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Speciation Professor Andrea Garrison Biology 3A Illustrations ©2010 Pearson Education, Inc. , unless otherwise noted Speciation • Natural selection drives evolution by favoring adaptive traits • Works on level of gene pool – Gene pool = all alleles for all genes in population – Affects entire population • Agents of change – Mutation (= chance changes in DNA) • Rare; 1 in100,000 to 1 in 1,000,000 events • Over evolutionary time scales, they add up – Sexual reproduction (mixes alleles; speeds up opportunities for selection of alleles) Speciation 2 Speciation • To understand how speciation can happen, and to see how to test it’s effects in experiments, it helps to understand what prevents speciation • This is a conceptual model that is assumed to work as a control – It provides a theoretical reference point against which observations can be evaluated Speciation 3 Speciation • Hardy-Weinberg Principle – Null model • Predicts what would happen if evolution has no effect • Specifies condition under which population of diploid organisms achieves genetic equilibrium – Genetic equilibrium = genotype and allele frequencies remain stable in succeeding generations – Population’s gene pool often has 2 or more alleles for each gene – Genotype frequency = % of individuals possessing each genotype • AA, Aa, aa • AA + Aa + aa = 1 (=100% of genotypes) – Allele frequency = relative abundance of each allele • For gene with 2 alleles, p = frequency of one allele, q = frequency of second allele • p + q = 1 (=100% of alleles) Speciation 4 Speciation Speciation; Table ©2014 Cengage Learning 5 Speciation • Hardy-Weinberg Principle – Null model • Genetic equilibrium possible only if all of the following are true – – – – – No mutations occurring No immigration from other populations Very large population All genotypes survive to reproduce equally well Mating is random • Under the above conditions, no microevolution occurs • Allele frequencies will never change, and genotype frequencies stop changing after one generation • If population’s genotype frequencies do not match model’s predictions, or if allele frequencies change over time, microevolution may be occurring Speciation 6 Speciation • Hardy-Weinberg equation p2 + 2pq + q2 = 1 (100% of population) If equation is true, population is at genetic equilibrium Speciation 7 Speciation For snapdragon population allele frequencies are p2 + 2pq + q2 = 1 p = 0.7 q = 0.3 0.72 + 2(0.7 x 0.3) + 0.32 = 0.49 + 0.42 + 0.09 = 1 Out of 1000 plants, Hardy-Weinberg predicts: 490 CRCR (red), 420 CRCW (pink), 90 CWCW (white) Observations showed 450 red, 500 pink, 50 white; so population is not in equilibrium Speciation; Table ©2014 Cengage Learning 8 Speciation • Allele frequencies will change over time if Hardy-Weinberg conditions are not met • Processes that foster microevolutionary change : – Mutation – Gene flow – Genetic drift – Natural selection – Nonrandom mating Speciation 9 Speciation • Mutation – Spontaneous change in DNA (causes microevoution only when hereditary, meaning in DNA of germ cells) – Have little or no immediate effect on allele frequency, but are available to be selected for by natural selection • Deleterious mutations: harmful, not usually passed on • Lethal mutations: kill all carriers (if dominant) or only homozygous carriers (if recessive), not usually passed on unless there is some advantage to heterozygote • Neutral mutations: are neither harmful nor helpful • Advantageous mutations: confer a benefit, natural selection may increase frequency Speciation 10 Speciation • Gene flow – Movement of organisms or their gametes (pollen) to a different population – Animals often migrate to other populations – Dispersal mechanisms provide gene flow • Wind pollination Speciation; photos ©2014 Cengage Learning 11 Speciation • Genetic Drift – Chance loss of alleles • Found in small populations Speciation 12 Genetic Drift • Founder Effect – Establishment of small population whose gene pool differs from parent population Speciation 13 Genetic Drift • Bottleneck Effect – Natural disasters decrease population to extent some alleles underrepresented by chance – Difficult to maintain healthy population Speciation 14 Speciation • Natural selection – Environment may select for specific phenotypes • Predators choose: – Smallest of prey population » Selects for larger individuals – Slowest of prey population » Selects for faster individuals Speciation 15 Speciation • Nonrandom mating – Many species mate nonrandomly – Selecting mate with specific phenotypes increases the alleles associated with that phenotype – Inbreeding (mating with closely related individuals) occurs within small populations • increases frequency of homozygous genotypes and decreases frequency of heterozygous genotypes • Recessive phenotypes often expressed Speciation 16 Patterns of Speciation • How does one species evolve from another? A → A1 non-branching evolution A1 A divergence (branching evolution) A2 Speciation 17 Speciation • Key to speciation is presence or absence of reproduction – Local subgroups may have chance mutations – Not new species until so different they cannot interbreed • Population = all members of a species within given geographic area (often taken to mean individuals interbreeding with one another) Speciation 18 Speciation • Non-branching evolution A → A1 – changes over time, A1 cannot interbreed with A • Divergence – A1 and A2 must be kept from interbreeding until they are so different they cannot interbreed A1 A A2 Speciation 19 Speciation • Divergence occurs because local populations breed only with their neighbors, and not those members of the same species on “the other side of the hill” – If members of two subgroups interbreed, no divergence • They share any chance mutations – If members of two subgroups do not interbreed, divergence can occur • No sharing of chance mutations – Chance mutations are different – Two different environments select differently for different mutations Speciation 20 Speciation • Some organisms recognized as separate species today are actually capable of interbreeding, and would converge back to one species if the opportunity existed • REPRODUCTIVE ISOLATING MECHANISMS – Prevent reproduction between two populations – Maintain separate species Speciation 21 Speciation 22 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Geographical isolation • Geographical ranges do not coincide • Species never encounter each other – Example: English Oak in Europe and Valley Oak in California • Never close enough to interbreed, although they could Speciation 23 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Ecological (habitat) isolation • Two species live in same area, but different habitats Speciation 24 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Temporal isolation • Closely related species mate at different times of the year Speciation 25 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Behavioral isolation • Closely related species with different mating rituals Speciation 26 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Mechanical isolation • Structural differences between species (copulatory organs, pollen grains, etc. Speciation 27 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Gametic isolation • Chemical differences between gametes (e.g., proteins on surface of gametes) Speciation 28 Speciation 29 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Reduced hybrid viability • Hybrid is weak or dies Speciation 30 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Reduced hybrid fertility • Hybrid is sterile Speciation; left illustration Cengage Learning 2014 31 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Hybrid breakdown • Hybrid reproduction produces weak offspring Speciation 32 Allopatric Speciation • Speciation while subpopulations separated – Geographical separation breaks population into two subgroups preventing gene flow – Mutations eventually cause reproductive isolation (mechanical, temporal, gametic) Speciation 33 Allopatric Speciation 1 At first, a 22 A geographical population is change, such as a distributed over a change in the river’s large geographical course, separates area. A river flows the original along one edge of population, creating the population’s a barrier to gene geographical range. flow. 33 In the absence of gene flow, the separated populations evolve independently and diverge into different species. 44 When the river later changes course again, allowing individuals of the two species to come into secondary contact, they do not interbreed. Figure 22-9, p. 486 Sympatric Speciation • Speciation while subgroups are not separated • Generally insects that live entire life (feed, mate, lay eggs) on same bush – Which bush determined by genetics or what larvae ate Speciation 35