Evolution of Populations Chapter 16

Gene Pool The combine genetic information of a particular population Contains 2 or more Alleles for each inheritable trait

Population A collection of individuals of the same species in a given area

Relative Frequency Is the number of times an allele occurs in a gene pool Compared with the number of times other alleles occur Example Relative Frequency: 70% Allele B 30% Allele b

Sources of Genetic Variation 2 main sources of Genetic Variations are Mutations and Genetic Shuffling that result from sexual reproduction Mutations: any change in DNA sequence ♦ Can occur because of: ♦ mistakes in replication ♦ environmental chemicals ♦ May or may not affect an organism’s phenotype

Genetic Shuffling Occurs during the formation of gametes Chromosomes move independently during…23 chromosomes Can produce 8.4 million different combo’s of genes Crossing over can also occur during meiosis further increasing the possible number of gene variation

Despite gene shuffling, the frequency of alleles does not change in a population. Explain why this is true. Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.

Gene Traits The number of phenotypes produced for a given trait depends on how many genes control that trait

Single Gene Trait Controlled by a single gene (2alleles) Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

Polygenic Trait Controlled by 2 or more genes can produce many genotypes and thus many phenotypes ♦ Examples: height, hair color, skin color, eye color Most human traits are polygenic.

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain. type: single gene why? Only two phenotypes possible Example: tongue roller or non- tongue roller type: polygenic why? Multiple (many) phenotypes possible Example: height range 4feet to 9 feet all

Natural Selection On single gene traits can lead to changes in allele frequency and thus to evolution Can effect the distribution of phenotypes in any of 3 ways: directional selection, stabilizing selection, or disruptive selection

Directional Selection Evolution causes an increase in the number of individuals with a certain trait at one end of the bell curve ♦ Individuals with highest fitness: those at one end of the curve ♦ Example: Galapagos finches – beak size

Stabilizing Selection Evolution causes individuals at the center of the curve to have a higher fitness than those at either end of the curve thus narrowing the curve – Individuals with highest fitness: near the center of the curve (average phenotype) – Example: human birth weight Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

Disruptive Selection individuals at both ends of the curve survive better than the middle of the curve Example: birds where seeds are either large or small Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness

3 Distribution of Natural Selection

Genetic Drift Random change in allele frequency that occurs in small populations Usually in smaller populations where the laws of probability are less likely to be predicted In small populations, individuals that carry a particular allele may leave more descendants than others by chance. Over time, this type of chance occurrence can cause an allele to be more common in a population a.Founder effect: allele frequencies change due to migration of a small subgroup of a population b.Example: fruit flies on Hawaiian islands

Two phenomena that result in small populations and cause genetic drift 1.Founder Effect 2.Bottleneck Effect

Founder effect Allele frequencies change due to migration of a small subgroup of a population

Fruit Flies on Hawaiian islands Founder Effect: : Fruit Flies on Hawaiian islands Sample of Original Population Founding Population A Founding Population B Descendants

Bottleneck Effect major change in allele frequencies when population decreases dramatically due to catastrophe ♦ Example: northern elephant seals decreased to 20 individuals in 1800’s, now 30,000 no genetic variation in 24 genes

Bottleneck Effect: Northern Elephant Seal Population ♦Hunted to near extintion ♦Population decreased to ♦ 20 individuals in 1800’s, ♦ those 20 repopulated so ♦today’s population is ~30,000 ♦No genetic variation in 24 genes

Evolution vs Genetic Equilibrium Hardy-Weinberg Principle – States that allele frequencies in a population will remain constant unless 1 or more factors cause the frequencies to change This situation is called Genetic Equilibrium If allele freq.’s do not change… the population will not evolve

5 Conditions are required to maintain genetic equilibrium form generation to generation: Random mating Large Population No movement into or out of the population No Mutation No Natural Selection If these conditions are not met then equilibrium is lost and the population will evolve

The Process of Speciation I. The formation of new biological species, II. Caused by natural selection and chance events that cause a change in relative allele frequencies III. As new species evolve, they become reproductively isolated from one another

Reproductive Isolation Is when 2 species evolve so that they cannot interbreed Populations now have separate gene pools Develop by Behavioral Isolation, Geographic Isolation, and Temporal Isolation

Behavioral Isolation Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

Example: Eastern and Western Meadowlark Eastern and Western Meadowlark populations overlap in the middle of the US

Geographic Isolation Two populations separated by a geographic barrier; river, lake, canyon, mountain range.

Temporal Isolation Populations reproduce at different times January

Speciation in the Galapagos islands finches occurred by founding of a new population, geographical isolation, changes in the new pop.’s gene pool, reproductive isolation, and ecological competition