16.1 Genes and Variations

The connection between heredity and evolution

Darwin did not know about Mendel’s findingsThis left 2 big gaps in his thinking:He didn’t know how traits were passed onHe didn’t understand why everyone and everything was differentEvolutionary biologists in 1930 made this connection

Natural selection and genes

Natural selection focuses on inheritable traitsTraits are determined by the inheritance of genes (dominant or recessive versions)People inherit different versions which lead to varietySome organisms inherit better versions of the trait than something else-why they surviveOrganisms 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 populationRelative Frequency-number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occurOut of 50 alleles, 20 are dominant and 30 are recessive. Evolution is any change in the relative frequency of alleles in a populationIf 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 pairsCaused by DNA replication errors or radiation/chemicalsNo effect, increase or decrease fitnessGene Shuffling-occurs during meiosisCrossing overCombining of different alleles during sexual reproductionLimitations: 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 phenotypesSingle gene trait-one gene controls a traitWidow’s peak/attached vs unattached earlobesPhenotypic ratios are determined by frequency of alleles and whether alleles are dominant or recessive

Polygenic traits

Traits controlled by more than one geneHeightBell shaped curve shows how many organisms have a certain phenotypeThe two extreme ends have low values and most organisms fall in the middle rangeNormal 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 geneNatural selection changes the allele frequency and evolution takes placePolygenic traits are affected in 3 waysDirectional, stabilizing, disruptive

Directional Selection

When individuals at one end of the curve have a higher fitness than the middleExample: finchesThicker beaks can feed more easily on harder thicker shelled seedsA food shortage may cause the supply of small and medium sized seeds to declineBirds 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 endHuman babiesSmaller babies are less likely to be healthyLarger babies have difficult being bornAverage 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 populationsIndividuals 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 EffectA situation in which allele frequencies change as a result of the migration of a small subgroup of a populationFruit flies on Hawaiian IslandsAll descended from the same mainland, but different habitats on different islands now have allele frequencies that are different from the original

Hardy-Weinberg principleExplains when no change takes place over timeAllele frequency in a population will remain constant unless one or more factors cause those frequencies to changeThe situation in which allele frequency stays the same is called genetic equilibrium5 conditionsRandom matingPopulation is largeNo mutations No natural selectionNo migration

Link between antibiotics and evolution

Antibiotics are used to kill bacteria.Many disease causing bacteria are evolving a resistance to antibioticsHow 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 resistanceCould this be a problem?

16.3 Ideas

Speciation-forming a new speciesReproductive isolation can make it happenCannot breed with their own kind and produce fertile offspring3 ways: behavioral, geographic, temporal

Behavioral isolationIndividuals are able to reproduce but have different reproductive strategiesCan be important since it prevents one species from mating with anotherCheetahs have a certain mating behavior that does not allow them to mate with other cats like lions and leopardsEastern 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 squirrelsAbert squirrel and Kaibab squirrel are very similar but have different fur colors

Temporal IsolationSpecies reproduce at different timesOrchid 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 blackbirdsAssumptions: Differences in beak size and shape produce different fitness that made natural selection take placeThere must be enough heritable variation

Tests for variation and findings

They caught individual birdsRecorded which lived and which diedRecorded 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 pickyWhen it is drier and food is scarce, they are pickierChanges in food supply can make it take place rapidlyDirectional selection

Turn to page 408

Hypothesis A suggests that Lake 1 and 2 are not relatedHypothesis B suggests they are relatedHypothesis A

Ways speciation occursFounding of a new populationFinches from South American mainland arrivedGeographic IsolationFlew to a different islandChanges in the gene poolReproductive isolationLike finches with same beak sizeEcological competition

Limitations

No formation of a new species

Why care about evolution?Understand things change and help us to respond to these changes