Processes of Evolution

Transcription

1 15 Processes of Evolution

2 Forces of Evolution

3 Concept 15.4 Selection Can Be Stabilizing, Directional, or Disruptive Natural selection can act on quantitative traits in three ways: Stabilizing selection favors average individuals. Directional selection favors individuals that vary in one direction from the mean. Disruptive selection favors individuals that vary in both directions from the mean.

4 Figure Natural Selection Can Operate in Several Ways

5 Concept 15.4 Selection Can Be Stabilizing, Directional, or Disruptive Stabilizing selection reduces variation in populations but does not change the mean. Example: Stabilizing selection operates on human birth weight. It is often called purifying selection, meaning selection against any deleterious mutations to the usual gene sequence.

6 Figure Human Birth Weight Is Influenced by Stabilizing Selection

7 Concept 15.4 Selection Can Be Stabilizing, Directional, or Disruptive In disruptive selection, individuals at opposite extremes of a character distribution contribute more offspring to the next generation. Results in increased variation in the population Can result in a bimodal distribution of traits Example: bill sizes in the black-bellied seedcracker (Pyrenestes ostrinus)

8 Figure Disruptive Selection Results in a Bimodal Character Distribution

9 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution In biology, evolution refers specifically to changes in the genetic makeup of populations over time. Population a group of individuals of a single species that live and interbreed in a particular geographic area at the same time. Individuals do not evolve; populations do.

10 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution The origin of genetic variation is mutation. Mutation any change in nucleotide sequences. Mutations occur randomly with respect to an organism s needs; natural selection acts on this random variation and results in adaptation.

11 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Mutations can be deleterious, beneficial, or have no effect (neutral). Mutation both creates and helps maintain genetic variation in populations. Mutation rates vary, but even low rates create considerable variation.

12 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Because of mutation, different forms of a gene, or alleles, may exist at a locus. Gene pool sum of all copies of all alleles at all loci in a population Allele frequency proportion of each allele in the gene pool Genotype frequency proportion of each genotype among individuals in the population

13 Figure 15.3 A Gene Pool

14 Figure 15.4 Mutations Accumulate Continuously

15 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Natural selection: Far more individuals are born than survive to reproduce. Offspring tend to resemble their parents but are not identical to their parents or to one another. Differences among individuals affect their chances of survival and reproduction, which will increase the frequency of favorable traits in the next generation.

16 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Adaptation a favored trait that evolves through natural selection Adaptation also describes the process that produces the trait. Individuals with deleterious mutations are less likely to survive, reproduce, and pass their alleles on to the next generation.

17 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Migration of individuals or movement of gametes (e.g., pollen) between populations results in gene flow, which can change allele frequencies.

18 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Genetic drift random changes in allele frequencies from one generation to the next In small populations, it can change allele frequencies. Harmful alleles may increase in frequency, or rare advantageous alleles may be lost. Even in large populations, genetic drift can influence frequencies of neutral alleles.

19 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Population bottleneck an environmental event results in survival of only a few individuals This can result in genetic drift and changing allele frequencies. Populations that go through bottlenecks lose much of their genetic variation. This is a problem for small populations of endangered species.

20 Figure 15.8 A Population Bottleneck

21 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Founder effect genetic drift changes allele frequencies when a few individuals colonize a new area It is equivalent to a large population reduced by a bottleneck.

22 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Nonrandom mating: Self-fertilization is common in plants. When individuals prefer others of the same genotype, homozygous genotypes will increase in frequency, and heterozygous genotypes will decrease.

23 Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution Sexual selection occurs when individuals of one sex mate preferentially with particular individuals of the opposite sex rather than at random. Some seemingly nonadaptive traits may make an individual more attractive to the opposite sex. There may be a trade-off between attracting mates (more likely to reproduce) and attracting predators (less likely to survive).

24 Figure 15.9 What Is the Advantage?

25 Figure Sexual Selection in Action (Part 1)

26 Figure Sexual Selection in Action (Part 2)

27 BLAST and Phylogenetics

28 BLAST is a Basic Local Alignment Search Tool What is it? Locates short matches between two or more sequences of a sequence of nucleotides or amino acids Why is it used?

29 How is it used? A query is a specific sequence of cdna, genomic DNA, or a peptide sequence. The database consists of one or multiple sequences. These sequences can be whole genomes, cdna libraries, or polypetide sequences. Query is searched against the Database (Like typing in Practice AP Exams in to Google)

30 Why is it used? - Identifying species - Locating domains of interest - Establishing phylogeny - DNA mapping on chromosomes - Comparison and gene identification

31 All of life is related through a common ancestor. This explains why the general principles of biology apply to all organisms. Phylogeny is the evolutionary history of these relationships. A phylogenetic tree is a diagrammatic reconstruction of that history.

32 An ancestor and its descendant populations form a lineage, shown as a line drawn on a time axis:

33 Concept 16.1 All of Life Is Connected through Its Evolutionary History When a single lineage divides into two, it is depicted as a split or node:

34 Concept 16.1 All of Life Is Connected through Its Evolutionary History As the lineages continue to split over time, the history can be represented in the form of a branching tree:

35 Concept 16.1 All of Life Is Connected through Its Evolutionary History A phylogenetic tree may portray the evolutionary history of: All life forms Major evolutionary groups Small groups of closely related species Individuals Populations Genes

36 Concept 16.1 All of Life Is Connected through Its Evolutionary History The common ancestor of all the organisms in the tree forms the root of the tree:

37 Concept 16.1 All of Life Is Connected through Its Evolutionary History The timing of splitting events is shown by the position of nodes on a time axis. The splits represent events such as: A speciation event (for a tree of species) A gene duplication event (for a tree of genes) A transmission event (for a tree of viral lineages)

38 Hardy Weinberg

39 Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies Evolution can be measured by changes in allele frequencies. Allele frequency: p = number of copies of allele in population total number of copies of all alleles in population

40 Figure Calculating Allele and Genotype Frequencies

41 Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies Hardy Weinberg equilibrium a model in which allele frequencies do not change across generations; genotype frequencies can be predicted from allele frequencies For a population to be at Hardy Weinberg equilibrium, there must be random mating and infinite population size, but no mutation, no gene flow, and no selection of genotypes.

42 Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies At Hardy Weinberg equilibrium, allele frequencies do not change. Genotype frequencies after one generation of random mating: Genotype: AA Aa aa Frequency: p 2 2pq q 2

43 Figure One Generation of Random Mating Restores Hardy Weinberg Equilibrium (Part 1)

44 Figure One Generation of Random Mating Restores Hardy Weinberg Equilibrium (Part 2)

45 Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies Probability of two A-gametes coming together: p p = p 2 = (0.55) 2 = Probability of two a-gametes coming together: q q = q 2 = (0.45) 2 = Overall probability of obtaining a heterozygote: 2pq = 0.495

46 Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies Populations in nature never meet the conditions of Hardy Weinberg equilibrium all biological populations evolve. The model is useful for predicting approximate genotype frequencies of a population. Specific patterns of deviation from Hardy Weinberg equilibrium help identify processes of evolutionary change.