BSC 2011- Week 2 Notes
BSC 2011- Week 2 Notes BSC 2011
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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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