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TEXAS A&M / Biology / BIOL 112 / Can evolution occur without genetic variation?

Can evolution occur without genetic variation?

Can evolution occur without genetic variation?

Description

School: Texas A&M University
Department: Biology
Course: Introductory Biology II
Professor: Christopher lee
Term: Spring 2017
Tags: The, evolution, and Of
Cost: 25
Name: Chapter 23
Description: Book notes
Uploaded: 01/28/2018
12 Pages 136 Views 2 Unlocks
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CHAPTER 23


Can evolution occur without genetic variation?



The Smallest Unit of Evolution

∙ Microevolution - change in allele frequencies in a population over generations

∙ 3 mechanisms that cause allele frequency to change 1. Natural Selection  

2. Genetic Drift (chance events that alter allele frequencies) 3. Gene Flow (the transfer of alleles between populations)

23.1 Genetic Variation Makes Evolution Possible ∙ Darwin proposed that natural selection was the primary cause for change in evolution of life on earth, that selection acts on the  differences which will lead to evolution of the species to better fit the  environment  If you want to learn more check out What is the unhealthy condition that would motivate pursuing having a child with two genetic mothers?

∙ Darwin did realize that variation in heritable traits is a  prerequisite for evolution he never knew how organisms pass down the  heritable traits to their offspring


How effectively a population can adapt when the environment changes?



∙ Mendel's paper on pea plants is what brought the idea of  understanding genetic differences on which evolution is based  

Genetic Variation  

∙ Individuals within a species vary in their specific characteristics  including physical and non physical traits (blood type)  If you want to learn more check out What does one go through the pre-production of film-making?

∙ Phenotypic variations are often reflected by genetic variation  ∙ Genetic Variation - differences among individuals within a  population

∙ Some variations occur based off of "either-or", and are  normally determined by a SINGLE gene locus with different  alleles producing distinct phenotypes

∙ Other phenotypic differences vary in gradations along a  continuum, which are results from the influence of two or more genes  on a single phenotypic characteristic (influenced by multiple genes)  


Why natural selection cannot fashion perfect organisms?



∙ Genetic variation can be measured at the gene level (gene  variability)  

∙ Genetic variation can be measured at the molecular level of DNA (nucleotide variability) - this rarely results in a phenotypic variation  because many of the differences occur within the introns that are taken  out of the DNA segment, and the exons rarely cause a change in the  amino acid sequence of the protein being coded for  

∙ Phenotypic variation does not result in genetic  differences in individuals  

∙ Phenotype is the product of an inherited genotype that has  many ENVIRONMENTAL influences  We also discuss several other topics like What kind of structures does rna have?

1. Example: body builder does not pass muscles to next  generation

Genetic variation provides the raw material for evolutionary  change and without it there would be no evolution

Sources of Genetic Variation  

∙ Genetic variation is dependent on mutations, gene duplication or other processes that cause new alleles and new genes  

∙ Genetic variation can occur rapidly with organisms that have  short lives and genetic variation can also occur with sexual reproduction because the existing genes are arranged in new ways  

Formation of New Alleles  We also discuss several other topics like What is the correct format for scientific genus and species' names?

∙ New alleles can arise by MUTATION, a change in the  nucleotide sequence of an organism

∙ In multicellular organisms, only mutations in cell lines that  produce gametes can be passed on to the offspring  

∙ In most animals, the majority of mutations occur in somatic cells  and are not passed on to the offspring  

∙ A point mutation can have a significant impact on phenotype ∙ Organisms phenotypes match well to their environments  because they reflect many generations of past selection  

∙ Most new mutations that alter phenotype can be harmful, and in  most cases natural selection removes these alleles  

∙ In diploid organisms harmful alleles that are recessive can be  hidden from selection and remain for generations due to heterozygous  individuals (the dominant allele masks the recessive and thus is stays in the population)  We also discuss several other topics like What was the philosophy of the bauhaus?

∙ These heterozygote protective mechanisms have a pool of alleles that may not be favorable in present conditions, BUT that could be  beneficial if the environment changes

∙ Mutations CAN BE BENEFICIAL or HAVE NO AFFECT  ∙ Neutral variation - difference in DNA sequence that do not cause  an advantage or a disadvantage  

Altering Gene Number or Position  

∙ Chromosomal changes that delete, disrupt or rearrange loci are  usually harmful, but large scale changes may not affect the genes  phenotype  

∙ Chromosomal rearrangements may even be beneficial  ∙ Duplication:  

1. In large segments are often harmful  

2. In smaller pieces of DNA it may not be harmful  3. Gene duplications that do not have a severe effect can  survive from generation to generation, which results in an  expanded genome with new genes that may take on new functions

∙ Increases in the gene number appeared to have played a major  role in evolution  We also discuss several other topics like What critique did capitalists like albert augustus pope make of competition?

1. Example: ancestors of mammals have a single gene that is  used for detecting odors, that has been duplicated over time and  now humans have 350 functional olfactory receptor genes and mice have 1,000  

2. This proliferation of olfactory genes has helped mammals  to detect faint odors and to distinguish between different smells  

Rapid Reproduction

∙ Mutation rates are slow for animals and plants (1 mutation every  100,000 genes per generation)

∙ These rates are even lower in prokaryotes, but prokaryotes have  more generations in a certain amount of time

1. This allows for mutations to generate genetic diversity  quickly in their populations 

2. Same with viruses  

3. This makes it difficult to develop a cure for this virus  because the resistant strains reproduce so rapidly that the new  medication that was developed would not be affective in 2 days  ∙ Example:  

1. HIV has a generation span of 2 days  

2. This means that when HIV is treated the mutant forms that  are resistant to the drug being used are able to reproduce quickly  3. The most affective treatment for AIDS is "cocktails" which a mixture of a lot of different drugs

∙ The shorter the generation period of a organism the faster it will  be able to develop genetic diversity  

Sexual Reproduction  

∙ Organisms that reproduce sexually, their genetic  variation in a population comes from the unique combination of  alleles the individual receives from its parents

∙ All the differences in these alleles have come from past  mutations  

∙ Sexual reproduction shuffles the alleles and deals them  randomly to produce an individuals genotype  

∙ Three mechanisms that contribute to shuffling:  

1. Crossing over  

2. Independent assortment of chromosomes 3. Fertilization  

∙ In meiosis, homologous chromosomes (one from each parent)  trade some of their alleles by crossing over. These homologous  chromosomes and alleles they carry are then distributed randomly into  gametes. Because of the countless number of mating combinations

exist in a population fertilization brings gametes that are likely to have  different genetic backgrounds  

∙ A combination of all three of these ensures that sexual  reproduction rearranges existing alleles into fresh combinations each  generation, providing genetic variation which makes evolution possible

∙ Individuals in a population must differ genetically for evolution to occur  

An Overview  

∙ Genetic Variation - refers to genetic differences of individuals  within a population  

∙ Nucleotide differences that provide the bases of genetic variation originate when mutations and genetic variants make new alleles and  genes  

∙ New genetic variants are produced quickly in organisms with  short generation times  

∙ In sexually reproducing organisms, most genetic diversity comes  from crossing over, independent assortment of chromosomes and  fertilization  

23.2: The Hardy-Weinberg Equation can be Used to Test  Whether a Population is Evolving  

∙ The presents of genetic variation doesn't guarantee that a  population will evolve, and for this to happen one of the factors that  cause evolution must be a work  

Gene Pools and Allele Frequencies  

∙ Population - a group of individuals of the same species that live  in the same area and interbreed producing fertile offspring  ∙ Some populations may be isolated geographically and exchange  material rarely

∙ Not all populations are isolated and members of a population  typically breed with one another and are closely related to each other  than to members of OTHER populations  

∙ Gene pool - consists of all copies of EVERY type of allele at every  locus in all members of the population  

∙ If only one allele exist for a certain locus in a population it is  considered fixed in the gene pool and all individuals are homozygous  for that allele  

∙ If there are two or more alleles for a particular locus in a  population, individuals may be homozygous or heterozygous  1. Example:

∙ 500 wild flowers with 2 alleles R and W for a locus  that codes for flower pigment

∙ These alleles show incomplete dominance - each  genotype has distinct phenotype and are diploid  

∙ RR = red flower

∙ WW = no red pigment so is white

∙ RW = some red and some white (pink)

2. Allele Frequency  

∙ 320 red, 160 pink and 20 white  

∙ Because they are diploid organisms these 500  individuals have 1000 copies of the gene for flower color  

∙ R = 800 (320 x 2 + 160 x 1 = 800)  

 Frequency = 800/1000=0.8  

∙ W frequency = p +q = 1 (p = R and q = W)   0.8 + q = 1  

 Q = 0.2

The Hardy-Weinberg Equation  

∙ To figure out if natural selection is occurring to cause evolution at a particular locus is to determine what the genetic make up would be if  was NOT evolving at that locus  

∙ We can compare this scenario with the data collected for the  population  

1. If there are NO difference then there is NO evolution occurring  

2. If there ARE differences then is suggest the population  may be evolving  

Hardy-Weinberg Equilibrium  

∙ Hardy-Weinberg Equilibrium - a population that is not evolving,  allele and genotype frequencies will remain the same from generation  to generation

∙ We will consider the combination of alleles in ALL of the crosses  in a population

∙ Selecting randomly from the gene pool - reproduction occurs by  selecting alleles at random from the gene pool  

∙ By viewing it in this way it is assumed that mating occurs at  random - meaning that male-female mating's are equally likely  ∙ A population is ONLY in Hardy Weinberg if the members  continue to mate RANDOMLY generation after generation and  the allele and genotype frequencies remain the same  

Hardy-Weinberg Equations  

∙ p + q = 1  

1. P = frequency of population that is dominant  

2. Q = frequency of population that is recessive  

∙ p2 + 2pq + q2 = 1

1. p2 = % of population that is homozygous dominant  2. 2pq = % of population that is heterozygous  

3. q2 = % of population that is homozygous recessive

Conditions for Hardy-Weinberg Equilibrium

o No Mutations: the gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated  

o Random Mating: If individuals do not randomly mate the  random mixing of gametes does not occur and genotype frequencies  will change

o No Natural Selection: differences in survival and  reproductive success of individuals carry different genotypes can alter  allele frequencies  

o Extremely Large Population Size: the smaller  population, the more likely it is that allele frequencies will fluctuate by  chance from one generation to the next (genetic drift)

o No Gene Flow: Moving genes in and out of the population, gene flow can alter allele frequencies  

o All of these conditions MUST be met other wise the  population will not be in Hardy Weinberg equilibrium  

o It is common for natural population to be in Hardy  Weinberg equilibrium for specific genes, if selection alters allele  frequencies at some loci but not others  

o Also, if the populations evolve so slowly that the changes  in their allele and genotype frequencies are difficult to distinguish form  those predicted for non-evolving population

Applying the Hardy-Weinberg Equation  

o Is the initial test of whether evolution is occurring in  a population  

o The equation also gives medical reasons, like estimating  the % of a population carrying the allele for an inherited disease o PKU

1. 1 in 10,000 babies  

2. Is a medical disorder carried by homozygous recessive  allele  

3. To apply the Hardy Weinberg equation one has to assume  that  

∙ no PKU mutations are being introduced

∙ People are neither choosing mates based on whether or not they carry the trait or mate with close relatives  

∙ Assume there are no effects of genetic drift

∙ Gene flow form other populations into the US  4. If these assumptions hold; then the frequency of  individuals with PKU will by q2 and we must also estimate the  number of heterozygotes rather than counting them directly

5. So q2 = 0.0001 --> q = 0.01  

6. P = 1-0.01 = 0.99  

7. 2pq = 2 x 0.99 x 0.01 = 0.0198  

8. This also suggest that harmful recessive alleles (that are  hidden) at this and other loci can be concealed in a population  because of the healthy heterozygotes  

Overview  

o

23.3: Natural Selection, Genetic Drift, and Gene Flow Can  Alter Allele Frequencies in a Population  

o Hardy-Weinberg equilibrium has to meet all 5 conditions 1. New mutations - can alter allele frequencies, but because  they are rare, the change from generation to generation would be  small  

2. Non-random mating - can affect frequencies of  homozygous and heterozygous genotypes but by itself has no  effect on allele frequencies in the gene pool (frequencies can  change if individuals with certain traits are more likely to obtain  mates)

Natural Selection  

o Based on differential success and reproduction  o Individuals in a population exhibit variations in their  heritable traits, those with traits that better suit the environment will  reproduce at a higher rate that those that don’t have those traits o Selection - alleles being passed to the next generation in  proportions that differ from the present generation

o An allele that confers resistance to an insecticide will  increase in frequency in a population that is exposed to that insecticide o Consistently favoring some alleles over other natural  selection can cause adaptive evolution  

o Adaptive evolution - evolution that results in a better  match between organisms and their environment  

Genetic Drift  

o Genetic drift - chance events that cause allele frequencies to  fluctuate unpredictably form one generation to the next (usually smaller  populations)

o There are different ways in which genetic drift and affect a  population  

1. Chance events associated with survival and reproduction  (large moose stepped on 3 baby moose that had the recessive allele  reducing the frequency of the recessive allele being passed down)

2. Chance events occurring during fertilization (a heterozygous  individual RW has a few children and by chance alone all the children  could ca R alleles)

The Founder Effect

o Founder Effect - when a few individuals become isolated from  larger population, the smaller group may establish a new population whose  gene pool differs from the source population  

o A big population > few individuals break off > form a new  population > have a different gene pool  

o May occur when a few members of a population are blown by a  storm to a new island  

o Genetic drift (chance events alter allele frequencies) will occur in such a case if the storm indiscriminately transports some individuals but not  others from the source (original) population

o Example:

1. Founders of Tristan da Cunha

2. One colonist carried a recessive allele for retinitis pigmentosa, a  progressive form of blindness that afflicts homozygous individuals 3. Of the colonist 240 descendants on the island 4 had the disorder 4. The frequency of the allele that causes this disease is 10 times  higher than that of which the founders are from

The Bottleneck Effect  

o Bottleneck Effect - the population size is drastically reduced and  the population is built from the remaining individuals

o By chance certain alleles may be overrepresented among the  survivors, or underrepresented or even absent  

o Ongoing genetic drift will have a lot of affect until the  population is large again and chance events will have less impact  o Will cause LOWER genetic diversity  

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken  o Illinois prairie chicken population decreased to below 50, with  low genetic diversity and less than 50% of their eggs hatched  

o This showed a loss in genetic diversity and an increase in the  frequency of harmful alleles (causing low hatching of eggs)  

o Researchers ended up finding that the current population had  fewer alleles per locus than the pre bottleneck population

o To counteract the affect some birds from neighboring  populations added to the Illinois population, which worked  

Effects of Genetic Drift: A Summary  

o Genetic drift is significant in small populations  1. Chance events occur in all size populations, but it tends to alter allele frequency substantially in smaller populations

o Genetic drift can cause allele frequencies to change at random  1. Genetic drift an allele frequency can change one year and then  decrease the next, it is not like natural selection where depending on the  environment the alleles that were favored went to the next generation,

genetic drift causes allele frequencies to change at RANDOM over time

o Genetic drift can lead to a loss of genetic variation within  population  

1. By causing allele frequencies to fluctuate randomly over time,  genetic drift can eliminate alleles from the population. Because of  these losses, it can influence how effectively a population can  adapt when the environment changes  

o Genetic drift can cause harmful alleles to become fixed  1. Alleles that are harmful nor beneficial could become fixed by chance through genetic drift. In smaller populations genetic drift can  cause harmful alleles to become fixed, this can cause a population's  survival to be threatened  

Gene Flow  

o Gene flow - the transfer of alleles into or out of a population due  to the movement of fertile individuals or their gametes

o Gene flow tends to reduce the genetic differences  between populations

o It can actually result in 2 populations combining into a single  population with a common gene pool  

o Alleles that are transferred through gene flow can also affect  how well a population can adapt to the environment (song birds central and  eastern. Eastern songbird survival rate is much higher than that of the central  due to genetic flow being higher in central than in the eastern areas. This  means that when songbirds go to the eastern part selection will reduce the  frequency of the bad genotypes)  

o Gene flow can also transfer alleles that improve the ability of  populations to adapt to the local conditions

o Humans gene flow has increase with the common  transportation: mating is more common between members of populations that previously had very little contact. Leading to exchange of alleles with fewer  genetic differences between those populations

23.4: Natural Selection is the Only Mechanism that Consistently causes adaptive  Evolution

o Evolution by natural selection:

1. Chance in the creation of new genetic variations (mutations) 2. Sorting as natural selection favors some alleles over others  o Due to this favoring of alleles the outcome is not random,  instead there is an increase in the frequency of alleles that provide  reproductive advantage leading to adaptive evolution  

Natural Selection: A Closer Look  

o Adaptive evolution comes about with relative fitness and  different ways that an organism's phenotype is subject to natural selection

Relative Fitness

o Reproductive success depends on many factors other than  battling

1. A moth may have more offspring than other moths in the same  population because its body color kept them from predators improving its  chance of survival long enough to produce more offspring  

o Relative Fitness - the contribution an individual makes to the  gene pool of the next generation relative to the contributions of other  individuals

o This selection acts more directly on the phenotype than the  genotype; it acts on the genotype indirectly by how the genotype affects the  phenotype  

Directional, Disruptive and Stabilizing Selection  

o Natural selection can alter allele frequency depending on which  phenotypes in a population are favorable  

o Directional Selection - when conditions favor individuals  exhibiting ONE EXREME of a phenotypic range  

1. Shifts the population's frequency curve for the phenotypic  character in one direction or the other

2. Common when a population environment changes or when a  member of a population migrate to new habitats  

o Disruptive selection - when conditions favor individuals of both  extremes of a phenotypic range and not the intermediate phenotypes  o Stabilizing Selection - Acts against both extreme phenotypes  and favors the intermediate  

1. Reduces variation and tends to maintain a particular phenotypic  characteristic  

2. Example: human babies weigh 6-8 pounds, anything above or  below increases the rate of morality  

o Selection favors individuals whose heritable phenotypic traits provide higher productive success than the other traits  

The Key Role of Natural Selection in Adaptive Evolution  

o Adaptations can arise gradually overtime as natural selection  increases the frequencies of alleles that enhance survival and reproduction  o Both genetic drift and flow, increase the frequencies of  alleles that improve the match between organisms and their  environment but neither do it CONSITANTLY  

o Genetic drift can cause frequencies of slightly beneficial to  increase but it can also cause beneficial genes to decrease  

o Gene flow may introduce beneficial genes or disadvantageous  genes into the population  

o Natural Selection is the only evolutionary mechanism that  consistently leads to adaptive evolution  

Sexual Selection  

o Sexual selection - form of natural selection in which individuals with certain inherited characteristics are more likely to find a mate  o Sexual dimorphism - difference in secondary characteristics  between male and females (size, color, ornamentations and behavior) o Intrasexual selection - selection within the same sex,  individuals of one sex compete directly for mates of the opposite sex

1. Mostly occurs among males but can occur in females  2. Single male may patrol a group of females and prevent other  males from mating with them  

o Intersexual selection - mate choice, individuals of one sex  (usually female) are picky in selection of mates from other sex

1. Females choice depends on the showiness of the males  appearance and behavior

2. If these characteristics help the male gain a mate and outweighs the risk from predation then both would be reinforced because they  enhance overall reproductive success  

3. Traits that females look for could include, "good genes" or  overall health of the male  

Balancing Selection  

o Diploid organisms may have unfavorable recessive alleles that  persist because they are hidden by a dominant trait of a heterozygote  o Selection itself may preserve variation at some loci, thus  maintaining two or more forms in a population  

o Balancing Selection - occurs when natural selection maintains  two or more forms in a population  

o type of selection includes heterozygote advantage and  frequency-dependent selection  

Heterozygous Advantage  

o Heterozygous Advantage - if individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes  o Is defined in terms of genotype not phenotype  o If it represents stabilizing or direction selection depends on the relationship between genotype and phenotype  

o If phenotype of a heterozygote is intermediate to the  phenotypes of both homozygotes, heterozygotes advantage is a form of  stabilizing selection  

o Example:  

1. Homozygous individuals with recessive allele at that locus  causes sickle-cell disease (blood is distorted in shape, sickled), and it  causes low oxygen conditions in the capillaries  

2. Some red blood cells become sickled in heterozygotes, not  enough become sickled to cause sickle cell disease

3. Heterozygotes for sickle cell allele are protected against  the most severe effects of malaria (infects the red blood cells) 4. This happens in heterozygotes because  

∙ The body destroys sickled red blood cells rapidly,  killing the parasites they harbor  

5. Protection against malaria is important especially in tropical  regions where the disease is a killer  

6. Selection favors heterozygotes over homozygote dominant  individuals (more vulnerable to the effects of malaria), and over  homozygous recessive individuals who develop sickle-cell disease

Frequency-Dependent Selection  

o Frequency-Dependent Selection - the fitness of a phenotype  depends on how common it is in the population  

o Example:

1. Scale eating fish  

2. Right and left mouths

3. Right mouth attacks from left side and the left mouth attacks  from the right side

4. The right side is dominant over the left  

5. Selection favors whichever mouth phenotype is least common  6. The frequency ends up balancing each phenotype to about 50%  

Why Natural Selection Cannot Fashion Perfect Organisms

o Selection can act ONLY on EXISTING variations  1. Natural selection favors the fittest in the current population,  which may not be the ideal traits  

2. New advantageous traits do NOT rise on demand  

o Evolution is limited by historical constraints  

1. Natural selection acts on the traits an organism already has and  adapts them to new situations  

2. Can't just grow a pair of wings, instead existing pair of limbs  took on new function for flight  

o Adaptations are often compromises  

1. Seals spend time on rock, it would be easier if they had legs to  walk, but then they couldn't swim as well

2. Human's are very versatile, but are prone to sprains, torn  ligaments and dislocations

∙ Structural reinforcement ahs been compromised for agility o Chance, natural selection and the environment interact 1.

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