1 Chapter 17 Origin of Species
2 KEY CONCEPTS • Although many species persist for long periods of time both genotypic and phenotypic changes occur over time. • Even if change is occurring, individuals that interbreed within a population(s) maintain the status of the population(s) as a species. • Conditions for speciation can be initiated when a population sub-divides or when a few individuals found a separate population in a similar or different environment. • Without the separated population occupying a different environment, speciation will be slow, on the order of millions of years in many groups. • Isolation of a group within a population also can lead to speciation, which can be much faster than speciation in geographically isolated populations. • In an isolated population, natural selection will lead to adaptation to the new environment as the isolate begins to differentiate from the original population. • Behavioral, seasonal or habitat differences can facilitate reproductive isolation of the isolate or of a group within a single population. • Subsequent reproductive isolation provides a barrier to interbreeding with the original population. • Reproductive isolation becomes complete and the population is reproductively isolated as a new species.
3 Overview Key QuestionsThe question of the origin of species is the question “How do new species arise?” What are allopatric and sympatric speciation and what role do they play in speciation? What are reproductive isolating mechanisms and what role do they play in speciation? Key Concept Speciation can be initiated when genetic exchange within or among populations (demes) is impeded in processes known as sympatric and allopatric speciation, respectively. Reproductive isolating mechanisms must occur either concurrently or subsequently with these processes. Note that throughout this chapter the biological species concept is used.
4 Species Can Change Without Speciation Being InitiatedMuch of evolution is about populations maintaining the integrated features that enabled them to speciate in the first place (don’t change what works-stasis). Over the short/long term, local adaptations may occur among the populations (demes) of a species leading to phenotypic changes in traits of morphology, physiology, behavior, etc. These phenotypic changes can lead to formation of subspecies, varieties, sibling species etc but are still a single species. Even when a species consists of many populations, gene flow between populations may slow or even inhibit local specializations and so promote continuance and stability of the species. greater the gene flow = more stable the species: less = accelerates formation of distinctive groups (varieties, subspecies, sibling species) Migratory vs. non-migratory birds and parental care modes in birds Thus, SPECIATION involves more than just phenotypic changes!
5 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 distinct from that from which it derived. But why should/does life diversify into discrete populations of species—2 proposed modes: initiated between populations following geographical isolation (allopatric speciation) initiated within a population (sympatric speciation). Allopatric speciation is usually followed by sympatric speciation within the isolated populations but sympatric speciation can be initiated in the absence of geographical isolation. Natural selection (environmental factors) acting on genetic variation in individuals (fitness-reproductive success) leading ultimately to changes in gene frequencies in populations (evolution) that optimize a population for the current environment (adaptation). Is it all starting to come together? Species of sexually reproducing organisms arise via reproductive isolation. Without reproductive isolation, there is no speciation!
6 Adaptation and DifferentiationEvidence supporting Darwin: Organisms that are closely related are usually connected geographically. Adaptive radiation signifies the rapid evolution of one or a few forms into many different species occupying a variety of habitats (niches) within a new geographical area. Examples Galapogos Finches (micro or macro?) Marsupials of Australia (macro—did this slip past you?) Persisted due to protection from placental mammals (speculative hypothesis-discussed next chapter) Parallel evolution with placental mammals on other continents
7 Macro- or Micro- evolutionary examples of adaptive radiation?
8 Patterns of Speciation that arise from mechanismsAnagenesis Single species transformed into a different species over the course of many generations—inferred by fossils-why? Cladogenesis—requires some form of reproductive isolation Division of a species into 2 or more species
9 Speciation Initiated by Geographical IsolationAllopatric speciation – geographic isolation Mechanisms that prevent interbreeding may be seasonal, behavioral, habitat preference, changes in reproductive biology, fertilization or embryonic development. Speciation is a rare event and requires more than just geographical isolation. Three steps required for speciation by allopatry: Geographical isolation Local adaptation (differential selection/fitness) Reproductive isolation (key requirement for speciation)
10 3 forms of Allopatric Speciation—differ with respect to degree of geographical isolationVicariance Vicariance speciation: when a population is divided because of a natural physical barrier or because intervening geographical populations become extinct
11 Peripatric speciation: when a population is divided because of the budding off of a small completely isolated founder colony from a larger population (founder effect). A small, isolated population would be subject to genetic drift and inbreeding and exposed to new adaptive landscapes and strong selection pressure. The combined effect of such forces can result in novel, coadapted gene combinations that affect behavioral, morphological and physiological traits, a necessary prelude to reproductive isolation from neighboring and/ or ancestral populations. Examples: Hawaiin Drosophila (box 17.1), Darwin’s finches
12 Parapatric speciation: when a population at the periphery of a species adapts to different environments but remains contiguous with its parent so that gene flow is possible between them Tennessee cave salamander California salamander (ring species)
13 Speciation Initiated by Geographical IsolationOnce isolated geographically, a population can become further isolated by processes that are seasonal, behavioral or habitat related, then by further restricted gene flow and reproductive isolation, which is the final phase in any speciation process. These changes of adaptation and differentiation, which occur within a population, are elements of sympatric speciation. Such changes within a population are secondary steps in speciation initiated by geographical isolation.
14 Speciation without Geographical IsolationAn important debate in evolutionary biology has been whether speciation can be initiated sympatrically by mechanisms that reduce gene flow within a population in the absence of initiating geographical isolation. Sympatric Speciation—the genetic divergence of various populations (from a single parent species) inhabiting the same geographic region, such that those populations become different species (controversial). Disruptive (bimodal) selection, morphs and phenotypic plasticity have all become important components of this process.
15 Figure 03A: Cichlid speciesCichlid fishes in African lakes have become an ideal model for the sympatric speciation concept. (read on own) Primarily new species of cichlids have arisen as they adapt to niches in the lakes and because of presumed high phenotypic plasticity. Cichlids are diverse ecologically and behaviorally. The combination of phenotypic plasticity, aggressive behavior, territoriality, ability to consume specialized diets and the availability of many microhabitats in the lakes provide “textbook” conditions for sympatric speciation. Homoplasy (convergent/parallel) evolution also appears when comparing species from the 2 different lakes—why? Developmental contraints?. Figure 03A: Cichlid species
16 Figure 03B: Cichlid species - GraphOther examples mentioned in text include apple maggots and Hawaiin Drosophila (box 17.1).
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18 Mechanisms Facilitating Reproductive IsolationSympatric speciation mechanisms divided into 2 broad categories—5 pre-zygotic and 4 post-zygotic mechanisms (book groups some together) Text gives several examples on page 336-bird song (Darwin’s finches), mating displays and calls, floral displays to attract pollinators Normally, however, the barriers separating species are not caused by a single isolating mechanism but rather a combination of 2 or more evolving concurrently. Reference Drosophila example on page 336 bottom of page
19 Prezygotic mechanisms—Factors which prevent individuals from mating.Temporal isolation: Individuals do not mate because they are active at different times (known as allochrony-which may arise from resource partitioning, character displacement, competitive exclusion). This may be different times of the day or different seasons. Species mating periods may not match up, so individuals do not encounter one another during their mating periods. Ecological isolation: Individuals only mate in their preferred habitat. They do not encounter individuals of other species with different ecological preferences. Behavioral isolation: Individuals of different species may meet, but one does not recognize any sexual cues that may be given. An individual, totally oblivious, chooses a member of its own species. Mechanical isolation: Copulation may be attempted but transfer of sperm does not take place. The individuals may be incompatible due to size or morphology (insignificant). Gametic incompatibility: Sperm transfer takes place, but the egg is not fertilized.
20 Reproductive IncompatibilityIsolation by season, habitat or behavior may, but need not, lead to reproductive incompatibility. The most common mechanisms by which reproductive incompatibility functions to isolate previously joined populations (which otherwise could form a hybrid) are the Postzygotic mechanisms —Genomic incompatibility, hybrid inviability or sterility. Zygotic mortality: The egg is fertilized, but the zygote does not develop. Hybrid inviability: Hybrid embryo forms (or offspring), but is not viable. Hybrid sterility: Hybrid is viable, but the resulting adult is sterile. Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) are inviable or sterile. Hybrid inferiority (form of breakdown): hybrids dilute ideal adaptations of both parents and so have lower fitness due to poor fit in environmental niches, reinforcing species separation.
21 Sexual isolation in sympatric and allopatric populations In the case of hybrid breakdown/inferiority: Because they are geographically close enough to produce deleterious hybrids, sexual isolation should be strongest among sympatric populations of related species. Because they are too distantly separated to produce such hybrids, sexual isolation should be weakest among allopatric populations. The textbook describes examples of this phenomenon in Drosophila and moths. Experimental culling of Drosophila and corn hybrids demonstrates that selection against the hybrid caused rapid selection for sexual isolation and reduction in hybrid formation. Experimentally this is meant to mimic natural selection against the hybrid. However, as will be discussed shortly, sometimes hybrids are better and promote speciation!
22 An extensive survey of populations in nature also shows that sexual isolation between pairs of Drosophila species is greater for sympatric species than for allopatric species pairs. The basis for the analysis is genetic distance, which is a measure of the evolutionary divergence between two species, usually determined on the basis of allele frequencies. The lower the genetic distance, the more closely related the species. As shown in the figure below, when species in a pair are closely related — genetic distance between them is small— they are more isolated from each other when sympatric than when allopatric. Furthermore, allopatric populations require much greater genetic isolation (greater genetic distance) to achieve reproductive isolation than do sympatric populations. Figure 05: Sexual isolation for pairs of allopatric and sympatric Drosophila
23 Speciation and Hybridization ZonesWhere species barriers break down to produce viable and fertile hybrids, as often occurs in plants, zones of hybridization or hybrid swarms can develop in which genotypes and phenotypes differ from both parental species. In some cases, fertile hybrids can act as intermediaries, introducing genes from one species into the other, enhancing a species’ ecological range and evolutionary flexibility—there is more variation. If a habitat exists to which the hybrids are better adapted than are the parents, the new population may eventually become isolated, a process that occurs in plants and in animals. Thus if the hybrid is better, speciation is reinforced.
24 Speciation and Hybridization ZonesHeld to be especially common in plants Between 40 and 70% of all plant species are polyploid and so could have arisen by hybridization, although chromosome number can change within a single species without hybridization, and hybridization can occur without polyploidy. © Cousin Avi/ShutterStock, Inc. Courtesy of Clarence A. Rechenthin and USDA NRCS Texas State Office © Colin D. Young/ShutterStock, Inc. Figure 06: Western US sunflower species show rapid evolution of a hybrid
25 Animals – far less common than in plantsDarwin’s finches—ground and cactus finches Hybrid reproduction and survival is under environmental control—rains and seed availability. Hybrids primarily backcross rather than form stable population, thus gene flow between two species is major benefit leading to increased genotypic/phenotypic variation.
26 Frequency and Impact of Hybridization in NatureEvolutionary consequence of episodic hybridization is to increase the genetic variation on which selection can later act. The frequency with which hybridization occurs among closely related species varies greatly but as generalization the ability to hybridize is most commonly seen in groups undergoing rapid radiation, where species are more likely to be closely related than in lineages that diverged longer ago. The impact of hybrids is controversial as to its ability to form new species and has been best supported in plants.
27 Genes and Speciation Are specific genes responsible for speciation?Search for “speciation genes” mostly focused on sexual traits. Sexual traits undergo greater selection for differences between populations during speciation, and greater selection for uniformity within species after attaining speciation. Changes in sexual traits are proposed to correlate with early speciation events.
28 Origin of Species: In summary…Once again, what is the fundamental concept for speciation? 2 parts: Species arise by some mode of isolating a part of a larger population and it diverges (differential selection and adaptation) until it no longer interbreeds with the main population.