Chapter 24 Species & Speciation

Chapter 24 in a nutshell Microevolution v. Macroevolution Speciation Prezygotic Postzygotic Gradualism v. Punctuated equilibrium

Species Morphological species concept Biological species concept Divisions of organisms into discrete units called species If two organisms look different enough, then they are considered different species Biological species concept Population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring This population is unable to produce viable, fertile offspring with members of another population

Micro- vs. Macro- Evolutions Microevolution Change in the genetic makeup of a population over generations Changes (adaptations) confined to a particular gene pool or population Macroevolution Changes (evolution) above the species level Changes used to define higher taxa

Speciation The process that produces a new species May be the result of microevolution OR macroevolution How could microevolution lead to speciation? How could a new species arise from macroevolution?

Reproductive Isolation Biological barriers that impede members of 2 species from producing viable, fertile offspring There are 2 types of barriers that lead to reproductive isolation Prezygotic Pre-mating Post-mating Postzygotic

Prezygotic Barriers Pre-Mating Post-Mating Habitat isolation Temporal isolation Behavioral isolation Post-Mating Mechanical isolation Gametic isolation

Prezygotic Barriers (Page 1) Pre-Mating Habitat isolation 2 species occupy different habitats in the geographic area Tree-dwelling species, Bush-dwelling species Temporal isolation Temporal = time Species breed at different times Different time of day, different seasons, or different years Behavioral isolation Certain signals or types of behavior are integral to courtship, and these signals/behaviors are unique to the species Other species do not respond to these signals or behaviors

Boobies!! (blue-footed) Different mating season – late winter or late summer Different habitats – water or terrestrial

Prezygotic Barriers (Post-Mating) Mechanical isolation Anatomical incompatibility Insect copulatory organs are elaborated for a particular species Gametic isolation Even if gametes meet, they are unable to fuse Sperm may not be able to survive in the reproductive tract of females from another species Purple + Red sea urchins cannot mate b/c gametes cannot fuse

Postzygotic Barriers Reduced hybrid vitality Reduced hybrid fertility Hybrid Breakdown

Postzygotic Barriers Reduced Hybrid vitality Reduced Hybrid fertility Although zygote forms, genetic incompatibility causes developmental cessation In some salamanders, hybrids form but do not complete development Reduced Hybrid fertility Sterile hybrids Donkey + Horse = mule (Sterile) Hybrid breakdown Although first-generation hybrids survive & reproduce, their offspring are feeble or sterile

 No Geographic Barrier

Allopatric Speciation Causes Geologic events or processes that fragment the population Emergence of a mountain range Formation of a land bridge Evaporation of a large lake into multiple smaller lakes Leads to a significant alteration of a gene pool

Sympatric Speciation Sympatric = sym (same) + patr (fatherland) Same country Geographically overlapping populations Major mechanism of sympatric speciation is Polyploidy Autopolyploidy Allopolyploidy Preferential Habitation

Polyploidy Extra set(s) of chromosomes Autopolyploidy Example > 2n Typically, nondisjunction (Meiosis) in Plants All chromosomes from the same species Example Tetraploid (4n) + diploid (2n) Produce triploid (3n) offspring which are sterile But plant tetraploids can self fertilize or mate with other tetraploids = viable, fertile polyploid offspring Tetraploids are reproductively isolated from the diploid population

Polyploidy (Page 2) Allopolyploidy Plants 2 different species interbreed = hybrid Hybrids cannot breed with either original species But hybrids can successfully mate with each other Self-pollination successfully occurs as well

Polyploidy (Page 3) Can happen in animals but not common In animals, sympatric speciation is usually due to part of the population switching to a new habitat, food source, or other resource

Adaptive Radiation Many new species arise from a common ancestor given new environmental opportunities or challenges Think Darwin’s finches & Galapagos islands As the finches moved from South America to the Galapagos, different environments and food sources were encountered Hawaiian Plants As the plants spread to Hawaiian islands, different environments on different islands = evolution into several distinct forms

Hawaiian Plants that had a common ancestor  Arose from Adaptive Radiation

Gradualism vs. Punctuated Equilibrium Gradualism – species descended from a common ancestor and gradually diverge in morphology (acquiring unique adaptations) until they are reproductively isolated Punctuated Equilibrium – long periods of apparent stasis punctuated by brief periods of sudden and rapid change Fossil record confirms the punctuated equilibrium model Also eliminates the necessity of finding “missing links”