2What Is a Species? Each species evolves independently Today, a species is defined as a group of populations that evolves independentlyThe biological species concept defines a species as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groupsA group that is reproductively isolated is unable to breed successfully outside the groupAlleles do not move between the gene pools.The most widely used standard is the biological species concept.Two major limitations:Based on sexual reproduction so determining asexual boundaries is difficult.It is not always possible to see whether members of two different groups interbreed.This is not the only definition for species because of its limitations.
3What Is a Species?Appearance can be misleading in determining a speciesOrganisms of similar appearance sometimes belong to different speciesNo visible difference between Asian mosquitoesOne spreads malaria and the other does notA butterfly considered for two centuries to be a single species known as the two-barred flasher butterfly is now known to be 10 separate speciesThe caterpillars differ in appearance
4Members of Different Species May Be Similar in Appearance Fig. 16-1The cordilleran flycatcher and Pacific slope flycatcher are so similar that birdwatchers can’t tell them apartThese birds were considered to be a single species, but research revealed that they do not interbreed and are in fact two different species
5Members of a Species May Differ in Appearance Northwestern garter snakes can be a range of colors, have stripes or not, the stripes may be different thicknesses or colors. But they are all the same species.These birds were thought to be separate but interbreed so are now considered one species.Just because the look slightly different doesn’t mean they are different species.Fig. 16-2
6How Is Reproductive Isolation Between Species Maintained? Traits that prevent interbreeding and maintain reproductive isolation are called isolating mechanismsMechanisms that prevent mating between species are called premating isolating mechanismsMechanisms that prevent formation of vigorous, fertile hybrids between species are called postmating isolating mechanismsIsolating mechanisms provide a clear benefit. To breed with another species may produce no offspring or offspring that is unfit or sterile.
7How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanisms include the followingGeographical isolation – physical barrier separates themEcological isolation – they occupy different habitatsTemporal isolation – they breed at different timesBehavioral isolation – different courtship and mating ritualsMechanical incompatibility – reproductive structures are incompatible
8How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanismsGeographically separated populations aren’t necessarily distinct speciesMechanism to allow new species to form not to maintain reproductive isolationThe physical barrier might disappear.The Kaibab and Abert squirrels living in geographically separate areas of the Grand Canyon are physically separated, but still very similarIt is currently unknown if the two populations have diverged into two species
9How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanismsEcological isolation occurs when species don’t mate because they occupy different habitatsWhite-crowned sparrows inhabit fields and meadows, while white-throated sparrows inhabit dense thicketsIn another example, each species of fig wasp breeds in the fruit of a different species of fig, and the wasps thus do not come into contact with one anotherOverlapping geographic ranges. May coexist within a few hundred yards of one another and yet seldom meet during the breeding season.This also prevents the fig species from interbreeding.
10How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanismsTemporal isolation occurs when species can’t mate because they breed at different timesIn nature, Bishop and Monterey pines are found near Monterey, CABishop pine pollination occurs in summerMonterey pine pollination occurs in early springEven if they occupy the same habitat.Spring and fall field crickets both occur in many areas of North America
11How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanismsBehavioral isolation occurs when species can’t mate because they have different courtship and mating ritualsMale frogs embrace any female regardless of speciesFemale frogs encountering males of a different species utter the “release call,” which causes the males to let goThe songs and plumage of male songbirds are species-specificThey attract females of the same speciesFemales of other species are unresponsiveFew hybrids are produced even with frogs
12How Is Reproductive Isolation Between Species Maintained? Premating isolating mechanismsMechanical incompatibility occurs when species cannot mate because their reproductive structures are incompatibleIn animals with internal fertilization, male and female sexual organs may not fit togetherFor example, snails of species whose shells have left-handed spirals may be unable to copulate successfully with snails whose shells have right-handed spiralsIn plants, differences in flower size or structure may attract different pollinatorsSnails – the body orientation matches that of the shell.
13How Is Reproductive Isolation Between Species Maintained? Postmating isolating mechanisms limit hybrid offspringHybrid – offspring of parents of different speciesPostmating isolating mechanisms include the followingGametic incompatibility – sperm cannot fertilize the eggHybrid inviability – offspring fail to surviveHybrid infertility – offspring are sterile or have low fertilityWhen premating mechanisms fail or have not yet evolved.
14How Is Reproductive Isolation Between Species Maintained? Postmating isolating mechanisms limit hybrid offspringGametic incompatibility may be an especially important in species such as marine invertebrates and wind-pollinated plants that reproduce by scattering gametes in the water or in the airSea urchin sperm cells contain a protein that allows them to bind only to eggs of their own speciesAmong plants, chemical incompatibility may prevent the germination of pollen from one species that lands on the stigma (pollen-catching structure) of the flower of another species
15How Is Reproductive Isolation Between Species Maintained? Postmating isolating mechanisms limit hybrid offspringHybrid inviability occurs when hybrid offspring fail to survive to maturityThe hybrid may abort early in development or die shortly after birthThe hybrid may be unable to reproduce because it displays behaviors that are mixtures of the two parental typesLovebird hybrids have great difficulty learning to carry nest materials during flight and probably could not reproduce in the wildLovebirds – one species carries nest material under its rump feathers and the other carries it in its beak. Hybrids try to tuck the nest material under their feathers but don’t release it from their beaks.
16How Is Reproductive Isolation Between Species Maintained? Postmating isolating mechanisms limit hybrid offspringHybrid infertility occurs when hybrid offspring are sterile or have reduced fertilityMule hybrids (a cross between a horse and a donkey) are sterileLiger hybrids (a zoo-based cross between a male lion and a female tiger) are sterileInfertility is often caused by the failure of chromosomes to pair properly during meiosis, so eggs and sperm never develop
17Review Define a species. Why is it difficult to tell species apart? What are the main types of premating reproductive isolation mechanisms?What are the main types of postmating reproductive isolation mechanism?1. Group of populations that evolves independently
18How Do New Species Form?Speciation is the process by which new species formSpeciation depends on two factorsThe isolation of populations must prevent gene flow from spreading between them and keeping them similarGenetic divergence of the populations must occur through the evolution of isolating mechanisms, driven by genetic drift or natural selectionSpeciation always requires isolation followed by divergence.Allopatric speciation – geographically separatedSympatric speciation – share same geographic area
19How Do New Species Form?Geographical separation of a population can lead to allopatric speciationspeciation occurs when two populations of a species become separated by a geographical barrierThe colonization of remote habitats, such as islands or reefs far out to sea, can lead to separate populationsGeological changes caused by such factors as climate shifts, volcanoes, earthquakes, continental drift, and change of course by rivers can lead to the separation of populations
20Allopatric Isolation and Divergence Part of a mainlandpopulation reachesto an isolated islandDivergence may eventuallybecome sufficient to causereproductive isolationThe isolated populations beginto diverge due to genetic driftand natural selectionSmall population – substantial genetic drift (founder event)Most evolutionary biologists believe that geographic isolation followed by allopatric speciation has been the most common source of new species, especially among animals.Fig. 16-8
21How Do New Species Form?Ecological isolation without geographical separation can lead to sympatric speciationSympatric speciation may occur when a geographical area contains two distinctly different habitats (ecological isolation)Through the action of natural selection, different members of the species begin to specialize in one habitat or the otherThe two isolated populations may diverge sufficiently genetically so that they can’t interbreed and are thus considered separate speciesTwo populations of a species living in the geographical area may occupy the two habitats
22Sympatric Isolation and Divergence Part of a fly population that lives only onhawthorne trees moves to an apple treeThe flies living on the apple treedo not encounter the flies livingon the hawthorne tree, so thepopulations divergeRhagoletis is a parasite of the American hawthorn tree.Apple trees were introduced to America from Europe.Why don’t they interbreed since the trees are found together and flies can fly?females typically lay their eggs in the fruit they developed in.males prefer the fruit they developed in.timing of ripe fruit is a couple of weeks off, so development of the flies are becoming different (some interbreeding still occurs).Fig
23How Do New Species Form? Mutations can lead to speciation New species may arise almost instantaneously as a result of mutations that change the number of chromosomes in their cellsThe acquisition of multiple copies of each chromosome is known as polyploidy and is a frequent cause of sympatric speciationIn general, polyploid individuals cannot mate successfully with normal diploid individuals and so are genetically isolated from the parent speciesPolyploid plants are more likely than polyploid animals to be able to reproduceMany plants can self-fertilize or reproduce asexually making it easier to become a founding member of a new species.
24How Do New Species Form?Under some conditions, many new species may arise in a short period of timeThe mechanisms of speciation and reproductive isolation lead to forking branches in the evolutionary tree of life, as one species splits into twoAdaptive radiation is the rise of many new species over a relatively short period of timeOne species invades a variety of new habitats and evolves in response to differing environmental pressures
25Interpreting Evolutionary Trees past(a) Evolutionary tree(b) Evolutionary tree representingadaptive radiationIn an adaptiveradiation, multiplespeciation eventsmay occur rapidlyenough thatbiologists cannotbe certain of theirorderpresentForksrepresentspeciationeventsEach linerepresentsa speciestimeFig
26Adaptive Radiation Fig. 16-11 (a) Ahinahina (b) Waialeale dubautia Finches colonized the Galapagos islands (13 species)cichlid fish reached isolated Lake Malawi in Africa (300 species)silversword plant species arrived at the Hawaiian Islands (30 species) (pictured)No competitors except other members of the same species.(c) Kupaoa(d) Na’ena’e ’ulaFig
27What Causes Extinction? Extinction is the death of all members of a speciesAt least 99.9% of all species that ever existed are now extinctThe immediate cause of extinction is probably always environmental changeEnvironmental changes that can lead to extinction include habitat destruction and increased competition among species
28What Causes Extinction? Localized distribution make species vulnerable in changing environmentsSpecies inhabiting extremely limited ranges may become extinct if the area is disturbedThe Devil’s Hole pupfish is found in only one spring-fed waterhole in the Nevada desertWide-ranging species normally do not succumb to local environmental catastrophes
29What Causes Extinction? Overspecialization increases the risk of extinction in changing environmentsSpecies that develop adaptations that favor survival in a specific environment are at risk of becoming extinctThe Karner blue butterfly feeds only on the blue lupine plantThe habitat of the lupine has been significantly reduced by developmentLoss of the lupine will lead to extinction of the Karner blue butterflyThese adaptations include specializations that favor survival ina particular and limited set of environmental conditions.
30What Causes Extinction? Interactions with other species may drive a species to extinctionSpecies that are unable to exploit resources more efficiently and effectively than their competitors may become extinct2.5 million years ago, a land bridge (the isthmus of Panama) formed between North and South AmericaNorth American species underwent adaptive radiation that displaced the vast majority of South American species, many of which became extinctCompetition and predation.
31What Causes Extinction? Habitat change and destruction are the leading causes of extinctionExtinctions due to prehistoric habitat change have had a significant effect on the evolution of organismsHuman activities are the primary cause of present-day habitat destructionClearing of tropical rainforests could lead to loss of up to half of all current species over the next 50 years
32Review What are the two general steps for speciation to occur? What is the difference between allopatric and sympatric speciation?What is adaptive radiation?What are the main causes of extinction?