THE ORIGIN OF SPECIES Chapter 24

Speciation Speciation The origin of a new species Speciation is a focal point of evolutionary theory Must explain how new species originate and how populations evolve Microevolution: Changes in allele frequency in a population over time Macroevolution: Broad patterns of evolutionary change above the species level

Biological Species Concept Biological Species Concept (used by Campbell’s Biology) defines a species as: A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring Gene flow between populations keeps the populations as one species Drawbacks: Can not use biological species concept to analyze fossil species Can not apply to species that reproduce asexually (all prokaryotes)

Other Definitions of Species Morphological Species Concept: Defines species by structural and anatomical differences and similarities Ecological Species Concept: Defines species by their ecological niche Phylogenetic Species Concept: Defines species as the smallest group of individuals on a phylogenetic tree

Getting Back to Biological Species Concept By defining a species as a group of individuals that can successfully reproduce, let’s focus on what keeps organisms from NOT reproducing Reproductive isolation: Existence of biological barriers that prevent two species from producing viable, fertile offspring Hybrids Offspring of crosses between species Can classify reproductive isolation as before or after fertilization

Prezygotic Barriers: Before Fertilization Block fertilization from ever happening by: Impeding different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful

Prezygotic barriers (a) (c) (e) (f) (g) (d) (b) Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Individuals of different species MATING ATTEMPT FERTILIZATION (a) (c) (e) (f) (g) Figure 24.3 Exploring: Reproductive Barriers (d) (b)

Postzygotic Barriers: After Fertilization Prevent hybrid zygote from developing into a viable, fertile adult by: Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Breakdown VIABLE, FERTILE OFFSPRING FERTILIZATION (h) (i) (l) Figure 24.3 Exploring: Reproductive Barriers (j) (k)

How Does Speciation Occur? Speciation can occur with or without geographic isolation Allopatric speciation (with isolation) Sympatric speciation (without isolation)

Allopatric speciation. A population forms a new species while Figure 24.5 Two main modes of speciation. (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A subset of a population forms a new species without geographic separation.

Allopatric Speciation Allopatric (“other country”) Gene flow is interrupted when population divided into geographically isolated subpopulations Geographic barrier prevents members of subpopulation from interacting Ability of population to disperse defines how much of a barrier is needed Separate populations may evolve independently through mutation, natural selection and genetic drift

Sympatric Speciation Sympatric (“same country”) Speciation takes place in geographically overlapping populations Although less common than allopatric, speciation can occur if gene flow is reduced by factors such as: Polyploidy Habitat differentiation Sexual selection

Polyploidy Polyploidy Mistakes in cell division result in extra sets of chromosomes Rare in animals but common in plants Many important crops such as oat and wheat are polyploids

Polyploidy Cell division error 2n = 6 Tetraploid cell 4n = 12 2n 2n Figure 24.UN01 In-text figure, p. 495 2n 2n New species (4n) Gametes produced by tetraploids

Habitat Differentiation Ecological niche Sum of an organism’s use of biotic and abiotic resources in an environment If some organisms can exploit a new niche from the parental population, they may eventually evolve into a new species

Hybrid Zones Hybrid zone Region in which members of different species mate and produce hybrids (incomplete reproductive barriers) Hybrids often have reduced fitness compared to either parental species

Hybrid Zones Over Time When closely related species meet in a hybrid zone, 3 possible outcomes: Reinforcement Hybrids less fit than parent species, fewer and fewer hybrids Fusion Hybrids as fit as parents, forms single species Stability Continued production of hybrids

Outcomes in Hybrid Zones Possible outcomes: Isolated population diverges Hybrid zone Reinforcement Figure 24.14 Formation of a hybrid zone and possible outcomes for hybrids over time. OR Fusion OR Gene flow Population Hybrid individual Barrier to gene flow Stability

How Long Does Speciation Take? Hard to say but patterns in fossil records, morphological data or molecular data can give some clues Fossil record shows some species appearing suddenly, remaining unchanged and then disappear Punctuated equilibrium Contrasts with model of gradual change Speciation by punctuated equilibrium model appears fast in geological time (still a long time!)

Speciation Patterns (a) Punctuated pattern Time (b) Gradual pattern Figure 24.17 Two models for the tempo of speciation. (b) Gradual pattern

How Long Does Speciation Take? Lots of variation in speciation time Interval can range from 4,000 years in some species (like cichlids) to 40 million years (some insects) Average is 6.5 million years