1124 General Biology I: Speciation PDF

1124 General Biology I Chapter 18: Speciation  Speciation Despite the unity of life, including a shared genetic program Only certain organisms can exchange genetic material by sexual reproduction These organisms are the same species ...

1124 General Biology I Chapter 18: Speciation  Speciation Despite the unity of life, including a shared genetic program Only certain organisms can exchange genetic material by sexual reproduction These organisms are the same species  Speciation Speciation is the formation of 2 new species from an original species Speciation forms a bridge between microevolution and macroevolution Speciation goes hand in hand with extinction  Speciation The biological species concept states: Individuals that can interbreed and produce viable, fertile offspring are the same species Members of the same species will share external and internal characteristics Can be misleading  Speciation Problems with this concept include: It does not work for asexual organisms It cannot be applied to extinct organisms Some species share a recent common ancestor that they may form viable, fertile offspring  Speciation A hybrid is a cross between two species This often results in an unsuccessful offspring and species remain apart Gene flow holds species together, organisms must be able to reproduce to pass on new traits to offspring  Speciation For speciation to occur, two populations must evolve from an original species in a way that make it impossible for them to interbreed Allopatric speciation Sympatric speciation  Allopatric Speciation In allopatric speciation, gene flow is interrupted when a population is divided into geographically isolated subpopulations Over long periods, allele frequencies in each population will uniquely change by mutation, natural selection and genetic drift Water level recession, geological division, colonize remote area E. thouarsi E. tribuloides  Allopatric Speciation Increases in geographical barriers, distance and time between populations increase likelihood of speciation events Genetic differences are too great to overcome  Allopatric Speciation Allopatry arises via two mechanisms: Vicariant processes occur when climate or geology causes populations to fragment Dispersal (founder effect) occurs when a few individuals disperse to a new geography  Allopatric Speciation Adaptive radiation occurs when a population of one species disperses throughout an area colonizing a distinct niche or isolated habitat The demands of the new environment may promote multiple speciation events Often results when an environment lacks competition  Sympatric Speciation In sympatric speciation, speciation takes place in the absence of geographical barriers, in the same geographic location Habitat differentiation, polyploidy, sexual selection  Sympatric Speciation Aquatic environments contain multiple niches North American maggot fly Hawthorne tree vs. apple tree  Sympatric Speciation Chromosomal error can provide opportunity for speciation Polyploidy Autopolyploidy results in increases in chromosome sets from its own species A 2n organism produces 2n gametes Self-pollination would result in 4n offspring  Sympatric Speciation Allopolyploid results from individuals from two different species reproducing viable offspring Requires 2 generations or two reproductive acts to occur  Sympatric Speciation The sunflower Helianthus anomalus originated from hybridization of two other sunflower species  Sympatric Speciation The selection pressures on males and females to obtain matings is known as sexual selection Can result in dramatic increases in phenotypes that improve opportunity of mating Individuals prefer mates that contain only those features  Reproductive Isolation Speciation requires populations to be reproductively separated long enough to affect the ability to interbreed Given enough time, genetic and phenotypic divergence among populations will affect reproduction Reproductive isolation is the existence of biological barriers that block interbreeding  Reproductive Isolation Reproductive isolation can manifest in many ways Prezygotic barriers block reproduction from taking place, blocks fertilization Postzygotic barriers are blocks that occur after fertilization  Prezygotic Barriers Habitat isolation: two species occupy different habitats Encounter each other rarely Temporal isolation: species breed during different times Day, seasons, years  Prezygotic Barriers Behavioral isolation: Courtship rituals Unique social behaviors that attract mates Mechanical isolation: morphological differences prevent mating Mating is attempted  Prezygotic Barriers Gametic Isolation: Sperm of one species cannot fertilize eggs of another species Sperm can’t survive reproductive tract, biochemical blocks  Postzygotic Barriers Hybrid inviability: genes of different parents incompatible Embryo failure or hybrid frailty Hybrid sterility: vigorous but sterile hybrids Meiosis fails Hybrid breakdown: 1st generation hybrids are viable and fertile The next generation is feeble or sterile  Reconnection of Species After speciation, two species may recombine or even continue interacting indefinitely The area where two species continue to interact is the hybrid zone  Reconnection of Species Over time, the hybrid zone may change due to the fitness of the hybrids and any reproductive barriers Reinforcement occurs when hybrids have reduced fitness compared with parental species  Reconnection of Species During reinforcement, reproductive barriers are stronger for sympatric than allopatric species  Reconnection of Species If reproductive barriers weaken fusion into one species will occur If reproductive barriers remain, but hybrids are fertile, stability occurs  Rates of Speciation How long does it take for new species to form How many genes and generations need to differ between species  Rates of Speciation Patterns in speciation can be studied using the fossil record, morphological and molecular data Gradualism Slow and steady changes Punctuated equilibrium Quick changes interrupted by long periods of no change  Alternative Speciation Concepts The morphological species concept Distinguishes species by morphology and structural characteristics The ecological species concept Defines a species in terms of its ecological niche The phylogenetic species concept A species is the smallest group of individuals that share a common ancestor Forming one branch on the tree of life