Chapter 24 ~ The Origin of Species

Macroevolution: evolutionary change above the species level: Speciation: the origin of new species Results in diversity of life forms Can be slow(gradualism) or occur in bursts (punctuated equilibrium). Macroevolution: describes evolution above the level of the species

Speciation and extinction: Mass extinctions are often followed by adaptive radiation Example: radial expansion of mammals after the extinction of dinosaurs (horse)

Two forms of speciation: 1- Anagenesis (phyletic evolution): accumulation of heritable changes Transforms a species into a species with different characteristics

Two forms of speciation: Cladogenesis: branching evolution The splitting of the gene pool into two or more separate pools, which give rise to one or more new species

Anagenesis vs. cladogenesis Which form of speciation is the basis for biological diversity?

How does the environment influence speciation How does the environment influence speciation? (physiological responses) Examples: Phototropism(plants and planaria) the response to the presence of light Photoperiodism(flowering plants, chickens and eggs) response in change in the length of the night

More examples: Diurnal/nocturnal Circadian rhythms: physiological cycle of about 24 hours Present in all eukaryotes Can persist in the absence of external cues Diurnal/nocturnal Seasonal responses- hibernation, estavation and migration

More examples: Quorum sensing in bacteria: regulation of gene expression in response to cell population and density

What is a species? Biological species concept (Mayr): a population or group of populations whose members have the potential to: Reproductive compatibility; Interbreeding, producing viable offspring Cannot produce viable offspring with members of other populations

Alternatives to the biological species concept: 1. morphological species concept 2. paleontological species concept 3. ecological species concept 4. phylogenic species concept

Reproductive Isolation (isolation of gene pools) New species arising when two populations diverge from a common ancestor and become reproductively isolated Must be maintained for species to remain distinct

Reproductive Isolation (isolation of gene pools) Prezygotic barriers: impede mating between species or hinder the fertilization of the ova Habitat (snakes; water/terrestrial) Behavioral (fireflies; mate signaling) Temporal (salmon; seasonal mating) Mechanical (flowers; pollination anatomy) Gametic (frogs; egg coat receptors)

Reproductive Isolation (cont.) Postzygotic barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity) Reduced hybrid fertility (mule; horse x donkey; cannot backbreed) Hybrid breakdown (cotton; 2nd generation hybrids are sterile)

Something to think about: Can prezygotic and postzygotic reproductive barriers break down? Are there real world examples of this??

Modes of speciation: (based on how gene flow is interrupted) Allopatric: populations segregated by a geographical barrier Separated from its parent population Adaptive radiation (island species) Occurs when potential to interbreed and produce fertile offspring no longer exists Examples: flycatchers

Modes of speciation (based on how gene flow is interrupted) Sympatric: reproductively isolated subpopulation in the midst of its parent population *no geographic isolation but reproductive barriers *Examples: polyploidy in plants; cichlids

Modes of speciation (based on how gene flow is interrupted)

Tempo of Speciation: Gradualism vs. Punctuated Equilibria Gradual divergence from a common ancestor Change in morphology over a long time as unique adaptations are acquired

Tempo of Speciation: Gradualism vs. Punctuated Equilibrium Sudden change Evolution in short bursts New species bud from parent species and then changes little for the rest of its existence