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Biology Chapter 16. Evolution Unit: Evolution of Populations. 16-1 Genes and Variation. As Darwin developed his theory of evolution, he was not aware of how _____________ passed from one generation to the next. heritable traits. and how variation appeared in organisms.
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Biology Chapter 16 Evolution Unit: Evolution of Populations
16-1 Genes and Variation • As Darwin developed his theory of evolution, he was not aware of how _____________ passed from one generation to the next heritable traits and how variation appeared in organisms.
B. Evolutionary biologists connected Darwin’s work and Mendel’s work during the 1930’s. 1. Changes in ________ produce heritable variation on which _______________ can operate. 2. genes natural selection Discovery of DNA demonstrated the molecular nature of mutation and genetic variation.
II. How Common is Genetic Variation? A. Individual fishes, reptiles, and mammals are typically heterozygous for between 4-8% of their genes. B. Variation and Gene Pools 1. Genetic variation is ______________ ________________ 2. Population: ____________________ _______________________________ studied in populations. a group of individuals of the same species that interbreed.
all genes, including all the • different alleles, that are present in a • population. 3. Gene pool:
4. Relative 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. Example: Fur color in a population of mice 40% B (black fur) 60% b (brown fur)
Relative Frequencies of AllelesFigure 16–2 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
Evolution is any change in the relative frequency of alleles • MICROEVOLUTION: ______________ __________________________________________ in a population. Microevolution refers to ___________ change in allele frequency over time. small scale
III. Sources of Genetic Variation A. Two sources of genetic variation 1. Mutation a. Ultimate source of variation. b. Any change in a sequence of DNA c. Most mutations are bad. Example: UV, radiation, toxins
d. Mutations that produce changes in an organism’s phenotype and increase an organism’s fitness, or its ability to reproduce in its environment, will be passed on.
2. Genetic shuffling that results from sexual reproduction. a. Independent assortment during meiosis produces 8.4 million possible combinations. b. Crossing-over.
IV. Single-Gene and Polygenic Traits A. The number of phenotypes produced for a given trait depends on how many genes control the trait. 1. Single-gene trait: Single gene that has two alleles. Example: Free earlobes (FF, Ff) or attached earlobes (ff). Free Attached
Phenotypes for Single-Gene Trait 100 80 60 40 20 0 Frequency of Phenotype (%) Free Earlobes (FF, Ff) Attached Earlobes (ff) Phenotype
Polygenic traits: Traits that are controlled by two or more genes. One polygenic trait can have many possible genotypes or phenotypes. Example: Height, eye color, skin color.
16-2 Evolution as Genetic Change I. Natural Selection on Single-Gene Traits A. Reminder: Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, evolve over time. B. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.
Effect of Color Mutations on Lizard Survival (Figure 16-5): 1. Organisms of one color may produce fewer offspring than organisms of other colors. Example: Red lizards are more visible to predators and therefore, may be more likely to be eaten and not pass on that red gene.
II. Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways: (1) directional selection (2) stabilizing selection (3) disruptive selection.
A. Directional Selection 1. One of the two possible extremes is favored. Example: Dark-colored peppered moths in regions of England with industrial pollution.
Directional Selection Figure 16–6 Section 16-2 Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce.
B. Stabilizing Selection 1. Intermediate characteristics are favored. Examples: Human babies with very high or very low birth weights have lower survival than babies with intermediate weights.
Stabilizing SelectionFigure 16–7 Section 16-2 Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight
C. Disruptive Selection 1. Natural selection moves characteristics toward both extremes, and intermediate phenotypes become rarest. Example: Populations of West African birds with either large or small, but not intermediate size beaks.
Disruptive Selection Figure 16–8 Section 16-2 Disruptive Selection Largest and smallest seeds become more common. Key Population splits into two subgroups specializing in different seeds. Low mortality, high fitness Number of Birdsin Population Number of Birdsin Population High mortality, low fitness Beak Size Beak Size
III. Genetic Drift • In small populations, an allele can become more or less common simply by chance. B. Genetic drift is a random change in allele frequency.
C.Two types of genetic drift: • Genetic bottleneck: If a population crashes, then there will be a loss of alleles from the population. Example: Northern Elephant Seals, Cheetahs.
2. Founder effect: A population can become limited in genetic variability if it’s founded by a small number of individuals. Example: Polydactyly in Amish.
Figure 16-9: Founder Effect Sample of Original Population Descendants Founding Population A Founding Population B
IV. Hardy-Weinberg and Genetic Equilibrium A. What would be necessary for no change to take place? 1. Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. 2. If allele frequencies remained constant then it there would be genetic equilibrium.
3. If allele frequencies do not change, THEN the population will not evolve. • Hardy-Weinberg Equation: p2 + 2pq + q2 = 1 a. The population is made of: homozygous dominant genotypes (p2) + heterozygous genotypes (2pq) + homozygous recessive genotypes (q2). b. The sum of the frequencies must always equal the entire population (100%).
90% c. Example: If 10% of the population exhibits attached earlobes (homozygous recessive phenotype: ff), then ___ of the population is FF or Ff and exhibits the ________ phenotype (____ earlobes). dominant free Solution Dominant Recessive p + q = 1 (free) + (attached earlobes) = 1 p + 10% = 100% p = 100% - 10% p = 90%
5. Five conditions necessary for Hardy-Weinberg Equilibrium NOTE: Hardy-Weinberg equilibrium rarely exists in natural populations but understanding the assumptions behind it gives us a basis for understanding how populations evolve.
Conditions necessary for Hardy-Weinberg Equilibrium a. The population is very large. b. The population isisolated (no migration of individuals, or alleles, into or out of the population). c. Mutations do not alter the gene pool. d. Mating israndom. e. All individuals are equal in reproductive success (no natural selection).
Small Tree Finch Large Ground Finch Woodpecker Finch
16-3 The Process of Speciation I. How do we get new species? A. What is a Species? 1. Species: This means that the individuals of the same species share a common gene pool. a group of interbreeding organisms that breed with one another and produce fertile offspring.
2. If a beneficial genetic change occurs in one individual, then that gene can be spread through the population as that individual and its offspring reproduce.
B. Isolating Mechanisms (Leads to a new species!) Reproductive Isolation – members of two populations cannot interbreed and produce fertile offspring.
PRE-Mating Reproductive Isolation – involves mechanisms which do not allow mating to occur in the first place. 1. Behavioral Isolation: Members of two populations are capable of interbreeding but have differences in mating displays or courtship rituals. a. b. c. specific scents (pheromones of insects). color patterns/strutting. specific sounds or calls.
