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Chapter 16 Population Genetics and Speciation Section 1: Genetic Equilibrium Section 2: Disruption of Genetic Equilibrium Section 3: Formation of Species Leopard Seal, Antarctica Section 1: Genetic Equilibrium Population Genetics (Microevolution) Study of evolution from a genetic point of view 1. Every population has some genetic variation that influences fitness - Evolution is potentially a continuing process in all populations 2. Changes in selective factors in the environment will almost always be met by evolutionary responses - leads to shifts in the frequencies of genotypes in a population 3. Rapid changes will often exceed the capacity of a population to respond by evolution - decline could lead to extinction Darwin’s finches • Peter and Rosemary Grant • The Galapagos Islands: normally dry • El Nino: increase rainfall, vegetation flourishes • La Nina: periods of drought • Reproductive success and survival of individuals differed between El Nino and La Nina years • Caused dramatic evolutionary change • Medium ground finch • Seeds, cracks with beak • La Nina (drought): – amount of seeds dropped, seeds became tougher – population dropped • 1400 in 1975 to 200 at end of 1977 – larger beaks could crack the larger seeds and survived better than those with smaller beaks – Average beak size increased Within a population individuals vary in observable traits. Traits cover a range that can be represented by a bell curve • El Nino (wet) 1983 – Small seeds in abundance – Those with smaller beaks handled the smaller seeds better, able to survive, produce more offspring than those with larger beaks – Average beak size returned to a lower value Population genetics studies the ways in which populations respond to such selective pressures with changes in allele frequencies Genetic Variation 1. Mutation: random change in a gene, passed on to offspring 2. Recombination: reshuffling of genes Independent assortment Crossing-over 3. Random Pairing of gametes Gene Pool: total genetic information available in a population When mate at random: all combinations of different alleles are possible Peccaries are small, tough relatives of the modern pig, whose lineage diverged about 40 million years ago. They live in southern Texas, Arizona, and New Mexico. The forces of evolution shape and change the composition of this gene pool and thus the nature of the population. Allele Frequency = # of a certain allele / total number of alleles B: Long bristles on the bodies b: short bristles 15 Individual peccaries in the population 30 alleles If 6 alleles in this population are b, and 24 are B Then the frequencies of these alleles are: 6/30 of the genes in the gene pool are b – a frequency of 0.2 24/30 of the gene in the gene pool are B – a frequency of 0.8 Phenotype Frequency = # of individuals with a particular phenotype / total number of individuals in the population Hardy-Weinberg Equilibrium Frequencies of alleles and genotypes remain constant from generation to generation in a population (No evolutionary change occurs through the process of sexual reproduction) Changes can result only from the action of additional forces on the gene pool of a population Ideal hypothetical population that is not evolving Set of Assumptions 1. No net mutations occur 2. Individuals neither enter nor leave the population 3. The population is large 4. Individuals mate randomly 5. Selection does not occur *any exception to these can result in evolution Section 2: Disruption of Genetic Equilibrium To be at Equilibrium No Net Mutations Occur • Spontaneous mutations occur constantly at very low rates • If exposed to mutagens, rates can increase significantly • Mutations can produce totally new alleles for a trait To be at Equilibrium Individuals can neither enter nor leave a population • Size of the population must remain constant • If individuals move, genes move too Immigration: movement into a population Emigration: movement out of a population Gene flow: process of genes moving from one population to another To be at Equilibrium Large Population Genetic Drift: phenomenon by which allele frequencies in a population change as a result of random events, or chance To be at Equilibrium Individuals Mate Randomly Nonrandom Mating • Influenced by geographic proximity • Select a mate with similar traits:Assortative Mating • Sexual selection: females tend to choose males based on certain traits (Planet Earth: Jungles, Birds of Paradise) Birds of Paradise-Planet Earth QuickTime™ and a decompressor are needed to see this picture. To be at Equilibrium No Natural Selection Ongoing process in nature Some members of a population are more likely than others to survive and reproduce and thus contribute their genes to the next generation Stabilizing Selection: individuals with the average form of a trait have the highest fitness Disruptive Selection: individuals with either extreme variation of a trait have greater fitness than those with the average form of that trait Directional Selection: individuals that display a more extreme form of a trait have a greater fitness than those with an average form Black dots represent individuals that die before passing on their genes. Hardy-Weinberg Conditions Animations http://nhscience.lonestar.edu/biol/hwe.html http://www.accessexcellence.org/AE/AEPC/WWC/1995/hardyweinberg.php Hardy-Weinberg Equation Used to discover the probable genotype frequencies in a population and to track their changes from one generation to the next p2 + 2pq + q2 = 1 p+q=1 p = frequency of the dominant allele (A) q = frequency of the recessive allele(a) p2 is the predicted frequency of homozygous dominant people (AA) 2pq is the predicted frequency of heterozygous people (Aa) q2 is the predicted frequency of homozygous recessive people (aa) Section 3: Formation of Species Speciation: process of species formation Species: a single kind of organism. Members are morphologically similar (external structure and appearance) and can interbreed to produce fertile offspring. (biological species concept) How do species give rise to other ones? 1. Geographic isolation: physical separation of members of a population. No longer experience gene flow, so the gene pools of each separate population may begin to differ due to genetic drift, mutations and natural selection Allopatric Speciation happens when species arise as a result of geographic isolation “different homeland” Reproductive Isolation: results from barriers to successful breeding between population groups in the same area. • Pre-zygotic: occurs before fertilization - • active at different seasons or times of day Post-zygotic: occurs after fertilization - zygote dies F1 hybrids have reduced fertility or sterility Example: mule from horse and donkey Sympatric Speciation: when two subpopulations become reproductively isolated within the same geographic area 200 years ago, ancestors of apple maggot flies laid their eggs only on hawthorns. Today, these flies lay eggs on hawthorns and domestic apples. Females choose to lay eggs on fruit they grew up in and males look for mates on the fruit they grew up on. Gene flow between parts of the population that mate on different types of fruit is reduced. Led to new species Rates of Speciation Happens at a regular, gradual rate (few million yrs) Sudden, rapid change followed by little to no change (few thousand years)