
increase in the population

adaptive evolutionary change
Selection
How Do Populations Change?
The 5 agents of evolutionary change
create opportunities for changes in allele
frequency in a population
 Population Genetics is the study of the
properties of genes within a population
 Using the Hardy Weinberg Rule a scientist
can set up a reference equilibrium point to
monitor genetic frequency changes in a
population.

Hardy Weinberg Equilibrium

Under the following conditions no evolution
should take place and allele frequencies should
remain constant in a population:





No Mutation (no modifications to the gene pool)
No Natural Selection (no differences in survival or
reproductive success)
Extremely Large Population (no genetic drift)
Random Mating (no one is more/less attractive)
Isolated Population/No Gene Flow (no Immigration
- entering or Emigration - exiting)
Hardy Weinberg Equilibrium
A mathematical equation exists to track
genotype and allele frequencies in a
population
 The Hardy Weinberg Equilibrium
Equation looks like this:


p2 + 2pq + q2 = 1
A
a
p2 + 2pq + q2 = 1

p2 = frequency of homozygous dominant
genotype


q2 = frequency of homozygous recessive
genotype


EX: 25% of population with “AA” genotype
EX: 25% of population with “aa” genotype
2pq = frequency of heterozygous
genotype

EX: 50% of population with “Aa” genotype
p+q=1
This is the second part of the Hardy
Weinberg equation
 p = frequency of dominant allele



EX: 50% have an “A” allele
q = frequency of recessive allele

EX: 50% have an “a” allele
Hardy-Weinberg Practice
Total # of students:
 # without Widow’s Peak:
 q2 =
q=
p=
 p2 =
 2pq =
