The Modern Synthesis. Causes of microevolution. The Modern Synthesis. Microevolution. Genetic Drift. Genetic drift example

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1 The Modern Synthesis Populations are the units of evolution Natural selection plays an important role in evolution, but is not the only factor Speciation is at the boundary between microevolution and macroevolution The Modern Synthesis Integrates ideas from many different fields: Comparative morphology & molecular biology Taxonomy relationships of taxa Paleontology study of fossils Biogeography distribution of species Population genetics Hardy-Weinberg Theorem Darwin Mendel All images Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Microevolution Generation-to-generation change in allele frequencies in populations The Hardy-Weinberg theory provides the baseline Microevolution occurs even if only a single locus in a population changes Causes of microevolution Genetic drift * Natural selection * Gene flow Mutation * The 2 most important factors All are departures from the conditions required for the Hardy-Weinberg equilibrium Genetic Drift Changes in gene frequencies due to chance events (sampling errors) in small populations Hardy Weinberg assumes reproduction works probabilistically on gene frequencies, (p + q = 1) Reproduction in small populations may not work this way Two similar situations lead to genetic drift Bottleneck effect Founder effect Genetic drift example Wildflower population with a stable size of only 10 plants Some alleles could easily be eliminated Fig. 23.4

2 Bottleneck Effect Large population drastically reduced by a disaster By chance, some survivor s alleles may be over- or under-represented, or some alleles may be eliminated Genetic drift continues until the population is large enough to minimize sampling errors Endangered species Bottleneck incidents cause loss of some alleles from the gene pool This reduces individual variation and adaptability Example: cheetah Genetic variation in wild populations is extremely low Similar to highly inbred lab mice! Founder effect New population starts with a few genetically unrepresentative of a larger source population. Extreme: single pregnant female or single seed More often larger sample, but small Genetic drift continues until the population is large enough to minimize sampling errors Natural selection Review: overpopulation, unequal reproduction, survival of the fittest, microevolution Only factor that generally adapts a population to its environment The other three factors may effect populations in positive, negative, or neutral ways Natural selection Fig 23.4 Used here to help illustrate natural selection Examples: Herbivory higher for white flowered plants than red flowered red-flowered alleles (R) increase Pollinators attracted by white flowers rather than red flowers white flower alleles (r) increase. Natural selection accumulates and maintains favorable genotypes Gene flow Genetic exchange due to migration of alleles Fertile individuals Gametes or spores Example: Wildflower population has white flowered plants only Pollen (with r alleles only) could be carried to another nearby population that lacks the allele. Gene flow tends to reduce differences between populations

3 Mutation Change in DNA Rare and random More likely to be harmful than beneficial Only mutations in cell lines that produce gametes can be passed along to offspring One mutation does not effect a large population in a single generation Very important to evolution over the long term The only source of new alleles Other causes of microevolution redistribute mutations Variation Natural selection More details Phenotypic Variation Combination of inheritable and nonheritable traits Phenotype is the cumulative product of: Inherited genotype Environmental influences Only the genetic component can be selected Fig Same genes, different seasons Genotypic variation Expressed in these ways: Quantitative (continuous multilocus?) ex. plant height Discrete (single locus?) ex. flower color Measured by: Gene diversity - % heterozygosity Human 14% DNA base diversity Human 0.1 % Geographic variation Between or within populations Natural selection working in response to differences in environment Genetic drift Cline = graded geographic change Fig Geographical distribution of variation in Yarrow plants Variation in isolated populations Discretely separated populations exhibit discrete differences Example: karyotypes of mice Fig House mice on Madiera

4 What keeps mutations? Heterozygote advantage Diploidy masks recessive alleles Hardy-Weinberg Equilibrium says that, without natural selection, gene frequencies remain the same A balance of recessive alleles can be kept even without Hardy-Weinberg Heterozygote advantage Frequency-dependent selection Sickle-cell allele Homozygous recessives unhealthy Heterozygotes protected from malaria Fig Sickle-cell allele and malaria Frequency-dependient selection Common morphs of snails more likely to die from parasites Rare morph less likely Fig Infection of snails by parasitic worms Neutral variation Have negligible impact on reproductive success Not selected by natural selection But their gene frequencies can change Hard to assess Some neutral alleles will increase and others will decrease by the chance effects of genetic drift May provide basis for future evolution How natural selection acts on allele frequency Directional Diversifying Stabilizing Fig Frequency of individuals showing a range of phenotype Directional Phenotype moves toward one end of the range Ex. Beak size in Galapago ground finch During dry years big beaks advantageous and increase in frequency Stabilizing selection is similar Fig

5 Diversifying Selects for two ends of a range Can result in balanced polymorphism Ex. Beak type in black-bellied seedcrackers Two types of seeds hard and soft Intermediate billed birds inefficient at feeding on either type Fig The evolution of species and larger taxa Evolutionary theory must also explain macroevolution Speciation is the keystone process in the origination of diversity of higher taxa Two types Anagenesis Cladogenesis Macroevolution Galapagos tortise Species Latin meaning kind or appearance Traditionally distinguished by morphological differences Today distinguished in addition by differences in body function, biochemistry, behavior, and genetic makeup Figure 1.17 Galapagos finches Biological species Concept emphasizes reproductive isolation Fig. 24.2a Similarity between species Fig. 24.2b Diversity within species How are biological species isolated? Prezygotic barriers impede mating habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation Postzygotic barriers prevent development reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown

6 Limitations of the biological species concept Impractical or impossible to assess: Fossils Many living species Asexual species (bacteria, fungi, protists) Alternative species concepts Ecological species defined in terms of its ecological niche Pluralistic species defined by combination of reproductive isolation and ecological niche Morphological species defined by structure Genealogical species defined as a set of organisms with a common and unique genetic history as shown by molecular patterns Allopatric speciation - geographic separation restricts gene flow Sympatric speciation - biological factors reduce gene flow Speciation Fig Allopatric speciation Geological processes that isolate populations Mountain ranges, glaciers, land bridges, or splintering of lakes Colonization of new, geographically remote areas How significant the barrier must be depends on the species Increases in small and isolated populations A. harrisi South Rim A. leucurus North Rim Fig species of antelope squirrel, Ammospermophilis near Grand Canyon Ring species Fig Ensatina escholtzii salamanders Adaptive Radiation The evolution of many diverselyadapted species from a common ancestor Seen in some island chains (Hawaii, Galapagos) Fig

7 Sympatric speciation Anagenesis Reproductive barriers must evolve between sympatric populations In plants, sympatric speciation often results from polyploidy In animals, sympatric speciation may result from gene-based shifts in habitat or mate preference Transformation of one species into another Fig. 24.1a Cladogenesis Tempo of speciation Creation of one or more new species from a parent species Promotes biological diversity by increasing the number of species Fig. 24.1b Gradualism Traditional view Not supported by fossil evidence Punctuated equilibrium Rapid appearance Slow to no change later Fig Evolution of complex structures Limpet Slit-shell Evolution does not have goals Continued modification of older structures Often fossil evidence of sequence not complete Nautilus Murex, snail Squid Fig Range of eye complexity in mollusks Fig

8 Remember, evolution is the touchstone of biology a criterion for determining the quality or genuineness of a thing a fundamental or quintessential part or feature