Chapter 17: Evolution of Populations Evolution of Populations 1. When Darwin developed his theory of evolution, he did not understand: • how heredity worked. This left him unable to explain two things: a. source of variation b. how inheritable traits pass from one generation to the next Evolution of Populations In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution. Vocabulary: Population: group of individuals of the same species living in the same area that breed with each other. gene pool combined genetic info. for all members of a population Allele: one form of a gene 2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent) Example Relative Frequency: 70% Allele B 30% Allele b 3. Sources of Variation: a. mutations: any change in DNA sequence ♦ Can occur because of: ♦mistakes in replication ♦ environmental chemicals ♦ May or may not affect an organism’s phenotype 3. Sources of Variation b. Gene Shuffling: recombination of genes that occurs during production of gametes ♦ ♦ ♦ ♦ Cause most inheritable differences between relatives Occurs during meiosis As a result, sexual reproduction is a major source of variation in organisms. 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. 4. Gene Traits: A) Single gene trait: controlled by single gene with two alleles ♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling (4. Gene Traits:) B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles ♦ 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 type: polygenic why? Only two phenotypes possible why? Multiple (many) phenotypes possible Example: tongue roller or non-tongue roller Example: height range 4feet to 9 feet all 5. Natural selection acts on phenotypes, not genotypes. Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white? Brown, more hidden. 5. If brown is dominant can a predator tell the difference between: BB Bb ? Mouse with highest fitness will have the most alleles passed on to the next generation. White mouse will have low fitness 5. Which mouse will have the lowest fitness? White, bb (recessive) BB Bb ? Will the fitness of BB and Bb differ? Why? No, Both BB and Bb have the same fitness advantage of being brown Natural Selection Three ways in which natural selection affects polygenic traits. •Directional Selection •Stabilizing Selection •Disruptive Selection Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait Key Food becomes scarce. Low mortalit High y, high mortalit fitness y, low fitness ♦ ♦ Individuals with highest fitness: those at one end of the curve Example: Galapagos finches – beak size Directional Selection Key Directional Selection Low mortality, high fitness Food becomes scarce. High mortality, low fitness Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve Stabilizing Selection Key Individuals with highest fitness: near the center of the curve (average phenotype) Example: human birth weight Low mortality, High high fitness mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place. Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve. Disruptive Selection High mortality, low fitness ♦ specializing in different seeds. Number of Birds in Population Individuals with highest fitness: both ends of Key Low mortality, Population splits curve high fitness into two subgroups Number of Birds in Population ♦ Largest and smallest seeds become more common. Beak Size Beak Size Example: birds where seeds are either large or small Stabilizing Selection Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place. Disruptive Selection Disruptive Selection Low mortality, high fitness High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Number of Birds in Population Largest and smallest seeds become more common. Beak Size However: No examples ever observed in animals A couple examples that may demonstrate speciation exist in plants and some insects. Genetic Drift random change in allele frequency that occurs in small populations Genetic Drift Two phenomena that result in small populations and cause genetic drift Founder Effect 2. Bottleneck Effect 1. Genetic Drift The results of genetic crosses can usually be predicted using the laws of probability. In small populations, however, these predictions are not always accurate. Founder effect Allele frequencies change due to migration of a small subgroup of a population Founder Effect Two groups from a large, diverse population could produce new populations that differ from the original group. 2. 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 Bottleneck Effect Original population Bottleneck Effect Catastrophe Original population Bottleneck Effect Catastrophe Original population Surviving population Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium? According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large (2) there can be no mutations (3) there must be random mating (4) there can be no movement into or out of the population (5) no natural selection can occur Genetic equilibrium = no evolution A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve. The Hardy-Weinberg principle describes the conditions under which evolution does not occur. The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change. Hardy-Weinberg principle 1. Large Population Genetic drift can cause changes in allele frequencies in small populations. Genetic drift has less effect on large populations. Large population size helps maintain genetic equilibrium 2. No Mutations If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change. Hardy-Weinberg principle 3. Random Mating All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random. In natural populations, however, mating is not random. •Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. •Such non-random mating means that alleles for those traits are under selection pressure. Hardy-Weinberg principle 4. No Movement Into or Out of the Population Individuals who join a population may introduce new alleles into the gene pool. (Immigration) Individuals who leave may remove alleles from the gene pool. (emigration) Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population. Hardy-Weinberg principle 5. No Natural Selection All genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another. Sexual Reproduction and Allele Frequency Meiosis and fertilization do not change the relative frequency of alleles in a population. The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population. The Process of Speciation The formation of new biological species, usually by the division of a single species into two or more genetically distinct one. Three Isolating Mechanisms: Isolate species forming subspecies and perhaps causing speciation. 1. 2. 3. Geographic Isolation Behavioral Isolation Temporal Isolation Example: Eastern and Western Meadowlark Male birds sing a matting song that females like, East and West have different songs. Females only respond to their subspecies song. 1. Geographic Isolation Two populations separated by a geographic barrier; river, lake, canyon, mountain range. Example: 10,000 years ago the Colorado River separated two squirrel populations. Kaibab Squirrel Abert Squirrel Kaibab Squirrel Abert Squirrel This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together. Example: Eastern and Western Meadowlark Eastern and Western Meadowlark populations overlap in the middle of the US 2. Behavioral Isolation Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ. 3. Temporal Isolation Populations reproduce at different times January 7 1 2 3 4 5 6 8 9 10 11 12 13 Example: Northern Leopard Frog & North American Bullfrog Mates in: April Mates in: July Conclusion: Geographic, Behavioral and Temporal Isolation are all believed to lead to speciation.