8.1_Diverge_speciation_S24.pdf

8.1 Divergence among populations and speciation TIME Evolutionary processes and patterns Today’s goal: Consider processes in single populations ongoing in multiple, separated populations and divergence among them    Intermating among individuals (esp. random mating) producing descendants results...

8.1 Divergence among populations and speciation TIME Evolutionary processes and patterns Today’s goal: Consider processes in single populations ongoing in multiple, separated populations and divergence among them    Intermating among individuals (esp. random mating) producing descendants results in genetic cohesion of a population. Considering multiple, distinct (sub)populations: gene flow between them tends to make them more similar in genetic composition. For alleles that have no effect on fitness (‘neutral alleles’), theory predicts that a single migrant between populations per generation results in the same frequency in each population. Darwin The general expected consequence of gene Evolutionary change flow between (sub)populations is to impede depends on genetic genetic divergence; homogenize populations. variation within populations Today’s goal: Connect processes in single populations with divergence among Galapagos finches populations Conversely, processes of population divergence, which may then lead to speciation, involve cessation of gene flow Darwin Evolutionary change depends on genetic variation Within populations Today’s goals: Contrast processes of divergence in allopatry vs. sympatry. Show how divergence among populations may result in speciation – Allopatric examples – Sympatric examples How are species limits maintained? Species concepts TIME Population divergence Overview of general concepts (idealizations) TIME Allopatric divergence between populations W W Assume that this large population is randomly mating. A geographic barrier arises, reducing gene flow between the E and W, allowing neutral genetic divergence due to drift. E Additionally, genetic divergence may proceed due to divergent selection between E and W. E Genetics of divergence in allopatry Pop 1 Pop 2 Alleles Assume that this large population is randomly Gene flow mating throughout the whole area it occupies. Here, physical barrier to gene flow, drift affecting Gene flow allele frequencies Fig. 8.3 FST is a measure of variance Gene flow tends to equalize allele of allele frequency among frequencies, whereas mating populations predominantly within each population results in differentiation. TIME Summary: Allopatric population divergence W W Assume that this large population initially mates at random. A geographic barrier arises reducing gene flow between E and W E can diverge from W by drift alone. Populations may further diverge due to adaptation to their local E environments, as well as drift, – Heterozygosity decreases E Variance among subpopulations in allele frequencies, FST, increases. TIME Population divergence TIME Sympatric population divergence Also in this case, assume that this large population is initially randomly mating. Disruptive selection may favor a distinctive phenotype (e.g. ‘green’ morphs have exclusive access to certain food types). Nevertheless, as long as intermating among the forms continues, the population remains cohesive (does not differentiate genetically). However, assortative mating may arise (even without a physical barrier); this tendency for mating between like phenotypes counters intermating between individuals with distinct phenotypes, such that subpopulations may diverge. TIME Population divergence Revisiting an example Mine site contaminated with heavy metals Pasture land Index of Zinc Tolerance Adaptation to Zn Antonovics and Bradshaw (1970) Adaptive process depends on heritable variation of Zinc tolerance in this grass. Early example of rapid evolutionary adaptation and genetic divergence despite ongoing gene flow. (Here, the wall does not completely prevent wind-dispersal of seeds and pollen) See also Fig. 8.11 Clinal variation: Interplay between selection and gene flow 1 2 Selection strong relative to gene flow Selection weak relative to gene flow Fig. 8.12 Divergence and Speciation TIME Gene tree Impediments/Barriers to gene flow (see Table 9.1) Variation in ancestral population Different alleles at a locus shown in different colors Divergence can proceed through drift, selection or both Today’s goals: Contrast allopatric and sympatric processes of divergence and speciation. – Allopatric divergence and speciation that depends on physical/external barriers to gene flow (by individuals or gametes). May involve either founder effects (drift) or adaptation to a new environment, or both. – Sympatric divergence and speciation depends on biological hindrances to gene flow that result in more frequent mating between individuals of like genotype. Mate preference plays a stronger role but habitat preference may also limit intermating between differentiating forms. Today’s goals: Contrast allopatric and sympatric processes of divergence and speciation. Species concepts: Classification of biological forms Show how divergence of populations leads to speciation – Allopatric examples – Sympatric examples How are species limits maintained? Species concepts How do we define species? (Section 9.1 in text) Recall Linnaeus (1707-1778) and his classification of species, ‘kinds’ Linnaeus’ system of classification accounted for and organized similarities within groups of organisms and differences among groups. Kingdom: plants, animals Phylum Class Order Family Genus Species Phenetic classification - organisms belonging to the same taxon look similar to one another AND distinct from those in other taxa How do we define species/higher taxa? Phenetic - organisms that belong to the same taxon look similar Discarded ‘Biological’ - Populations that are reproductively connected (and isolated from others) Phylogenetic Monophyletic groups, i.e. share common ancestor Summary: we may use both Phylogenetic Monophyletic groups, i.e. share common ancestor Biological - Populations that are reproductively connected (and isolated from others) Gene trees help clarify divergence and extent of reproductive isolation among species Today’s goals: Contrast allopatric and sympatric processes of divergence and speciation. Species concepts Show how divergence of populations leads to speciation – Allopatric examples – Sympatric examples How are species limits maintained? Allopatric speciation, given a geographic barrier arrows show ocean currents See Fig. 18.10 Closely related species may look very different Allopatric speciation, following emergence of a geographic barrier arrows are ocean currents Today’s goals: Contrast allopatric and sympatric processes of divergence and speciation. Delimiting species: : Classification of biological forms Show how divergence of populations leads to speciation – Allopatric examples - physical barriers can readily lead to divergence of populations and evolution of species differences Both drift and adaptation (to new environments) likely involved Because of physical barriers, hybridization between diverging populations is unlikely – Sympatric examples California Mimulus (monkeyflowers) M. lewisii M. cardinalis Non-random mating: due to pollinator preferences and fidelity More matings occur between similar flowers than between different flowers * A pre-zygotic reproductive isolating mechanism, impediment/barrier to hybridization (mating between the distinct forms) Reproductive isolation in sympatry can manifest at different points in the life cycle Pre-zygotic reproductive barriers Pollinator preference Mating system or mating preference Host preference Post-zygotic reproductive barriers Fertilization occurs Hybrids have lower fitness than progeny of conspecific matings Sympatric speciation of African cichlid fish See also Fig 9.1 (Meier et al. 2016) A hybridization event and subsequent rapid divergence in cichlid fish Sympatric speciation in African cichlids Cichlids in African rift lakes Dr. Catherine Wagner (U of WY) Initial divergence: visual cues and feeding differences How did incipient species further diverge? - Mating preferences based on visual cues Species diversity under threat – Cloudy water diminishes visual cues to mating preferences, collapse of species California Mimulus (monkeyflowers) Schemske et al. made crosses Habitat differences – a pre-zygotic barrier to hybridization Hybrids have lower fitness – This is a post-zygotic barrier to hybridization Of course, both processes may occur Hawaiian honeycreepers See Fig. 18.9 How is species divergence maintained? Spatial and even subtle habitat differences may limit gene flow Large scale Much smaller scale - Apple maggot fly species (Rhagoletis) use different hosts but share many alleles Phylogenetic species concept Looking backward in time, gene genealogies coalesce within species, in a more recent common ancestor than the divergence between species Fennessey et al. 2016 Evolutionary processes over differing time scales TIME Phylogenetic species delimited by divergence of genes Phylogenetic species: monophyletic group that all derive from a recent common ancestor that differs from the common ancestor of other species. Passerine super-radiation Keith Barker EEB Carl H. Oliveros et al. PNAS 2019;116:16:7916-7925 Rapidly diverging species are defined using these phylogenetic criteria Biological species concept Ernst Mayr (1942): “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Biological species concept But how much actual interbreeding (gene flow)? What is “potential” interbreeding? Cichlid fish in rift lakes were reproductively isolated but, more recently, have hybridized in polluted lakes. Species that at one time were distinct may ‘collapse’ into a single one through interbreeding. Biological and phylogenetic species concepts can often be reconciled Ernst Mayr (1942): “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Sp1 Sp2 Sp3