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Microevolution – BioH Ch 16
Where did all organisms come from?
Why such variety?
Early Beliefs
• Supernatural intervention (BC & early AD)
– “Gods”
• Naturalism (1300’s)
– Chain of Being
– “Small” known world, small number of species
– World exploration led to
many more species
More Early Beliefs
• Biogeography (1500’s)
– Organization of
organisms based on
geographic distribution
• Comparative morphology (1700’s)
– Perfection
•Evolution theory
Modification over time
“New Theories”
• Growing evidence
– Fossil record abrupt changes
– Discontinuity between sedimentary layers
• “Catastrophism”
• Charles Darwin
– Influenced by geology
– Gradual, uniform
Darwin’s Theory of Evolution by
Natural Selection
 Individual organisms differ; some differences are heritable
 Organisms produce more offspring than can survive; many that do
survive do not reproduce
 Because more organisms are produced than can survive, they
compete for limited resources
 Each unique organism has advantages & disadvantages in the
struggle for existence. “Survival of the fittest”. These organisms pass
on those advantageous traits to their offspring. Those that do not
have this advantage either die out, or leave fewer offspring.
 Species alive today descended with modification from ancestral
species that lived in the distant past. This process, by which all
diverse species evolve from common ancestors, unites all
organisms on earth into a single “tree of life”.
Key Concepts
• Populations evolve, not individuals
– A group of individuals of same species in
same area
– Two or more different forms of traits
• Gene pool
– All genes in population
– Different gene forms = alleles
– Allele frequency
Which alleles end up in which gamete
and eventually in which new individual?
Depends upon five factors:
 Gene mutation (producing new alleles)
 Meiosis crossing over (producing new combinations of alleles)
 Meiosis independent assortment (producing random mix of
paternal & maternal genes)
 Fertilization (producing random combination of parental alleles)
 Chromosomal mutations (producing changes in chromosome
number or chromosome gene positions)
Stable allele frequencies = “genetic equilibrium”
NOT exhibiting evolution
All five factors do not usually happen at the same time
Population Changes and Microevolution
Changes in combinations of alleles lead to variations in a
populations’ phenotypes. (This could be structural, functional and/or
behavioral changes).
MICROEVOLUTION refers to small-scale changes in allele
frequencies as caused by mutations, gene flow & genetic drift. These
all shuffle alleles into, through or out of populations.
Allele mutations can be lethal, neutral or beneficial.
Gene flow involves the natural movement of individuals into, through
or out of a population – moving their specific alleles with them.
Genetic drift is the natural, random change in allele frequencies over
time caused by chance alone
Natural Selection and Population
Change for Polygenic traits
Natural Selection = an outcome (result) of the differences in survival and
reproduction among individuals that show variation in heritable traits.
With time, natural selection can lead to increased fitness (better
adaptation to the environment).
Directional Selection
Allele frequencies change in one
direction in response to environmental
changes or changes caused by
16.6 16.7
Peppered moths and the Industrial Revolution
Pre-Industrial Revolution – light moths common on light tree trunks and
darks moths were rare
Post-Industrial revolution – moth population depended upon how close
the trees were to an industrial site (and soot)
Stabilizing and Disruptive Selection
Stabilizing selection – forms of a trait
are favored by environment conditions.
The trait becomes prominent
Disruptive selection – the
intermediate form of the trait is
NOT favored. The extreme forms
of the trait become prominent
Selection types
• A bright red feathered bird is just as
successful at mating than a black
feathered bird, but any colors in
between have a hard time finding
mates. = disruptive selection
• Birds with medium sized beaks are
most common in their population while
birds with small or large beaks are a
rarity. The birds with medium sized
beaks are able to get food most
efficiently making them able to survive.
= stabilizing selection
• There used to be only small seeds
available for the finches to eat, but
when only large seeds were available,
their beak size changed from small to
the large. = directional selection
Other types of Selection
 Sexual dimorphism – distinct
male and female phenotypes
resulting in selective breeding
 Balanced polymorphism – where nonidentical alleles for a trait remain
somewhat constant
Gene Flow
 Over time, individuals within the same species move about, so
that alleles are mixed from population area to population area
 Immigration
 Emigration
 Physical flow of alleles into, through
and out of a population = gene flow
 Helps to keep the separated
populations genetically similar
Genetic Drift
 Random change in allele frequencies due to chance alone
 Significant effect in small populations
 Insignificant effect in very large populations
In small populations, genetic drift may lead to a homozygous
condition with the loss of genetic diversity
Genetic Drift Conditions
 Bottleneck condition – severe reduction in
population size caused by intense selection
pressure (contagious disease, hunting,
natural disaster)
 Results in altered allele frequencies
 Founder effect – a few individuals
leave a population for a new location
and start a new population
 Results in altered allele
 Inbreeding – non-random
breeding causing homozygous