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 Overview: That “Mystery of Mysteries” In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Video: Galápagos Tortoise Video: Galápagos Tortoise

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Speciation, the origin of new species, is at the focal point of evolutionary theory 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. Ex: how groups of organisms such as mammals evolved. Animation: Macroevolution Animation: Macroevolution

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 24.1: The biological species concept emphasizes reproductive isolation Species is a Latin word meaning “kind” or “appearance” Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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; they do not breed successfully with other populations Gene flow between populations holds the phenotype of a population together

Fig. 24-2a (a) Similarity between different species Although the eastern and western meadowlarks look similar, they are distinct species because their songs and behaviors prevent interbreeding should they meet in the wild.

Fig. 24-2b (b) Diversity within a species Although they look different, all human beings belong to the same species.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reproductive Isolation The formation of new species hinges on reproductive isolation. – Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Such barriers block gene flow and limit the formation of hybrids. – Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Prezygotic barriers include: – Habitat Isolation – Temporal Isolation – Behavioral Isolation – Mechanical Isolation – Gametic Isolation

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers Water-dwelling Thamnophis Terrestrial Thamnophis Two species of garter snake occur in the same geographic areas, but one lives in water and the other on land.

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 Eastern spotted skunk (Spilogale putorius) Western spotted skunk (Spilogale gracilis) The ranges of these two skunks overlap, but the eastern skunk mates in winter and the western skunk mates in the summer.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Video: Blue-footed Boobies Courtship Ritual Video: Blue-footed Boobies Courtship Ritual Video: Giraffe Courtship Ritual Video: Giraffe Courtship Ritual Video: Albatross Courtship Ritual Video: Albatross Courtship Ritual Female blue-footed boobies will only choose a mate who performs the correct courtship display. This provides for the proper “mate recognition”

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Mechanical isolation: Morphological differences can prevent successful mating Bradybaena with shells spiraling in opposite directions As a result of their shells spiraling in opposite directions, these two species of snails cannot physically mate. Their genital openings, indicated by arrows, do not line up.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species Sea urchins Sperm and eggs from sea urchins are released into the water where they fuse and form zygotes. However, sperm and eggs from the red and purple sea urchins seen here are unable to fuse because their surface proteins cannot bind to each other.

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development Ensatina hybrid Some salamander subspecies will occasionally hybridize but the hybrids do not usually complete their development, and those that do are usually very frail

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile The hybrid offspring of a donkey and a horse is a mule…it is sterile.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Hybrid cultivated rice plants with stunted offspring (center) Both of the healthy hybrid parents carry recessive alleles that show up in the offspring.

Fig Prezygotic barriers Habitat Isolation Individuals of different species Temporal Isolation Behavioral Isolation Mating attempt Mechanical Isolation Gametic Isolation Fertilization Reduced Hybrid ViabilityReduced Hybrid Fertility Postzygotic barriers Hybrid Breakdown Viable, fertile offspring (a) (b) (d) (c)(e) (f) (g)(h) (i) (j) (l) (k) Recap Summary of Pre/Postzygotic barriers

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) Thus, we have alternative species concepts that are useful in some situations.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Other Definitions of Species 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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 (organisms adapting to different environmental conditions) 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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 24.2: Speciation can take place with or without geographic separation Speciation can occur in two ways: – Allopatric speciation (with geographic separation) – Sympatric speciation (without geographic separation) – (In both cases: gene flow is interrupted)

Fig (a) Allopatric speciation (b) Sympatric speciation

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Process of Allopatric Speciation The definition of barrier depends on the ability of a population to disperse. Ex: birds can travers the Grand Canyon but squirrels cannot..thus, two species of antelope squirrels have evolved on the rims of the canyon. Separate populations may evolve independently through mutation, natural selection, and genetic drift

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Evidence of Allopatric Speciation Regions with many geographic barriers typically have more species than do regions with fewer barriers

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reproductive isolation between populations generally increases as the distance between them increases Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Fig EXPERIMENT RESULTS Initial population Some flies raised on starch medium Mating experiments after 40 generations Some flies raised on maltose medium Female Starch Maltose population 1 population 2 Male Starch Maltose Male Starch population 1 population Mating frequencies in experimental group Mating frequencies in control group

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations – It can occur if gene flow is reduced by such factors as polyploidy, habitat differentiation and sexual selection.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division. A new species may originate if an accident during cell division produces a polyploid individual. An autopolyploid is an individual with more than two chromosome sets, derived from one species

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue.

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n2n Gametes produced are diploid..

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n2n Gametes produced are diploid.. 4n4n Offspring with tetraploid karyotypes may be viable and fertile—creating a new biological species

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings An allopolyploid is a species with multiple sets of chromosomes derived from different species:

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10 Allopolyploids are fertile when mating with each other, but cannot interbreed with either parent species; thus they represent a new biological species.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Polyploidy is much more common in plants than in animals Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Habitat Differentiation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees – Natural selection has favored the apple-feeding populations and they are now beginning to show temporal isolation from each other. They are still considered subspecies—but gene flow is being reduced and they appear to be on their way to forming two new species.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Fig EXPERIMENT Normal light Monochromatic orange light P. pundamilia P. nyererei

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Allopatric and Sympatric Speciation: A Review In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet Hybrids 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)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: – Strengthening of reproductive barriers – Weakening of reproductive barriers – Continued formation of hybrid individuals

Fig Gene flow Population (five individuals are shown) Barrier to gene flow

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid Possible outcomes: Reinforcement OR Fusion Stability

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Reinforcement: Strengthening Reproductive Barriers The 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

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

Fig Pundamilia nyerereiPundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water

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 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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

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

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

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Studying the Genetics of Speciation The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation Depending on the species in question, speciation might require the change of only a single allele or many alleles

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings From Speciation to Macroevolution Macroevolution is the cumulative effect of many speciation and extinction events

Fig. 24-UN1 Original population Allopatric speciation Sympatric speciation

Fig. 24-UN2 Ancestral species: Triticum monococcum (2n = 14) AABB Wild Triticum (2n = 14) Product: AA BB DD T. aestivum (bread wheat) (2n = 42) Wild T. tauschii (2n = 14) DD

Fig. 24-UN3

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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.Explain how polyploidy can cause reproductive isolation 5.Define the term hybrid zone and describe three outcomes for hybrid zones over time