Lesson Overview 17.1 Genes and Variation

OBJECTIVES: Define evolution in genetic terms. Identify the main sources of genetic variation in a population. State what determines the number of phenotypes for a trait.

Genetics Joins Evolutionary Theory Heritable traits are controlled by GENES!!!! Changes in GENES / CHROMOSOMES result in VARIATION!!!! Genotype  combination of alleles it carries; produces a phenotype Phenotype  all physical, physiological, and behavioral characteristics   Natural selection acts directly on PHENOTYPE, not genotype. Ex. Some individuals have phenotypes that are better suited to their environment, so they will survive and reproduce, passing on more copies of their genes to the next generation.

Populations and Gene Pools group of individuals of the same species that mate and produce offspring. gene pool all the genes, including all the different alleles for each gene, that are present in a population Ex. B = black fur and b = brown fur The relative frequency of an allele is studied in gene pools Number of times a particular allele occurs in a gene pool compared with the number of times the other allele appears in the gene pool Ex. How many times does B appear versus b????  

Ex. this diagram shows the gene pool for fur color in a population of mice.  

Populations and Gene Pools Evolution is any change in the relative frequency of alleles in the gene pool of a population over time. Natural selection acts on POPULATIONS not INDIVIDUALS.  

Mutations Mutations Can produce changes in genotypes, and therefore phenotypes May or may not affect fitness (some are lethal and some are beneficial) Can affect evolution ONLY if the mutation is heritable (passed from generation to generation)  must be in the egg or sperm cell

Single-Gene Traits single-gene trait a trait controlled by only one gene Have only 2 or 3 distinct phenotypes Most common forms are dominant or recessive

Polygenic Traits Polygenic traits traits controlled by two or more genes Each gene has two or more alleles  leads to MANY possible genotypes and phenotypes Ex. AaBBccDd …..for height  

17.2 Evolution as Genetic Change in Populations Lesson Overview 17.2 Evolution as Genetic Change in Populations

OBJECTIVES: Explain how natural selection affects single-gene and polygenic traits. Describe genetic drift. Explain how different factors affect genetic equilibrium.

Natural Selection on Single-Gene Traits Can lead to changes in allele frequencies and then leads to evolution Ex. Deer Population   

Natural Selection on Polygenic Traits Can affect the range of phenotypes and hence the shape of the bell curve Results in: Directional Selection Stabilizing Selection Disruptive Selection

Directional Selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts to one extreme of the bell curve

Stabilizing Selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. Narrows the overall bell curve, but keeps the center of the curve at its current position

Disruptive Selection Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle acts against individuals of an intermediate type and can create two distinct phenotypes

Genetic Drift An allele becomes more or less common simply by chance or a random change in allele frequency Occurs in small populations

Genetic Bottlenecks bottleneck effect change in allele frequency following a dramatic reduction in the size of a population Ex. After a natural disaster  

The Founder Effect founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population

Evolution Versus Genetic Equilibrium When a population’s allele frequencies remain the same…..NO EVOLUTION TAKES PLACE. Due to: Large populations Less likelihood of genetic drift No mutations No new alleles are introduced into the population Lack in Random Mating Some species prefer to mate with others having specific traits No Movement into or out of a Population no alleles flow into or out of the gene pool to change frequencies No Natural Selection All phenotypes have equal likelihood of survival and reproduction

17.3 The Process of Speciation Lesson Overview 17.3 The Process of Speciation

OBJECTIVES: Identify the types of isolation that can lead to the formation of new species. Describe the current hypothesis about Galapagos finch speciation.

Isolating Mechanisms Speciation formation of a new species (population whose members can interbreed and produce fertile offspring) Due to: Reproductive Isolation Behavioral Isolation Geographic Isolation Temporal Isolation

Reproductive Isolation occurs when a population splits into two groups and the two populations no longer interbreed. Can result in evolution of two separate species

Behavioral Isolation Behavioral isolation when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors.

Geographic Isolation Geographic isolation occurs when two populations are separated by geographic barriers Ex. Rivers, mountains, bodies of water

Temporal Isolation Temporal isolation two or more species reproduce at different times Ex. Species of orchids….flowers that last only one day and must be pollinated on that day to produce seeds…..if they bloom on different days, they cannot cross-pollinate.  

Testing Natural Selection in Nature Darwin hypothesized that the Galápagos finches had descended from a common ancestor. Natural selection shaped the beaks of different bird populations as they became adapted to eat different foods.  

Founders Arrive Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands  founder effect allele frequencies of this founding finch population could have differed from those in the South American population

Geographic Isolation Populations on one island will separate to other islands due to competition for resources

Changes in Gene Pools Over time, populations on each island adapted to local environments.   Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.

Behavioral Isolation When species B returns to its previous island, behaviors for mating may have changes to prevent reproduction with species A

Competition and Continued Evolution Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E).