Evolution in Populations Chapter 17 Evolution in Populations

Section 1 Genes and Variation

Genetics Joins Evolutionary Theory Darwin’s original ideas can now be understood in GENETIC TERMS. Researchers discovered that TRAITS are controlled by GENES and that many genes have at least TWO FORMS, or alleles. The combination of DIFFERENT ALLELES is an individuals GENOTYPE. Natural selection acts on PHENOTYPE, not genotype.

Genetics Joins Evolutionary Theory GENETIC VARIATION and EVOLTUION are studied in populations. Members of a population share a common group of GENES, called a GENE POOL. Allele frequency is the NUMBER of times an ALLELE occurs in a gene pool compared with the number of times other ALLELES for the same gene occur.

Genetics Joins Evolutionary Theory In genetic terms, evolution is any CHANGE in ALLELE FREQUENCY.

Sources of Genetic Variation The three main sources of genetic variation are MUTATIONS, GENETIC RECOMBINATION during sexual reproduction, and LATERAL GENE TRANSFER. A mutation is any CHANGE in a SEQUENCE of DNA. TAATATCAC  TAATATGAC

Sources of Genetic Variation Most heritable differences are due to genetic recombination during SEXUAL REPRODUCTION. The occurs during MEIOSIS when each chromosome in a pair moves INDEPENDENTLY. Genetic recombination also occurs during CROSSING-OVER in meiosis.

Sources of Genetic Variation Lateral gene transfer is the PASSING of GENES from one organism to another organism that is NOT ITS OFFSPRING.

Single-Gene and Polygenic Traits The number of different PHENOTYPES for a given TRAIT depends on how many GENES control the trait. A SINGLE-GENE trait is controlled by ONE gene. An example in SNAILS is the presence or absence of DARK BANDS on their shells.

Single-Genes and Polygenic Traits A POLYGENIC trait is controlled by TWO or more genes, and each gene often has TWO or more ALLELES. An example of a human polygenic trait is HEIGHT.

Evolution as Genetic Change in Populations Section 2 Evolution as Genetic Change in Populations

How Natural Selection Works Natural Selection on a SINGLE-GENE trait can lead to changes in ALLELE FREQUENCY and changes in PHENOTYPE FREQUENCIES.

How Natural Selection Works For POLYGENIC traits, populations of PHENOTYPES for a trait. When graphed, this range usually forms a BELL CURVE with FEWER individuals exhibiting the extreme phenotypes than those with the average (in the case of BEAK SIZE, the extremes may be TINY and LARGE beaks).

How Natural Selection Works Natural selection on polygenic traits can cause SHIFTS in the bell curve depending upon which phenotype is selected for. DIRECTIONAL selection takes place when individuals at one end of the bell curve have HIGHER fitness than those near the middle or at the other end of the curve. For example, when large seeds are PLENTIFUL, LARGE-beaked birds in a population may be selected for.

How Natural Selection Works STABILIZING selection takes place when individuals near the MIDDLE of the curve have higher FITNESS than individuals at either end.

How Natural Selection Works DISRUPTIVE selection takes place when individuals at the UPPER and LOWER ends of the curve have higher fitness than the individuals near the MIDDLE.

Genetic Drift In SMALL populations, alleles can become MORE or LESS common simply by CHANCE. This kind of change in alleles frequency is called GENETIC DRIFT. The BOTTLENECK EFFECT is a change in allele frequency following a dramatic REDUCTION in the SIZE of a population.

Genetic Drift The FOUNDER EFFECT is a change in allele frequency that may occur when a FEW INDIVIDUALS from a population MIGRATE to and COLONIZE a new habitat.

Evolution Versus Genetic Equilibrium If allele frequencies in a population do not change, the populations is in GENETIC EQUILIBRIUM. Evolution is NOT TAKING PLACE.

Evolution Versus Genetic Equilibrium The HARDY-WEINBERG Principle states that allele frequencies in a population should remain CONSTANT unless one or more factors cause those frequencies to CHANGE. These factors include: NON-RANDOM MATING, SMALL population size, IMMIGRATION or EMIGRATION, MUTATIONS, and NATURAL SELECTION.

Evolution Versus Genetic Equilibrium Non-random mating: select mates based on heritable traits, such as size, strength, or coloration (Sexual Selection) Immigration: movement of individuals into a population Emigration: movement of individuals out of a population Natural Selection: genotypes provide different fitness

Evolution Versus Genetic Equilibrium Hardy Weinberg Equation: p2 + 2pq + q2 = 1 and p + q = 1 (frequency of AA) + (frequency of Aa) + (frequency of aa) = 100% (frequency of A) + (frequency of a) = 100%

Evolution Versus Genetic Equilibrium Hardy Weinberg Equation: Example: p=.40 q=.60 Frequency of AA= .16 or 16% .402+2(.40x.60)+.602 Frequency of Aa= .48 or 48% Frequency of aa= .36 or 36%

Evolution Versus Genetic Equilibrium Hardy Weinberg Equation: p2 + 2pq + q2 = 1 and p + q = 1

Evolution Versus Genetic Equilibrium Populations are RARELY in genetic equilibrium. Most of the time, evolution is occurring. For example, many species exhibit non-random mating patterns.

Evolution Versus Genetic Equilibrium SEXUAL SELECTION, or the process in which an individual chooses its mate based on HERITABLE TRAITS (such as SIZE or STRENGTH), is a common practice for many organisms.

The Process of Speciation Section 3 The Process of Speciation

Isolating Mechanisms SPECIATION is the formation of a new species. For one species to evolve into TWO NEW SPECIES, the gene pools of two populations must become SEPARATED, or reproductively isolated.

Isolating Mechanisms REPRODUCTIVE ISOLATION occurs when members of two populations do not INTERBREED and produce FERTILE OFFSPRING.

Isolating Mechanisms Reproductive isolation can develop through behavioral, geographic, or temporal isolation. BEHAVIORAL isolation occurs when populations have different COURTSHIP rituals or other behaviors involved in REPRODUCTION

Isolating Mechanisms GEOGRAPHICA isolation occurs when populations are SEPARATED by geographic barriers, such as MOUNTAINS, or RIVERS.

Isolating Mechanisms TEMPORAL isolation occurs when populations reproduce at DIFFERENT TIMES.

Speciation in Darwin’s Finches PETER and ROSEMARY Grant’s work supports the hypothesis that SPECIATION in the Galápagos finches was, and still continues to be, a result of FOUNDER EFFECT and NATURAL SELECTION.

Speciation in Darwin’s Finches Speciation in Galápagos finches may have occurred in a SEQUENCE of events that involved the FOUNDING of a new population, GEOGRAPHIC isolation, changes in the GENE POOL, behavioral ISOLATION, and ECOLOGICAL COMPETITION.

Speciation in Darwin’s Finches For example, a few finches may have FLOWN from mainland SOUTH AMERICA to one of the islands. There, they SURVIVED and REPRODUCED.

Speciation in Darwin’s Finches Some birds may have crossed to a second island, and the two populations became GEOGRAPHICALLY ISOLATED.

Speciation in Darwin’s Finches Seed sizes on the second island could have FAVORED birds with larger beaks, so the population on the second island EVOLVED into a population with larger beaks.

Speciation in Darwin’s Finches Eventually these LARGE-BEAKED birds became REPRODUCTIVELY ISOLATED and evolved into a new species.