Population Genetics. Chapter 16

Transcription

1 Population Genetics Chapter 16

2 Populations and Gene Pools Evolution is the change of genetic composition of populations over time. Microevolution is change within species which can occur over dozens of generations examples: antibiotic resistant bacteria, mosquitoes evolving resistance to DDT, HIV strains evolving resistance to antiviral medicines Macroevolution involves longer periods of time and formations of new species Population Genetics is the field of biology that studies microevolution Macroevolution refers to evolution of groups larger than an individual species.

3 Gene Pool A gene pool consists of all the genes of a local population of organisms A gene pool only refers to one population not an entire species Organisms in the same geographical region make up a local population and the unit of evolution

4 Genetic Variation Differences between individuals in a population Mutations and crossingover are two sources of genetic variation Variation is the Raw material for evolution

5 Why is genetic variation important? variation global warming survival EXTINCTION!! 5 no variation

6 Why do populations change? Hardy and Weinberg The Hardy-Weinberg Principal describes a hypothetical situation where there is no change in the gene pool SO no evolution. An important way of discovering why real populations change with time is to construct a model of a population that does not change. 6

7 Hardy-Weinberg Model Hardy & Weinberg developed this mathematical model to study populations Used to describe a non-evolving population under certain conditions Natural populations are not expected to be in Hardy-Weinberg equilibrium Understanding the conditions necessary for consistent allele frequencies helps us understand why populations change 1=p+q (allele frequencies) 1=p 2 +2pq+q 2 (Genotype frequencies) Deviation usually results in Evolution

8 Hardy-Weinberg Assumptions Their Assumptions The organism is diploid Reproduction is sexual Generations are non overlapping Population size is very large Mutation is negligible Gametes unite at random Migration is negligible Natural Selection does not operate

9 Hardy-Weinberg conditions Large breeding population The larger the population the less likely they will be affected by chance fluctuations in allele frequencies (eg. Hurricane, firestorm)

10 Hardy Weinberg Conditions Populations exhibits random mating Gametes unite at random Assortative mating reduces the number of heterozygous individuals Sexual selection would steer the gene pool toward the desired phenotypes and genotypes.

11 Hardy Weinberg Conditions Changes in allelic frequency due to mutation are negligible Any mutation in a particular gene would change the balance of alleles in the gene pool.

12 Hardy Weinberg Conditions Migration is negligible No new alleles can come into the population, and no alleles can be lost. Both immigration and emigration can alter allelic frequency.

13 Hardy Weinberg Conditions Natural Selection does not operate No alleles are selected over other alleles If selection occurs, those alleles that are selected for will become more common.

14 Hardy Weinberg Conditions: These conditions are the absence of the things that can cause evolution. In other words, if no mechanisms of evolution are acting on a population, evolution will not occur--the gene pool frequencies will remain unchanged. However, since it is highly unlikely that any of these seven conditions, let alone all of them, will happen in the real world, evolution is the inevitable result.

15 Hardy Weinberg Equilibrium Allele frequencies of nonevolving populations are stable over generations The Equation: 1.0= p 2 +2pq+q 2 P 2 = frequency of AA genotypes 2pq= Aa +aa genotypes q 2 = frequency of aa genotypes

16 Example use of H-W theorem 1000-head sheep flock. No selection for color. Closed to outside breeding. 910 white (BB or Bb) 90 black (bb) Start with known: 90bb/1000 =.09=q 2 q=.09 =.3 (allele frequency) p+q = 1 so p = 1 q =.7 = p p 2 +2pq+q 2 to figure out genotypic frequencies: Frequency of (BB) = p 2 = (.7) 2 =.49 or 49% BB Frequency of (Bb) = 2pq = 2(.7)(.3) =.42 or 42% Bb Frequency of (bb) = q 2 = (.3 )2 =. 09 or 9% bb Phenotypic frequency =.09/9% black &.91/91% white 16

17 Hardy-Weinberg Equilibrium

18 Hardy-Weinberg Applied The allele for black coat is recessive to the allele for white coat. Can you count the number of recessive alleles in this population? p = the frequency of the dominant allele (represented here by A) q = the frequency of the recessive allele (represented here by a) For a population in genetic equilibrium: p + q = 1.0 (The sum of the frequencies of both alleles is 100%.) Genotypic frequencies represented by: p 2 + 2pq + q 2 = 1 p 2 = frequency of AA (homozygous dominant) 2pq = frequency of Aa (heterozygous) q 2 = frequency of aa (homozygous recessive)

19 How To Solve The Problems??? Calculate q 2 Count the individuals that are homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q 2 Answer: Four of the sixteen individuals show the recessive phenotype, so the correct answer is 25% or Find q. Take the square root of q 2 to obtain q, the frequency of the recessive allele. Answer: q = 0.5 Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the dominant allele? Answer: p = 1 - q, so p = 0.5 Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat: Answer: 2pq = 2(0.5) (0.5) = 0.5, so 50% of the population is heterozygous.

20 Hardy-Weinberg calculations Allele frequencies p+q=1 p=dominant ALLELE frequencies q=recessive ALLELE frequencies Genotype frequencies p 2 +2pq+q 2 =1 p 2= Homozygous Dominant GENOTYPE frequency 2pq=Heterozygous GENOTYPE frequency q 2 =Homozygous recessive GENOTYPE frequency

21 Main Factors that Affect Gene Pools Natural Selection Gene Flow Mutations Genetic Drift

22 Natural Selection English Peppered Moth (Biston bitularia) In pre-industrial England; birches had white bark (due to white lichens) White moths predominated over black moths 95% white, 5% black In industrial England; birch bark was coated with soot (lichens died) Black moths predominated over white moths Percentages almost reversed Natural Selection changed the frequencies in the gene pool

23 Natural selection: Sickle Cell Anemia Sickle cell anemia is a disease in which your body produces abnormally shaped red blood cells. Hemoglobin is the protein in red blood cell that carries oxygen throughout the body Most people have normal hemoglobin Allele (A) One amino acid difference caused by a change in one nucleotide in the DNA produces and alternative defective sickle allele (S)

24 Sickle Cell Advantage (SS) individuals have Sickle cells anemia and serious health problems Sickle Cell disease is most common in regions with high incidence of Malaria Sickled cells provide resistance to malaria! (SS)-have Sickle cell Anemia but are resistant to Malaria (AA)- are healthy but vulnerable to Malaria (AS)- Enough healthy blood cells to not have true symptoms and enough sickled cells to be resistant to Malaria Natural Selection favors Heterozygotes where there is Malaria maintaining both alleles in the population Malaria

25 Three Modes of Natural Selection: Directional Selection occurs when selection favors one extreme trait value over the other extreme. Result= a change in the mean value of the trait under selection. Disruptive Selection occurs when selection favors the extreme trait values over the intermediate trait values. Result = the variance increases as the population is divided into two distinct groups. Disruptive selection plays an important role in speciation. Stabilizing Selection occurs when selection favors the intermediate trait value over the extreme values. Result= a decrease in the amount of genetic variation for the trait under selection.

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27 When Populations change

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30 Gene Flow Gene Flow Refers to Migration of individuals between populations Potentially introduces new alleles into a population or alters existing allele frequencies Reduces genetic differences among populations; thus slowing genetic differentiation

31 Mutations Mutations are changes in genetic information The occur spontaneously at a very low rate Usually result from a slight error in DNA replication Most have bad or no effects Some are beneficial and favored by natural selection causing them to increase in frequency over generations

32 Genetic Drift Random changes in allele frequencies in small populations Substantial effect on small populations but little effect on large populations Two factors cause Genetic Drift Founder Effect Bottleneck Effect

33 Genetic Drift due to Founder Effect Genetic drift that follows the colonization of a new habitat Allele frequencies can be different than source populations and can sometimes exclude diversity of original population Who settles the new population has great effect on the following genetic diversity and allele frequencies

34 Genetic Drift due to Bottleneck Populations Drastic reduction in populations for a few generations Allele frequencies can be different than source populations and can sometimes exclude diversity of original population Ex: American Bison experienced a severe population bottleneck in the19 th century due to over hunting

35 Genetic drift >>>Inbreeding Genetic Drift can also lead to loss of genetic variation Gradual increase in Homozygosity is called Inbreeding Inbreeding depression Ferity and survival rate are relatively reduced Small populations have problems maintaining stable numbers A typical human has an estimated 7 alleles that would be lethal if homozygous

36 Artificial Selection Involves Breeders who select only plants and animals with desired traits for breading This process allows breeders to manipulate animal populations over generations Natural selection vs. artificial selection: Nature provides the genetic variation but In Natural Selection nature selects which individuals reproduce In Artificial Selection the breeder (people) selects which individuals reproduce

37 Speciation A species is a group of populations who s individuals can breed and produce fertile offspring Speciation is the formation of new species One species may split into two or more species A species may evolve into a new species Requires very long periods of time

38 Speciation these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.