Random Change
In terms of genetics, it is any change in allele frequencies within a
population.
 The H-W, provided conditions that evolution would not occur, thus the
following are key for evolution to occur:
I.
When a population is small, chance fluctuations can cause changes in
allele frequencies
II.
When mating is nonrandom, individuals preferred as mates will pass
on their alleles in greater numbers then those who are not preferred
III.
When genetic mutations occur, new alleles can be created or old ones
changed into new ones, effectively changing allele frequencies in both
new and original alleles
IV.
When individuals migrate they remove their alleles from on population
and add them to the other
V.
When natural selection occurs, individuals with certain alleles have
better reproductive success than others, thus increasing the frequency
of their alleles in the next generation

Example: 2 % of cricket frogs carry a certain allele, C. If the
population was large, say 10, 000, you would expect 200 frogs
to carry the allele. If severe weather conditions caused 50% of
them to die, then you would expect 100 of 500 surviving frogs
to carry the allele. But in this case the species is endangered
and there are only 100 frogs. In this case only 2 carry the C
allele. If 50% of the frogs died then there would be a good
chance that both of those frogs would die (eliminating the C
allele forever) or both could survive doubling the frequency of
the C allele (2/50 is 4%).
 This may be an extreme example (much more pronounced in
small populations) but it demonstrates genetic drift, which is a
change in the genetic makeup of a population due to chance

Fixation of alleles results in completely homozygous individuals, reducing
genetic diversity
Occurs when a severe event drastically reduces the
number of individuals in a population, resulting in
significant genetic drift

Results when a few individuals from a large
population leave to establish a new population
(genetic drift)
 Allele frequencies will be different from parent
population
 This is common in nature, for example-seeds carried
away by birds or wind==in self pollinating species an
entire population can establish from a single seed

Members of the Amish population living in
Pennsylvania are descendents of about 30 people
from Switzerland who emigrated in 1720
 One member had a rare recessive allele causing
short limbs
 Today the frequency of that allele is 7% vs. 0.1% in
most other Amish populations

In 1982, two scientists were working on Daphne
Major of the Galapagos
 They observed a population of finches that would
visit the island every year
 One year they witnessed 3 males and 2 females
remaining on the island to breed
 They produced 17 young birds which became the
founders of the new population on the island
 They have remained ever since and upon further
investigation this population is now genetically
different from the original population

The movement of alleles
from one population to
another==migration
 Example: prairie dogs live in
large populations that do not allow new members in
 However in late summer, male pups are permitted into
adjacent populations, affecting the gene pool
 Genetic information can also be shared if the individuals
don’t move permanently, instead only breeding and
leaving
 This is different from genetic drift, as it tends to reduce
genetic differences between populations

Mutations are the only new source of genetic
material and alleles
 Only concerned with mutations in a gamete since
these can be passed on and enter the gene pool
 What effects can mutations have?
 How frequently do they occur?

They can be neutral, harmful or beneficial
Because they are random changes to the genetic
code they are more likely to be neutral or harmful
 Neutral mutation: one that has no immediate effect
on an individual’s fitness, usually silent in the noncoding portion of the DNA
 Note: fitness refers to the reproductive success of
an individual


Harmful mutation: reduces fitness, occurring when
a cell loses the ability to produce proteins or when
chromosomal changes adversely affect meiosis and/or
mitosis
 Beneficial mutation: occurs when a cell gains the
ability to produce a new or improved protein,
increasing fitness

Different types of mutations vary in their ability to affect
phenotypes and thus evolution
 Point mutations: a single change in a DNA base pair
 They are neutral when occurring in the non-coding
regions
 If it occurs in the coding portion, it could be lethal or
may have no significant effect. Rarely will it will result in a
beneficial mutation
 Insertions/deletions: these almost always produce nonfunctioning genes when in coding areas==harmful
 Do not play an important role in evolution since they are
usually never beneficial

Gene duplication: leads to the production of an
extra copy of a gene locus, usually the result of
unequal crossing over during meiosis
 Important because it is the source of new genes
 Initially just a redundancy in the genome, but over
time it has the ability to mutate and maybe gain a new
function
 This can result in gene “families” having similar
structures, located close together, but have altered
functions==histones

1/10, 000 in small genomes, such as in bacteria
1/gamete in species with large genomes
However usually result in unobservable
traits==death of the gamete before birth



Pgs 550-554
Have a Good WEEKEND!