Genetic Variation Within a Population Population: A group of organisms, all of the same species, which interbreed and live in the same place at the same time. A population shares a common gene pool.

Genetic variation in a population increases the chance that some individuals will survive. Genetic variation leads to phenotypic variation. Phenotypic variation is necessary for natural selection. Genetic variation is stored in a population’s gene pool.

Allele frequencies measure genetic variation Genetic variation is how common is allele in population.

Figure 16–2 Relative Frequencies of Alleles Section 16-1 Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

Sources of Genetic Variation Mutations = any change in a sequence of DNA Remember: mutations result as a mistake during replication or toxin (chemicals/radiation) Some mutations effect phenotypes (physical characteristics), which can effect an organism’s fitness (ability to survive) Gene shuffling = different gene combinations inherited during gamete production creating different genotypes (genetic makeup), different phenotypes and more variation Crossing over increases number of different genotypes Does not change the relative frequency of alleles in a population Think of a deck of cards – there are many possible combinations, but frequency remains same

Single-Gene vs. Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait. If it is a trait controlled by a single-gene with 2 alleles, there will be only 2 phenotypes. Phenotypes of a single-gene is represented by a bar graph If it is a trait is polygenic with 2 or more alleles, there will be many genotypes and even more phenotypes. Phenotypes of a polygenic trait is represented by a normal distribution (bell curve) Frequency of Phenotype (%) Phenotype Frequency of Phenotype Phenotype (height)

Evolution as Genetic Change Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time. Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals.

Natural selection acts on distributions of traits. A normal distribution graphs as a bell-shaped curve. highest frequency near mean value frequencies decrease toward each extreme value Traits have a normal distribution when not undergoing natural selection. Example: human height.

Microevolution is evolution within a population. observable change in the allele frequencies can result from natural selection As this map1 shows, sparrows in colder places are now generally larger than sparrows in warmer locales. Since these differences are probably genetically based, they almost certainly represent microevolutionary change: populations descended from the same ancestral population have different gene frequencies.

Natural selection can change the distribution of a trait in one of three ways or paths. Directional selection favors phenotypes at one extreme.

Natural selection can take one of three paths. Stabilizing selection favors the intermediate phenotype.

Natural selection can take one of three paths. Disruptive selection favors both extreme phenotypes.

Types of Selection Designate the colors of the two generations on your note sheet.

Gene flow is the movement of alleles between populations. Gene flow occurs when individuals join new populations and reproduce in new population. Lots of gene flow results in genetically similar populations. Limited gene flow results in genetically different populations. Gene flow in beetles Gene flow (wind movement) in plants

Genetic drift is a change in allele frequencies due to chance. It is most common in small populations. Genetic diversity can increase or decrease. A population bottleneck can lead to genetic drift. The bottleneck effect is genetic drift that occurs after an event drastically reduces population size.

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

The Founder Effect is when a few individuals migrate and start a new population. Due to chance only.

Genetic drift has negative effects on a population. Loses genetic variation preventing some from adapting Harmful alleles can become more common due to chance alone Ex: Amish population has higher incidence of Ellis-van Creveld syndrome (form of dwarfism involving: short stature, polydactyly (extra fingers or toes), abnormalities of the nails and teeth, and occasionally a hole between the two upper chambers of the heart.

Camouflage Natural selection that favors either blending in with the specie’s natural environment to hide, confuse or distract predators. Can include: special appendages, coloration and the ability to change color to blend with surroundings as needed.

Mimicry 2 basic types: natural selection favoring coloration to imitate toxic/dangerous species. Natural selection creating a body part that can mimic another object to increase survival

Sexual selection occurs when certain traits increase mating success and become more common. Females prefer males with certain traits which become exaggerated in each generation males produce many sperm continuously females are more limited in potential offspring each cycle

There are two types of sexual selection. intrasexual selection: competition among males intersexual selection: males display certain traits to attract females

Hardy-Weinberg equilibrium describes populations that are not evolving. Used as a model to compare with real data. Tests factors leading to evolution.

Genotype frequencies stay the same if five conditions are met. Hardy-Weinberg equilibrium describes populations that are not evolving. Genotype frequencies stay the same if five conditions are met. very large population: no genetic drift no emigration or immigration: no gene flow no mutations: no new alleles added to gene pool random mating: no sexual selection no natural selection: all traits aid equally in survival

Hardy-Weinberg equilibrium describes populations that are not evolving. Real populations rarely meet all five conditions. Real population data is compared to a model. Models are used to studying how populations evolve.

The Hardy-Weinberg equation is used to predict genotype frequencies in a population. used for traits in simple dominant-recessive systems p2 + 2pq + q2 = 1 What it means: p is dominant allele q is recessive allele

There are five factors that can lead to evolution. Genetic drift changes allele frequencies due to chance alone. Gene flow moves alleles from one population to another. Mutations produce the genetic variation needed for evolution. Sexual selection selects for traits that improve mating success. Natural selection selects for traits advantageous for survival.

In nature, populations evolve. expected in all populations most of the time respond to changing environments

Speciation Speciation = formation of new species Species = group of organisms that breed with one another and produce fertile offspring (share a common gene pool) As new species evolve, populations become reproductively isolated from each other: When 2 populations can’t breed and produce fertile offspring, resulting in separate gene pools Behavioral isolation: Capable of breeding, but have different courtship rituals or behaviors Geographic isolation: Separate by geographic barriers Temporal isolation: Reproduce at different times

Speciation of Darwin’s Finches Speciation in the Galapagos finches occurred by: Founding a new population: A small population of finches migrates to a different island Geographic isolation: Finches don’t usually fly over open water, so stayed on own island (separate gene pool) Changes in the new population’s gene pool: Adapted to new environment (directional selection) to be more fit Reproductive isolation: Differences in phenotypes and mating rituals may turn different finches off to one another Ecological competition: Similar finches compete, so individuals that are most different from each other have the highest fitness, because less competition. Continued Evolution: Process repeats and over many generations, it produced the 13 different finch species

Evolution through natural selection is not random. Natural selection can have direction. The effects of natural selection add up over time.

Convergent evolution describes unrelated species becoming similar due to common environment. Ex. Dolphins, sharks and penguins

Divergent evolution describes close related species become dissimilar. How do convergent and divergent evolution illustrate the directional nature of natural selection? ancestor Kit fox Red fox

Species can shape each other over time. Two or more species can evolve together through co-evolution. evolutionary paths become connected species evolve in response to changes in each other

Coevolution can occur in beneficial relationships. Both species benefit Ex. Ant and acacia plant

Co-evolution can occur in competitive relationships, sometimes called evolutionary arms race.

Species can become extinct. Extinction is the elimination of a species from Earth. Background extinctions occur continuously at a very low rate. occur at roughly the same rate as speciation usually affects a few species in a small area caused by local changes in environment

Mass extinctions are rare but much more intense. destroy many species at global level thought to be caused by catastrophic events at least five mass extinctions in last 600 million years

Speciation often occurs in patterns. A pattern of punctuated equilibrium exists in the fossil record. theory proposed by Eldredge and Gould in 1972 episodes of speciation occur suddenly in geologic time followed by long periods of little evolutionary change revised Darwin’s idea that species arose through gradual transformations

Many species evolve from one species during adaptive radiation. ancestral species diversifies into many descendent species descendent species usually adapted to wide range of environments