03 Origins of Species 2025 PDF

Unit 1: Evolution and Biodiversity What is Evolution? Phylogenetic Trees Evolution of Populations...

Unit 1: Evolution and Biodiversity What is Evolution? Phylogenetic Trees Evolution of Populations Origin of Species History of Life Biodiversity: Bacteria, Archaea, Protists, Fungi & Plants The Tree of Life/ The Phylogeny of Life Spring 2025 Today’s Topic: Origin of Species Ch. 24  Speciation Key points from today’s lecture 1) Speciation, the process by which one species evolves into two species, usually involves geographic isolation and results from accumulated genetic differences between populations. 2) The biological species concept defines a species as a group of potentially interbreeding populations that is reproductively isolated from other such groups. 3) Reproductive isolating mechanisms maintain or reinforce genetic differences between populations and can be categorized as pre- zygotic or post-zygotic. 4) A goal of systematics, the study of evolutionary history, is to build monophyletic evolutionary trees termed phylogenies. Synopsis for today Understand the role of sexual selection Identify reasons why polymorphisms are maintained in populations Differentiate pattern (species) and process (speciation) Understand how scientists define species Understand and describe ways in which speciation can occur Flightless cormorant (Phalacrocorax harrisi)  Are these different species? gobotany.nativeplanttrust.org A. Yes A. Yes B. No B. No Are these different species? 1. How do we recognize and name species? 2. How could we define the process giving rise to new species (“speciation”)? 3. Is there necessarily a relationship between how we define what a species is and the process of speciation?  Are these different species? Sterile… Donkey Horse Mule 1. How do we recognize and name species? 2. How could we define the process giving rise to new species (“speciation”)? 3. Is there necessarily a relationship between how we define what a species is and the process of speciation? Fig. 22-UN1 Patterns of Natural Selection When natural selection acts on a population, phenotypes can change in 3 ways (altering mean and variance): shift mean increase variance decrease variance Speciation links Microevolution & Macroevolution leads to… Microevolution Macroevolution Change in allele frequency between Long-term & large scale change in generations populations/species Peppered moth - Biston betularia  What Is a Species? Latin for “kind” or “appearance” Species often distinguished on the basis of morphology, physiology, biochemistry, behavior and/or genetic sequence Western Meadowlark Eastern Meadowlark Differences in geographic distributions, songs, and behaviors prevents interbreeding What about… Homo sapiens most recent common ancestor of ALL humans ~200,000 years ago originated in Africa; more genetic diversity between individuals in sub-Saharan Africa than between individuals from anywhere else (“Out of Africa” hypothesis)! no biological barriers to interbreeding between any groups vast majority of our genomes are identical BUT individuals are unique (intraspecific variation!) phenotypic characters associated with “race” are under control of many genes (quantitative traits) perceptions of race often not = ancestry  Biological Species Concept “Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups” (E. Mayr, 1942) 1. Produce viable, fertile offspring (at least potentially) 2. Breeding with other species is prevented (reproductive isolation) But some species reproduce asexually, and we cannot tell if extinct “species” could have viable offspring…  The Species “Problem” Other Definitions of Species: Morphological species concept (look different) Ecological species concept (unique environmental niche) Phylogenetic species concept (monophyletic groups only identified) etc…  Biological Species Concept + = Is a mule a species? A. Yes B. No  Biological Species Concept + = Is a mule a species? NO - according to the biological species concept  Morphological Species Concept Species = unique combination of traits Historically used physical traits (e.g., Linnaeus) Disadvantages: Subjective choice of traits Cactus Euphorbia Non-morphological trait differences? Convergent evolution can lead to incorrect estimates of common ancestry  Phylogenetic Species Concept Species = monophyletic groups identified by shared, derived traits traits that are unique to descendants of a common ancestor Disadvantages: phylogenies not available for all taxa many more species than currently recognized  Speciation due to Selection Speciation: evolutionary process of divergence of one species into two species Directional and disruptive selection can split one species into two or more species Requires reproductive-isolating mechanisms to prevent breeding (i.e., gene flow) between divergent populations  Reproductive-Isolating Mechanisms Prezygotic barriers Postzygotic barriers Habitat Temporal Behavioral Mechanical Gametic Reduced Reduced Hybrid isolation isolation isolation isolation isolation hybrid hybrid breakdown viability fertility VIABLE, Individuals of MATING FERTILI- FERTILE different ATTEMPT ZATION OFF- species SPRING (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) (k) prevent gene flow between populations  Pre-zygotic Barriers Impede mating or fertilization if mating occurs Ecological isolation – ecological (habitat) isolation Horned lizards Thorny devils – temporal isolation – behavioral isolation – morphological (mechanical) isolation – isolation of gametes (gametic) Behavioral isolation bird of paradise.jpg Temporal isolation  Post-zygotic Barriers Hybrid inviability prevent a hybrid zygote from developing into a viable, fertile adult – hybrid inviability: hybrid does not make it to reproductive age Hybrid sterility – hybrid sterility – hybrid breakdown: offspring of the hybrid have reduced viability or fertility  Modes of Speciation Allopatric speciation (most common) – geographical isolation through the formation of a physical barrier Ex. 1 Uplift of mountain range splits a population Ex. 2 New river forms & splits a population Ex. 3 Lowering of water level of a lake produces 2 smaller lakes 23 Allopatric speciation – 15 pairs of sibling species of snapping shrimp (Alpheus) – separated by the Isthmus of Panama species originated 9 to 13 million years ago Uplift of Isthmus of Panama separated the Atlantic and Pacific Oceans  Modes of Speciation Sympatric speciation (rarer) – NO geographical isolation – Requires special conditions How can this occur? – Polyploidy (genome duplication) Rare in animals, common in plants – Strong divergent selection Habitat differentiation (space & time) Sexual selection Polyploidy Sequencing of whole Allopolyploid speciation genomes has helped Autopolyploid speciation (Allos = other) uncover ancient (Autos = self) 2 or more chromosome sets from polyploid events in 2 or more chromosome sets from different species form asexual hybrid single species ancestors of most plants (and animals!) Duplicate copies of genes may allow for evolutionary innovations Polyploidy is very important in plant breeding Examples: grapes, citrus, apples, wheat, potato, cotton, etc. 26  Sexual Selection (nonrandom mating) Hypothesis: Females mate only with males of the same species, based on coloration Species A Species B 2 sympatric cichlid species from Lake Victoria  Sexual Selection (nonrandom mating) Results: Consistent with the hypothesis that females identify and mate with their own species based on color Lamp of orange light = Lamp of natural light = random mating Non-random mating Species A Species A Species B Species B (Currently experiencing loss of species, possibly due to water pollution and thus murkiness)  Hybrid Zones Exist Between Separate Species Toad example: Ranges 4000-km long hybrid zone, but just 10 km wide Zone is along altitudinal gradient but toads can easily cross zone Species separation maintained because hybrid offspring have developmental defects & low survival  Hybrid Zones Separate Species Ranges What happens when two species come back into contact? Gene flow Population Barrier to gene flow Formation of a Hybrid Zone & Possible Outcomes Isolated population diverges. Gene flow Population Barrier to gene flow Formation of a Hybrid Zone & Possible Outcomes Isolated population Hybrid diverges. zone Gene flow Population Barrier to gene flow Hybrid individual Formation of a Hybrid Zone & Possible Outcomes Isolated Possible outcomes: population Hybrid diverges. zone Reinforcement – Selection to prevent hybrid formation Fusion – Species fuse back Gene flow into one species Population Barrier to gene flow Stability – Hybrids Hybrid continue to form, don’t individual leave narrow zone  How Fast is Speciation? Gradualism: species arise due to gradual accumulation of different traits (Darwin: observations of fossil record) Why Aren’t There More Intermediate Fossils? ? Taphonomy New species may have arisen allopatrically (different place) Rates of speciation aren’t always slow!  Punctuated Equilibrium Model Stephen J. Gould: hypothesis that species apparently remain relatively phenotypically constant over long periods of geologic time, then diverge rapidly & change little for long periods of geologic time Geologic time Change in form Next: Adaptive Radiation & Phylogeny Lancelet (outgroup) Lamprey Tuna Vertebral column Salamander Hinged jaws Turtle Four walking legs Amniotic egg Leopard Hair