Speciation Lecture Notes PDF

Lecture 5: Species & Speciation Radiometric dating can inform the age of past events with precision The Cretaceous–Paleogene (or K–Pg) transition marks a major mass extinction event 65 million years...

Lecture 5: Species & Speciation Radiometric dating can inform the age of past events with precision The Cretaceous–Paleogene (or K–Pg) transition marks a major mass extinction event 65 million years ago caused by massive asteroid impact. The white layer corresponds to K-Pg ejecta layer exposed along Interstate 25 in southern Colorado. Ejecta layer was formed when rocks and dust blasted into atmosphere, causing a global winter, and killing off many species that couldn’t adapt. Pollen and spores from some Cretaceous plants are found immediately below this layer but not above it. Same pattern is observed for dinosaurs and other animal groups. Credit: Kirk Johnson, Denver Museum of Nature & Science, and Wikipedia Calculating Origination and Extinction Rates α = Origination Rate. Ω = Extinction Rate. α – Ω = Net Diversification Rate. Zimmer & Emlen Calculating Origination and Extinction Rates α Ω α–Ω 6 9 -3 8 12 -2 10 6 4 24 0 24 α = Origination Rate. Ω = Extinction Rate. α – Ω = Net Diversification Rate. Zimmer & Emlen  Origination, Extinction, and Diversification Rates of marine invertebrates (genera) α = origination rate Five mass extinctions observed In O, D, P, Tr and K by spikes in extinction rates. Ω = extinction rate Following mass extinctions, there are increases in origination rates Net Diversification Both α and Ω have declined Rate= α - Ω over time but Net Diversification rate remains constant over time.  Goals Species Concepts The Origin of New Species (Speciation)  Speciation: Outline Species concepts and definitions: – Biological Species Concept (BSC) – Phylogenetic Species Concept (PSC) Speciation: – Speciation and the role of geography. – Types of reproductive isolating barriers. – Hybrid sterility. – Reinforcement.  Early naturalists classified life’s diversity Carl Linnaeus (1707-78): Developed modern taxonomy. Long before the dawn of modern biology, and well before any concepts of evolution were available, we have been naming species.  Species concept v. Species diagnosis Species concept: what we mean by species. Species diagnosis: Defining criteria that can be used to delimit a particular species, and the traits that separate that species from other similar species. An example of a Species diagnosis Ramírez, 2005 Zootoxa  What is a species? eastern screech owl (Megascops asio). western screech owl (Megascops kennicottii). https://ebird.org/species/easowl1 https://ebird.org/species/wesowl1  Are species even real? “The so-called species problem can be reduced to a simple choice between two alternatives: Are species realities of nature or are they simply theoretical constructs of the human mind?” Ernst Mayr (1982) POLL: 1. Are Species Real? What observations support the notion that species are real entities that exist in nature. What observations support the notion that species are arbitrary human constructs that exist only in our minds.  Species concepts At least 24 different species concepts, the most common being: Biological Species Concept (BSC): Species are groups of interbreeding populations that are reproductively isolated from other such groups. Phylogenetic Species Concept (PSC): smallest possible group descending from a common ancestor recognizable by unique, derived traits. All species concepts share the notion that species are evolutionary independent lineages.  Biological species concept (BSC) Species are groups of actually or potentially interbreeding populations in nature, reproductively isolated from other such groups. Critical observations that contributed to the formulation of BSC: 1. Variation within species. 2. Geographic variation. Ernest Mayr 3. Existence of sibling/cryptic species (e.g. ( 1904-2005) screech owl).  Biological Species Concept (BSC) Species are groups of actually or potentially BSC: interbreeding populations in nature, reproductively isolated from other such groups. Reproductive isolation means that any of several biological differences between the groups greatly reduces gene exchange between them, even if they are not geographically isolated. BSC does not require 100% reproductive isolation (some genetic “leakage” through hybridization can happen between species). Problems: Can’t deal with fossils: Impractical for taxonomically diverse lineages How to assess allopatric populations? Crossable in lab is not enough What level of interbreeding between species is permissible? Cannot include asexual Species.  Elk and red deer: two species? The greatest practical limitation of the BSC is in determining whether populations that geographically separated (allopatric) belong to the same species or not. The BSC requires that we make judgement calls as to whether they would interbreed if they came into contact under natural conditions. Experimental or zoo populations can provide insight, but not always Some researchers argue that these two populations should be lumped in the same possible. species. Others contend that they should be considered two separate species.  Phylogenetic Species Concept (PSC) Phylogenetic Species Concept (PSC): smallest PSC: possible group descending from a common ancestor and recognizable by unique, derived traits. Greya moth The above pattern would be expected if a local population of ancestral G. piperella evolved reproductive isolation and became the distinct biological species G. mitellae. According the the BSC G. piperella and G. mitellae are two distinct species. According to the PSC, if G. mitellae is a separate species, the various populations of G. piperella must be considered different species.  The Process of Speciation Speciation is the evolution of biological barriers to gene flow. Study of speciation bridges the evolution of populations and the evolution of taxonomic diversity. Each branching point (node) in the tree of life marks a speciation event (origin of two species from one). Speciation can be considered a tug-of-war between two sets of forces: Selection & Recombination/Gene Flow  The Process of Speciation In Space These two examples illustrate how easter and western populations were geographically separated and diverged in the recent past. This is an example of allopatric speciation: The evolution of genetic barriers between Populations that were geographically isolated.  Geography of Speciation All three scenarios requires reduction of gene flow for new species to evolve In allopatric speciation, populations diverge while separated by a geographic barrier that that prevents/reduces gene flow between populations. After the two populations become so different that reproductive isolation evolves, the two can coexist without interbreeding. In parapatric speciation, neighboring populations diverge while still exchanging some genes (but not freely). Adjacent populations adapt to different habitats/niches and selection favors divergence. Reduction of gene flow is due to strong selection preventing colonization of the other habitat or the formation of hybrids. In sympatric speciation, two new species emerge from a single ancestor without any geographic isolation and while exchanging genes freely. This requires extremely strong divergent selection. This speciation mode is highly controversial and unlikely to happen in nature.  Sympatric speciation less likely than allopatric speciation: recombination breaks down genetic combinations that might form new species Suppose there are two bird morphs: thin bills specialize on insects and stout bills specialize on seeds. Both forms have high fitness and intermediates have lower fitness. In addition,m birds select mates on the basis of color: red mates with red, and blue mates with blue. In sympatry, recombination erodes linkage disequilibrium between the loci that affect color and bill size. In allopatry, geographically isolated populations can diverge both in traits that affect prezygotic isolation (such as color) and in traits that affect postzygotic isolation (such as bill size). If the differences are sufficient, the two populations will stay genetically distinct when they come into secondary contact. New species have formed.  Clarification about different types of barriers Geographic and reproductive barriers are not mutually exclusive, and instead often work together during the speciation process. Reproductive Barriers to Gene Flow Most important distinction of Reproductive Isolation Barriers: Prezygotic and Postzygotic Zimmer & Emlen