Five Easy Pieces: The Premises of Natural Selection PDF

Five Easy Pieces: The Premises of Natural Selection 1 Darwin A lifelong Naturalist "You care for nothing but shooting, dogs, and rat- catching, and you will be a disgrace to yourself and all your family.” Darwin’s Dad 2...

Five Easy Pieces: The Premises of Natural Selection 1 Darwin A lifelong Naturalist "You care for nothing but shooting, dogs, and rat- catching, and you will be a disgrace to yourself and all your family.” Darwin’s Dad 2 1  Darwin A lifelong Naturalist Trained in medicine and for the clergy 1831 - Boards the Beagle at age 22 for a five year voyage Not yet an evolutionist Interested in:  Variation  Biogeography 3 Darwin In a few years, worked out the basic argument Variation - observable, but under appreciated Heritable variation - Already known that this could happen, but not known how Struggle - Influenced by Malthus 4 2  Darwin In a few years, worked out the basic argument Variation - observable, but under appreciated Heritable variation - Already known that this could happen, but not known how Struggle - Influenced by Malthus Differential reproduction based on heritable variation Changes in characteristics of the population - evolution 5 Darwin In a few years, worked out the basic argument Slow to publish But was part of a large intellectual group “Like confessing a murder” Amassed a great deal of evidence and experiment over the next 15 years. 6 3  Darwin and Wallace Wallace independently came up with essentially the same idea Co-published with Darwin Interesting character  See Quammen, Song of the Dodo 7 Darwin’s Accomplishments Established evolution  An old idea, but Darwin provided overwhelming evidence and a very clear argument  Widely accepted after 1859 Proposed a plausible mechanism for evolutionary change and design  Huxley - “How stupid of me not to have thought of that”  Not widely accepted at first 8 4  Natural Selection Evolution - Changes in genetic characteristics of a population over time Natural selection - one agent of evolution  Theonly one that leads to a fit between characteristics of organisms and their environment Other agents of evolution include  Mutation  Migration  Genetic drift 9 Natural Selection - Five easy pieces Natural Selection 1) Variation 2) Heritable variation 3) Struggle 4) Differential reproduction based on heritable variation 5) Changes in heritable characteristics of the population - evolution as a necessary consequence 10 5  Example - Darwin’s finches Intensively studied by the Grants, et al. An extremely well documented example of Natural selection.  SeeThe Beak of the Finch by JonathanWeiner 11 Darwin’s finches 14 species Similar in appearance, but differ in bill dimensions 12 6  Directional Selection - Darwin’s finches Intensively studied by the Grants, et al. We will examine one species, Geospiza fortis, with respect to: Variation Heritable variation Struggle Differential reproduction based on heritable variation Changes in heritable characteristics of the population - evolution 13 G. fortis on Daphne Major Daphne major A small island in the Galapagos  About the size of 80 football fields 14 7  G. fortis on Daphne Major Daphne major Geospiza fortis  Medium ground finch  Primarily eat seeds  On average, about 1200 individuals on the island  This population tends to stay put  Essentially all the individuals have been caught and measured 15 G. fortis on Daphne Major - Variation 1) Is there variation in beak dimension?  Yes 16 8  G. fortis on Daphne Major - Heritability 2) Is this variation heritable? What is heritability?  The proportion of the variation that is due to genes  Varies from 0 to 1 0 - variation entirely determined by the environment regardless of the genes  1 - variation entirely determined by the genes regardless of the environment A relatively easy, but imperfect, estimate can be made by comparing the characteristics of the offspring to their parents 17 G. fortis on Daphne Major - Heritability 2) Is this variation heritable? What is heritability? Tall What would you expect if heritability was high? Low? Offspring Short Tall Average of parents (e.g. height) 18 9  G. fortis on Daphne Major - Heritability What is heritability? Is there a heritable component to variation in beak size in G. fortis? Yes Not in text 19 G. fortis on Daphne Major - Struggle 3) Is there a struggle for existence? Always the case over the long term  The Earth would be 7 feet deep in the descendants of a single bacterium in only 48 hrs! In G. fortis, droughts lead of starvation  Major drought in 1977  Drop in food abundance  Major die-off of finches - almost 85% 20 10  G. fortis on Daphne Major - Struggle Is there a struggle for existence? In G. fortis, droughts lead of starvation Not in text  Major drought in 1977  Drop in food abundance  Major die-off of finches - almost 85%! 21 G. fortis on Daphne Major - Differential Survival/Reproduction 4) Differential survival/Reproduction: Did differences in beak dimension influence the outcome of the struggle? Fewer small, soft seeds are available in periods of drought Larger beaks are better for eating large seeds 22 11  G. fortis on Daphne Major - Differential Survival/Reproduction YES: Finches with larger beaks survived the drought in greater proportions than birds with smaller beaks Inevitable consequence of 1-4 = 5) Fig. 24.13b 23 G. fortis on Daphne Major - Differential Survival/Reproduction YES: Finches with larger beaks survived the drought in greater proportions than birds with smaller beaks Inevitable consequence of 1-4 = 5) Evolution by NS See Fig. 23.2 24 12  G. fortis on Daphne Major - Evolution The Grants demonstrated 1) variation; 2) heritability; 3) struggle; 4) differential survival/reproduction based on heritable variation That is evolution by natural selection! The average beak size of the offspring of the survivors of the drought were larger than the average of the population before the drought 25 Summary Natural selection is a very powerful agent of evolutionary change  Other agents include mutation, migration, drift Evolution is an inevitable consequence of four conditions  Variation, Heritability, Struggle, Differential reproduction  All of these can be studied  In the field - e.g. finches  In the lab - e.g. Drosophila 26 13