Evolution

Evolution

1. Natural Selection - the process in which organisms best suited to their environment survive to produce offspring

   • Stabilising - selection for those individuals with average phenotypes and against those with extreme phenotypes
     • Population tends to become more uniform
   • Directional - selection for one end of phenotypic range at expense of the other
     • Population tends to change in that direction
   • Disruptive - selection against average and for extremes
     • Population tends to become bimodal for that phenotypic characteristic

2. Speciation - any process that promotes divergence leading to new species

   • Species: a group of organisms capable of successfully reproducing (producing fertile offspring) in nature. Genetic barriers keep species distinct.
     • Demes - local population with limited gene flow with other demes. Phenotypic variations reflect local n.s. pressures
     • Clines - geographical gradient in phenotypic characteristics of individuals of same species. Altitudinal / latitudinal clines = common in plants
   • Allopatric- speciation occurs in different geographical areas (gene flow prevented by water etc.)
     • Isolated populations subject to different natural selection pressures. Accumulated difference in two gene pools result in reproductive isolation persisting even if two populations / species become sympatric again
   • Sympatric- speciation occurs in same geographical area (gene flow prevented by behavioural, ecological etc. mechanism)
     • e.g. plant mutation that changed its flowering period cannot cross-polinate with plants of the same species, so would become isolated reproductively
   • Instantaneous speciation by polyploidy - new species in 1 generation (special case of sympatric). Polyploids = 3+ sets of homologous chromosomes, increased vigour, pest / animal resistance, can cope with extreme environments.
     • Autopolyploidy - all sets of chromosomes are from same ancestral species (new species has 1 parent species)
     • Allopolyploidy - all sets of chromosomes originate from 2+ ancestral species (new species arose from otherwise sterile hybrid)
     • Amphiploidy - doubling of chromosome number that occurs at first mitosis after fertilisation. Sterile (2n) hybrid between 2 species becomes a fertile (4n) allopolyploid.

3. Patterns of Evolution

   • Gradualism - evolutionary change that occurs relatively slow + steady. Includes transitional forms that have accumulated adaptive characteristics.
   • Convergent - species with similar niches tend to evolve similar adaptations despite having different ancestors.
     • Ecological equivalents: species with different origins developed similar adaptations because they occupy similar niches in different geographic areas.
     • Analogous organs: organs have same basic structure + function but evolved independently from different ancestral organs in unrelated species.
     • Parallel evolution: similar features evolve in unrelated or related species because both have been subject to similar selection pressures.
   • Punctuated equilibrium - short periods of rapid change interspersed with longer periods of relatively slow change (stasis)
     • Rapid change - invasion of new habitat with niches available, sudden environmental change allows one group to diverge rapidly, evolution of a new characteristic with a distinct advantage that allows one group to out-compete +replace the other.
     • During stasis the gene pool is in 'equilibrium' with environmental selection pressures so little evolutionary change occurs.
   • Divergent - when 2+ related groups evolve different adaptations because they occupy different niches.
   • Adaptive radiation - divergence of a number of species from common ancestor. Each occupies different niche + evolved different adaptations.
     • Describe ancestor, likely niche, appearance + adaptions, when it existed, possible migrations
     • Describe new habitats into which ancestor/ descendants spread.
     • Explain of how new natural selection pressures in different habitats affected each isolated species and the adaptations each evolved.
     • Discuss relevant reproductive isolating mechanisms that operate to keep gene pools separate.
     • Homologous organs - found in related species evolved from common ancestral organ, adapted (diverged) to different functions, but retain common structure.
   • Co-evolution - the reciprocal evolutionary effect that two interacting species can have on each other. Each species acts as a natural selective influence on the other.

4. Reproductive Isolating Mechanisms - a factor that prevents successful reproduction between 2 species (prevents gene flow between 2 gene pools)

   • Geographic (oceans isolate gene pools) or temporal (timing of reproduction)
   • Ecological (habitat - closely related species occupy different niches within same geographical area, ethological (behavioural)
   • Structural e.g. plants flower shape determines which pollinators gain entry.
   • Gamete incompatibility - biochemical differences prevent zygote being formed, sperm/pollen does not respond to chemicals produced by egg.
   • Hybrid inviability: chromosomal incompatibility leads to developmental problems + low survivorship of hybrids. Hybrid sterility: hybrids viable but sterile. Hybrid breakdown: F1 hybrids are fertile but F2 individuals are not.