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Transcript
MICROEVOLUTION THE SUBTLE CHANGES IN A GENE POOL Genetics of Populations • Continuous variation can be quantified – Eye color and height – Bell curve – Refer to pages 186 - 187 • Polygenic inheritance – Additive effect of 2 or more genes on a single phenotype character Environmental conditions can modify the expression of a gene • Hydrangea colors: blue or pink – Only true for that individual – Not inherited Inherited Alleles depend on • • • • • Gene mutation Crossover Independent assortment Fertilization Change in chromosome number or structure Tracking the Rate of Genetic Change • Calculate the allele frequency • Compare with the ideal population as outlined in H-W Rule – Involves a population in genetic equilibrium – Frequencies are stable generation after generation 5 Conditions for an Ideal (life-population) • • • • • Random mating No mutations Very large Isolation Equal successful reproduction – No natural selection – If reproductive success were different, it would alter the frequencies in the gene pool Calculation “particulars” • Use p + q = 1 – when finding the frequency of the allele • Use p2 + 2pq + q2 – When finding the frequency of the genotype 500 flowered plants Pink (A) white (aa) • 20 white • 480 pink – 320 AA – 160 Aa • Find the frequency of the A allele – 640 + 160 = 800 – 800 / 1000 = .8 • Allele “a” has frequency of 0.2 – Now let’s apply this information to specific genotypes • Each allele will occur in the same frequency as the original population – Gamete that will have “A” is 0.8 • Calculate the frequencies of the 3 possible genotypes in the next generation • Frequency of AA? – 0.8 x 0.8 = .64 or 64% • Frequency of aa? – 0.2 x 0.2 = .04 or 4% • Frequency of Aa? – 0.2 x 0.8 = 0.16 • BUT THERE IS ALSO “aA” possible – That’s why the formula is 2pq – Heterozygote frequency is 0.32 or 32 % • If you only knew the genotype information, you can still find the allele frequency – Use square root to solve for p and/or q • Imagine 1 in 10,000 births has PKU – – – – – – Q2 = 0.0001 Q = 0.01 P= 1-q P= 0.99 Carriers? 2pq = 2 (0.99)(.01) = 0.0198 which is nearly 2% of the population • In a population with 2 alleles B and b, the frequency of the allele B is 0.7. What would be the frequency of heterozygotes? – – – – – A. 0.7 B. 0.49 C. 0.21 D. 0.42 E. 0.09 • Answer D • If 16% of the individuals in a population show the recessive trait, what is the frequency of the dominant allele? – – – – – A. 0.84 B. 0.36 C. 0.6 D. 0.4 E. 0.48 • Answer C Microevolution • Most common driving forces away from equilibrium: – Natural selection – Gene flow – Genetic drift • Others – Gene mutations • Only source of new alleles in a population • Rare enough not to have an immediate effect on the allele frequency • What would decrease the frequency of an allele? – Lethal mutation • A mutation that turns out to be an advantage can be maintained through natural selection • Mutations create new alleles BUT • Natural selection, gene flow, and genetic drift change the frequencies of alleles in the gene pool Genetic Drift Change in allele frequencies over the generations – Gene pool will change – Especially true if population is 100 or less – Negligible in a very large population • Due to chance alone – Just like rolling dice or flipping coins Bottleneck Effect Genetic Drift Example • Segments of a population are destroyed by disasters or hunting • Usually reduces genetic variability • Serious threat to the survival of a species – Cheetahs –ice age victims 10.000 years ago then hunted near extinction 1900’s – Northern elephant seal had been hunted down to 20; now 30,000 but electrophoresis shows no variability in genes Founder Effect • Small sample of a population colonizes a new habitat – Darwin’s finches strayed from S. A. – Inherited disorders among humans • Retinitis pigmentosa frequency higher among an isolated population due to colonists carrying the gene Gene Flow • Alleles enter and leave a population as an outcome of immigration and emigration – Most populations are certainly not closed systems • Wind carries pollen – Result is that over time gene flow reduce differences between populations • Neighboring populations may have been affected by natural selection • Gene flow will eventually amalgamate the neighbors • Human migration reduces the variability Mutations • Usual rate is 1 in a million • Not significant source of variation in a gene pool • But it is the original source of variation • Serves as raw material for natural selection Nonrandom mating • In reality, we do NOT mate randomly • Assortative mating—select partners like themselves in certain characteristics • Inbreeding • Extreme “selfing” is self-fertilization of plants! • Frequencies of genotypes shows a decrease in heterozygotes • Wildflower population – Self-fert will increase the frequency of homozygotes at the expense of heterozygotes – AA begets AA – aa begets aa – “Aa” selfs; only half of their offspring will be heterozygous – Each generation decreases that number • Result? • More hom recessive – Greater than H-W prediction would be • However the p and q frequency of alleles remains the same – p+q=1 Natural Selection • H-W says everyone has to be equal in their ability to produce viable, fertile offspring • Reality is differential success – Some have more offspring – Maybe red flowers produce more offspring that white flowers (white visible to predators) – Freq of “A” would increase • This is the only agent of microevolution that can be adaptive – Accumulates and maintains favorable genotypes in a population – If the environment changes, selection responds by favoring genotypes adapted to the new conditions Causes of Microevolution • • • • • Genetic Drift Gene Flow Mutation Nonrandom mating Natural selection • As a mechanism of microevolution, natural selection can be most closely equated with – – – – – A. Assortative mating B. Genetic drift C. Differential reproductive success D. Bottlenecking of a population E. Gene flow • Answer next page • C Differential reproductive success • • • • • • Selection acts directly on A. Phenotype B. Genotype C. The entire genome D. Each allele E. The entire gene pool • A Phenotype • Most of the variation we see in coat coloration and pattern in a population of wild mustangs in any generation is probably due to – – – – – A. New mutations in the preceding generation B. Sexual recombination C. Genetic drift D. Geographic variation within the population E. Environmental effects • B Sexual recombination • The most likely effect of assortative mating on the frequencies of alleles and genotypes would be – – – – – A. Decrease in p2 compared to q2 B. Trend toward zero for q2 C. Convergence of p2 and q2 toward equal values D. A change in p and q E. A decrease in 2pq below the value expected by H-W • E decrease in 2pq – Reduce heterozygotes – Does not affect p and q • A founder event favors microevolution in the founding population mainly because – A. Mutations are more common in a new environment – B. Small population is subject to sampling error in the composition of its gene pool – C. The new environment is likely to be patchy, favoring diversifying selection – D. Gene flow increases – E. Members of a small population tend to migrate • B small population leads to sampling error Adaptive Evolution Modes of Natural Selection • Response to the environment • Adaptive mutation – Allele frequency shifts – Phenotype frequency shifts Directional Selection • Most common during periods of environmental change • Or when members migrate to a new habitat with different environmental conditions • Frequency shifts in one direction – Favors rare individuals that deviate from the average Peppered Moth • Case of industrial melanism • Bird predation contributed to the selection Protective Coloration • Rabbits • Mice • Tigers Antibiotic Resistance • Antibiotics “killed” susceptible cells • Also allowed (favored) CELLS THAT ARE RESISTANT • All this in the last 60 years Pesticide Resistance • Kills insects, worms, etc. • Has allowed resistant forms to increase • Genetically engineered plants – Pesticide resistant – Will still trigger pests to evolve – Coevolution • Biological controls – Natural enemies of the pests are raised and released on a particular area – Allows pests (prey) and “predator” to coevolve Stabilizing Selection • Culls the extreme variants from the population • Reduces phenotypic variation • Maintains the status quo – Favors the “average” for a trait – Human birth weight around 7 lbs – Mortality increases for higher or lower values Gall Stories • Page 262 was about a crown gall caused by bacterium Agrobacterium tumefasciens • Page 288 is a gall caused by a fly larvae – – – – Fly lays eggs on stem Egg develops into pupa then larva Larva bores into stem Eats plant juice and tissues (yum) • Plant cells respond by rapid growth of tissue which forms a tumor (gall) • These galls can vary in range depending on the phenotypes of the fly • Small galls—not favored because a wasp will puncture it and lay its eggs • Those eggs develop into larvae and eat. . . – Eat the fly larvae! – Reduces that phenotype of fly larvae – Fly larvae that make small galls are selected against • Large galls are not favored because birds will chip into the gall and eat the larvae • Flies natural enemies (wasp and birds) act as predators • Cause stabilizing selection in favor of intermediate sized galls Disruptive Selection • Phenotypes of two extremes are favored • Intermediates are selected against • This category also includes sexual selection. – Females are main agents of selection. – It’s their reproductive success that matters most! • Balanced polymorphism includes sickle cell story. • Heterozygotes favored • Malaria is the selective force in tropical and subtropical habitats. • African Finch—balanced polymorphism – Only found small or large billed species – Why? • Available food (sedge—grasslike plant) • Wet season: Two species of sedge grow – One has hard seeds, other soft • Birds mate • Both species of sedge available • Birds with both sizes of beaks • Dry season? – Sedge with soft seeds decrease in population – Birds with large bills are favored – Small-billed may not survive Beak of the Finch • Darwin’s finches follow the same natural selection path that the story of African finches do. • Darwin’s finches were the first “natural” (in the wild) evidence for natural selection. • Artificial selection was already in place. – Breeding of domesticated animals • The volcanic Galapagos give rise to a rich diversity of environmental habitats. • Darwin maintains that the beaks are adaptations to different food sources. – Question of seed-beak compatibility if you will. – Harder the seed, the larger and stronger the bill. • Wet years, the ground finch prefers small seeds – Easier no matter what the bill size is – Selection favors those finches with smaller bills and the population evolves – Directional stabilization – Just don’t bother with the large, hard seeds • Dry years, the small and large seeds are less plentiful. • Finches with larger bills are favored because they simply have an advantage of being able to eat more food. • Current research includes work done by Princeton researchers Peter Grant and his wife. • I have one of their books if you want to see it! • Island with most species. . .Daphne major – Trivia that someone may want to know 1858 • Alfred Wallace had been researching in the East Indies and was ready to submit his findings. – He asked Darwin (whom he respected) to read it and forward it to Lyell for publication. – Lyell included excerpts on Darwin’s 1844 unpublished essay • In 1859, On the Origin of Species, was finally published • Darwin developed and supported the theory much more extensively than Wallace. • Darwin’s notebook also collaborated that he had developed the theory 15 years before Wallace. • Even Wallace felt that Darwin deserved most of the credit. • Darwin only collected data from a few different species. Later learned of the 13 species that inhabit the island chain. • He also thought that the changes took place s-l-o-w-l-y through gradual adaptations. • That’s because of the geologic gradualism that he was reading from Lyell. • “Evolution” or “change through time” was not the debated issue as many think today. • It has taken many years of genetic evidence (hence post Mendelian time) for the theory to be accepted. The Darwinian View of Life • One facet is that evolution is the basis of unity and diversity of life • Darwin never used the word evolution in The Origin of Species • He DID use the terms “descent with modification” • He perceived that all organisms are related through descent from some unknown prototype that lived in the remote past. • Descendants spilled into various habitats over millions of years and accumulated diverse modifications (adaptations). • History of life is like a tree. • At each fork is an ancestor common to all lines of evolution branching from that fork. • This was called “common descent”. • Reality is that most branches of evolution are dead ends. • 99% of all species that have ever lived are extinct. • Darwin actually devoted very little space to the origin of species. • He really concentrated on how populations became better adapted to their local environments through natural selection. • This is the theory that spurred controversy. • Natural selection is a mechanism Darwin proposed to explain the facts of evolution documented by fossils, biogeography, and other types of historical evidence.