Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 24 The Origin of Species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Speciation, “the origin of new species”, is the focal point of evolutionary theory – Species -group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring Microevolution consists of adaptations that evolve within a population, confined to one gene pool Macroevolution - evolutionary change above the species level The Biological Species Concept

Fig (a) Similarity between different species (b) Diversity within a species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reproductive Isolation Reproductive isolation - existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids - offspring of crosses between different species Reproductive isolation can be classified by whether factors act before (prezygotic) or after fertilization (post zygotic)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Mechanical isolation: Morphological differences can prevent successful mating Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: – Reduced hybrid viability – Reduced hybrid fertility – Hybrid breakdown Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Other Definitions of Species The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) Morphological species concept defines a species by structural features – It applies to sexual and asexual species but relies on subjective criteria

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Allopatric (“Other Country”) Speciation Allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations Sympatric speciation, speciation takes place in geographically overlapping populations

Fig (a) Allopatric speciation (b) Sympatric speciation

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Polyploidy Polyploidy - presence of extra sets of chromosomes due to accidents during cell division Autopolyploid - individual with more than two chromosome sets, derived from one species Allopolyploid - species with multiple sets of chromosomes derived from different species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation Hybrid zone - region in which members of different species mate and produce hybrids – can occur in a single band where adjacent species meet – often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region

Fig EUROPE Fire-bellied toad range Hybrid zone Yellow-bellied toad range Yellow-bellied toad, Bombina variegata Fire-bellied toad, Bombina bombina Allele frequency (log scale) Distance from hybrid zone center (km)

Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid Possible outcomes: Reinforcement OR Fusion Stability When closely related species meet in a hybrid zone, there are three possible outcomes: 1.Strengthening of reproductive barriers 2.Weakening of reproductive barriers 3.Continued formation of hybrid individuals

Fig Sympatric male pied flycatcher Allopatric male pied flycatcher Pied flycatchers Collared flycatchers Number of females (none) Females mating with males from: Own species Other species Sympatric males Own species Other species Allopatric males Reinforcement of barriers occurs when hybrids are less fit than the parent species Over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

Fig Pundamilia nyerereiPundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water Fusion: Weakening Reproductive Barriers If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Stability: Continued Formation of Hybrid Individuals Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 (a) Punctuated pattern (b) Gradual pattern Time