Evolution of Populations

Individual organisms do not evolve. This is a misconception. While natural selection acts on individuals, evolution is only apparent in the changes in a population of organisms over time

Microevolution Microevolution: a change in allele frequencies in a population over generations Microevolution is evolution on the smallest scale Microevolution can be caused by natural selection, genetic drift, and gene flow Only natural selection improves the match between organisms and their environments

Genetic variation is responsible for making evolution possible Genetic variation: differences among individuals in the composition of their DNA sequences/genes Genetic variation often causes phenotypic variation However, some phenotypic variation is the result of environmental influences, not heredity

Sources of Genetic Variation 1.Formation of new alleles 2.Altering gene number or position 3.Rapid reproduction 4.Sexual reproduction

Sources of Genetic Variation 1.Formation of new alleles -New alleles arise by mutation 2.Altering gene number or position -Chromosomal changes that delete, disrupt, or rearrange many loci at once may be beneficial -An important source of variation is the duplication of genes due to errors in meiosis -Ex. Certain species can smell well because their olfactory genes have been duplicated many times

Sources of Genetic Variation 3. Rapid reproduction -Mutations can quickly generate genetic variation in populations of organisms that reproduce rapidly Ex. Prokaryotes, viruses, etc. 4. Sexual reproduction -Most of the genetic variation in a population that reproduces sexually comes from the unique combination of alleles that each individual receives from its parents -Sexual reproduction rearranges existing alleles into fresh combinations each generation

The Hardy-Weinberg Equation Used to test whether evolution is occurring in a population Population: a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring Gene pool: all the copies of every type of allele at every locus in all members of the population Used to characterize a population’s genetic makeup Every allele has a frequency (proportion) in the population If an allele is fixed in the gene pool, that means only one allele exists for a particular locus in a population, and all individuals are homozygous for that allele When studying a locus with two alleles, p is used to represent the frequency of one allele and q is used to represent the frequency of the other

The Hardy-Weinberg Equation One way to assess whether evolution is occurring at a particular locus in a population is to determine what the genetic makeup of the population would look like if evolution were not occurring and compare the two Hardy-Weinberg principle: describes the gene pool of a population that is not evolving The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that natural selection is not acting upon that population A gene pool that is not evolving is said to be in Hardy-Weinberg equilibrium

Conditions for Hardy-Weinberg Equilibrium 1.No mutations 2.Random mating -Allows for random mixing of gametes 3.No natural selection 4.Extremely large population size -Allele frequencies fluctuate more in smaller populations 5.No gene flow. -If one of these requirements is not met, evolution usually occurs. -It is common for a population to be in H-W equilibrium for a specific gene, not necessarily all of its genes at once.

Hardy-Weinberg The existence of genetic variation in population does not necessarily mean that evolution is occurring We use HW to assess whether evolution is occurring at a particular locus in a population The HW equation represents the genetic frequencies in the population if evolution was not occurring p 2 + 2pq + q 2 = 1  says that at a locus with two alleles, the three possible genotypes will appear in these proportion if evolution is not occurring

Hardy - Weinberg We use the HW equation to practice identifying the allele frequencies and genotypic frequencies in hypothetical populations Scientists use the HW equation to determine what a population would look like if evolution were not happening so that they can compare the data they collect in the field to figure out if evolution is happening in that particular population

Hardy-Weinberg equilibrium can only exist if any all of the following is occurring in the population: 1.No mutations 2.Random mating -Allows for random mixing of gametes 3.No natural selection 4.Extremely large population size -Allele frequencies fluctuate more in smaller populations 5.No gene flow. What things cause evolution?

Natural selection, genetic drift, and gene flow are the three factors that cause the most evolutionary change in a population Any time a population deviates from the 5 conditions required for H- W equilibrium, evolution can occur. Three mechanisms alter allele frequencies directly and cause evolutionary change: natural selection, genetic drift, and gene flow

Natural Selection Natural selection occurs when individuals have different levels of success in survival and reproduction This allows a selection of alleles to be passed on to the next generation in proportions that differ from those in the present generation

Genetic Drift Genetic drift: chance events that can cause allele frequencies to fluctuate unpredictably from one generation to the next Affects smaller populations more than bigger ones

Genetic Drift Founder effect: a type of genetic drift that occurs when a few individuals become isolated from a larger population and they establish a new population whose gene pool differs from the original population

Genetic Drift Bottleneck effect: a type of genetic drift that occurs when a severe drop in population size creates a new population with a different gene pool than the original

Effects of genetic drift 1.Genetic drift is significant in small populations 2.Genetic drift can cause 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

Gene Flow Gene flow: the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes Gene flow tends to reduce the genetic differences between populations because alleles are being transferred between two populations If it is extensive enough, gene flow can result in two populations combining into a single population with a common gene pool

Natural selection is the only mechanism that consistently causes adaptive evolution Adaptive evolution: evolutionary changes that are adaptive to the given environment Natural selection doesn’t create perfectly fit individuals over time; it creates individuals that have greater relative fitness Relative fitness: the survival of an individual relative to that of other individuals Three types of natural selection: directional selection, disruptive selection, and stabilizing selection

Directional selection: when conditions favor individuals at one extreme of a phenotype

Disruptive selection: when conditions favors individuals at both extreme of a phenotype

Stabilizing selection: when conditions favor the intermediate phenotype over the extremes

The key role of natural selection is adaptive evolution Natural selection causes the match between a species and its environment to improve However, an organism’s physical environment is constantly changing, so adaptive evolution is a continuous, dynamic process Natural selection is the only evolutionary mechanisms that consistently leads to adaptive evolution; genetic drift and gene flow are not consistent in doing this

Sexual Selection Sexual selection: a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates Sexual selection can result in sexual dimorphism Sexual dimorphism: a difference in secondary sexual characteristics between males and females of the same species Types of sexual selection: Intrasexual selection: selection within the same sex where individuals compete for mates (usually among males) Intersexual selection: individuals of one sex (usually females) are choosy in selecting their mates from the other sex

Sexual Selection Types of sexual selection: Intrasexual selection: selection within the same sex where individuals compete for mates (usually among males) Intersexual selection: individuals of one sex (usually females) are choosy in selecting their mates from the other sex

The Preservation of Genetic Variation Diploidy and balancing selection preserve genetic variation when directional and stabilizing selection reduce it Diploidy: When two alleles are contained for one trait In diploidy, a considerable amount of genetic variation is hidden from selection the form of recessive alleles

The Preservation of Genetic Variation Balancing selection: when natural selection maintains two or more forms in a population. Two types: Heterozygote advantage: when individuals who are heterozygous at a particular locus have greater fitness than either type of homozygote Frequency-dependent selection: when the fitness of a phenotype depends on how common it is in the population

Why natural selection cannot make perfect organisms 1.Selection can only act on existing variations 2.Evolution is limited by historical constraints 3.Adaptations are often compromises 4.Chance, natural selection, and the environment interact