ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Twenty-Two Outline INTRODUCTION Many Reasons for Changes in Natural Populations Best documented are caused by human activities Similar changes in untouched populations Result from changes in allele frequencies Promote survival in diverse habitats Change traits to fit new areas or new conditions Many Causes for Population Changes Frequency of sickle-cell anemia affected by locale fig 20.2 Increased with migration to areas where malaria is prevalent Remained rare in populations in other areas Climate changes with continental glaciation THE NATURE OF SPECIES Recognition of the Species Category: Ray Individuals breed, progeny still of that species Dogs, pigeons each an individual species Carp and goldfish in separate species fig 22.1 Intergradation of Species: Darwin Species catalogued and understood fig 22.2 Similarities in features supported evolution Species distinction in terms of an individual`s niche Occurs in particular location Displays certain activities at certain times Specific habitat Species Definition of Population Geneticists: Mayr Groups of interbreeding populations, reproductively isolated from others Hybrids between species rare in nature No true barriers between some groups Strong barriers between other groups Recognition primarily by differences in features fig 22.3 Variation in Populations Within a Species Populations from separate areas appear different fig 22.4 Populations from same area have mixed appearance Classified as subspecies or varieties Populations intergrade with one another Possess features of both groups Species in nature do not intergrade, may hybridize THE DIVERGENCE OF POPULATIONS Development of Local Populations Geographical separation or dissimilar conditions Infrequent interbreeding fig 22.5 Limited exchange of genetic material Characteristics optimized to that environment Rate of change correlated to strength of selective forces Allopatric speciation: differentiation of geographically isolated populations into species Sympatric speciation: splitting of populations in a common area into species Distinction between these terms is misleading Response to Combination of Selective Factors Unpredictable Phenotypic appearance due to interaction of many genes Integration of developmental processes Different complement of alleles in each population ECOLOGICAL RACES Divergence in Population Creates Races Individuals from different areas appear different fig 22.4 Intermediate stage in evolution of species Appearance of clones differs with their environment Ecotypes in Plants Differences in appearance have genetic basis Isolated races from varying environments Grew under common conditions Most unique features maintained Differences in physiology genetic as well Ecological Races in Animals Morphological or physiological differences fig 22.4 Almost exclusively genetically determined BARRIERS TO HYBRIDIZATION Isolation May Eventually Alter Niche of Population Exploit different resources in different ways Remain distinct if reassociated with others Become distinct species Retention of Species Identity Prezygotic mechanisms: prevent formation of zygotes Postzygotic mechanisms: prevent functioning and development of zygotes Prezygotic Isolating Mechanisms Geographical isolation Individuals physically separate from one another Hybridization may occur in zoos, gardens Example: species of oaks fig 22.6 Example: lion/tiger matings Ecological isolation Same area, but different habits and habitats Example: overlapped range of Indian lions and tigers Example: California oaks Temporal isolation Breeding periods at different times Example: wild lettuce Example: Rana species Behavioral isolation Species specific mating rituals Example: Hawaiian Drosophila fig 22.7 Hybrids fully fertile Mechanical isolation General structural differences Specific differences in copulatory organs Example: arthropods Example: bees pollinating various plants Prevention of gamete fusion Sperm not attracted to eggs of other species Sperm incapable of penetrating eggs Growth of pollen tubes impeded Postzygotic Isolating Mechanisms Hybrids develop improperly Example: sheep/goat hybrids Example: leopard frogs fig 22.8 Experimental manipulation of plant hybrids Embryo removed from parent Cultured with special growth supplements Hybrids weaker than parents, are eliminated in nature Strong hybrids may be sterile Abnormal development of sex organs Fertility generally lower than normal Example: donkey x horse = mule fig 22.1 Reproductive Isolation: An Overview tbl 22.1 Formation of species is a continuous process Partially differentiated populations may interbreed Various isolating mechanisms exist to limit hybridization Selective factors may limit success of hybrids Individuals that do not hybridize may be more fit than those that do CLUSTERS OF SPECIES Adaptive Radiation Existence of closely related species within a genera Evolved recently from common ancestor Most pronounced in sharply discontinuous habitats Darwin`s Finches fig 22.9 Thirteen species inhabit Galapagos and Coco Islands Islands have distinct variety in ecology Limited competition from other bird species Descendants occupy different kinds of habitats fig 22.10 New arrivals filled unoccupied niches Selective pressures diversified populations Resulted in three primary groups of finches Ground finches: six species fig 1.10a Feed on seeds Bill size related to seed size Tree finches: six species Four feed on insects, related to bill size One feeds on buds and fruit One uses twigs to probe like a woodpecker Warbler finch: one species Ancestor to all resembled blue-black grassquit fig 1.10b Hawaiian Drosophila Species selective for host plants and plant parts Older species invade new islands, new species evolve Over 800 individual species evolved from single ancestor fig 22.7 Morphological and behavioral differences between species Sexual Selection and the Origin of Species Differential reproduction resulting from variable success in obtaining mates Social factors more important than ecological factors Specific courtship behavior necessary for reproduction Rapid Evolution More pronounced on islands Accelerated in mainland areas with diverse climates Example: variety of California Example: regions damaged by recent catastrophes THE ROLE OF HYBRIDIZATION IN EVOLUTION Hybrids Can Be Successful Better suited to an environment than either parent Greater opportunity to recombine alleles Sterile Hybrids Can Be Successful Plants reproduce vegetatively Pieces of stems and roots become established Likely to occur if clone well-adapted to environment Animals and plants reproduce via parthenogenesis Egg cells directly give rise to new individuals Somatic cells function in fertilization Example: dandelions Formation of polyploid individuals Chromosome number of original sterile hybrid is doubled Polyploidy results in individuals with more than two sets of chromosomes