The Origin of Species

Speciation: Formation of new species

What is a Species? “morphospecies” – species are defined by their anatomical features -- used by Linnaeus

Biological Species Concept Proposed by Ernst Mayr (1942) Biological species: groups of interbreeding natural populations that are reproductively isolated from other such groups Reproductive barriers preserve species integrity Largest unit in which gene flow is possible Defined by reproductive isolation from other species in natural environments

Cannot be applied to: 1. organisms that are completely asexual in their reproduction 2. extinct organisms represented only by fossils 3. geographically segregated populations

Reproductive Barriers Any factor that impedes two species from producing fertile hybrids Prevents interbreeding between closely related species

Prezygotic Barriers Impede mating or fertilization 1. habitat isolation: 2 species living in different habitats within same area may not encounter each other very often

2. temporal isolation: 2 species that breed at different times of day, seasons or years (e.g. brown trout breed in fall, rainbow trout breed in spring)

3. behavioral isolation: species-specific behavior and elaborate courtship behavior; (e.g. blinking patterns in fireflies, -- pheromones (insect scents) -- bird songs pie-billed grebe great blue heron

4. mechanical isolation: anatomical incompatibility

5. gametic isolation: the gametes of different species are rarely able to fuse

Postzygotic Barriers Prevent development of viable, fertile hybrid 1. reduced hybrid viability: genetic incompatibility may abort hybrid as an embryo or hybrid is very frail

2. reduced hybrid fertility: hybrids are usually sterile

3. hybrid breakdown: first generation hybrid is fine, but next generation is feeble or sterile

Allopatric Speciation Geographical barrier physically isolates the population (mountain ranges emerge, glaciers move, land bridges form)

Conditions Favoring Allopatric Speciation: Small, isolated population Geographical isolation of a small population usually occurs at fringe of parent population’s range (“peripheral isolate”) Gene pool of peripheral isolate is probably different from parent pool Genetic drift, natural selection may go in different direction

Sympatric Speciation Formation of new species within the range of the parent population Reproductive isolation evolves without geographical isolation Many plant species have evolved from improper cell division that results in extra sets of chromosomes (“polyploidy”)

A) autopolyploidy: an organism that has more than 2 chromosome sets, all derived from a single parent species -- mutants can’t breed with diploid parents because hybrids would have odd number of chromosomes  postzygotic barrier

B) allopolyploid: a polyploid hybrid resulting from contributions by two different species More common than autopolyploidy 2 species interbreed to produce hybrid

Habitat Differentiation: Another way for sympatric speciation to occur A subpopulation uses a habitat or resource not used by the parent population

For example, the North American apple maggot fly’s original habitat was the hawthorn tree, but some populations colonized apple trees about 200 years ago Apples mature more quickly than hawthorn fruit, so the natural selection has pressured the apple tree flies to develop more rapidly Temporal isolation has now occurred between the populations

If 2 closely related populations come together after a lengthy separation, They may: 1. interbreed freely (no speciation has occurred) OR 2. reproductive barriers may prevent interbreeding (speciation) 3. hybrid zone: region of interbreeding where geographical regions overlap

Hybrid Zones Occur where two species have incomplete reproductive barriers 3 possible outcomes:

1. Reinforcement When hybrids are less fit than parent species Natural selection strengthens prezygotic barriers to reproduction This reduces formation of unfit hybrids

2. Fusion Occurs when barriers to reproduction are weak Lots of gene flow occurs The hybridizing species may eventually fuse into a single species

3. Stability Hybrids survive or reproduce better than members of either parent species, at least in certain circumstances

Punctuated Equilibrium Eldredge and Gould Long periods of stasis are punctuated with episodes of rapid speciation