Post on 25-Dec-2015
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