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