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BSC 2011- Week 2 Notes

by: Anton Nguyen

BSC 2011- Week 2 Notes BSC 2011

Anton Nguyen

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These are notes from Chapter 23
Bio II- Biological Diversity
Chantale Begin
Class Notes
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This 10 page Class Notes was uploaded by Anton Nguyen on Monday September 5, 2016. The Class Notes belongs to BSC 2011 at University of South Florida taught by Chantale Begin in Fall 2016. Since its upload, it has received 4 views. For similar materials see Bio II- Biological Diversity in Biology at University of South Florida.


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Date Created: 09/05/16
Chapter 23  The Evolution of Populations  The Smallest Unit of Evolution o One common misconception is that individuals evolve  However, natural selection acts on an individual, with its traits  affecting its survival and reproductive success,   But the evolutionary impact of natural selection is only seen in  the changes found in the population over time o Microevolution: evolution on the smallest scale, in the form of allele  frequency changes over generations  Three main mechanisms of microevolution:  Natural selection  Genetic drift (random events that alter allele frequencies)  Gene flow (transfer of alleles between populations)  Each mechanism has a distinct effect on a population but only  natural selection improves the match between organisms and  their environments, through adaptations  Genetic Variation o Differences among individuals in the genetic composition of their  genes and other DNA sequences  Gene variability: can be quantified as the average percentage  of loci that are heterogeneous (having two different alleles for  the same gene). Can be seen as genetic variation at the whole­ gene level  Nucleotide variability: genetic variation at the molecular level  of DNA, although this is barely seen through phenotypic  variations  Due to introns, noncoding segments of DNA, being in  between exons, regions leftover in messenger RNA  (mRNA) after RNA processing, which contain  information for a certain gene o Some phenotypic variations don’t occur from genetic differences  among individuals  Phenotypes are the product of inherited genotype and many  environmental influences  As genetic variations provide raw materials for evolutionary  change: without it, evolution can’t occur  Sources of Genetic Variation o Formation of New Alleles  Can arise from mutations, changes in the nucleotide sequence  of an organism’s DNA  They happen randomly and can’t be accurately predicted  A change of even one single nucleotide of a gene, called  “point mutations” can have an enormous impact on the  phenotype of the organism  And with phenotypes reflecting past generations, they tend to  be well­fitted for the present environment  As a result, mutations can sometimes change the  potentially harmful phenotype, but can be removed by  natural selection o Although such harmful changes can still persist  due to a gene being heterozygous  Neutral variation: point mutations in noncoding regions, with  differences in DNA being neither an advantage or a  disadvantage o Altering Gene Number or Position  Occur through a number of ways such as chromosomal changes which:  Delete  Disrupt   Or rearrange many loci  But when the genes are still intact from large­scale changes,  they might not affect a phenotype  An important key of variance are:  Gene duplications during meiosis o Although large amounts of DNA duplication can  be harmful, o While small amounts may be beneficial o With those not as harmful persisting over  generations, allowing mutations to accumulate  Slippage during DNA replication o  Where a sequence of repetitive nucleotides are  found at a replication site, which lead to mistakes  being made o Rapid Reproduction  Occur mainly in prokaryotes and viruses where they can have  multiple mutations form in a short amount of time, due to the  lack of RNA repair, which would normally correct any  mistakes that occurred o Sexual Reproduction   Most genetic variations that result in a population are from the  unique combination of alleles that each offspring receive from  its parents  At the nucleotide level, all differences among alleles were from  past mutations  From which sexual reproduction shuffles existing alleles  and randomly shuffles them to produce individual  genotypes  Three mechanisms that contribute:  Crossing over o Where homologous chromosomes, one from each  parent, trade alleles through said mechanism  Independent assortment of chromosomes o Then the homologous chromosomes and the alleles carried, are distributed randomly into gametes, or  sex cells  And fertilization o And then the sex cells become one, rearranging  existing alleles into new combinations for the new  offspring o Gene Pools and Allele Frequencies  Population: a group of individuals of the same species that  lives in the same area and interbreed, producing fertile  offspring  Isolation is common due to geographical boundaries, and  are usually more closely related to each other than those  of a different population o Although in some cases, very rarely, do different  populations mate and exchange genetic material   Gene pools: are an entire population’s genetic makeup,  consisting of all copies of every type of allele at every locus in  all members of the population  If there is only one allele, then it’s considered fixed, and  homozygous, but if there’s two or more alleles, then  organisms may be homozygous or heterozygous.  Each allele has a frequency, or proportion, in the  population, in which the allele can be found  o Hardy­Weinberg Equilibrium  A situation in which a population is not evolving and  allele/genotype frequencies are constant from generation to  generation  Instead of using Punnett squares to determine offspring,   combinations of alleles in ALL crosses in a population are  considered  We use the following equations in which o P stands for the dominate allele and q stands for  the recessive allele, while 2pq stands for the  heterozygote frequency   o P + q = 1 o P  + 2pq + q  = 1  And from these equations, we use them to determine both the  o allele frequency   (Number of copies of a specific allele) / (the  total number of alleles) o and the genotype frequency  (Number of individuals with a gene) / (the  total number of population)  Conditions for H­W Equilibrium  1) No mutations­ the gene pool must be constant with no  changes of any sort  2) Random mating­ doesn’t lead to genotype frequencies  changing, due to an absence of random mixing of  alleles/gametes  3) No natural selection­ Differences in survival and  reproductive success of individuals having different  genotypes can change allele frequencies  4) Large population size­ The smaller a population, the  more likely allele frequencies can change, while a large  population won’t noticeably change  5) No gene flow­ Moving alleles in and out of a  population, gene flow can change allele frequencies  Three mechanisms that affect allele frequencies directly:  Natural selection  Genetic drift  Gene flow o Natural Selection   Basically boils down to those individuals with traits that are  more suited for their environment tend to survive and produce  more offspring than those with traits not well suited for that  environment  In terms of genetics, alleles being passed down are different  than those in the current population  Adaptive evolutions: Evolutions that results in a better match  between organisms and their environment  Due to natural selection consistently favoring some  alleles over others o Genetic Drift  Chance events that can cause allele frequencies to fluctuate  unpredictably from one generation to the next, especially in  small populations  Two examples are:  The founder effect o Occurs when a few individuals become isolated  from a larger population and may establish a new  population, with a different gene pool from the  original population o Also can happen from chance events alter allele  frequencies   The bottleneck effect o A sudden event, such as a fire or flood, may  drastically reduce the size of the population  o This might lead to an abundance of certain alleles,  while a reduced amount of a different type, or not  a single one of a different kind o Such events will have a substantial effect on the  gene pool until the population becomes large  again, but even then, there would be low levels of  genetic variation for a long period of time  Effect of Genetic Drift: A Summary  1) Genetic drift is significant in small populations o Can cause alleles to be disproportionately over­ or  underrepresented   2) Genetic drift can cause allele frequencies to change at  random o Allele changes from year to year is not predictable, and change randomly over time  3) Genetic drift can lead to a loss of genetic variation  within populations o Due to random fluctuation, genetic drift can  eliminate alleles from a population and can  influence how effectively a population can adapt to change in the environment   4) Genetic drift can cause harmful alleles to become  fixed o Alleles that are neither harmful nor beneficial can  be lost or become fixed by chance o And because of this, harmful alleles can become  fixed and the population’s survival can be  threatened o Gene Flow  Definition: The transfer of alleles in or out of a population due  to the movement of fertile individuals or their gametes  Transferred alleles can affect how well populations are  adapted to the local environment o And can possibly improve the ability to adapt to  local conditions  And gene flow has become increasingly important in  human populations as humans move more freely  throughout the world, unlike in the past  o As a result, mating is more common between  different populations, which leads to:  An exchange of alleles  And fewer genetic differences between  populations  Natural Selection  It consistently increases frequency of alleles that provide  advantages  o Relative Fitness  Definition: The contribution an individual makes to the gene  pool of the next generation relative to those of others  Selection acts more directly on phenotype than on genotype o Selection  Three Types  Directional:  o Occurs when the population begins to favor  variants at one extreme, shifting the phenotype in  one direction or another o Common when an environment changes or the  population migrates to a new and different habitat  Disruptive: o Occurs when the current conditions of the  environment cause the population to favor both  ends of the distribution   Stabilizing: o Happens when the population begins to favor the  intermediate variations of a phenotype, while  rejecting the two extreme variations   Regardless of the type of selection, the basics are the same.  Selection favors individuals with traits that guarantee a higher  chance of reproductive success than others  Role of Natural Selection in Adaptive Evolution o Adaptations can arise gradually over time as natural selections traits  that promise a high reproductive success rate o And with genetic drift and gene flow, both can increase the  frequencies of alleles but neither do so consistently  Genetic drift can cause a beneficial allele to either increase or  decrease  And gene flow can introduce alleles that are either  advantageous or disadvantageous o Sexual Selection  Definition: A form of natural selection in which individuals  with certain inherited characteristics are more likely than others to get a mate  Can result in sexual dimorphism, which is a difference in  secondary sexual characteristics between males and females of  the same species  Several ways in which sexual selection can operate  Intrasexual selection: where individuals of the same sex directly compete for mates of the opposite sex  Intersexual selection: also called, mate choice, where  individuals of one sex, usually females, choose their mate from the other sex o Balancing Selection  Definition: Includes heterozygote advantage and frequency­ dependent selection  Heterozygote advantage: where an individual who’s  heterozygote has greater fitness than those with both types of  homozygotes  In which case, natural selection tends to maintain two or  more alleles at that locus  This is defined in terms of genotype, not phenotype  And can either represent stabilizing or directional  selection, depending on the geno/phenotype relationship  Frequency­Dependent Selection: where the fitness of a  phenotype depends on how common it is in a population  As a result, the frequency of a phenotype can oscillate  over time, and balancing selection, due to frequency  dependence, keeps the frequency of each phenotype close to 50%  Why Natural Selection Can’t Make Perfect Organisms o Though it leads to adaptation, here are some reasons why it can’t  make the perfect organisms 1. Selection can only act on existing variations  Natural selection favors only the fittest phenotypes  among the current gene pool, even if they’re not ideal  And new advantageous alleles don’t arise on demand 2. Evolution is limited by historical restraints  Each species has a legacy of descent with modification  from ancestral forms  Evolution doesn’t get rid of the ancestral anatomy and  build a new structure from nothing, instead it co­opts  existing structures and adapts them to new situations  And it operates on traits an organism already has 3. Adaptations are often compromises  Each organism must do many different things  o Example: humans and our versatility and  athleticism to our hands and flexible limbs; but we  are prone to sprains and other types of injuries  Structural reinforcement was compromised  for agility 4. Chance, natural selection, and the environment interact  Chance events can affect the evolution of populations  And sometimes not all of the alleles present in the  founding population’s pool maybe suited for the new  environment  While the environment somewhere can change  unpredictably from year to year, limiting adaptive  evolution to where the organism is well­suited for the  current environment conditions


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