Speciation: outline

Species concepts & problems Biological, Phylogenetic, Morphological Speciation Speciation: Types of reproductive isolation Origins of reproductive isolation allopatry or sympatry (w/ or w/o gene flow) What is a species? How and why do species Hybridization and possible outcomes evolve? incompatibility, reinforcement, introgression, speciation, How long does it take? polyploidy What roles do drift, Adaptive radiations selection, mutation, and non- random mating play? Reading: Freeman and Herron Chapter 16 336-15 1 336-15 2

Major patterns of evolutionary change Multiple species concepts exist.

Three key: • Evolutionary change within lineages over time (anagenesis). biological species concept (BSC) phylogenetic (genealogical) species • Splitting of lineages over time concept (PSC) – (cladogenesis). morphological (typological) species concept (MSC)

336-15 3 336-15 4 The Biological Species Concept (BSC) Problems with the BSC

“A species consists of groups of actually or potentially • How do you characterize interbreeding populations of organisms that are reproductively potential mating opportunity isolated from other such groups.”(Mayr, 1942) in geographically separate populations?

– Species are breeding units that are evolutionarily independent • Cannot be applied to fossil from other breeding units taxa

• Does not apply to asexual Reproductively isolated = no exchange of genetic material. taxa (bacteria, viruses)

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BSC and hybrids Phylogenetic Species Concept (PSC)

How much reproductive isolation is necessary? • A species is the smallest set of populations (if 1 in 10,000 of wild caught, pregnant Drosophila sexual) or lineages (if asexual) that shares unique, derived features pseudo-obscura had mated with D. persimilis Hybrids between Helianthus annuus and H. • All descendants of a common ancestor petiolaris are fertile in back-crosses

• Shared, derived features (synapomorphies) indicate actual interbreeding groups of sexual “Good” species? organisms

336-15 7 336-15 8 Importance of shared derived traits Problems with the PSC

Taxa that are not Derived traits should • How much must a population differ from other interbreeding may only be shared if the populations to be a species? share ancestral traits lines are interbreeding

Different populations may “splitter”: every “lumper”: require differ in one or two traits difference defines many differences but still reproduce and a new species for a new species share most other traits

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Morphological species concept Species Continuum

• Species are defined by differences in At what point do we morphological characters Different evolutionary processes recognize two species? result in different patterns over • Widely used in practice time. Initial changes may affect • Workable for all kinds of organisms morphology – extinct or extant – sexual or asexual Hyla chrysoscelis (2n) and H. versicolor (4n) ecology reproduction

• Major Difficulties: – no tie to evolution – genetic differences may not be reflected in morphology 336-15 11 336-15 12 Species are dynamic Lake Victoria Cichlids

• 500 species in one lake What species concepts? Populations that appear to be • Very recent origin ca. “good” species may not persist 12,000 years • Distinct colour patterns • Can hybridize, usually do not . . .

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Applying Species Concepts: the eastern wolf Hypothetical canid distribution

black: eastern wolf (Canis lycaon – N. Am origin) speckle: eastern wolf coyote (Canis red wolf latrans – N. Am. origin) Data: gray: gray -many red wolves and eastern wolves have coyote wolf (Canis mtDNA lupus – -red wolves and eastern wolves hybridize with coyotes Eurasian origin) 336-15 15 336-15 16 -gray wolves rarely hybridize with coyotes Eastern Wolf or Coyote? Species concept used to study speciation? • Biological species concept (very loosely) – must be some barriers to gene flow – some hybridization may still occur Coyote (Canus latrans) • What processes lead to reproductive BSC: similarity to coyote may indicate hybridization isolation? MSC: specimens from before 1930 suggest eastern wolf distinct – different processes establish reproductive isolation in different ways PSC: some DNA from pelts <1930 suggest that the eastern wolf is a hybrid

Should the eastern wolf be saved? 336-15 17 336-15 18

Reproductive isolation Prezygotic Isolation • Pre-zygotic Geographic or Ecological Isolation – occurs before hybrid embryo forms – differences in space – behavioral, ecological – individuals do not overlap in space either • Post-zygotic on a landscape or microenvironment scale – occurs after hybrid embryo forms – genetic, ecological Temporal Isolation – differences in time – individuals do are not reproductively active at the same time or season 336-15 19 336-15 20 Prezygotic Isolation Ethological Isolation Pre-zygotic isolation – sexes of the two populations are not • Different flowers may have attracted (they don’t speak the same different pollinators language) • Mechanical isolation: genitalia between different species may not fit • Gametes may not survive in reproductive system • Sperm may not have correct protein to fertilize eggs (sea urchins)

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Why speciate? Post-zygotic isolation Adaptive landscape After reproduction Hybrid Inviability – hybrid zygotes are formed but are inviable or have extremely reduced fitness; may be at F1 or F2 – may be intrinsic, or dependent on the environment

Hybrid Sterility – hybrids of one sex or the other do not produce functional gametes – heterogametic sex is usually sterile “Haldane's rule” (male in mammals: XY)

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