Speciation & Species Concepts PDF

## Speciation & Species Concepts ### Speciation - **Process** in which a species splits into 2+ species - "Daughter" species share characteristics (common ancestor) - nested groups (Linnaeus, etc.) - **Irreversible endpoint**: two taxa can't form fertile hybrids ### Species Concepts - Understand...

## Speciation & Species Concepts ### Speciation - **Process** in which a species splits into 2+ species - "Daughter" species share characteristics (common ancestor) - nested groups (Linnaeus, etc.) - **Irreversible endpoint**: two taxa can't form fertile hybrids ### Species Concepts - Understanding how new species arise ### Biological Species Concept (BSC) - Groups that interbreed and produce fertile offspring in nature - i.e. - Same species if they are reproductively compatible. - Diff species if reproductively isolated. Gene flow possible! ### Biological Features: Reproductive Barriers - Biological features: reproductive barriers that reduce/prevent gene exchange with other groups ### Advantages - Biologically relevant (lack of gene exchange and flow, evolving independently) - Straightforward approach to studying speciation ### Disadvantages - Not for asexual taxa (e.g. prokaryotes, fossils) - Hard to assay reproductive barriers. - Natural selection maintains divergence despite gene flow → some species make hybrids! ### Other Species Concepts - **Morphological Similarity** (misses cryptic species) - **Ecological Niche** - Conditions required - Can't apply to fossils - Subjective ## Reproductive Barriers ### Prezygotic - **Before mating**: - Habitat isolation - Temporal isolation - Behavioral/pollinator isolation - **After mating**: - Mechanical isolation - Gametic isolation (incl. sperm competition & preference) ### Postzygotic - **Genetic incompatibilities**: genes of parents interact such that viability and/or fertility of offspring is reduced. - Can arise due to any speciation mechanism - e.g. mule - **Ecologically-dependent Isolation**: reduction in hybrid fitness bc of mismatch between phenotype & environment & falls between divergent natural selection niches. ## Geography ### Allopatric Speciation - Gene flow interrupted with a barrier. - Pops. evolve independently & accumulate reproductive barriers. #### Biogeographic Evidence - Isolated habitats harbor endemic species - Young sister species tend to be allopatric ### Sympatric Speciation - Speciation in absence of a barrier (same region!) - Hard to overcome unrestricted gene flow. - Can occur form: - Polyploidization - Strongly disruptive selection. - Positive assortive mating (mating occurs within, not between, incipient species. Sympatric is RARE!) ## Mechanisms ### Ecological Speciation - Reproductive isolation arises as a by-product of adaptation. - Any geographical context. - Divergent natural selection. ### Polyploidization - Increase in # of set of chromosomes. - Postzygotic can be instant (unbalanced # of chromosomes). - Sympatry. - Common in flowering plants (self-compatible). #### Two Types - **Autopolyploid**: 2+ chromosomes from single species. - **Allopolyploid**: 2+ chromosomes from diff species ### Reinforcement - Strengthening of prezygotic barrier by natural selection. - Response to reduced hybrid fitness. - Can complete speciation initiated by any mechanism. - Stronger prezygotic isolation in sympatry than allopatry. - Need at least partial sympatry. ## Studying Speciation ### Laboratory Studies - Test feasibility of various mechanisms of speciation. - Explore conditions they are more/less likely to occur. ### Experimental Evolution - Multiple replicate populations derived from common ancestor. - Evolution tracked under controlled conditions. ### Ecological Speciation - Predicts stronger behavioral isolation between pops in diff envts, compared to same envt. ## Speciation in Nature ### Testing for Ecologically-dependent postzygotic Isolation - Occurs when hybrid fitness is reduced - In absence of intermediate envt, ecological speciation is expected. - Tested via reciprocal transplant of parental species & hybrids in both habitats.