Transcript Slide 1
SPECIATION Boy evolves into lamb A rare mid-speciation event caught on camera Types of Selection • environmental selection external pressure from environment driving selection ex. predators, diseases, food resources, climate • sexual selection selection based on mating preferences In almost all species (other than humans) it is the male that becomes showy & decorated to attract females. ex. Irish Elk: females selected for male elk with largest antlers antlers became so large over time, contributed to elk’s extinction over time, changes in allele frequencies can lead to SPECIATIONthe divergence of one species into 2 or more populations of one species (gold) time A time B time C time D populations of a daughter species (green) What is a species ???? one “type” or “kind” of organism how do you decide what is one “type” of organism? a fish? a rainbow trout? a trout? a rainbow trout in Lake Tahoe? Biological Species Concept: populations or organisms belong to the same species if they can: Problems? NOW: we’re replacing the B.S.C. with genetic comparisons between organisms– use DNA differences to tell how evolutionarily different organisms are. Macroevolution Evolutionary changes that create new species Occurs by accumulation of microevolutionary changes (changes in a single gene) Speciation begins as gene flow ends between populations. Sexually reproducing species attain and maintain separate identities through reproductive isolation. Prevent gene flow between species 1. Intrinsic Isolating mechanisms a. Prezygotic barriers • Prevent mating or fertilization b. Postzygotic barriers • Reproductive failure after fertilization 2. Extrinsic Isolating mechanisms Populations of one species can become different species through: Reproductive Isolation intrinsic reproductive isolating mechanisms = any heritable feature of: • body plan • function • or behavior that PREVENTS interbreeding between populations Lake filled with golden trout that mate in Sept. ONE POPULATION OF GOLDEN TROUT several decades of drought: lake level falls how many populations now? 2 after a few decades (centuries?): a mutation occurs in the games of one female fish: all her offspring are June maters this is a successful adaptation & is selected for (increases fitness) centuries later: climate changes, increased rains once again create one large lake. How many species now? 2 Why? Can not interbreed! TYPES OF REPRODUCTIVE ISOLATING MECHANISMS… 1.Behavioral Isolation: change in behavior that reproductively isolates one group of organisms from others Ex. change in courtship dances in birds Changes in song Long, flutelike song short simple whistles 2. Temporal Isolation: change in timing of mating 1. Mating at different times of year 2. Mating at different times of day 3. Mechanical Isolation: change in body form or shape that prevents reproduction with rest of population/ species Ex. change in flower structure such that insect pollinator no longer “fits” flower Mechanical isolation example Incompatible flowers in two Salvia species Salvia mellifera Only small bees can land on the petal of the black sage Flowers in early spring Salvia apiana Only large bees brush against the stamens of the white sage Flowers in late spring 4. Ecological Isolation species occupy different microenvironments Habitat Isolation Species occupy different habitats in the same geographic range. Ex: Flycatchers Empidonax Least Flycatcher open woods and farmlands Alder Flycatcher Alder swamps and wet thickets Yellow-bellied Flycatcher coniferous forests and cold bogs Hybrid Sterility (postzygotic) Horse (2N=64) X Donkey (2N=62) = Mule Hawaiian honeycreeper ancestor is believed to be the Eurasian rosefinch (arrived on the island ~ 3-7 mya) Fig. 25.8 Since that time at least 54 species of honeycreepers have evolved on the islands to fill available niches on the islands The Herring Gull Populations of the Herring Gull form a ring around the Arctic. • populations living next to each other are able to interbreed, yet populations at either end of the range have accumulated too many genetic differences between each other and cannot interbreed and are, thus, reproductively isolated North America Siberia Great Britain Mechanisms in speciation Underlying cause of speciation is the accumulation of genetic changes that ultimately promote enough differences so that we judge a population to constitute a unique species