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