16.1 Genes and Variations
The connection between heredity
and evolution
• Darwin did not know about Mendel’s
findings
• This left 2 big gaps in his thinking:
– He didn’t know how traits were passed on
– He didn’t understand why everyone and
everything was different
• Evolutionary biologists in 1930 made this
connection
Natural selection and genes
• Natural selection focuses on inheritable traits
• Traits are determined by the inheritance of genes
(dominant or recessive versions)
– People inherit different versions which lead to variety
• Some organisms inherit better versions of the trait
than something else-why they survive
• Organisms are typically Bb for traits
Gene Pool and Relative
Frequency
• Gene pool-made up of all the genes, including all
the different alleles, that are in a population
• Relative Frequency-number of times an allele
occurs in a gene pool compared with the number
of times other alleles for the same gene occur
– Out of 50 alleles, 20 are dominant and 30 are recessive.
• Evolution is any change in the relative frequency
of alleles in a population
– If the relative freq. of the B in the mouse population
changed over time to 30%, the population is evolving.
Why are organisms genetically
different?
• Mutations-change in the DNA base pairs
– Caused by DNA replication errors or
radiation/chemicals
– No effect, increase or decrease fitness
• Gene Shuffling-occurs during meiosis
– Crossing over
• Combining of different alleles during sexual
reproduction
– Limitations: does not change the relative frequency of
alleles in a population
Single Gene and Polygenic traits
• The number of genes that control a trait
determine the # of phenotypes
• Single gene trait-one gene controls a trait
– Widow’s peak/attached vs unattached earlobes
• Phenotypic ratios are determined by
frequency of alleles and whether alleles are
dominant or recessive
Polygenic traits
• Traits controlled by more than one gene
– Height
• Bell shaped curve shows how many
organisms have a certain phenotype
– The two extreme ends have low values and
most organisms fall in the middle range
– Normal distribution
16.2 Evolution as Genetic
Change
Genetics of Evolution-How does
evolution work on polygenic and
single gene inheritance
• Single gene trait-controlled by one gene
– Natural selection changes the allele frequency
and evolution takes place
• Polygenic traits are affected in 3 ways
– Directional, stabilizing, disruptive
Directional Selection
• When individuals at one end of the curve
have a higher fitness than the middle
• Example: finches
– Thicker beaks can feed more easily on harder
thicker shelled seeds
– A food shortage may cause the supply of small
and medium sized seeds to decline
– Birds that have larger beaks will survive
because they have higher fitness
Stabilizing selection
• When individuals near the center have
higher fitness than individuals at either end
• Human babies
– Smaller babies are less likely to be healthy
– Larger babies have difficult being born
– Average babies have the best chance
Disruptive Selection
• When individuals at
the upper and lower
ends have the highest
fitness
• Birds with big and
small beaks are more
fit
Genetic Drift
• Random change in allele frequency (number
of times you see a certain letter for a gene)
that occur in small populations
• Individuals carry a particular allele may
leave more descendants than other
individuals, just by chance.
• Over time, chance can cause an allele to
become common in a population
Founder Effect
• A situation in which allele
frequencies change as a
result of the migration of a
small subgroup of a
population
• Fruit flies on Hawaiian
Islands
– All descended from the
same mainland, but
different habitats on
different islands now
have allele frequencies
that are different from
the original
Hardy-Weinberg principle
• Explains when no change takes place over time
• Allele frequency in a population will remain
constant unless one or more factors cause those
frequencies to change
– The situation in which allele frequency stays the same
is called genetic equilibrium
• 5 conditions
–
–
–
–
–
Random mating
Population is large
No mutations
No natural selection
No migration
Link between antibiotics and
evolution
• Antibiotics are used to kill bacteria.
• Many disease causing bacteria are evolving a
resistance to antibiotics
• How did this happen?
– One or two bacteria have a genetic mutation which
allows it to be unaffected by bacteria; reproduction
happens and eventually all bacteria have this resistance
• Could this be a problem?
16.3 Ideas
• Speciation-forming a new species
• Reproductive isolation can make it happen
– Cannot breed with their own kind and produce
fertile offspring
– 3 ways: behavioral, geographic, temporal
Behavioral isolation
• Individuals are able to reproduce but have
different reproductive strategies
• Can be important since it prevents one
species from mating with another
• Cheetahs have a certain mating behavior
that does not allow them to mate with other
cats like lions and leopards
• Eastern and western meadowlark have
different calling songs even though they are
in the same area
Geographic Isolation
• Barriers separate mating
• The Colorado River split and separated two
types of squirrels
– Abert squirrel and Kaibab squirrel are very
similar but have different fur colors
Temporal Isolation
• Species reproduce at different times
• Orchid species in the rainforest
• Rana aurora - breeds January - March
Rana boylii - breeds late March - May
Unique about Darwin’s birds
• They were all finches; he thought they were
robins, warbler, and blackbirds
• Assumptions:
– Differences in beak size and shape produce
different fitness that made natural selection take
place
– There must be enough heritable variation
Tests for variation and findings
• They caught individual birds
• Recorded which lived and which died
• Recorded anatomical characteristics (bell
shaped curve)
• Found there was tons of diversity amongst
inheritable traits
How and when do finches
specialize
• During rainy season, food is plentiful so
they are NOT picky
• When it is drier and food is scarce, they are
pickier
• Changes in food supply can make it take
place rapidly
– Directional selection
Turn to page 408
• Hypothesis A suggests that Lake 1 and 2 are
not related
• Hypothesis B suggests they are related
• Hypothesis A
Ways speciation occurs
• Founding of a new population
– Finches from South American mainland arrived
• Geographic Isolation
– Flew to a different island
• Changes in the gene pool
• Reproductive isolation
– Like finches with same beak size
• Ecological competition
Limitations
• No formation of a new species
Why care about evolution?
Understand things change and help us to
respond to these changes