Chapter 23 biology notes PDF

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CHAPTER 23 The Evolution of Populations 23 THE SMALLEST UNIT OF EVOLUTION  Microevolution- evolutionary change below the species level; change in the allele frequencies in a pop. over generations o Natural selection (NS) o Genetic drift: chance events that alter allele freq (GD) o Gene flow (GF): transfer of alleles between populations 23 GENETIC VARIATION MAKES EVOLUTION POSSIBLE 23 GENETIC VARIATION  Differences  Genetic variation: mutation, gene duplication, sexual reproduction → existing genes arranged in new way  Gene variability: avg % of loci that are het 23 FORMATION OF NEW ALLELES  Suited for life  Most new mutations that alter are at least slightly harmful  Neutral variation: difference in DNA sequence that don’t confer a selective adv. Or disadv. The redundancy in the gen. Code is another source of neutral variation o In most animals, majority of mutation occur in somatic cells and aren't passed to offspring 23 ALTERING GENE NUMBER OR POSITION  Key sources of variation = errors in meiosis e.g. unequal crossing over, slippage during DNA rep. ; large CMO dup = riskier than small  Gene duplications that don’t have severe effects can persist over generations, allowing mutation to accum. o Olfact 23 RAPID REPRODUCTION  Prokaryote  E.g. HIV

 Adaptive radiation is rapid speciation under conditions in which there is little competition  High mut. Rate per lack of RNA repair mechanisms  No one source → cocktail of meds 23 SEXUAL REPRODUCTION  Shuffles existing alleles and deals at random to produce individual genotype o Crossing over, independent assortment of CMOs, and fertilization 23 THE HARDY-WEINBERG EQUATION CAN BE USED TO TEST WHETHER A POPULATION IS EVOLVING  Presence of genetic variation doesn’t guarantee that a population will evolve 23 GENE POOLS AND ALLELE FREQUENCIES  A population = group of indiv. Of the same species that live in the same area and interbreed and produce fertile offspring o Member of population breed with each other = usually more closely related  Gene pool - all copies of every type of allele at every locus in all members of the population o One allele exists o One allele fixed in population all individuals = homozygous for that allele o 2 or more alleles for a particular locus in a population (het o hom) o Diploid organisms… 500 individuals→ 1000 copies of gene for flower color 23 THE HARDY-WEINBERG EQUATION  Way to assess if nat. select/other factors are causing evo. 23 HARDY- WEINBERG EQUILIBRIUM  Gametes formed by select alleles at random… prob. That egg/sperm contains c^n or C^r = to frequency of alleles in the bin  Like deck shuffled… aces don't grow… 23

CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM Condition 1. No mutations

Consequence if condition doenst hold gene pool is modified if mutual occur or if entire genes are deleted or duplicated

2. Random mating

If individuals mate within a subset of the population (e.g. near neighbors/close relatives (inbreeding)) random mixing of gametes does not occur and genotype frequencies change

3. No natural selection

Allele frequencies change when individuals with different genotypes show consistent dif. In their survival or reproduction success

4. Extremely In small population, allele frequencies fluctuation large by chance over time (a process called genetic drift) population size 5. No gene flow

By moving alleles into/out of populations gene flow can alter allele frequencies 23

APPLYING THE HARDY-WEINBERG EQUATION  Inbreeding = non-random  Helps understand population prob. For PKU 23 NATURAL SELECTION, GENETIC DRIFT, AND GENE FLOW CAN ALTER ALLELE FREQUENCIES IN A POPULATION  New mutations can alter allele frequency o Are rare-change from 1 gen to next probability = small  Non-random mating can affect freq of hom. And het genotype but by itself has no effect on allele frequency in the gene pool  3 mechanisms that alter allele frequency directly: o Natural selection o Genetic drift o Gene flow 23 NATURAL SELECTION

 Those with traits better suited to environment tend to produce more offspring  Adaptive evolution - process in which “enhancing” traits tend to increase over time 23 GENETIC DRIFT  E.g. illinois greater prairie chicken  Smaller # of coin flips = more likely that chance alone will cause a deviation  Chance causes allele freq. To fluctuate unpredictably from 1 gen to next, especially in a small population = genetic drift o Environment or unlucky change in reproduction  Examples of genetic drift? o Founder effect o Bottleneck effect 23 THE FOUNDER EFFECT (ISO)  Few indiv. Become isolated from larger pop→ smaller group → new pop. → gene pool differs from source population = founder effect o E.g. British colonists eye disease on Tristan de Canha 23 THE BOTTLENECK EFFECT (wipeout)  Severe drop in pop. 23 EFFECTS OF GENETIC DRIFT: A SUMMARY 1. GD is significant in small population 2. GD can cause allele frequency to change at random 3. GD can lead to loss of gen variation within populations 4. GD can cause harmful alleles to become fixed 23 GENE FLOW  Transfer of alleles into or out of a population due to mvmt of fertile indiv. Or their gametes o E.g. banded snakes mainland swim island… o Pop. previously had very little contact (humans) now mate more = exchange of alleles and fewer GD between populations 23 NATURAL SELECTION IS THE ONLY MECHANISM THAT CONSISTENTLY CAUSES ADAPTIVE EVOLUTION

 Natural selection = blend of chance and sorting: chance in create of new gen var. Favors certain alleles - outcome does not = random, increase freq. Of alleles with reprod. Adv.  Cline - grad. Change in a trait along a geog axis 23 RELATIVE FITNESS  The contribution an indiv makes to the gene pool of next gen relative to the contributions of other individs o Selection acts more direct on phenotype 23 DIRECTIONAL, DISRUPTIVE, AND STABILIZING SELECTION (how NS alters… )  Directional selection: favors indiv with 1 ext. Of a phen of range o Common when populations environment environmental changes/when indiv. Go to now habitat  Disruptive selection: favor both ext over intermed (beaks/seed)  Stabilizing selection- favors intermed. Variants  Paedomorph = retention of juv. Character in adult 23 SEXUAL SELECTION  Indiv. With certain inherit. Charact. More likely than other indiv. Of same sex to obtain mates o Can result in sexual dimorphism- dif. In 2ndary sexed charact. Between m/f oof same species  Intrasexual selection: among males e.g. among same sex…. Combat  Intersexual selection (mate choice)- when one sex (usually good genes f) are choosy…. Depend on m appearance/behav---> good genes 23 BALANCING SELECTION  Natural selection that maintains 2-0- more phenot. Forms in a pop. 23 FREQUENCY- DEPENDENT SELECTION  Select. In which fitness of a phenot. Depends on how common phenot. Is in pop (e.g. fish side of mouth)  Keeps frequency close to 50-50 23 HETEROZYGOTE ADVANTAGE  E.g. sickle-cell disease 23

WHY NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS 1. Selection can act only on existing variants 2. Evo. limited by historical constraints 3. Adaptations are often compromises (organisms must do many dif. things) 4. Chance, natural selection and env. Interact...