IV. Natural Selection A. Important points (1) Natural selection does not cause genetic changes in individuals (2) Change in allele frequency occurs in populations (3) Fitness!" Reproductive Success = survival to reproductive maturity, passing on genes, > # of offspring (4) Evolutionary changes are not goal oriented mutations are random environments/selective forces constantly change>> (5) Of the 5 agents of Evolutionary Change, ONLY selection produces adaptive evolutionary change C. Components of Fitness (1) Competition: scarce resources favor the best-adapted individuals (2) Predator/Prey Interactions one organism (the predator) eats another (the prey) coevolution of predator and prey (3) Symbiosis leads to the most intricate coevolutionary adaptations 1
(4) Sexual selection (access to sexual partners) " Favors traits that help an organism attract a mate " Usually, all females get to breed, but only some males " Result is sexual dimorphism 2 mechanisms: Male-male combat (large structures for fighting; large male size) Female choice (males have attractive structures to lure females) ***Under both scenarios females get good genes for offspring C. Components of Fitness (cont.) (5) Kin Selection Actions of an individual increase survival and reproductive success of relatives Who have some of the same alleles! Prairie Dogs Spadefoot toad tadpoles 2
Forms of Selection Favors one extreme Favors average Favors both extremes V. Forms of Selection: What type of Selection? Percent of births in population 20 15 10 5 100 70 50 30 20 10 7 5 3 2 Percent infant mortality 2 3 4 5 6 7 8 9 10 Birth weight in pounds 3
V. Forms of Selection: What type of Selection? Peppered Moths and Industrialized Melanism " Until the mid-nineteenth century, peppered moths, Biston betularia, had predominately light-colored wings. Subsequently, dark individuals became predominant " Industrial smog helped turn lichens on tree trunks dark. " Contrasting colors between trunk color and moth color led to differential predation by birds. VII. Constraints on Evolutionary Perfection " Historical constraints Evolution tinkers with existing structures " Adaptations are compromises Seal flippers on land and water " Chance and selection interact No prediction of future conditions " Selection can edit only existing variation Mutation is random 4
I. What Is a Species? groups of actually or potentially interbreeding natural populations that are reproductively isolated from other species maintain connectedness over geographic distances gene flow maintains a species production of fertile, viable offspring II. Isolating Mechanisms Species maintain their genetic distinctiveness through barriers to reproduction A) Prezygotic Isolating Mechanisms 1. Geographical Isolation 2. Ecological Isolation 3. Temporal Isolation Rana aurora - breeds January - March Rana boylii - breeds late March - May 5
A) Prezygotic Isolating Mechanisms 4. Behavioral Isolation different courtship rituals 5. Mechanical Incompatibility Structural differences prevent mating 6. Gametic Incompatibility Sperm from one species are unable to fertilize eggs of another B) Postzygotic Isolating Mechanisms: hybrid zygotes fail to develop or develop abnormally, or hybrids cannot establish 1. Hybrid Inviability offspring fail to survive to maturity due to developmental defects 2. Hybrid Infertility hybrid offspring unable to produce normal sperm or eggs Animal hybrid examples: mules (horse + donkey) ligers (lion + tiger) often sterile chromosomes don t pair properly during gamete formation (i.e. meiosis) 3. Ecological inviability hybrids develop normally, but cannot find a suitable ecological niche 6
III. Speciation Similar populations of one species diverge until reproductive isolation results! Separate species 1. Populations adapt to their environment or random changes may occur in separate populations. 2. Adaptation or random changes result in the accumulation of many differences between the populations. 3. Isolating mechanisms prevent population hybridization. 4. The populations are no longer capable of interbreeding successfully. III. Speciation A. Requirements of speciation (1) restricted gene flow " very low or no gene flow (migration) between diverging populations (2) genetic divergence large genetic differences accrue between populations (3) reproductive isolation must evolve to maintain the differences " can no longer interbreed " can no longer produce normal offspring B C A 7
B. Mechanisms of Speciation (1) Allopatric Speciation: populations geographically separated no or little gene flow Differences accumulate Eventually, cannot interbreed B. Mechanisms of Speciation (1) Allopatric Speciation: Long distance colonization Barriers to movement Extinction of intermediate populations 8
B. Mechanisms of Speciation (2) Sympatric Speciation: populations share the same geographic locality but with no (or little) gene flow What reduces gene flow in these cases? (2) Sympatric Speciation: What reduces gene flow? Fruit fly: Parasite Lays eggs in fruit a. Ecological isolation / Disruptive selection HAWTHORN TREES (native) Rhagoletis pomonella APPLE TREES (introduced) Flies have different emerging times Tree loyalty 9
III. Speciation C. Speciation produces Evolutionary Trees = Canines D. Adaptive Radiation One species new species: A new species: B new species: C new species: D Typically occurs when populations of a single species... invade a variety of new habitats, evolve under different environmental pressures (selective forces) Examples: Darwin s Galapagos Finches, Hawaiian Silverswords 10
D. Adaptive Radiation IV. Pace of Evolution Gradualism phenotypic change Evolutionary change is slow and imperceptible from generation to generation Over the course of thousands and millions of years, major changes could occur Punctuated equilibrium Evolution normally proceeds in spurts, with long periods of little evolutionary change in between. The proposed stasis would be expected in large populations experiencing stabilizing selection over long periods of time. 11
V. Extinction Extinction = death of all the members of a species How and why does it happen? (1) Localized distributions & overspecialization Increase an organism s likelihood of extinction Devil s Hole pupfish: Lives in isolated water pools in Mojave Desert What Causes Extinction? (2) Interactions with other organisms - competition, parasitism, predation EX. the Panama land bridge: connected 2 continents, formed during the Pleiocene (~3 mya) N. American species & South American species mixed " For the first time in history Many S. American species were out-competed " went extinct 12
What Causes Extinction? (3) Habitat change / destruction leading causes of extinction tropical forests ALSO: Habitat Fragmentation What Causes Extinction? (3) habitat change / destruction What can we do? BOZEMAN PASS WILDLIFE LINKAGE 13
VI. Speciation and Extinction Through Time There have been 5 major mass extinctions interspersed within relatively consistent extinction patterns. most famous " end of Cretaceous period (65 mya); dinosaurs went extinct Mammals quickly experienced evolutionary radiation. Biological diversity tends to rebound after mass extinctions 14