Northwestern Garter Snake (Thamnophis. ordinoides)

Northwestern Garter Snake (Thamnophis ordinoides)

Speciation I. What is a species? II. Isolating Mechanisms III. Models of Speciation

Aristotle coined the term species. Genus -a group that shares generic properties Species -a group that shares specific properties Aristotle (384-322 322 BC)

Linnaean System of Classification Carl von LinnJ a.k.a. Linnaeus (1707-1778) 1778)

Definitions of Species Morphological species Biological species Evolutionary species

Morphological Species A species is a group of individuals that are structurally similar. Two species or one?

Cryptic species of parasitic flies (Apocephalus paraponerae) Some flies feed and lay eggs in an injured worker ant. Larvae develop inside the ant and consume their host. Species of flies are separated by the ability to use specific species of ants. Distantly related flies have converged on the same species of ants. The two species of flies are genetically distinct, but phenotypically indistinguishable.

Biological Species A species is a group of potentially interbreeding individuals. Note: Not all individuals have to interbreed, but they must have the potential to do so. Ernst Mayr 1927 [Source: Mayr 1942, Systematics and the Origin of Species,, Columbia University Press]

The Ring

Biological Species Concept A species is a group of potentially interbreeding individuals. But what about asexual organisms? Oat aphid (Rhopalosiphum padi)

Evolutionary Species Concept A species is a lineage that maintains its integrity through space and time. A B Ancestor [Source: Wiens 2004, The American Naturalist 163: 914-923] 923]

Speciation depends on ecological distributions.? Allopatry -two groups don t occur in the same place Sympatry -two groups do occur in the same place

Allopatric speciation Sympatric speciation geographic isolation Evolutionary species Evolutionary and Biological species concepts evolution of reproductive isolation evolution of morphological differences Biological species Morphological species

Allopatric speciation Sympatric speciation geographic isolation Evolutionary species evolution of reproductive isolation Evolutionary & Biological species evolution of reproductive isolation evolution of morphological differences Biological species Morphological species

Isolating Mechanisms Spatial Temporal Behavioral Mechanical Gametic

Spatial & Temporal Isolating Mechanisms Tradescantia canaliculata -full sun on edge of cliffs -flowers in spring Tradescantia subaspersa -full shade of deep forest -flowers in summer Spider wort (Tradescantia spp.) Both spatial and temporal isolation prevent interbreeding.

Behavioral Isolating Mechanisms In Drosophila,, a female attracts a male with chemicals on the surface of her body. Fruit flies courting The chemicals on females of Drosophila melanogaster are unattractive to males of Drosophila simulans (and vice versa).

Mechanical Isolating Mechanisms Pollination of Sage A B A) Only large pollinators can pick up pollen from the anthers of S. apiana. Salvia mellifera B) Only small pollinators can land on the flower of S. mellifera. Salvia apiana

Gametic Isolating Mechanisms Sperm or egg is destroyed, or sperm cannot fertilize eggs. Fruit flies mating For example, when a male fly inseminates a female of another species, her vagina swells and prevents fertilization.

Isolating Mechanisms Pre-zygotic Spatial Temporal Behavioral Mechanical Gametic

Hybridization

Isolating Mechanisms Spatial Temporal Pre-zygotic Post-zygotic Behavioral Mechanical Gametic Hybrid Inviability Hybrid Sterility

Hybrid Inviability Many hybrids die either in the embryonic or juvenile stage. Green frog For example, frogs of the genus Rana hybridize but the resulting offspring die as embryos.

Hybrid Sterility Some hybrids survive just fine, but cannot reproduce. A mule killing a mountain lion For example, mules are the sterile hybrids of horses and donkeys. Yet, they are quite viable as hybrids (see left).

Pre- & Post-zygotic Isolating Mechanisms Drosophila pseudoobscura -low elevation (warm) -mates in evening Drosophila persimilis -high elevation (cool) -mates in morning Fruit flies Even if interbreeding occurs, hybrids have very low fertility.

How do new biological species form? OR How do isolating mechanisms evolve?

Mayr s Model of Allopatric Speciation A single species over a broad range. A geographic barrier arises that prevents gene flow. Populations evolve independently. The barrier disappears but the populations are no longer capable of interbreeding.

Seven Pairs of Species (P =Pacific; C= Caribbean) snapping shrimp (Alpheus spp.) [Source: Knowlton et al. 1993, Science 260:1629-1632]

Reinforcement When contact reoccurs between populations, selection can produce isolating mechanisms that finalize speciation. Usually, pre-zygotic isolating mechanisms evolve to reinforce post-zygotic isolating mechanisms. Why do you think reinforcement occurs?

Sympatric Speciation Speciation between populations living in the same area Proposed in the 1930 s s but not appreciated until the mid 1980 s Usually requires strong disruptive selection [Source: Via 2001, Trends in Ecology & Evolution 16:381-390] 390]

Disruptive Selection & Assortative Mating more fit least fit more fit Assortative mating Assortative mating

On Isla Santa Cruz, beaks sizes of Darwin s finches vary greatly. two birds caught in the same net Source: Hendry et al. 2006, Proc. R. Soc. Lond.

Beak size constrains the song of a bird.

Large-beaked male Small-beaked male Time (s) Frequency (khz) Frequency (khz)

Factors Promoting Sympatric Speciation Habitat choice and mate choice are each determined by one gene Genes for habitat choice and mating choice are physically linked (i.e., on the same chromosome). Even better, one gene could affect both habitat choice and mate choice. [Source: Via 2001, Trends in Ecology & Evolution 16:381-390] 390]

Sympatric Speciation by Polyploidy (duplication of chromosomes within a species) Most sexually reproducing organisms are diploid (two copies of each gene = 2N). These copies segregate during meisosis. Diploid cell Hybrids are sterile Tetraploids (4 N) are formed when chromosomes do not segregate during meiosis. Tetraploid cell