CHAPTER 24: THE ORIGIN OF SPECIESWhat is a species?---Species is a group whose members possess similar anatomical characteristicsand have the ability to interbreedMany species were originally distinguished by morphology- Two organisms that look alike must be the same speciesHowever, biological species are based on the ability to reproduce.- Individuals may look alike but not reproduce and therefore are not thesame species OR- Individuals may look different but can reproduce and therefore are thesame species. (See fig 24.2)The species category can be conceptualized in many ways:1. Biological Species Concept: a population or group of populationswhose members have the potential to interbreed in nature and produceviable, fertile offspring, but are not able to produce viable, fertile offspring with members of other populations2. Morphological Species Concept: measurable anatomical criteria 3. Ecological Species Concept: in terms of ecological niche, the sum of how members of the species interact with the nonliving and living parts of their environment4. Phylogenetic Species Concept: as the smallest group of individuals that share a common ancestor, forming one branch on a tree of life---Biological Species are based on the ability to reproduce- The organisms can produce viable, fertile, offspring; but they cannot produce viable, fertile, offspring with other groups of organisms.- What about organisms which reproduce asexually or where the reproduction hasn’t been observed?---Morphological Species defines a species by measurable anatomical criteria- Many species were originally distinguished by morphology- If two organisms look alike, they must be the same species- As the DNA for more organisms is studied, the placement of organisms within the taxonomic tree becomes based more on the DNA.However, biological species are based on the ability to reproduce.- Individuals may look similar but not reproduce and therefore are not the same species (See fig 24.2)Or- Individuals may look different but can reproduce and therefore are the same species. (Ex. Different breeds of dogs)---Ecological Species: defines a species in terms of ecological roles (niches), based on behavior---Phylogenetic Species defines a species as the smallest group of individuals that shares a common ancestor sharing an evolutionary history- But how do we “know” the evolutionary history?- The ability to examine small but meaningful differences between organisms has resulted in changes in the way organisms are classified.1Fossils indicate the previous existence of many species- Most now extinct- Many have no living descendantsEvolution is the force behind speciation---Speciation development of a newspecies where there was one beforeTwo main components1. Isolation of the population2. Genetic diversity>>>>>>Reproductive barriers separate species. (see fig. 24.3)---Reproductive barriers impedemating between species or preventfertilization if members of differentspecies attempt to mateReproductive Isolation is a prerequisite for the formation of species- A barrier prevents two species from producing fertile hybrids- Most species have more than one barrier>>>Prezygotic isolating mechanisms - hinders zygote formation 1. Geographical isolation: Prevents populations from interbreeding but if the species are put together they may mate.2. Ecological (Habitat) Isolation: Isolation because of where they choose to live, tree top versus forestfloor3. Temporal Isolation: breeding seasons occur at different times4. Behavioral Isolation: courtship rituals are unique to a certain species5. Mechanical incompatibility: physiological barrier, Great Dane and Chihuahua6. Gametic Incompatibility or isolation: sperm or one species cannot fertilize egg of another species>>>Postzygotic isolating mechanism - hinders zygote development and propagation 1. Hybrid Viability: genes of the different parent species may interact and impair the hybrid’s development, can’t build a proper nest2. Hybrid Fertility: hybrid is sterile or has greatly diminish fertility3. Hybrid Breakdown: some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the nextgeneration are feeble or sterile>>>>>>The mechanisms of speciation 1. Allopatric speciation (see fig 24.6): The gene pools mustbe geographically isolated2. Sympatric speciation: The population shares the same geographic area>>>Geographical isolation can lead to the origin of species: Allopatric speciation---Allopatric speciation the formation of new species in populations that are geographically isolated fromon each other2- The type of necessary barrier depends on the species>>>A new species can originate without geographicseparation: Sympatric speciation.---Sympatric speciation the formation of a new species inpopulations that live in the same geographic area- Usually result from radical mutations Polyploidy: an extra chromosome set(s), it is a result ofan accidental cell division- Autopolyploidy: individual has more than twochromosome sets, all derived from a single species- Allopolyploid: a fertile individual that has more than twochromosomes sets as a result of two different speciesinterbreeding and combining their chromosomes- common among plants - Offspring survive just fine or even better but can’t reproduce- No sperm or eggsHybrid Zones provide insight to factors that cause reproductionisolation- Hybrid Zones: regions where two related populations thatdiverged after becoming geographically isolated makessecondary contact and interbred where their geographicranges overlap>>>>>>Gradualism versus Punctuated EquilibriumGradualism is the traditional view of evolution.- New species evolve from gradual accumulationof characteristics However, paleontologists frequently do not findtransitional forms: species appear and disappear"suddenly"Punctuated Equilibrium: speciation occurs duringbrief periods of rapid change, followed by a periodof little changeRarity of transitional fossils
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