Speciation Topic 6 PDF

Topic 6 SPECIATION 1  Learning objectives Define speciation, reproductive barriers, and reproductive isolation Outline the different species concepts, list their advantages and disadvantages, and discuss why, when speciation is considered as a process, they may not always agr...

Topic 6 SPECIATION 1  Learning objectives Define speciation, reproductive barriers, and reproductive isolation Outline the different species concepts, list their advantages and disadvantages, and discuss why, when speciation is considered as a process, they may not always agree Describe the various forms of pre- and postzygotic barriers and be able to classify examples correctly Define allopatric and sympatric with respect to the geography of populations and of speciation Explain the different mechanisms of speciation (i.e., how/why does reproductive isolation evolve under each) and the geographic context in which each can occur Summarize the technique of experimental evolution and discuss how lab experiments can employ it to provide insight into our understanding of speciation Outline conceptually how the existence of ecologically-dependent postzygotic isolation provides evidence for ecological speciation and explain how to test for this via a reciprocal transplant experiment 2 Outline 6.1 Speciation & species concepts 6.2 Reproductive barriers 6.3 Geography of speciation 6.4 Mechanisms of speciation 6.5 Studying speciation 3  Speciation Is the process by which one species splits into two or more species Much of the biological diversity of life exists among species How new species arise is one of the most intensively studied questions in evolutionary biology 4  Speciation When speciation occurs, the new ‘daughter’ species share characteristics because they are descended from a common ancestor Species are therefore related to one another to differing degrees, like individuals in a family tree This explains why species can be classified into nested groups based on similarities (i.e., Linnaeus and subsequent taxonomists) Speciation links microevolutionary processes with macroevolutionary patterns 5 Peter Halasz,Public domain, via Wikimedia Commons Species concepts – What is a species? ‘Species’ is Latin for ‘kind’ or ‘appearance’ Speciation is a process and pairs of taxa vary in how far they are along; it eventually reaches an irreversible endpoint when two taxa cannot form fertile hybrids – i.e., they are entirely separate gene pools between which alleles cannot move When the process has not yet reached an irreversible endpoint, different definitions will not necessarily agree Credit: Edward, Ben Himme Different definitions are not right or wrong, but can be more or less useful in a given context >20 different definitions have been proposed (e.g., biological, phylogenetic, cohesion, recognition, ecological, internodal, morphological)! Our goal is to try to understand how new species arise 6  Biological species concept (BSC) Biological species are groups that actually or potentially interbreed and produce fertile offspring in nature, and which do not form fertile offspring with members of other groups – i.e., two populations are part of the same species if they are reproductively compatible, and they are different species if they are ‘reproductively isolated’ The ability to form fertile offspring between two groups (i.e., reproductive compatibility) means gene flow is possible, and the resulting exchange of alleles slows or prevents the accumulation of genetic differences Reproductive isolation is the opposite of compatibility; it is the result of reproductive barriers, defined as biological features of organisms that reduce or prevent gene exchange with members of other groups 7  Dis/advantages of the BSC Biologically relevant: lack of gene exchange between populations is the yardstick for species status and lack of gene flow means they are evolving independently (i.e. they are separate gene pools) Provides a straightforward approach to studying speciation: understanding how reproductive barriers evolve But… Can’t be applied to asexual taxa (e.g., prokaryotes) or fossils Can be hard to assay reproductive barriers (compared to morphology or genetics) **Many ‘good species’ exchange genes, with natural selection maintaining divergence despite ongoing gene flow (e.g., current estimate is that ~16% of birds hybridize with at least one other species in nature, some with multiple other species) 8 Mallard x Red-crested Pochard hybrid © Julen Santa Cristina; The Cornell Lab of Ornithology  Other species concepts Morphological – defines species based on morphological similarity (i.e., as groups that are morphologically distinct from one another are different species) – Can be applied to fossils and asexual taxa; but is subjective and misses cryptic species Ecological – defines species based on their ecological niche (the set of physical and environmental conditions it requires, including interactions with other species); i.e., groups that inhabit the same niche – Can be applied to asexual taxa but not fossils; is subjective (niches are hard to quantify) There are many others, but for understanding speciation in sexual taxa, the BSC has proven the most useful by far and is nearly universal in its use 9 Case study: freshwater threespine sticklebacks Video correction: pelvic Limnetic SPINES (not fins) to protect them from gape- limited predators Benthic L1 B1 L2 B2 How many species? 10 Case study: ring species Greenish warbler Dibyendu Ash, CC BY-SA 3.0 via Wikimedia Commons 11 Irwin (2001) https://doi.org/10.1038/35053059 Outline 6.1 Speciation & species concepts 6.2 Reproductive barriers 6.3 Geography of speciation 6.4 Mechanisms of speciation 6.5 Studying speciation 12 Reproductive barriers Can be distinguished by whether they occur before or after fertilization (i.e., formation of the zygote), yielding prezygotic (before fertilization) and postzygotic (after fertilization) Prezygotic – block fertilization by reducing the likelihood of mating, by preventing an attempted mating from being successful (i.e., from transferring sperm), or by impeding fertilization after a successful sperm transfer Postzygotic – reduce the survival/or and reproductive success of hybrids (i.e. reduce the fitness of the hybrid zygote) 13 Prezygotic barriers Benthic females Before mating: habitat isolation temporal isolation Limnetic females behavioural/pollinator isolation threespine sticklebacks Echenopa treehoppers Ryan Hodnett, CC BY-SA via Wikimedia Commons 14  Prezygotic barriers After mating: mechanical isolation gametic isolation (including sperm competition & sperm preference) Copyright © 2006 Bill Claff orchid pollen adhering to different parts of a moth’s head Gametic isolation in ground crickets (Allonemobius spp.) Futuyma, Evolutionary Biology, 3rd ed. 1998. Sinauer Howard et al. 1998. Evolution 15  Postzygotic barriers 1) Genetic incompatibilities – genes of the two parent species interact in ways that reduce the viability (i.e., survival) and/or fertility (i.e., sterility) of F1 or later generation hybrids – independent of the ecological context of the hybrid © 2004 Nature Publishing Group Wu, C. & Ting, C. Genes and speciation. Nature Reviews Genetics 5, 118 (2004). All rights – can arise due to any mechanism of speciation (due to any microevolutionary process) male horse × = hinny R. sylvatica Rana pipiens female donkey (wood frog) (leopard frog) male donkey × = mule female horse F1 hybrids are inviable F1 hybrids are sterile  Postzygotic barriers 2) Ecologically-dependent postzygotic isolation – a reduction in hybrid fitness due to a mismatch between their phenotype and their environment (i.e., hybrids ‘fall between niches’) – can only be produced by divergent natural selection lab wild Outline 6.1 Speciation & species concepts 6.2 Reproductive barriers 6.3 Geography of speciation 6.4 Mechanisms of speciation 6.5 Studying speciation 18 Geography of populations Allopatric – occurring in separate, non-overlapping geographic areas Sympatric – occurring in the same, overlapping geographic area (Parapatric – something in between; i.e., partial overlap) Fig. 23.5, Campbell Biology, 3rd Canadian Edition. 2021. Pearson 19 Geographic context of speciation 1) Allopatric speciation – Gene flow is interrupted by some extrinsic barrier – Populations evolve independently, eventually accumulating reproductive barriers – If secondary contact occurs, populations are reproductively isolated 20  Allopatric speciation Biogeographic evidence – Many closely related species pairs have geographic ranges that abut one another, often Prof. Nancy Knowlton, Smithsonian Museum separated by some sort of geographic barrier; e.g., shrimp across the Isthmus of Panama (see https://youtu.be/39ewdLEuScU for a more detailed explanation of this example) – Young sister species tend to be allopatric, not sympatric – Isolated habitats (e.g., islands, mountaintops, lakes) often harbour endemic species – Reproductive isolation among populations often increases with geographic distance between them 21 Anker A, Grave S, Alpheus cedrici holotype, dorsal view - ZooKeys-183-001-g003A, CC BY 3.0 Allopatric Speciation downloaded from PBS LearningMedia, https://www.pbslearningmedia.org/resource/tdc02.sci.life.evo.allopatric/allopatric-speciation  Geography of speciation 2) Sympatric speciation – Speciation in the absence of any extrinsic or geographic barrier (i.e., between populations that inhabit the same region) – Difficult to overcome the initially unrestricted gene flow – Can occur via polyploidization or via strongly disruptive selection alongside some mechanism for positive assortative mating (i.e., so mating tends to occur within, and not between, each incipient species) – Restrictive conditions, and biogeographic evidence of allopatric speciation, suggest sympatric speciation is rare 22  Sympatric speciation - example Lord Howe palms kentia or thatch palm (Howea forsteriana) and curly palm (H. belmoreana) are two endemic species molecular evidence indicated that they diverged from a single common ancestor edited by M.Minderhoud, Public domain, via Wikimedia Commons well after the island formed specialized on two different soil types which causes temporal isolation as a by-product tiny size of the island (