The Evolution of Populations

Transcription

1 The Evolution of Populations What you need to know How and reproduction each produce genetic. The conditions for equilibrium. How to use the Hardy-Weinberg equation to calculate allelic and to test whether a population is. - change in allele frequencies over generations Mutation and sexual reproduction produce the genetic variation that makes evolution possible Two processes produce the variation in gene pools that contributes to differences among individuals o o reproduction Not all phenotypic variation is, some are Most species exhibit variation, differences between gene pools of separate populations or population subgroups Some examples of geographic variation occur as a, which is a graded change in a trait along a geographic axis. are changes in the nucleotide sequence of DNA o Mutations cause genes and alleles to arise o Only mutations in cells that produce gametes can be to offspring o A mutation is a change in one base in a gene! The effects of point mutations can vary: in regions of DNA are often harmless in a might not affect protein production because of redundancy in the genetic code Mutations that result in a change in protein production are often Mutations that result in a change in protein production can sometimes the fitness between organism and environment

2 The Hardy-Weinberg equation can be used to test whether a population is evolving A is a localized group of individuals capable of interbreeding and producing fertile offspring A pool consists of all the alleles for all loci in a population A is fixed if all individuals in a population are homozygous for the same allele The frequency of an allele in a population can be calculated o For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 o The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles o If there are 2 alleles at a locus, p and q are used to represent their frequencies the frequency of all alleles in a population will add up to 1! For example The principle describes a population that is not evolving o If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving o where p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype The five conditions for nonevolving populations are rarely met in nature: o No o mating o No o Extremely population size o No flow Lets s Practice If brown (B) is dominant and white (b) is recessive, answer the following questions. TO BE SUPER CONFUSING, in Hardy-Weinberg, p represents the dominant and q will represent the recessive. There are 11,237 members of a population where 1124 individuals show the recessive condition. What is percentage of each trait? show the dominant while show the recessive. ALWAYS REMEMBER-USE THE RECESSIVE FIRST.

3 If 10% of the population show the recessive, that is a frequency of.10. Remember that there are TWO recessive alleles, so to find only one, you must take the square root of that number. What is the frequency of ONLY 1 of the recessive alleles? Now that you know that, you can determine p. What does p = Now use the formula p 2 + 2pq + q 2 = 1 to solve for either the frequency or percentage of each variation. What did you get? If 1/10,000 people have PKU, what percentage of the population is heterozygous? Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Three major factors alter allele frequencies and bring about most evolutionary change: o o o Genetic describes how allele frequencies fluctuate unpredictably from one generation to the next o Genetic drift tends to genetic variation through losses of alleles How is it possible that only red plants will be on the population after only 3 generations?

4 o The effect occurs when a few individuals become isolated from a larger population! Allele frequencies in the small founder population can be from those in the larger parent population o The effect is a sudden reduction in population size due to a change in the environment! The survivors are! The resulting gene pool may no longer be reflective of the original population s gene pool! If the population remains, it may be further affected by genetic drift How did scientists save the prairie chicken?

5 o Effects of Genetic Drift: A Summary! Genetic drift is significant in populations! Genetic drift causes frequencies to change at! Genetic drift can lead to a of genetic variation within populations! Genetic drift can cause alleles to become fixed Gene consists of the movement of alleles among populations o Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) o Gene flow tends to differences between populations over time o Gene flow is more likely than mutation to allele frequencies directly o Gene flow can the fitness of a population How do mosquitos become resistant to insecticide and why is that bad for humans? Natural selection is the only mechanism that consistently causes adaptive evolution Only natural selection consistently results in evolution Natural selection brings about adaptive evolution by acting on an organisms Relative is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain by acting on the of certain organisms Three modes of selection: o selection favors individuals at one end of the phenotypic range o selection favors individuals at both extremes of the phenotypic range o selection favors intermediate variants and acts against extreme phenotypes

6 Label the picture selection is natural selection for mating success o It can result in sexual, marked differences between the sexes in secondary sexual characteristics selection is competition among individuals of one sex (often males) for mates of the opposite sex selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates maintains genetic variation in the form of hidden alleles selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population advantage occurs when heterozygotes have a higher fitness than do both homozygotes

7 o Natural selection will tend to two or more alleles at that locus o The sickle-cell allele causes mutations in but also confers malaria resistance Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can only on existing variations 2. Evolution is by historical constraints 3. Adaptations are often 4. Chance, natural selection, and the environment You should now be able to 1. Explain why the majority of mutations are harmless 2. Explain how recombination generates genetic variability 3. Define the terms population, species, gene pool, relative fitness, and neutral variation 4. List the conditions of Hardy-Weinberg equilibrium 5. Apply the Hardy-Weinberg equation to a population genetics problem 6. Explain why natural selection is the mechanism that consistently produces adaptive change 7. Explain the of population size in genetic drift 8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 9. List reasons why natural selection cannot produce perfect organisms