Presentation on theme: "How does it work? What does it mean?"— Presentation transcript:
1 How does it work? What does it mean? Evolution IIHow does it work? What does it mean?
2 topics Patterns Mechanisms Speciation Macroevolution trends Evolution and how it worksEvidence:FossilsHomologyEmbryologyMis-conceptions
3 Example 1: beetles on a diet % of a population of beetles is green;a % is brownOne year, a drought limits food supplyAs a result of this environmental stress,the next generation of beetles are smallerAfter another generation, the % of green andbrown beetles changesIs this an example of evolution? NO! Why?
4 All changes -even those from environmental stress or limited resources - are not evolution. What do you need to see to know you are looking at evolution?Change in the “gene pool” - the total genetic makeup of a population (the actively interbreeding portion of a species; a species consists of interbreeding or potentially interbreeding individuals)So if changes in the gene pool define evolution, what causes change in this?
5 Changes in the gene pool (the genetic composition of a population) are caused by:1. Mutation2. Gene flow, or migration3. Genetic drift4. Natural selection
6 Introduction of genetic variation: Mutation - changes in DNA, usually from a “copying error” but also from external causes (ex, radiation). A single mutation can have a large effect, but usually accumulation of mutations over time produces change. Most mutations are neutral. Mutations are randomGene flow - introducing new genes to a population from emigrants, including:sexual recombination - shuffling genes during fertilization and cell division
7 What is genetic drift?In every generation someindividuals leave behindslightly more offspring (andtheir genes) than otherindividuals. As a result, the nextgeneration has more of the genesof the reproducing ancestors than genesof the ancestors with feweroffspring. As these descendentsin turn create offspring, so more oftheir genes will be transmitted tothe next generation than thoseancestors who had fewer offspring. Noadaptations are involved, it’s just amatter of statistical change over time.
8 Natural SelectionVariation in traits; green and brown beetlesVariation in reproductive success. If one color ofbeetle is preferentially eaten by a predator, more of theother color will remain to reproduceHeredity: more of the brown beetles will pass theirgenes on to offspringEnd result: as these 3 steps continue, greater and greater numbers of brown beetles will appear in successive generations; the composition of the gene pool changes over time = evolution
9 Some questions:does the beetle in our example need to change color in order to survive?Is a brown beetle “better”?How is our green beetle/brown beetle example of natural selection different from “inheritance of acquired characteristics” of Lamarck?
10 An important feature of natural selection is “fitness” What does this mean?A genotype’s fitness is defined by its ability to successfully pass onits genes to the next generation.What defines reproductive success?survival to reproduceacquire a matesuccessful reproductionbehavior (in mate acquisition and parenting)gentoo“survival of the fittest” does not necessarily mean big, strong,mean, etc, etc. It may refer to protective coloration, metabolicneeds, lengthy parenting, etc, etc.
11 What is “adaptation”?Modification of the phenotype that enhances fitness or reproductive successThere are limits on the amount of adaptation in a population…rates on the processes that produce genetic variation are low, and there are also constraints on the phyletic history. Ex, a lobster can’t adapt by losing it’s exoskeleton…that’s “hard wired” into what defines an arthropod.Every generation that reproduces IS adapted; there aren’t “losers”.Adaptation doesn’t happen FOR a reason…it can only be viewed historically; there is no genetic modification to produce a “goal” of adaptation
12 What is microevolution? The change in gene frequency in a population which may lead to speciation (creation of a new species)Ex: our green and brown beetles: if you determine the genetic makeup of the beetle population in successive years and notice a change in ratios of genes, you are studying microevolutionThis important question is,“How did this happen?”
13 Mechanisms for microevolution: Mutation (although it’s not likely thatyou would see this after only 1generation)Gene flow - introduction ofnew genes into thepopulation by emigration orsexual recombinationNatural selection - brown beetles escapepredation and reproduce more frequentlygenetic drift - random changes inthe % of various genes
14 species = a population of Speciationspecies = a population ofinterbreeding or potentiallyinterbreeding individualsSpeciation = a lineage-splitting event that produces 2 speciesSpeciation happens through geographicisolationThe gene pool for a species becomesgeographically separated and due to subtleenvironmental differences between the tworegions, the gene pool diverges over time.Geographic isolation can also happen withouta physical barrier. If the geographic distributionof a species is very wide, those populations onone “side” won’t actively interbreed withpopulations on the other “side,” and over time,genetic drift will result in their divergence.
15 Do we see speciation happening today? 3 examples: The 1995 Hurricane Marilyn washed severaltree logs, and resident lizards, to a new Caribbean island (Anguilla). Iguana iguana had not been recorded on Anguilla before. Biologists are waiting to see if these new emigrants survive and reproduce,and how, over time I.iguana’s gene pool maychange (= evolution) and if that will result ina new species of iguana. Stay tuned!The spotted owl, Strix occidentalis, iswidespread in the western U.S. Thepopulations in the south are slightlydifferent from those in the Pacific NW.Current studies on owl geneticssuggests that the gene pools arediverging. Speciation appears to be happening.
16 Experimental results….. Drosophila populations physicallyseparated are fed different foods.After several generations they arereintroduced to one another to see if theymate or are reproductively isolated.Fruit flies fed one food source preferredto mate with other fruit flies fed the samefood (the food source altered theirfeeding behavior). Studies continue tosee if their gene pools have diverged (ifspeciation has occurred yet).
17 What speciation would look like in the fossil record:Species accumulate morphologicchange over time, much of which isrepresented in unconformities andbedding planes. Sometimes theentire sequence of intermediateforms IS preservedThe “gaps” or missingintermediary steps are real, reflectingthe very rapid burst of evolution thatcreates a new species, followed bylong periods of stasis.
18 How phyletic gradualism happens: Incremental morphologic change over time.results from changes in the gene pool over time.The graph represents an imaginary distributionof genes in a species (think of the humangenome project for all humans!).If an environmental change applies a stresson a species, those having certain genesthat enable them to adapt willsuccessfully reproduce and individuals withmore of those genes will appear in subsequentgenerations. Over time the entire gene pool willaccumulate these changes and the distribution will change.This type of gradual change in the genotype willresult in incremental changes in thephenotype.
19 Another model for speciation: The slow change in the genetic makeup of a population as a result ofreduced gene flow.Perhaps this happens as a result of a population exploiting a newniche or food source, changing its behavior as a result, and diminishedinterbreeding. Continued over time, reduced gene flow will isolatethe populations.Are we seeing this type of speciation now?Apple maggot flies have historically eaten/laid eggs in apples. More recently they have also started to eat/reproduce in hawthorne apples. Since insects tend to matewith other insects that eat the same food,apple maggot flies are interbreeding with other apple maggot flies and not with hawthorne maggot flies. Over time will these gene pools isolate?
20 Another mode of speciation: “punctuated equilibrium” Morphology (and gene frequency) stays relatively constant over long periods of time followed by rapid change and the appearance of a new species.Populations living in the geographic limits of the species develop slightly different genotypes because they are living in less than ideal environments (temp extremes, water depth extremes, etc).Environmental change stresses the populations living in the optimal environments..they have difficulty successfully reproducing. If the geographically distant populations are more successful reproducing under these new conditions, their genotype will allow for more successful reproduction.
21 Thanks to the Berkeley Museum website for these illustrations