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).

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

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: 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 interbreed and produce viable, fertile offspring (genetic exchange is possible and that is genetically isolated from other populations)

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)

Other definitions of species: emphasize unity within a species: 1. morphological species concept: characterizes a species by its body shape, size, and other structural features. 

Other definitions of species: emphasize unity within a species: 2. paleontological species concept: focuses on morphologically discrete species known only from the fossil record

Other definitions of species: emphasize unity within a species: 3. ecological species concept: views a species in terms of its ecological niche, its role in a biological community.

Other definitions of species: emphasize unity within a species: 4

Other definitions of species: emphasize unity within a species: 4. phylogenetic species concept: defines a species as a set of organisms with a unique genetic history- a branch on a tree of life

Modes of speciation:(based on how gene flow is interrupted) Allopatric: populations segregated by a geographical barrier Separated from its parent population can result in 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 *(change in genome); polyploidy in plants; cichlid fishes

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 Equilibria Punctuated equilibrium: Sudden change Evolution in short bursts New species bud from parent species and then changes little for the rest of its existence