Natural Selection Major mechanism of evolution Environment is always changing Acts upon the phenotype of the population Based on Darwin’s idea that resources are limited and that there is competition for those resources. Adaptation = a genetic variation favored by natural selection. When allele frequencies shift, speciation occurs Thus, the frequency change is NOT RANDOM AP Biology Effects of Selection Changes in the average trait of a population DIRECTIONAL SELECTION AP Biology giraffe neck horse size STABILIZING SELECTION DISRUPTIVE SELECTION human birth weight rock pocket mice AP Biology Natural selection in action Resistance… NOT immunity! AP Biology MRSA Heterozygote Advantage Keeps the recessive allele in the population Ex: Sickle Cell Anemia aa – dies of sickle cell anemia Aa – some side affects BUT resistant to malaria! AA – no disease present BUT prone to malaria AP Biology Hidden variations can be exposed through selection! Terminal bud Lateral buds Cabbage Artificial selection Brussels sprouts Leaves Flower cluster Kale Cauliflower Stem Flower and stems Broccoli AP Biology Wild mustard Kohlrabi In addition to natural selection, evolutionary change is also driven by random processes… AP Biology Genetic Drift Chance events changing frequency of traits in a population not adaptation to environmental conditions not selection founder effect small group splinters off & starts a new colony it’s random who joins the group bottleneck a disaster reduces population to AP Biology small number & then population recovers & expands again but from a limited gene pool who survives disaster may be random Ex: Cheetahs All cheetahs share a small number of alleles less than 1% diversity 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat AP Biology Conservation issues Bottlenecking is an important Peregrine Falcon concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Breeding programs must consciously outcross AP Biology Golden Lion Tamarin Human Impact on variation How do we affect variation in other populations? Artificial selection/Inbreeding Animal breeds Loss of genetic diversity Insecticide usage Overuse of antibiotics resistant bacterial strains AP Biology Hardy Weinberg: Population Genetics Using mathematical approaches to calculate changes in allele frequencies…this is evidence of evolution. AP Biology Hardy-Weinberg equilibrium Hypothetical, non-evolving population preserves allele frequencies natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a population measuring evolutionary change G.H. Hardy AP mathematician Biology W. Weinberg physician Evolution of populations Evolution = change in allele frequencies in a population hypothetical: what conditions would cause allele frequencies to not change? 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit) H-W occurs ONLY in non-evolving populations! AP Biology Populations & gene pools Concepts a population is a localized group of interbreeding individuals gene pool is collection of alleles in the population remember difference between alleles & genes! allele frequency is how common is that allele in the population how many A vs. a in whole population AP Biology H-W formulas Alleles: p+q=1 B Individuals: p2 + 2pq + q2 = 1 BB BB AP Biology b Bb Bb bb bb Hardy-Weinberg theorem Counting Alleles Frequencies are usually written as decimals! assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add to 1 (100%), so: p+q=1 BB AP Biology Bb bb Hardy-Weinberg theorem Counting Individuals frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 BB AP Biology Bb bb Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p2=.36 BB q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 2pq=.48 Bb q2=.16 bb MustWhat assume are population the genotype is in frequencies? H-W equilibrium! AP Biology Using Hardy-Weinberg equation p2=.36 Assuming H-W equilibrium: Expected data Observed data How do you explain the data? AP Biology 2pq=.48 q2=.16 BB Bb bb p2=.20 =.74 BB 2pq=.64 2pq=.10 Bb q2=.16 bb Origin of the Equation Assuming that a trait is recessive or dominant Allele pairs AA, Aa, aa would exist in a population p+q=1 The probability that an individual would contribute an A is called p The probability that an individual would contribute an a is called q Because only A and a are present in the population the probability that an individual would donate one or the other is 100% p2 + 2pq + q2 AP Biology Male Gametes A(p) Male Gametes a(q) Female gametes A(p) AA p2 Aa pq Female Gametes a(q) Aa pq aa q2 Example of an evolving population: Peppered moth AP Biology Variation of colors in the population existed (Black, Peppered, White) As environmental conditions changed the frequency of the recessive allele increased. This was seen as an adaptation to the environment that allowed the species to continue to live. The Origin of Species Mom, Dad… There’s something you need to know… I’m a MAMMAL! AP Biology 2010-2011 Speciation • Changes in allele frequency are so great that a new species is formed • Can be slow and gradual or in “bursts” • Extinction rates can be rapid and then adaptive radiation follows when new habitats are available Correlation of speciation to food sources Seed eaters Flower eaters Insect eaters Rapid speciation: new species filling niches, because they inherited successful adaptations. AP Biology radiation Adaptive So…what is a species? • Population whose members can interbreed & produce viable, fertile offspring • Reproductively compatible Distinct species: songs & behaviors are different enough to prevent interbreeding Eastern Meadowlark Western Meadowlark How do new species originate? When two populations become reproductively isolated from each other. Speciation Modes: allopatric geographic separation “other country” sympatric still live in same area “same country” AP Biology Allopatric Speciation Physical/geographical separation of two populations Allele frequencies diverge After a length of time the two populations are so different that they are considered different species If the barrier is removed interbreeding will still not occur due to pre/post zygotic isolation Sympatric Speciation Formation of a new species without geographic isolation. Causes: – Pre-zygotic barriers exist to mating – Polyploidy (only organism with an even number of chromosomes are fertile…speciation occurs quickly) – Hybridization: two different forms of a species mate in common ground (hybrid zone) and produce offspring with greater genetic diversity than the parents….eventually the hybrid diverges from both sets of parents Sympatric Speciation Gene flow has been reduced between flies that feed on different food varieties, even though they both live in the same geographic area. Pre-zygotic Isolation Sperm never gets a chance to meet egg •Geographic isolation: barriers prevent mating •Ecological isolation: different habitats in same region •Temporal isolation: different populations are fertile at different times •Behavior Isolation: they don’t recognize each other or the mating rituals •Mechanical isolation: morphological differences •Gamete Isolation: Sperm and egg do not recognize each other PRE-Zygotic barriers Obstacle to mating or to fertilization if mating occurs geographic isolation AP Biology behavioral isolation ecological isolation temporal isolation mechanical isolation gametic isolation Post Zygotic Isolation • Hybrid Inviability – the embryo cannot develop inside the mothers womb • Hybrid Sterility – Adult individuals can be produced BUT they are not fertile • Hybrid Breakdown – each successive generation has less fertility than the parental generation Evolutionary Time Scale • Microevolution – changing of allele frequencies in a population over time. • Macroevolution – patterns of change over geologic time. Determines phylogeny – Gradualism – species are always slowly evolving – Punctuated equilibrium – periods of massive evolution followed by periods with little to no evolution Patterns of Evolution • Divergent Evolution (adaptive radiation) • Convergent Evolution – AP Biology Two or more species that share a common environment but not a common ancestor evolve to be similar Is it a shark or a dolphin?? Coevolution Two or more species reciprocally affect each other’s evolution predator-prey disease & host competitive species mutualism pollinators & flowers AP Biology Mass Extinctions • At least 5 mass extinctions have occurred throughout history. • Possible causes: dramatic climate changes occurring after meteorite collisions and/or continents drift into new and different configurations. AP Biology Origin of the Earth What must Earth have been like before living things took over? AP Biology The Primitive Earth Atmosphere: All chemicals/compounds necessary are thought to have originated on earth Inorganic precursors: Water vapor Nitrogen Carbon dioxide Small amounts of hydrogen and carbon monoxide These were the monomers for forming more complex molecules. Experiments have shown that it is possible to form organic from inorganic. AP Biology Electrodes discharge sparks (lightning simulation) Origin of Organic Molecules Abiotic synthesis 1920 - Oparin first molecules formed by strong energy sources 1953 - Miller & Urey test hypothesis Water vapor CH4 NH3 Mixture of gases ("primitive atmosphere") H2 Condenser Water formed organic compounds amino acids adenine AP Biology Heated water ("ocean") Condensed liquid with complex, organic molecules Key Events in Origin of Life Origin of Cells (Protobionts) lipid bubbles separate inside from outside metabolism & reproduction Origin of Genetics RNA is likely first genetic material multiple functions: encodes information (selfreplicating), enzyme, regulatory molecule, transport molecule (tRNA, mRNA) makes inheritance possible makes natural selection & evolution possible Origin of Eukaryotes AP Biology endosymbiosis Timeline Key events in evolutionary history of life on Earth 3.5–4.0 bya: life originated 2.7 bya: free O2 = photosynthetic bacteria 2 bya: first eukaryotes AP Biology ~2 bya First Eukaryotes Development of internal membranes create internal micro-environments advantage: specialization = increase efficiency natural selection! infolding of the plasma membrane plasma membrane endoplasmic reticulum (ER) nuclear envelope nucleus DNA cell wall Prokaryotic cell AP Biology Prokaryotic ancestor of eukaryotic cells plasma membrane Eukaryotic cell 1st Endosymbiosis Evolution of eukaryotes origin of mitochondria engulfed aerobic bacteria, but did not digest them mutually beneficial relationship natural selection! internal membrane system aerobic bacterium mitochondrion Endosymbiosis Ancestral AP Biology eukaryotic cell Eukaryotic cell with mitochondrion 2nd Endosymbiosis Evolution of eukaryotes Eukaryotic cell with mitochondrion origin of chloroplasts engulfed photosynthetic bacteria, but did not digest them mutually beneficial relationship natural selection! photosynthetic bacterium chloroplast Endosymbiosis Eukaryotic cell with AP Biology chloroplast & mitochondrion mitochondrion Theory of Endosymbiosis Evidence structural mitochondria & chloroplasts resemble bacterial structure genetic Lynn Margulis mitochondria & chloroplasts have their own circular DNA, like bacteria functional mitochondria & chloroplasts AP Biology move freely within the cell mitochondria & chloroplasts reproduce independently from the cell Cambrian explosion Diversification of Animals 543 mya AP Biology within 10–20 million years most of the major phyla of animals appear in fossil record