genotype: A A genotype: A B genotype: B B

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2 Beak Length among the Finches is a simple (Mendelian) trait determined by two alleles, Aand B. Homozygotes for the B allele have short beaks, homozygotes for the Aallele have long beaks, and heterozygotes (AB) have intermediate beaks. genotype: A A 0 genotype: A B Calories per Fruit genotype: B B short beak medium beak long beak Fruit Ripeness 0 Beak Length affects the amount of calories an individual gets from each fruit. Unripe green fruits are hardest, and short-beaked finches get more energy out of them. Longbeaked finches get more energy from red, ripe fruits which are very chewy. Medium-length beaked birds get the most energy from yellow, medium ripe fruits. The averagenumber of calories for each fruit are shown above. The graph at left shows the energy gained for each beak length as ripeness increases.

3 Each Finch eats 8fruits. The number of calories gained from the fruits depends on their ripeness and the bird s beak length. + = 4670cal. Finches only reproduce once. The energy gained determines the number of offspring produced. Each nesting pair of Finches combines their energy to reproduce (males and females contribute equally). For ever 4000 calories gained, the pair produces one offspring. nesting pair 3535 calories 4892 calories total energy: 8427calories = 2 offspring

4 You might want to Minimize the Ribbon to improve viewing. When you open Darwin s Finches, click Options and choose Enable this content The middle of the screen shows each individual s genotype and phenotype, the fruits each individual ate, the energy each individual gained, and the offspring produced by each nesting pair. Here, the first nesting pair consisted of a BB and an BA individual. The pair gained enough energy to produce one offspring. Fruit Eaten Energy Gain Offspring 1904 cal cal. 1

5 Starting Conditions set the population size and allele frequencies for Generation 0. Rainfall determines average fruit ripeness more rain means riper fruit. Energy/Offspring determines the calories needed by a nesting pair to produce one offspring. These numbers and graphs tell you the population size, allele frequencies, and genotype frequencies of the current generation of adult finches. Thebuttonsbelow movethesimulation ahead intime, orreset it to the starting or default conditions. 1 GENERATION moves the game 1 generation forward in time; 10 GENERATIONS moves it 10 generations forward. STABLE POPULATION moves the game 1 generation forward in time while maintaining a constant population size. RESET resets the game to Generation 0, using the Starting Conditions (Population Size and Allele Frequencies) that the user enters under Starting Conditions. DEFAULT resets the Starting Conditions and Variables to their default values Population size and allele frequencies of the offspring generation are shown here. The alleles of the offspring are determined by those of their parents following Mendels Rule of Segregation: each offspring has a 50% chance of inheriting a given allele from each parent.

6 When you click the Generationbutton, time moves ahead one generation: the offspring become the adults*. *The total number of offspring produced becomes the population size of adults in the subsequent generation, and the allele frequencies in the offspring population determine the allele frequencies in the subsequent adult population. For example, offspring population size and allele frequencies in Generation 0 determine population size and allele frequencies in Generation 1. Allele frequencies in the offspring population are determined by the reproductive success of each adult: if a BB individual has 4 offspring,itcontributes4b stotheoffspringgenepool;ifanabindividualhas3offspring,itcontributes1.5a sand1.5b s,andsoon. Allele frequencies in the adult population, shown in blue and yellow, determine the probability that an adult will have an A or B allele followingthehardyweinbergmodel.ifthefrequencyofallelebis30%,theneachindividualhasa30%chanceofhavingaballeleona given chromosome. They have a 9% chance of having a B allele on both chromosomes (30% x 30% = 9%). Conversely, since the frequencyofainthisexamplemustbe70%,theyhavea49%chanceofbeingaa(70%x70%=49%).theyhavea42%chanceofbeing oneaandoneb[2(70%x30%)=42%]. Note that allele frequencies determine the probability of inheriting a given allele, but the process is a random event like flipping a coin. Whenweflipacoinseveraltimes,weexpecttheoutcometobe50%headsand50%tails,butweknowtheobservedoutcomemightbe somewhat different. Similarly, while we would expect 9% of the population in the example above to be BB, the observed frequency of that genotype, shown in the lower blue/green/yellow chart, might be somewhat different.

7 With the default amount of Rainfall (1000 mm/yr) there will be an even amount of unripe, medium-ripe, and ripe fruit; all beak lengths will have an equal chance of foraging success. With no selection pressure on beak length, allele frequencies will be subject to genetic drift, but there will be no selection for longer or shorter beaks. The main graph records the allele frequencies and population size for each generation. Changing the amount of Rainfall affects fruit ripeness. With increased rainfall, more of the fruit are ripe(red) and Long-beaked finches are more able to find enough food. With decreased rainfall, most fruit is unripe (green) and Short-beaked finches have the advantage. These selective advantages are reflected in allele frequency changes. Rainfall: mm/yr Rainfall: 1200 mm/yr Rainfall: 800 mm/yr Rainfall can be changed at any time. But be careful! Small changes in rainfall have big effects.

8 Energy/Offspring determines the amount of calories needed to produce one offspring. At the default setting of 3,950 calories and Rainfall at 1000 mm/yr nesting pairs will generally obtain enough food for two offspring. Population size will generally remain stable. The % of adults that were able to establish a nest is shown here. The island can only hold 1,500 nests. Like Rainfall, Energy/Offspring can be changed at any time. Increasing Energy/Offspring makes it more difficult for nesting pairs to reproduce, and the population will likely decline. Conversely, reducing Energy/Offspring will make it easier for each pair to reproduce and lead to population growth. The island only has enough space for 1,500 nests once the population exceeds 3,000 (1,500 nests), some adults won t be able to establish a nest. Available nesting space therefore sets a carrying capacity for the Island: it can only hold the offspring of 1,500 nesting pairs. carrying capacity Decreasing the Energy/Offspring will also make the population more resilient to changes in Rainfall Rainfall: mm/yr rapid population growth Energy/Offspring: 3000 calories Energy/Offspring: 3000 calories