Chapter 24 The Origin of Species 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
Overview: That Mystery of Mysteries In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Video: Galápagos Tortoise
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 Animation: Macroevolution
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
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. 24-2 (a) Similarity between different species (b) Diversity within a species
Fig. 24-3 EXPERIMENT Example of a gene tree for population pair A-B Gene flow event Allele 1 B Population 2 3 4 A A A Allele 1 is more closely related to alleles 2, 3, and 4 than to alleles 5, 6, and 7. Inference: Gene flow occurred. 5 6 7 B B B Alleles 5, 6, and 7 are more closely related to one another than to alleles in population A. Inference: No gene flow occurred. RESULTS Pair of populations with detected gene flow A-B K-L A-C B-C F-G G-I C-E Estimated minimum number of gene flow events to account for genetic patterns 5 3 2 3 2 2 2 1 2 Distance between populations (km) 340 720 1,390 1,190 760 1,110 1,310
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
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
Fig. 24-4 Prezygotic barriers Postzygotic barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Individuals of different species Mating attempt Fertilization Viable, fertile offspring (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) (k)
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 Video: Albatross Courtship Ritual Video: Giraffe Courtship Ritual Video: Blue-footed Boobies Courtship Ritual
Fig. 24-4g (e) Courtship ritual of bluefooted boobies
Mechanical isolation: Morphological differences can prevent successful mating
Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
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
Limitations of the Biological Species Concept The biological species concept can t be applied to fossils or asexual organisms (including all prokaryotes)
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
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
Fig. 24-5 (a) Allopatric speciation (b) Sympatric speciation
Allopatric ( Other Country ) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
The Process of Allopatric Speciation The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift
Fig. 24-6 A. harrisi A. leucurus
Evidence of Allopatric Speciation Regions with many geographic barriers typically have more species than do regions with fewer barriers
Fig. 24-7 Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 80 60 40 20 0 1 2 3 Millions of years ago (mya) 1 2 3 India Madagascar 88 mya 65 mya 56 mya
Reproductive isolation between populations generally increases as the distance between them increases
Fig. 24-8 Degree of reproductive isolation 2.0 1.5 1.0 0.5 0 0 50 100 150 200 250 300 Geographic distance (km)
Barriers to reproduction are intrinsic; separation itself is not a biological barrier
Maltose Starch population 2 Male Male Starch Starch population 1 Fig. 24-9 EXPERIMENT Initial population Some flies raised on starch medium Mating experiments after 40 generations Some flies raised on maltose medium RESULTS Female Female Starch Starch Starch Maltose population 1 population 2 22 9 18 15 8 20 12 15 Mating frequencies in experimental group Mating frequencies in control group
Sympatric ( Same Country ) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations
Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division An autopolyploid is an individual with more than two chromosome sets, derived from one species
Fig. 24-10-3 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n Gametes produced are diploid.. 4n Offspring with tetraploid karyotypes may be viable and fertile.
An allopolyploid is a species with multiple sets of chromosomes derived from different species
Fig. 24-11-4 Species B 2n = 4 Meiotic error Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Species A 2n = 6 Normal gamete n = 3 Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10
Polyploidy is much more common in plants than in animals Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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
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. 24-12 EXPERIMENT Normal light Monochromatic orange light P. pundamilia P. nyererei
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
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
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
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
Allele frequency (log scale) Fig. 24-13 EUROPE Yellow-bellied toad, Bombina variegata Hybrid zone Yellow-bellied toad range Fire-bellied toad range 0.99 Fire-bellied toad, Bombina bombina 0.9 0.5 0.1 0.01 40 30 20 10 0 10 20 Distance from hybrid zone center (km)
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. 24-14-4 Isolated population diverges Hybrid zone Possible outcomes: Reinforcement OR Gene flow Population (five individuals are shown) Barrier to gene flow Hybrid Fusion OR Stability
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
Fig. 24-15 Number of females Sympatric male pied flycatcher Allopatric male pied flycatcher 28 24 20 Pied flycatchers Collared flycatchers 16 12 8 4 0 Females mating with males from: Own species (none) Other species Own species Other species Sympatric males Allopatric males
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. 24-16 Pundamilia nyererei Pundamilia pundamilia Pundamilia turbid water, hybrid offspring from a location with turbid water
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
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
The Time Course of Speciation 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
Fig. 24-18 (a) The wild sunflower Helianthus anomalus H. anomalus Chromosome 1 Experimental hybrid H. anomalus Chromosome 2 Experimental hybrid H. anomalus Chromosome 3 Experimental hybrid Key Region diagnostic for parent species H. petiolaris Region diagnostic for parent species H. annuus Region lacking information on parental origin (b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids
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
Fig. 24-20 (a) Typical Mimulus lewisii (b) M. lewisii with an M. cardinalis flower-color allele (c) Typical Mimulus cardinalis (d) M. cardinalis with an M. lewisii flower-color allele
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: AA Triticum monococcum (2n = 14) BB Wild Triticum (2n = 14) DD Wild T. tauschii (2n = 14) Product: AA BB DD T. aestivum (bread wheat) (2n = 42)