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T HE E VOLUTION O F P OPULATIONS. E VOLUTION AND V ARIATION Microevolution- small scale evolution; change in allele frequencies in a population over generations.

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Presentation on theme: "T HE E VOLUTION O F P OPULATIONS. E VOLUTION AND V ARIATION Microevolution- small scale evolution; change in allele frequencies in a population over generations."— Presentation transcript:

1 T HE E VOLUTION O F P OPULATIONS

2 E VOLUTION AND V ARIATION Microevolution- small scale evolution; change in allele frequencies in a population over generations. Discrete Characters- classified on an either-or basis Quantitative Characters- vary along a continuum Average Heterozygosity- (gene variability) the average percent of loci that are heterozygous. Nucleotide Variability- comparing DNA sequences of two individuals Geographic Variation- differences in genetic composition of separate populations.

3 M UTATION Mutation- the ultimate source of new alleles Point mutations- a change in one base in a gene Neutral and Beneficial Mutations Mutations Rates Plants/Animals- 1/100,000 genes per generation Prokaryotes- fewer mutations, shorter generation span, more genetic variation Viruses- more mutations, shorter generation span, RNA genome with fewer repair mechanisms

4 G ENE P OOLS AND A LLELE F REQUENCY Population- a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. Gene pool- all of the alleles for all the loci in all individuals of the population. Fixed- only one allele exists for a particular locus and all individuals are homozygous for that allele

5 H ARDY -W EINBERG P RINCIPLE H-W Equilibrium describes a constant frequency of alleles within a gene pool. p pq + q 2 = 1 where p 2 and q 2 represent the frequencies of the homozygous genotypes and 2 pq represents the frequency of the heterozygous genotype Frequencies of alleles Alleles in the population Gametes produced Each egg:Each sperm: 80% chance 80% chance 20% chance 20% chance q = frequency of p = frequency of C R allele = 0.8 C W allele = 0.2

6 H ARDY -W EINBERG A SSUMPTIONS 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow *Departure from any of these conditions usually results in evolutionary change.

7 P RACTICE H ARDY -W EINBERG P ROBLEM For a locus with two alleles (A and a) in a population at risk from an infections neurodegenerative disease, 16 people had genotype AA, 92 had genotype Aa, and 12 had genotype aa. Use the Hardy-Weinberg equation to determine whether this population appears to be evolving.

8 MECHANISMS THAT ALTER ALLELE FREQUENCY Natural selection Leads to adaptive radiation Genetic drift Founder Effect Bottleneck Effect Gene flow

9 Fig Generation 1 C W C R C R C W C R C R C W p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W C W C R p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R

10 Fig Original population Bottlenecking event Surviving population

11 E FFECTS OF G ENETIC D RIFT 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed

12 MECHANISMS THAT ALTER ALLELE FREQUENCY Natural selection Leads to adaptive radiation Genetic drift Founder Effect Bottleneck Effect Gene flow the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

13 N ATURAL S ELECTION AND A DAPTIVE E VOLUTION Relative Fitness- the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals. Natural selection is the only evolutionary mechanism that continually leads to adaptive evolution.

14 D IRECTIONAL S ELECTION Occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the frequency curve for the phenotypic character in one direction or another. Original population (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

15 D ISRUPTIVE S ELECTION Occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. Fig b Original population (b) Disruptive selection Phenotypes (fur color) Frequency of individuals Evolved population

16 S TABILIZING S ELECTION Acts against both extreme phenotypes and favors intermediate variants. Fig c Original population (c) Stabilizing selection Phenotypes (fur color) Frequency of individuals Evolved population

17 Sexual Selection a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. Sexual Dimorphism marked differences between the two sexes in secondary sexual characteristics, which are not directly associated with reproduction or survival. Intrasexual Selection Selection within the same sex. Individuals of one sex compete directly for mates of the opposite sex. Intersexual Selection mate choice- individuals of one sex (usually females) are choosy in selecting their mates from the other sex.

18 P RESERVATION OF G ENETIC V ARIATION Diploidy Hides genetic variation from selection in the form of recessive alleles Balancing Selection Occurs when natural selection maintains two or more forms in a population. Heterozygote Advantage Individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes Frequency-Dependent Selection The fitness of a phenotype declines if it becomes too common in the population Neutral Variation Has no selective advantage or disadvantage

19 W HY N ATURAL S ELECTION C ANNOT F ASHION P ERFECT O RGANISMS Selection can only act on existing variations. Evolution is limited by historical constraints. Adaptations are often compromises Chance, natural selection, and the environment interact.

20 E XIT S LIP Of all the mutations that occur in a population, why do only a small fraction become widespread among the populations members? If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time? Explain.


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