Speciation Chapter 14 March 2014

Defining Species

14.1 The origin of species is the source of biological diversity Microevolution is the change in the gene pool of a population from one generation to the next Speciation is the process by which one species splits into 2 or more species Every time speciation occurs, the diversity of life increases The many millions of species on Earth have all arisen from an ancestral life form that lived around 3.5 billion years ago

14.2 There are several ways to define a species The Biological Species Concept defines a species as Group of populations Whose members have the potential to interbreed in nature Produce fertile offspring Therefore, members of a species are similar because they reproduce with each other

14.2 There are several ways to define a species Reproductive Isolation Prevents members of different species from mating with each other Prevents gene flow btwn species and Maintains separate species Therefore, species are distinct from each other because they do not share the same gene pool

14.2 There are several ways to define a species The Morphological Species Concept Classifies organisms based on observable physical traits Can be applied to Asexual organisms Fossils However there is some subjectivity in declining which traits to use

14.2 There are several ways to define a species The Ecological Species Concept Defines a species by its ecological role and Focuses on unique adaptations to particular roles in a biological community For example, 2 species may be similar in appearance but distinguishable based on: What they eat Where they live

14.3 Reproductive barriers keep species separate Serve to isolate the gene pools of species and Prevent interbreeding Depending on whether they function before or after zygotes form, reproductive barriers are categorized as Prezygotic Postzygotic

5 types of Prezygotic Barriers Prevent Mating or Fertilization btwn Species 1. Habitat Isolation – 2 species live in the same general area but not in the same kind of place 2. Temporal Isolation – 2 species breed at different times (seasons, times of day, years) 3. Behavioral Isolation – little or no mate recognition btwn females and males of different species 4. Mechanical Isolation – female and male sex organs are not compatible 5. Gametic Isolation – female and male gametes are not compatible

3 Types of Postzygotic Barriers Operate After Hybrid Zygotes Have Formed 1. Reduced Hybrid Viability – most hybrid offspring don’t survive 2. Reduced Hybrid Fertility – hybrid offspring are vigorous but sterile 3. Hybrid Breakdown The 1st generation hybrids are viable and fertile but The offspring of the hybrids are feeble or sterile

Mechanisms of Speciation

14.4 In Allopatric Speciation, Geographic Isolation Leads to Speciation In Allopatric Speciation, populations of the same species are geographically separated, isolating their gene pools A Gene Pool is the total collection of genes in a population at any one time Isolated populations will no longer share changes in allele frequencies caused by Natural Selection Genetic Drift And/or Mutation Gene flow btwn populations is initially prevented by a geographic barrier. For example: The Grand Canyon and Colorado River separate 2 species of antelope squirrels And the Isthmus of Panama separates 15 pairs of snapping shrimp

14.6 Sympatric Speciation takes place without Geographic Isolation Sympatric Speciation occurs when a new species arises within the same geographic area as a parent species How can reproductive isolation develop when members of sympatric populations remain in contact with each other? Gene flow btwn populations may be reduced by Polyploidy Habitat Differentiation Or Sexual Selection Polyploidy = more than regular number of chromosomes (usually double)

14.6 Sympatric Speciation takes place without Geographic Isolation Many plant species have evolved by Polyploidy, in which cells have more than 2 complete sets of chromosomes Sympatric Speciation can result from polyploidy Within a species (by self-fertilization) or Btwn 2 species (by hybridization)

14.8 Isolated islands are often Showcases of Speciation The evolution of many diverse species from a common ancestor is Adaptive Radiation The Galapagos Archipelago Located 900 km west of Ecuador One of the world’s great showcases of adaptive radiation Has many species of plants and animals found nowhere else in the world