David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu B – Le basi molecolari della vita e dell’evoluzione Evolution and Its Mechanisms

What facts form the base of our understanding of evolution? Evolution and Its Mechanisms What facts form the base of our understanding of evolution? What are the mechanisms of evolutionary change? What evolutionary mechanisms result in adaptation? How is genetic variation maintained within populations? What are the constraints on evolution?

Population genetics has three main goals: Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Population genetics has three main goals: explain the origin and maintenance of genetic variation; explain patterns and organization of genetic variation; understand mechanisms that cause changes in allele frequencies.

Different forms of a gene are called alleles. Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Different forms of a gene are called alleles. The gene pool is the sum of all copies of all alleles at all loci in a population.

Populations have genetic variation for many characters. Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Populations have genetic variation for many characters. Artificial selection for different characters in a single species of wild mustard produced many crop plants.

Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Calculating Hardy–Weinberg genotype frequencies (part 1)

Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Calculating Hardy–Weinberg genotype frequencies (part 2)

For generation 1, probability of two A alleles coming together is: Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? For generation 1, probability of two A alleles coming together is: Probability of two a alleles:

There are two ways of producing a heterozygote: Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? There are two ways of producing a heterozygote: The Hardy-Weinberg equation:

Evolution and Its Mechanisms - What facts form the base of our understanding of evolution? Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. But, it is useful in predicting genotype frequencies from allele frequencies; and, because the model describes conditions that would result in no evolution, patterns of deviation from the model help identify specific mechanisms of evolution.

Known evolutionary mechanisms: mutation; gene flow; genetic drift; Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent. Known evolutionary mechanisms: mutation; gene flow; genetic drift; nonrandom mating; natural selection.

Mutation is the origin of genetic variation. Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Mutation is the origin of genetic variation. Mutation is any change in DNA; it appears to be random with respect to the adaptive needs of an organism. Most mutations are harmful or neutral, but if conditions change, could become advantageous.

Mutations can also restore alleles that other processes remove. Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Mutations can also restore alleles that other processes remove. Mutation rates are low—about one per locus in a million zygotes. Creates a lot of variation because of the number of genes that can mutate, chromosome rearrangements that can change many genes simultaneously, and large numbers of individuals.

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Because mutation rate is low, mutations in themselves result in only minor deviations from Hardy-Weinberg equilibrium. If large deviations are found, it is appropriate to look for other mechanisms.

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Gene flow is a result of the migration of individuals and movements of gametes between populations. New alleles can be added to the gene pool, or allele frequencies changed.

Genetic drift results from random changes in allele frequencies. Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Genetic drift results from random changes in allele frequencies. In large populations, genetic drift can influence frequencies of alleles that don’t affect survival and reproduction. If populations are reduced to a small number of individuals—a population bottleneck, genetic drift can reduce the genetic variation.

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? A population bottleneck

Founder effect—equivalent to a bottleneck. Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Genetic drift also effects small populations that colonize a new region. Colonizing population is unlikely to have all the alleles present in the whole population. Founder effect—equivalent to a bottleneck.

Example of founder effect: Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Example of founder effect: Populations of European fruit fly D. subobscura began in Chile, and then in Washington state. Both populations grew and expanded their ranges. These populations have very similar genetic structure, and much less variation than the European populations.

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? A founder effect

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Nonrandom mating occurs when individuals choose mates with particular phenotypes. If individuals choose the same genotype as themselves, homozygote frequencies will increase. Nonrandom mating in primroses (Primula): two flower types—pin and thrum. Pollen from one type can fertilize only flowers of the other type.

Evolution and Its Mechanisms - What are the mechanisms of evolutionary change? Flower structure fosters non-random mating

Natural selection acts on phenotype. Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Natural selection acts on phenotype. Fitness is the reproductive contribution of a phenotype to subsequent generations and it is determined by the average rates of survival and reproduction of individuals with that phenotype. Changes in the relative success of different phenotypes in a population leads to change in allele frequencies.

stabilizing selection preserves average phenotype; Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Natural selection can act on characters with quantitative variation in three ways: stabilizing selection preserves average phenotype; directional selection favors individuals that vary in one direction; disruptive selection favors individuals that vary in opposite directions from the average.

Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Natural selection can operate on quantitative variation in several ways (part 1)

Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Natural selection can operate on quantitative variation in several ways (part 2)

Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Natural selection can operate on quantitative variation in several ways (part 3)

Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Sexual selection (part 1)

Evolution and Its Mechanisms - What evolutionary mechanisms result in adaptation? Sexual selection (part 2)

Evolution and Its Mechanisms - How is genetic variation maintained within populations? Frequency-dependent selection: A polymorphism can be maintained when fitness depends on its frequency in the population. Example: a scale-eating fish in Lake Tanganyika. “Left-mouthed” and “right- mouthed” individuals are both favored; the host fish can be attacked from either side.

Evolution and Its Mechanisms - How is genetic variation maintained within populations? A stable polymorphism

Environmental variation also helps to preserve genetic variation. Evolution and Its Mechanisms - How is genetic variation maintained within populations? Environmental variation also helps to preserve genetic variation. Example: Colias butterflies live in an environment with temperature extremes. The population is polymorphic for an enzyme that influences flight at different temperatures. Heterozygotes are favored because they can fly over a larger temperature range.

Evolution and Its Mechanisms - How is genetic variation maintained within populations? A heterozygote mating advantage (part 1)

Evolution and Its Mechanisms - How is genetic variation maintained within populations? A heterozygote mating advantage (part 2)

Evolution and Its Mechanisms - What are the constraints on evolution? Lack of genetic variation can prevent evolution of potentially favorable traits. If the allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be favored by natural selection.

cell size, constrained by surface area-to- volume ratios; Evolution and Its Mechanisms - What are the constraints on evolution? Evolution must work within the boundaries of universal constraints such as: cell size, constrained by surface area-to- volume ratios; protein folding, constrained by types of bonding that can occur; laws of thermodynamics that constrain energy transfers. Developmental processes also constrain evolution.

Example: two lineages of bottom-dwelling fishes. Evolution and Its Mechanisms - What are the constraints on evolution? All evolutionary innovations are modifications of previously existing structures. Example: two lineages of bottom-dwelling fishes. Skates and rays evolved from a common ancestor with sharks. They started with a flattened body plan, and can swim along the ocean floor. B) Sole and flounder evolved from laterally flattened bony fishes. They can’t swim well, but lie still on the bottom. The eyes gradually shifted to one side.

Evolution and Its Mechanisms - What are the constraints on evolution? Two solutions to a single problem