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Chapter 16.1 & 16.2 Class Notes: Population Genetics and Speciation
What causes evolution (brainstorm)?
 the development of new species from older ones
 the minor changes within a species from generation to generation over long periods of time that can
result in the gradual transition to new species.
the sum total of the genetically inherited changes in the individuals who are the members of a
population's gene pool.
It is clear that the effects of evolution are felt by individuals, but it is the population as a whole that
actually evolves
Population: a group of individuals of the same species that routinely interbreed.
 Population genetics: study of evolution from a genetic point of view.
 Genetic material of organisms consists of many alleles (variations) of many genes that code for various
 Smallest unit at which evolution occurs.
Draw a picture of the bell curve in the box below:
What does the bell curve illustrate?
(within a population individuals vary with certain observable traits (doesn’t necessarily mean that evolution
occurred), so the norm for equilibrium in these traits is the bell curve.
Illustrates that most members of a population have
Similar values for a given, measurable trait. Only a
Few individuals display extreme variations of the
Traits. In nature many quantitative traits in a
Population- such as height and weight- ten to show
Variation that follows a bell curve.
(fish example in text -317 – fish of a single species in
A pond may vary in size, few are short, few long,
Most are average.
What causes this slight variation in the observable traits?
 Environmental factors (ex. quality of food)
 Heredity
 Range of phenotypic possibilities (range of body sizes)
 Specific phenotypes (two possible flower colors)
What is the Hardy-Weinberg Genetic Equilibrium?
Scientists were wondering what happened to allele frequencies over generations???
German physician- Wilhem Weinberg and British mathematician, Godfrey Hardy (late 1800s-early
1900s) independently showed that (DEFINITION) genotype frequencies in a population tend to remain
the same from generation to generation unless acted on by outside influences
Also phenotype frequencies can change dramatically from gen. to generation.
TRUE equilibrium is a theoretical state.
In other words, if no mechanisms of evolution are acting on a population, evolution will not occur--the
gene pool frequencies will remain unchanged. However, since it is highly unlikely that any of these
seven conditions, let alone all of them, will happen in the real world, evolution is the inevitable result.
What are their five conditions for equilibrium? Give examples of what can happen if they are disrupted.
Stress to the students that Hardy-Weinberg Equilibrium is only achieved if:
1. No net mutations occur; alleles remain the same
 If there are mutations totally new alleles are produced in a population
 After being shuffled in various combinations with the rest of the gene pool, these provide the raw
material on which natural selection can act.
Mutations are defined as random changes in cellular DNA. They change the genetic code for amino acid
sequence in proteins, thus introducing biochemical errors of varying degrees of severity. Mutations have been
classified as deletions (loss of DNA bases), insertions (gain of DNA bases), and missense or nonsense
(substitution of a DNA base).
2. Individuals neither enter nor leave the population – no migration or isolation.
 migration enables gene flow: the movement of genes from one population into another.
 If the two populations originally had different gene frequencies and if selection is not operating,
migration (or, to be exact, gene flow) alone will rapidly cause the gene frequencies of the different
populations to converge.
3. Having a Large Population
Small populations- less people to mate with, not too much gene variation.
Inbreeding depression is usually taken to mean any immediate harmful effect, on individuals or the
population, of a decrease in either type of genetic variation.
One major example of Inbreeding was the royal European families in the middle ages. There was so
little genetic diversity between these royal airs that they contracted diseases like Hemophlia.
4. Individuals mate randomly
Most populations DO NOT mate randomly
Mating of related individuals can amplify certain traits and can result in offspring with disorders caused
by recessive genes.
If Individuals may select a mate that has similar traits to their own ->similar genes (assortative mating).
Nonrandom mating affects which alleles will be combined with individuals but does not affect overall
allele frequencies.
5. Selection does not occur for a particular allele
Natural selection is evident when the distribution of traits in a population change over time, shifting from the original
bell curve.
Sexual Selection
males have bright colors and plumages (ex. Frigate bird,
booby dance, albatross dance) these traits attract females
to the mate with them. Females may thing that the specific
male has better fitness/genes because of his bright colors
and choose to mate with them. MUST BE SUCCESSFUL
reproduces to be successful in natural selection.
Example: peacocks, bird plumage, bird calls, fireflies
blinking releasing pheromones.
Stabilizing Selection (Natural Selection)
Define: intermediate phenotypes thrive, both
extreme phenotypes die off.
individuals with the average form of a trait have the
highest fitness. The average represents the optimum for
most traits.
Example: lizards
larger lizards are easily spotted by predators.
smaller lizards cannot run fast enough to escape
Medium = just right
Disruptive Selection (Natural Selection)
Define: both extreme phenotypes thrive,
intermediate phenotypes die off
individuals with either extreme variation of a trait have
greater fitness than individuals with average form of trait.
(limpet color, either white or dark brown, not mottled –
keeps them safe from predators).
Example: limpets
White ones hide on rocks near barnacles
Dark ones hide on dark rocks, blending in.
Predators (birds) cannot see them on the dark rocks
or distinguish them from barnaclzes.
Directional Selection (Natural Selection)
Define: one extreme phenotype thrives.
individuals that display a more extreme form of a trait
have greater fitness than those with an average form of
the trait (ant eaters and tongue length- must be very, very
long to reach ants)
Example: if termites build deeper nests, ant eaters
need longer tongues.
Neck of giraffes: males “neck” to display strength for
females- if have longer neck, bigger head- stronger hit
to combat with other males