Darwin’sFinchesandNaturalSelection-Casestudy.ppt

1、Darwins Finches and Natural Selection,by Cheryl Heinz, Dept. of Biological Sciences, Benedictine University, and Eric Ribbens, Dept. of Biological Sciences, Western Illinois University,1,The Galapagos Islands,Located approximately 1000km from the coast of Ecuador, South America.This is just a little

2、 closer than the distance between Chicago and Philadelphia.Mostly ground between the two U.S. cities.Mostly deep water between the Galapagos Islands and the coast of South America.,2,3,Implications,Terrestrial species on these islands wont have many relatives nearby. Neighboring islands will have cl

3、ose relativesbut new terrestrial species wont arrive on these islands from the South American mainland very often.most of the island species have had plenty of time to differentiate from their nearest living relatives.Another way of saying this is that there is very little gene flow between the isla

4、nds and the mainland.,4,CQ1: Limited gene flow means:,A: Birds rarely move between the mainland and the islands.B: Birds on the island have the same genes as birds on the mainland.C: Birds on the mainland dont like birds from the islands.D: Birds on the mainland wont evolve, but birds on the islands

5、 might.,5,Charles Darwin,Darwin explored these islands from April through October 1835.Entire voyage of The Beagle: Dec 1831 - Oct 1836When and where he started thinking about what was to become his theory of evolution by natural selection.He did not publish his thoughts until the publication of The

6、 Origin of Species in 1859.,6,Charles Darwins Ideas,Biological evolution is change in species over time.This was not a new idea at the timeBut there were no good mechanisms to explain how these changes occurredNatural selection is just such a mechanism, and this is what Darwin contributed.,7,Galapag

7、os Endemics,The Galapagos today is an amazing place.Animals live there that are found nowhere else on earth.This makes them endemicPerhaps the most famous of the endemic birds are the finches, of which there are 13 different speciesThe islands are a natural laboratory, and one in which evolution can

8、 be observed.,8,Endemics,Among the kinds of animals found here and nowhere else: 1 penguin species1 giant tortoise species1 marine iguana species7 species of lava lizard14 species of sea cucumber1 species of sea lion1 species of hawkseveral species each of mockingbirds, doves, owls, flycatchers, and

9、 yellow warblers,9,CQ2: Endemic means:,A: The end is imminent.B: The species isnt found anywhere else.C: The species has very specific habitat requirements.D: The species needs to be protected.E: The species is extinct.,10,The Finches,The 13 finch species include: 6 species of ground finches3 specie

10、s of tree finches1 woodpecker finch1 vegetarian finch1 mangrove finch1 Coco Island finchA warbler finch that looks more like a warbler than a finch (one of the tree finches).The woodpecker finch actually uses cactus spines to dig grubs out of branches!,11,Peter and Rosemary Grant,Scientists Peter an

11、d Rosemary Grant have studied many of these species for the past thirty years.Spend months at a time on the islandsOften know every finch on an islandLets look at some of their data.,12,Graph showing the distribution of beak depths for medium ground finches in Year 1,13,CQ3: What is the average dept

12、h of the finches beaks in Year 1?,A: about 7mmB: about 8mmC: about 9.5mmD: about 10mmE: about 11mm,14,CQ4: How much was the biggest difference in beak depth?,A: 2 mmB: 4 mmC: 6 mmD: 8 mmE: 10 mm,15,A Change in the Weather,Year 2Like most years, some rain fell the first week of January.The rest of Ja

13、nuary, there was one small shower.The total rainfall for the entire year: 24mm.In a normal year, 130mm of rain would fall.In Year 1, 137mm of rain fell.,16,A Change in the Weather,The ground finches feed on seedsYear 1 June: 1m2 of lava on the island has over 10 grams of seeds.Year 2 June: 6 grams o

14、f seeds per m2.Year 2 December: 3 grams of seeds per m2.In the drought, the plants conserved their resources and did not produce new seeds.Similarly, the finches did not mate and did not produce eggs in Year 2,17,Seeds,A variety of seeds are produced on the island.Finches prefer the softest seeds, w

15、hich are the easiest to open. The seeds above are seeds of a plant called Caltrop, in the genus Tribulus.These are among the hardest to eat.It takes a medium ground finch with a beak at least 11mm long to open one.Ground finches with beaks that are 10.5mm long or less havent even been seen trying to

16、 eat them.,18,Prediction?,What do you think will happen to the size of the finch population between Years 1 and 3? (Remember, Year 2 is a drought year.) Sketch a rough graph of your prediction,19,CQ5: What do you think a graph of population size would look like for Year 1 to Year 3?,A:,Time,Time,Tim

17、e,Time,B:,C:,D:,20,Another Year of Change,On one day in January of Year 3, more than 50mm of rain fell on the island .The plants finally flowered and produced new seeds.The Grants and their colleagues returned to the Galapagos.They found the finch population had been decimated.No new finches hatched

18、 in Year 2.Only one finch born in Year 1 survived to Year 3.,21,Year 3 Data,22,CQ6: What was the average beak depth in 1978? (Remember that the average beak depth in 1976 was 9.5 mm.),A: Just under 7mmB: About 8mmC: About 9mmD: Just under 10mmE: Just under 11mm,23,Evolution is:,A change in the frequ

19、ency of an allele, such as an allele for beak depth, is the basic definition of evolution.,24,CQ7: Did the finch population evolve from 1976 to 1978?,A: YesB: NoC: MaybeD: I dont know,25,Evolution by natural selection,The Grants first went to the Galapagos to take a quick snapshot of finch diversity

20、.Within only a few years, they saw natural selection.In the course of one season, the beaks got 0.54mm deeper and 0.39mm longer.The sex ratio changed, too.,26,Evolution by natural selection,The beak size and shape was changing, right before the Grants eyes!This is definitely evolution as we defined

21、it earlier.,27,Surprises?,Two things surprised the Grants:Evolution could occur quickly enough to observe within a few field seasons.Darwin believed that we did not have a long enough lifespan to observe evolution.A single weather event can cause evolution, if there are traits that affect survival a

22、nd if there is variation in those traits.,28,Surprises?,2. Evolution can occur at very small scales.The Grants measurements were very careful.The birds werent used to humans, and so were easy to catch and measureThey couldnt see a difference in even 1mm between two finches, but their measurements co

23、uldAnd due to those measurements, they could find that 0.5mm was enough to make a difference between survival and death in a drought year,29,CQ8: Which do you find more surprising?,A: Evolution occurs at a rate that we humans can observe.B: As little as half of a millimeter can make the difference b

24、etween life and death.C: Both surprise me.D: Neither was particularly surprising.,30,Heritability,Its important to note that beak size and shape is heritable in these finches.A bird with a large, deep beak will have offspring with large and deep beaks.Natural selection can occur without heritability

25、, but evolution by natural selection cannot!(think about that for a minute),31,Evolution by Natural Selection,Steps:Individuals vary in some traits.2. Some of the differences in traits are passed along to offspring.This requires a genetic basis to the traitThe trait is thus heritable (more),32,Evolu

26、tion by Natural Selection,3. Different individuals produce different numbers of surviving offspring.Produce different numbers, orDifferent numbers survive.4. The particular value of a trait is connected to the number of offspring produced.Traits that allow for more offspring to be produced are said

27、to be “naturally selected.”,33,CQ9: If beak depth increased during the drought, primarily due to selective mortality, can we really say that this natural selection was driven by environment favoring the survival of birds with deeper beaks?,A: No. Beak depth changed due to birds dying, not to birds surviving.B: Yes. Birds with deeper beaks survived at a higher rate than birds with shallower beaks.C: Im really confused.,34,Want to find out more of the story? Check out ,The Beak of the Finch: A story of evolution in our time, by Jonathan Weiner (New York:Knopf,1994).,35,