Species and Speciation Module PDF

Species and speciation Module  Species & Speciation What Species Are and How They Arise How do these processes affect information flow?  Evolution, review What five processes cause evolution? So, what processes are likely to cause individual...

Species and speciation Module  Species & Speciation What Species Are and How They Arise How do these processes affect information flow?  Evolution, review What five processes cause evolution? So, what processes are likely to cause individuals to look different in different parts of the species range? What processes are likely to cause individuals to look alike?  Speciation: the origin of new species from existing species The fossil record reveals two patterns of speciation – Anagenesis (phyletic speciation) Accumulation of heritable changes in a population associated with speciation – Cladogenesis (branching speciation) New species arises from a population that buds from a parent species Basis for biological diversity I. What is a species? (This is much more difficult to answer than it seems)  Species concepts Morphological species concept Group of individuals united by similarities that distinguish them from all other individuals. Aristotle, Carolus Linnaeus…  Species concepts Biological species concept (BSC) Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such populations Theodosius Dobzhansky, Ernst Mayr (1940)...  Biological Species Concept: elements and Issues “…actually or potentially interbreeding…” – regards geographically separated populations as same species if presumed to be capable of interbreeding – relies on the criterion of interfertility – Concept is restricted in its generality, in that it does not apply to asexually reproducing species  Is the BSC useful for all creatures? Does the BSC correctly identify what we like to call species? Certainly fossil and asexual species are left out of the definition. What about species that have barriers to reproduction that don’t involve genetic intersterility?  Consider most plant species… Tens (hundreds?) of thousands of plant species do not hybridize in nature, but can be forced to in greenhouse conditions Does this mean they are not evolutionarily distinct? Shall we lump all entities that are compatible under the same species name?  Biological Species Concept: Elements and Issues Speciation itself is a temporal process: from initiation of population differentiation to the evolution of complete reproductive barriers can take a very long time. The BSC represents the end of this time period. What do we call entities undergoing this process? This is why there are >20 other species concepts floating in the literature. Ultimately we need to decide what we want the term “species” to mean…. What do YOU think? Reproductive Isolation and Isolating Mechanisms As an incipient new species diverges behaviorally, physiologically, morphologically from the parent species, those very differences may preclude the two from reproducing successfully; i.e., the two may become “good biological species”…or not! Reproductive barriers: Evolved traits that preclude production of fertile, viable hybrid offspring. Biologists distinguish between prezygotic and postzygotic isolating mechanisms.  Prezygotic Barriers: reproductive isolating mechanisms that operate before fertilization, some before mating Spatial, behavioral, temporal, mechanical and Habitat or Spatial Isolation  Behavioral Isolation Western meadow lark Eastern meadow lark  Temporal Isolation 13-year cicada 17-year cicada Mechanical Isolation C. wapleri Thompson Creek, W. Feliciana Parish  Gametic isolation ovum  Reduced hybrid viability hybrid zygote formed, but the hybrid dies some time before it reaches reproductive age due to genetic incompatibility e.g., hybrids produced by sheep × goat crosses die as embryos (miscarriages).  Reduced hybrid fertility hybrid offspring survives, but is unable to reproduce either with other hybrids or with parental species e.g., horse (female) × ass (male) cross produces a mule -- healthy organism that is sterile Should be selection to promote pre- zygotic barriers. Why?  Hybrid breakdown If species cross-mate, the first generation hybrid may be viable and fertile, but when these hybrids mate with one another or with the parent species, offspring will be feeble or sterile. Summarizing Reproductive Isolating Mechanisms Both Pre-zygotic and Post- zygotic…  Reproductive Barriers Mechanical Habitat Temporal Behavioral isolation isolation isolation isolation Individual s Mating of attempt different species HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION  Reproductive Barriers Gametic Reduce Reduc Hybrid isolation hybrid e breakdown viabilit hybrid y fertility Viable Fertilization fertile offsprin g REDUCED REDUCED HYBRID GAMETIC ISOLATION HYBRID BREAKDOWN HYBRID FERTILITY VIABILITY  II. SPECIATION The process by which new species arise  The basic elements of speciation Species with populations Barrier to gene flow distributed throughout the species range Reproductive Genetic isolation differentiation Summary  Gene flow is interrupted Why do populations change? Agents of evolutionary change; variously operate on the isolated gene pools Selection (Adaptation) Mutation Drift Non-random mating What evolutionary processes have allowed divergent evolution? Degree of reproductive isolation evolves Complete reproductive isolation may be a consequence of independent evolution  Two general modes of (processes) of speciation Allopatric Speciation – Etymology: “allo-patric” = “different-country” – Gene pools isolated via geographic separation – Prevalent mechanism of speciation in animals – review dispersal and vicariance routes to isolation Sympatric Speciation – Etymology: “sym-patric” = “same-country” – Gene pools isolated via process other than geographic separation – via polyploidy, particularly in plants – via behavior, ecology and to a lesser extent polyploidy in animals – Common in plants (>70% of species via sympatric speciation) – Growing understanding of this process as it applies to animals  Allopatric speciation: Geographic isolation via dispersal or vicariance  Allopatric speciation Peripheral Isolates may be predisposed to (Allopatric) Speciation – Peripheral Isolate; small, isolated population at edge of species’ range (see slide 75) – Environment, and therefore selection pressures may differ substantially compared to parent population – Gene pool likely to show incipient differentiation. Why??? – Genetic drift will drive random changes in genetic structure – Also called peripatric speciation.  Example: The Galapagos Finches Founder events (dispersal, in the example to follow) may lead to speciation  Ten species of Darwin’s Finches from Isla Santa Cruz, each with different bill size and feeding habits.  Adaptive radiation model for evolution of Darwin’s Finches Founding population inhabits San Cristobal Period of dispersal and differentiation Secondary contact; immi- grants from Santa Cruz are reproductively isolated from the ancestral stock on San Cristobal The consequence of repeated speciation events, such as is thought to have occurred with Darwin’s finches, is referred to as Adaptive Radiation – Emergence of numerous species from common ancestor that diversify to occupy different ecological zones  Allopatric speciation and adaptive radiation on ‘island chains’ dispersal and colonization adaptation of colonizing population to environment on colonized island genetic differentiation underlies phenotypic adaptations if, following repatriation of ancestral and descendant lineages, those lineages are reproductively isolated, we recognize those lineages as distinct species Why did I put ‘island chains’ in quotes? How could this be generalized? Allopatric speciation: vicariance (geographic isolation without dispersal) Real example  Ammospermophilus Ammospermophilus harrisi leucurus S N Allopatric speciation of antelope ground squirrels in the Grand Canyon  Sciurus aberti kaibabensis Sciurus aberti aberti S N Allopatric speciation of Abert’s and Kaibab squirrels in the Grand Canyon  Allopatric speciation: important considerations The effectiveness of a geographic barrier to impede or eliminate gene flow depends on the locomotion and other characteristics of individuals  Allopatric speciation: important considerations The effectiveness of a geographic barrier to impede or eliminate gene flow depends on the locomotion and other characteristics of individuals  Allopatric speciation: important considerations We consider speciation to have occurred if and when two gene pools have diverged to the point that, should secondary contact occur, the individuals of each are reproductively isolated from each other; they can no longer interbreed and produce fertile offspring.  Allopatric speciation: important considerations Recognized to be prominent in animals as closely related species usually do not have largely overlapping ranges. AND these ranges are often separated by barriers to dispersal.  Sympatric speciation Sympatric speciation - a mode in which a new species arises in the geographic midst of its progenitor species.  Sympatric speciation Uncommon, examples are hard to establish with confidence. Why? More common in plants than animals. Why? (hint: plants are commonly hermaphroditic; why would this matter?) Polyploidy is a common route (especially fish, amphibians, lizards, and many plants). Sympatric speciation in animals - East African Cichlids (sick-lids)  Sympatric speciation in Cichlids Lake Victoria in Africa is a very large lake Cichlid fish may have separated by habitat (edges, open water, deep water, etc.) or food (algae, plants, fish, insect larvae, detritus, etc.) or some other factor 500 species emerged in the past 15,000 years (The original genetic variability was due to two very different cichlids (Congo R. and Upper Nile R. drainages) hybridizing more than once)  How to keep recently diverged species apart when they live together Pundamilia pundamilia Pundamilia nyererei a) Normal b) Monochromatic light orange light Non-random mating, in which females select mates having a preferred appearance, is the main reproductive barrier keeping these two species separate in nature. This is an example of the maintenance of sympatric speciation through sexual selection.  Instant Sympatric Speciation: Polyploidy Polyploids have multiple complete sets of chromosomes > 2n Polyploidy more common in plants than animals – Because some plants undergo self-fertilization, easier to propagate a new polyploid generation Polyploidy found more often in harsh environ-ments, e.g., desert or polar areas Polyploids cannot reproduce with diploid parental types, so reproductive isolation achieved immediately  Results of Polyploidy If viable, polyploids are often vigorous Among animals, often larger size Most common agricultural plants were domesticated through polyploidy – Wheat – Corn – Rice Goldfish are polyploids Sympatric speciation requires the emergence of some sort of reproductive barrier that isolates the gene pool of a population subset without geographic separation from the parent population. Barriers to gene flow other than geographic isolation Species with populations Barrier to gene flow distributed throughout the species range Reproductive Genetic isolation differentiation Hybrid Zones  Challenging Reproductive Isolation When reproductive isolation is incomplete, hybridization may occur Regions where different species co- occur and mate are known as hybrid zones The key to understanding hybrid zones is gene flow Figure 24.14-1 Gene flow Population Barrier to gene flow Figure 24.14-2 Isolated population diverges Gene flow Population Barrier to gene flow Figure 24.14-3 Isolated population Hybrid diverges zone Gene flow Population Hybrid Barrier to individual gene flow Figure 24.14-4 Possible outcomes: Isolated population Hybrid diverges zone Reinforcement OR Fusion Gene flow OR Population Hybrid Barrier to individual gene flow Stability  Reinforcement In the following slide, we see two European flycatcher (bird) species that probably speciated allopatrically In the different regions which each species inhabits, the males look identical In the area of overlap, the males look different; so the reproductive barrier is stronger in areas of sympatry Mating choice is by females of males  Females choosing between Females choosing between these males: these males: 28 Sympatric pied male Allopatric pied male 24 Sympatric collared male Allopatric collared male Number of females 20 16 12 8 4 (none) 0 Own Other Own Other species species species species Female mate choice Female mate choice 10th ed., Figure 24.15  Fusion Where gene flow cannot be prevented, the two incipient species re-merge (fuse); then speciation fails In the following slide, we see two cichlid fish species from Lake Victoria; the males are quite different In areas where the water is turbid due to pollution, females cannot see the differences in color pattern and mate indiscriminately Figure 24.15 Pundamilia nyererei Pundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water  Stability In the following example, fire-bellied toads live at low altitudes and yellow- bellied toads live at high altitudes; there is a narrow area where both are found and hybridize Hybrids show developmental abnormalities and survival is low, so gene flow is restricted and the species remain separate However, hybrids continue to be produced, apparently because the zone of overlap is habitable by both species Figure 24.13 EUROPE Fire-bellied Fire-bellied toad, Bombina bombina toad range Hybrid zone Yellow- bellied toad range 0.99 B. variegata-specific allele Hybrid zone 0.9 Yellow-bellied Frequency of 0.5 Yellow-bellied Fire-bellied toad, Bombina toad range toad range variegata 0.1 0.01 40 30 20 10 0 10 20 Distance from hybrid zone center (km) Figure 24.13b B. variegata-specific allele 0.99 Hybrid zone 0.9 Frequency of 0.5 Yellow-bellied Fire-bellied toad range toad range 0.1 0.01 40 30 20 10 0 10 20 Distance from hybrid zone center (km) Figure 24.14-4 Possible outcomes: Isolated population Hybrid diverges zone Reinforcement OR Fusion Gene flow OR Population Hybrid Barrier to individual gene flow Stability  Hybridization Can Cause the Introduction of New Alleles In Africa, two mosquitos, Anopheles gambiae and A. coluzzii, carry malaria A. gambiae (only) carried a recessive allele of a sodium channel gene, kdr, that conferred resistance to DDT, used to kill mosquitos In 2006, DDT-laced bed nets were introduced leading to strong directional selection in A. gambiae 2006 also saw hybridization of both mosquitos Beginning only in 2006, the resistant allele showed up in A. coluzzii, followed by selection favoring the allele  Figure 24.UN02a You should be able to calculate p2, 2pq, and  Mode and Tempo in Evolution Paleontologist G.G. Simpson’s important 1944 book, Tempo and Mode in Evolution, indicated that both factors were important In the 1970s, two of his graduate students, Stephen Jay Gould and Niles Eldridge, noted that a century after Darwin, many gaps in the fossil record remained They developed an explanation for this based on the tempo of evolution Their theory contradicted the gradualism that was accepted by Darwin, influenced by uniformitarianism (Lyell), and believed by most evolutionists  Punctuated Equilibrium Argues that most species show very little evolution for long periods (stasis), punctuated by sudden bursts in significant evolution Being often a product of allopatric speciation, this burst often found in populations at the edge of the geographic range, exposed to extreme conditions Because fossils are hard to form and difficult to find, intermediates are rarely seen – The populations in which they occur are small – The events are relatively fast This explanation conforms to the fossil record itself Common Interpretation of Evolutionary Tempo  Is All Change Adaptive? For a long time, Darwinists have argued that change is a function of adaptation; i.e., all change must be adaptive or would not occur This is challenged by “neutral selection” whereby change does not affect fitness It was also challenged by Stephen Jay Gould and Richard Lewontin in their article, The Spandrals of San Marcos and the Panglossian Paradigm You should now be able to address the following: Why are there species? Discuss the differences among species concepts and what those differences reflect Contrast the different modes of speciation Distinguish among the different reproductive isolating barriers. Discuss the origin and consequences of reproductive barriers. Explain why hybridization occurs, and its possible long-term outcomes.