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Today: •The Black Death…The Dramatic Conclusion •Population Genetics Continued •Origin of Species! Microevolution: • generation-to-generation change in a population’s frequencies of alleles • Below the species level! Four Primary Causes of Microevolution: 1. Genetic Drift 2. Natural Selection 3. Gene Flow 4. Mutation 2.Natural 2.Natural Selection: Types of Selection: Examples?? Natural selection sorts through variations, increasing the frequencies of certain genotypes and “fitting” organisms to their environment. It is the entire organism that is “sorted”, as natural selection acts on phenotype. Diversifying Selection: An Example Looking at GENETIC VARIATION, the Foundation of Natural Selection: •Variation occurs both within and between populations •Not all variations are heritable! (Phenotype) The female Papilio butterflies exist in several morphs, some of which resemble two other species which are noxious. Intermediate butterflies do not gain the advantage of mimicry and thus are more likely to be preyed upon. Seasonal variations of Arachnia levana, the map butterfly. Individuals that emerge in spring are orange and brown; individuals that emerge in late summer are black and white. 1 Variation may consist of QUANITATIVE or DISCRETE Characters Quantitative variation usually indicates polygenic inheritance If two + discrete characters are present in a population, it is POLYMORPHIC. Genetic Diversity may be measured most directly as GENE DIVERSITY or NUCLEOTIDE DIVERSITY Gene Diversity: the percent of loci that are heterozygous. Fruit flies typically have ~14% gene diversity. Nucleotide Diversity: percent difference of the nucleotide sequence. Fruit flies typically have ~1% sequence diversity. (humans = ~0.1%) GEOGRAPHIC VARIATION may also be observed BOTH within and between populations. Cline Geographic Variation may also occur in DISCRETE CHARACTERS Variation is generated by MUTATION and SEXUAL RECOMBINATION Variation is generated by MUTATION and SEXUAL RECOMBINATION •Many single point mutations have little or no effect • However, organisms that reproduce rapidly, or sloppily (i.e. HIV) can achieve massive variation through mutation. •Those that do have serious effects of the protein are usually harmful •Thus random change rarely improves the genome, especially in a stable environment •Sexual reproduction alters the combination of alleles an individual receives 2 Preserving Variation: 1. Diploidy- “hides” genetic variation from selection Frequency-dependent selection-survival and reproduction of any one morph declines if that form becomes too common. 2. Balanced Polymorphism- natural selection favoring 2+ phenotypes 1. Heterozygote advantage- Sickle Cell Disease 2. Frequency-dependent selection- survival and reproduction of any one morph declines if that form becomes too common. 3. Neutral Variation- no known selective advantage “shields” from natural selection. Sexual selection Darwin: "...the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction“ or Sexual Selection frequently involves a TRADEOFF between survival and attracting mates. Sexual selection arises because one sex is a limiting resource for another. Selection that arises from differential mating success Two Types of Sexual Selection: Two Types of Sexual Selection: 1. INTRASEXUAL selection 2. INTERSEXUAL SELECTION (Mate Choice) • Well documented Females are usually more selective in mate choice than males (why?) • Arises (typically) from competition among males for access to females or mating sites • Often results in larger body sizes or weaponry that may be advantageous in battle More Problematic! A male and female Bowerbird. Males build ornate nests to compete for mates. 3 Female preference can be a complex trait! Preference may serve to increase her fitness (examples?) Fitness may not be affected by her preference (if not, why should she have a preference?) If females are generally the "choosy" ones, how might you explain some of the exceptions (like sea horses) where males are the ones who are choosy? Three Hypotheses to Explain Intersexual Selection: 1. 2. 3. Direct BenefitsBenefits- increased food, protection, etc Indirect BenefitsBenefits- good genes? Sensory BiasBias- artifact of other selective force Reminders: Natural Selection is limited! Defining a Species: the Biological Species Concept The Next Step: Macroevolution & the Formation of Species Evolution limited by historical constraints Adaptations are often compromises Not all evolution is adaptive (Chance!) Selection can only sort existing variations Defines a species as… a population (s) whose members can potentially interbreed in nature to produce viable, fertile offspring, but who can’t produce viable, fertile offspring with members of other species 4 Barriers leading to Biological Species: Types of Prezygotic Barriers: Habitat IsolationIsolation- two species rarely encounter each other, despite living in same area ExEx- Two species of Manzanita, share geographic range in the Sierra. One species is better adapted to more sheltered areas; the other to exposed, rocky hillsides. 1. Barriers can be PREZYGOTIC or POSTZYGOTIC Types of Prezygotic Barriers: Fireflies. Males use patterns of flashes to signal to females. 3. Temporal IsolationIsolation- two species breed during different times and thus do not interbreed despite sharing the same geographic area. 2. Behavioral Isolation: specialized signals and behaviors that prevent mating between species. Blue-footed boobies performing a mating ritual. Types of Prezygotic Barriers: Ex. Many birds use complex displays including tactile, visual, and acoustical components to attract and recognize mates. Male fire flies use distinct flashes of light to attract and recognize mates. The periodical cicada, Magicicada septendecim Types of Prezygotic Barriers: 4. Mechanical IsolationIsolationanatomical differences prevent mating. Can be direct or indirect. Ex. Ex. The zebra orchid has evolved a precise, anatomical fit with the wasp species that pollinates it, an indirect mechanical isolation. Ex. Ex. This periodical cicada emerges from the ground to reproduce every 17 years. A sibling species shares the same habitat, emerging every 13 years to reproduce Types of Prezygotic Barriers: Balanophora fungosa pollen grain. 5. Gametic IsolationIsolationdifferences in the structure of the gametes or reproductive structures prevent gametes of different species from fusing. Ex. Ex. Molecular recognition enables many flowers to discriminate between pollen grains from different species. 5 Types of POSTzygotic Barriers: 1. Reduced Hybrid ViabilityViabilityHybrid zygotes may have genetic incompatibility that interferes with development. Even hybrids who do complete development may be frail. Ex.Ex.- Leopard frogs frequently hybridize, but offspring that survive are frail. Even Darwin’s finches may occasionally form viable hybrids. Types of POSTzygotic Barriers: Reduced Hybrid Fertility2. FertilityTwo species mate, produce vigorous offspring, but hybrids are sterile ExEx- Mules are robust but sterile hybrids between a horse and donkey The leopard frog, Rana pipiens Types of POSTzygotic Barriers: Hybrid Breakdown3. BreakdownFirst generation hybrids are robust and fertile. Their offspring are feeble or sterile. The BREAKDOWN of the BIOLOGICAL SPECIES CONCEPT Many limitations, including: •Impractical for many existing, and all extinct species! •Doesn’t allow for classification of asexual organisms! ExEx- Many species of plants, cotton, produce fertile hybrids that “die off”. A BETTER MODEL?? ALTERNATE MODELS: The ECOLOGICAL species conceptconceptDefines a species in terms of its ecological niche -Emphasizes the ROLE of an organism -Accommodates asexual organisms ALTERNATE MODELS: The Pluralistic Species ConceptConceptDefines a species based on variable characters on a case by case basis Hookworm SEM 6 ALTERNATE MODELS: The Morphological Species ConceptConceptCharacterizes each species in terms of a unique set of structural features -Easy and most common -Doesn’t address evolutionary relationships ALTERNATE MODELS: The Genealogical Species Concept Defines a species as a set of organisms with a unique genetic history Modes of Speciation Biological Model is perhaps most useful in understanding the causes of SPECIATION. SPECIATION driven by reproductive barriers to gene flow. Two primary Modes: ALLOPATRIC and SYMPATRIC Speciation Modes of Speciation: Allopatric Speciation Allopatric (“other homeland”) Speciation occurs in populations with geographically separate ranges Gene flow is interrupted or reduced due to spatial separation (geological remodeling). Likelihood increased in SMALL, ISOLATED populations– populations– WHY? 1 A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat. Later in time, a few 1 individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. Speciation may also follow colonization of 1 islands 3 and 4. And it may follow invasion of island a by genetically different descendants of the ancestral species. 3 2 4 ALLOPATRIC SPECIATION on an isolated archipelago ADAPTIVE RADIATION 2 3 2 4 Ex- Hawaii, the Galapagos, but not the San Juans– WHY?? 7 Hawaiian Honeycreepers: KONA FINCH extinct Key Question: Did speciation occur during geographic isolation? KAUAI AKIALAOA LAYSAN FINCH AMAKIHI AKIAPOLAAU IIWI MAUI PARROTBILL APAPANE insect and nectar eaters fruit and seed eaters FOUNDER SPECIES Allopatric Speciation in Process can Produce RING SPECIES Expected pattern of genetic relatedness? Two Important Misunderstandings: 1. 2. Geographic isolation prevents interbreeding, but doesn’t necessarily equal reproductive isolation! Reproductive barriers usually evolve by coincidence. (No selection pressure!) In fact, when hybridization can occur, it usually confers a selective ADVANTAGE! Used with permission; www.denniskunkel.com Modes of Sympatric Speciation Red viscacha rat from Argentina = 4n One Possible ALLOPOLYPLOID Mechanism: A new species can also arise in the same geographic area as the parent species. Modes: Polyploid Speciation in Plants (and animals!) Autopolyploids have 2+ chromosome sets from a single species. Allopolyploids have chromosome sets from two different species. Note: This causes a SPECIATION EVENT in a single generation! 8 Other Modes of Sympatric Speciation •Populations in the same area can become isolated due to resource use (habitat, food source, etc.) •Rigid mate preference can also make populations diverge. 9