Experimental transplant of guppies. Predator: Pike-cichlid; preys mainly on large guppies

Transcription

1 Hypothesis: That killfish predation will lead to changes in size among the prey population. EXPERIMENT Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools and measured the average age and size of guppies at maturity over an 11-year period (30 to 60 generations). Pools with killifish, but not guppies prior to transplant Predator: Killifish; preys mainly on small guppies Experimental transplant of guppies Guppies: Larger at sexual maturity than those in pike-cichlid pools Predator: Pike-cichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in killifish pools Experimental variable Organism being investigated Dependent variable Killfish Guppy Size

2 Control Group Pike-cichlid pool Experimental Group Killfish pool EXPERIMENT Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools and measured the average age and size of guppies at maturity over an 11-year period (30 to 60 generations). Pools with killifish, but not guppies prior to transplant Predator: Killifish; preys mainly on small guppies Experimental transplant of guppies Guppies: Larger at sexual maturity than those in pike-cichlid pools Predator: Pike-cichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in killifish pools What is the selective agent? Predation behaviours of pike-cichlid or killfish.

3 Control what variables for method? Use identical pools Use identical water quality Identical pool temperatures Begin experiment with populations of similar phenotype. Begin experiment with identical numbers in original population Control what variables for reliable data gathering? Establish a standard measuring practice Use a fair & reasonable sample size Sample across multiple generations Sample organisms of the same age Sample at the same time of year or moment in the breeding cycle. Random sampling Sample from more than one pool location Sample both populations from similar pool locations Calculate averages for data

4 Hypothesis: That adaptive divergence will be supported by preferential mating in Drosophila EXPERIMENT Diane Dodd, of Yale University, divided a fruit-fly population, raising some populations on a starch medium and others on a maltose medium. After many generations, natural selection resulted in divergent evolution: Populations raised on starch digested starch more efficiently, while those raised on maltose digested maltose more efficiently. Dodd then put flies from the same or different populations in mating cages and measured mating frequencies. Initial population of fruit flies (Drosphila Pseudoobscura) Some flies raised on starch medium Mating experiments after several generations Some flies raised on maltose medium Experimental variable Supplied medium Organism being investigated Drosophila Dependent variable Mating preference

5 Weight of guppies at maturity (mg) Age of guppies at maturity (days) RESULTS After 11 years, the average size and age at maturity of guppies in the transplanted populations increased compared to those of guppies in control populations Males Females Males Females Control Population: Guppies from pools with pike-cichlids as predators Experimental Population: Guppies transplanted to pools with killifish as predators CONCLUSION Reznick and Endler concluded that the change in predator resulted in different variations in the population (larger size and faster maturation) being favored. Over a relatively short time, this altered selection pressure resulted in an observable evolutionary change in the experimental population. What selective advantage was conferred by the phenotypes displayed in each pool?

6 Control Group mating identically adapted Drosophila from different populations Experimental Group mating differently adapted Drosophila from different populations RESULTS When flies from starch populations were mixed with flies from maltose populations, the flies tended to mate with like partners. In the control group, flies taken from different populations that were adapted to the same medium were about as likely to mate with each other as with flies from their own populations. Male Maltose Starch Female Starch Maltose Male Same population Different populations Female Same population Different populations Mating frequencies in experimental group Mating frequencies in control group CONCLUSION The strong preference of starch flies and maltose flies to mate with like-adapted flies, even if they were from different populations, indicates that a reproductive barrier is forming between the divergent populations of flies. The barrier is not absolute (some mating between starch flies and maltose flies did occur) but appears to be under way after several generations of divergence resulting from the separation of these allopatric populations into different environments. What is the selective agent? Nutrient medium provided

7 Control what variables for method? Use identical breeding vessels Use concentrations of nutrients with identical energy contents Ensure an identical variety of nutrient is available (if other than monosaccharide-based) Begin experiment with populations of similar phenotype. Begin experiment with identical numbers in original population Identical light, temperature, ventilation conditions Control what variables for reliable data gathering? Establish a standard counting practice for successful matings Use a fair & reasonable sample size Sample across multiple generations Sample organisms of the same age Sample at the same time of year or moment in the breeding cycle. Random sampling

8 RESULTS When flies from starch populations were mixed with flies from maltose populations, the flies tended to mate with like partners. In the control group, flies taken from different populations that were adapted to the same medium were about as likely to mate with each other as with flies from their own populations. Male Maltose Starch Female Starch Maltose Male Same population Different populations Female Same population Different populations Mating frequencies in experimental group Mating frequencies in control group CONCLUSION The strong preference of starch flies and maltose flies to mate with like-adapted flies, even if they were from different populations, indicates that a reproductive barrier is forming between the divergent populations of flies. The barrier is not absolute (some mating between starch flies and maltose flies did occur) but appears to be under way after several generations of divergence resulting from the separation of these allopatric populations into different environments. What selective advantage was conferred by the phenotypes displayed in each group?

9 SAC Exam style questions Focus is on Area of Study 2 Read case studies very carefully Read the questions very carefully Provide clear explanations using the appropriate terminology Pay attention to experimental design where it is required Do complete the activities and readings listed in the preparation handout Be clear in your understanding of natural selection and be prepared to explain how it works. Remember that mutations contribute to evolution- know how they may affect inidividuals Remember that allele frequencies in populations will require an understanding of how alleles affect phenotypes. ARRIVE EARLY SO WE CAN START STRAIGHT AWAY