Chapter 24 The Origin of Species PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Overview That Mystery of Mysteries In the Gal pagos Islands Darwin discovered plants and animals found nowhere else on Earth Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of adaptations that evolve within a population confined to one gene pool Macroevolution refers to evolutionary change above the species level Concept 24 1 The biological species concept emphasizes reproductive isolation Speciation the origin of new species is at the focal point of evolutionary theory Species is a Latin word meaning kind or appearance Biologists compare morphology physiology biochemistry and DNA sequences when grouping organisms The Biological Species Concept The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring Gene flow between populations holds the phenotype of a population together a Similarity between different species The eastern meadowlark Sturnella magna left and the western meadowlark Sturnella neglecta right have similar body shapes and colorations Nevertheless they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild b Diversity within a species As diverse as we may be in appearance all humans belong to a single biological species Homo sapiens defined by our capacity to interbreed Figure 24 3 A B Reproductive Isolation Reproductive isolation is the existence of biological factors barriers that impede two species from producing viable fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization Reproductive Isolation Prezygotic barriers block fertilization from occurring by Impeding different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful Habitat Temporal Behavioral Mechanical and Gametic Isolations Prezygotic barriers Prezygotic barriers Habitat Isolation Temporal Isolation Individuals of different species a Mechanical Isolation Gametic Isolation Mating attempt c d b Behavioral Isolation e f Fertilization g Postzygotic barriers prevent the hybrid zygote from developing into a viable fertile adult Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown Postzygotic barriers Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Viable fertile offspring Fertilization h i j k l Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms including all prokaryotes Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual species but it can be difficult to determine the degree of difference required for separate species Concept 24 2 Speciation can take place with or without geographic separation Speciation can occur in two ways Allopatric speciation Sympatric speciation Figure 24 5 A B a Allopatric speciation A b Sympatric speciation A small population becomes a new species population forms a new species while geographically without geographic separation isolated from its parent population Allopatric Other Country Speciation In Allopatric Speciation Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations Once geographic separation has occurred One or both populations may undergo evolutionary change during the period of separation Allopatric Other Country Speciation The magnitude of geographic isolation is a result of the Type of barrier and the ability of an organism to move about and overcome the barrier A harrisi Figure 24 6 A leucurus Sympatric Same Country Speciation In Sympatric Speciation Speciation takes place in geographically overlapping populations Mechanisms include chromosomal changes and nonrandom rating i e behavioral changes that reduces gene flow Sympatric speciation Can also result from the appearance of new ecological niches In cichlid fish Sympatric speciation has resulted from nonrandom mating due to sexual selection Allopatric and Sympatric Speciation A Summary In allopatric speciation A new species forms while geographically isolated from its parent population In sympatric speciation The emergence of a reproductive barrier isolates a subset of a population without geographic separation from the parent species Concept 24 4 Speciation can occur rapidly or slowly and can result from changes in few or many genes Many questions remain concerning how long it takes for new species to form or how many genes need to differ between species Broad patterns in speciation can be studied using the fossil record morphological data or molecular data Patterns in the Fossil Record The fossil record includes examples of species that appear suddenly persist essentially unchanged for some time and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change The punctuated equilibrium model contrasts with a model of gradual change in a species existence Fig 24 17 a Punctuated pattern Time b Gradual pattern Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid The interval between speciation events can range from 4 000 years some cichlids to 40 000 000 years some beetles with an average of 6 500 000 years You should now be able to 1 Define and discuss the limitations of the four species concepts 2 Describe and provide examples of prezygotic and postzygotic reproductive barriers 3 Distinguish between and provide examples of allopatric and sympatric speciation 4
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