Speciation. Mechanisms of Speciation. Title goes here. Some Key Tenets of the Modern Synthesis

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1 Carol Eunmi Lee 11/10/16 Speciation Increasing genetic distance Fitness Mating between different species Mating between relatives Inbreeding Depression Hybrid Vigor Outbreeding Depression 2 Darwin s Origin of Species did not actually discuss the origins of species, but rather mechanisms of natural selection. So, how does one go from evolutionary mechanisms within populations to the creation of entirely new species (i.e. speciation)? Mechanisms of Speciation So How can microevolutionary processes of Genetic Drift, Natural Selection, Mutations, etc. lead to more macroevolutionary processes such as the formation of new species? Some Key Tenets of the Modern Synthesis This gap in understanding was addressed somewhat during the Evolutionary Synthesis Title goes here Populations are the units of Evolution Mendel vs Darwin: Continuous traits are also coded by particulate genes, but many genes Mutation vs Selection: Mutations are sources of genetic variation upon which Selection acts Natural Selection and Mutation are not the only evolutionary forces. Example: Genetic Drift, Recombination Microevolutionary processes, such as Drift, Selection, Mutation, lead to Macroevolutionary changes (such as speciation) 1

2 Common Mechanisms of Speciation in different taxa Although, at the time of the Evolutionary Synthesis, the genetic mechanisms of speciation were not well understood Animals: Outbreeding depression, breakdown of coadapted gene complexes (Dobzhansky-Müller incompatibilities) à Reproductive Isolation Plants: Polyploidization Prokaryotes: Horizontal Gene Transfer but, it s hard to define what speciation is for a prokaryote Mechanisms of Speciation (1) Genetic Models: (Book doesn t discuss sufficiently, but important) The roles of: Mutations Natural Selection Genetic Drift The interaction between these three evolutionary mechanisms could cause two populations to become two different species (1) Mechanisms of Speciation (1.1) Genetic Models: How do Genetic Drift, Natural Selection, Mutations, etc. create new species? Are there speciation genes? (1.2) Species Concepts: What exactly is a species??? 10 Today s OUTLINE: Focusing on Genetic Mechanisms of Speciation: Genetic Model of Speciation: Dobzhansky-Müller Incompatibilities Covered in 16.3 in your book (Freeman & Herron) but with the details glossed over How population genetic processes lead to speciation Dobzhansky-Müller incompatibilities Common mechanisms in plants: Polyploidization The link between microevolution and macroevolution This particular model of speciation applies only to sexual species Title goes here 2

3 Genetic Model of Speciation: Dobzhansky-Müller Incompatibilities Speciation: the process by which populations become different species Microevolution: results from evolutionary mechanisms (mutation, drift, selection, etc) acting within a population Macroevolution: refers to evolutionary change above the species level Reproductive isolation occurs as a byproduct of genetic divergence between two populations Mutations accumulate in each population These mutations become fixed by natural selection or genetic drift in each population While the mutations are functional on their normal genetic backgrounds, they don t work well together when they are brought together in hybrids (negative epistasis) Causing failure to mate or hybrid sterility or inviability ( hybrid breakdown ) 14 As ancestral populations diverge, each lineage evolves independent substitutions that are not detrimental in their native genomic contexts. However, because the derived alleles have never been tested together, they can cause genetic incompatibilities when combined in a hybrid. As ancestral populations diverge, each lineage gains different mutations (shown as lower case letter alleles in the figure) that are not detrimental in their native genomic contexts. However, because the new derived alleles have never been tested together (not yet taken out by selection), they can cause genetic incompatibilities when combined in a hybrid. Explanation in an Intro Bio Textbook pg=pa484&dq=dobzhansky+muller&source=bl&ots=czoh8tsjf9&si g=cboen10k2m6mr3xdmdndmp9ohtg&hl=en&sa=x&ei=mzozv PrOJI2dyASgi4LQCg&ved=0CFkQ6AEwCDge#v=onepage&q=dob zhansky%20muller&f=false 16 Genetic Model of Speciation: Dobzhansky-Müller Incompatibilities For instance, an enzyme might have 4 subunits. A new mutation in one subunit might make that subunit no longer functionally compatible with a new mutation in another subunit of the enzyme (encoded by a different gene). If these incompatible mutations arise in different populations, they might be fine without the other mutation... But, if the two populations come together, and the two incompatible mutations are now brought together in the same individual, the hybrid might now be inviable (die) or sterile (infertile). 17 A population with two loci 18 Title goes here 3

4 The population becomes divided and geographically separated (allopatric) The population becomes divided and geographically separated (allopatric) New mutation AaBB AaBB AABb AABb New mutation Natural Selection and Speciation Selection favors different alleles in each population Natural Selection and Speciation The different alleles become fixed in each population New mutation AaBB AaBB AABb AABb A different New mutation Natural Selection and Speciation Natural Selection and Speciation The different alleles become fixed in each population In some cases, these new fixed alleles will not function with loci in the other genetic background Title goes here 4

5 Natural Selection and Speciation Some Hybrids will be inviable aabb Natural Selection and Speciation Across multiple loci across the genome, such incompabilities will lead to inviability of hybrids aabb Crossing between two divergent populations Across the genome, the impact of hybrid incompatibilities will differ at different loci, depending on the magnitude of incompatibility between genes, dominance at each locus (which alleles are expressed), etc. 27 Case 1: The new alleles are A and B and they are Dominant at their own respective loci And If A and B are alleles that are incompatible between the two loci Parents: x F1: AaBb x AaBb If any of the incompatible alleles are dominant, selection will act very quickly on the F1 generation 28 Crossing between two divergent populations Case 2: At loci where the alleles are codominant at their own loci (both alleles expressed) And if a and b are alleles that are incompatible across the two loci Parents: x F1: AaBb x AaBb Hybrid breakdown could proceed at the first generation (though selection might not act as strongly as in Case 1) as the incompatible alleles will both be expressed 29 Crossing between two divergent populations Case 3: At loci where there is dominance, the new mutations are more commonly recessive at their own loci And if a and b are alleles that are incompatible between the two loci Parents: x Aabb F1: AaBb x AaBb F2 (different combos, numbers not indicated):, AaBB,, AABb, AaBb, aabb,, Aabb, aabb Hybrid breakdown most commonly appears at the F2 generation because dominance typically masks deleterious alleles at the F1 generation 30 Title goes here 5

6 Crossing between two divergent populations Case 3: At loci where there is dominance, and the new alleles are typically recessive, and if the new mutations are incompatible with each other Parents: x F1: AaBb x AaBb F2 (different combos, numbers not indicated):, AaBB,, AABb, AaBb, aabb,, Aabb, aabb At any one locus, only a fraction of the hybrids are likely to show deleterious effects 31 Crossing between two divergent populations Case 3: At loci where the new mutations are recessive and incompatible with each other Parents: x F1: AaBb x AaBb F2 (different combos, numbers not indicated):, AaBB,, AABb, AaBb, aabb,, Aabb, aabb If you sum across the entire genome, many many individuals will show F2 hybrid breakdown across some of their loci. So, you will see many inviable F2 offspring in divergent 32 populations So, eventually the two populations can no longer come together (due to hybrid breakdown) and they become different species Speciation could happen gradually in that some genes might become more diverged and unable to function in the genetic background of the other population more quickly than others So, during the process of speciation, some hybrid individuals might by chance be more viable than others, depending on the combination of alleles that the individual happens to have Genetic Models Genetic Drift and Speciation Genetic drift can work the same way, but often takes longer (even weak selection can speed rate of speciation; Gavrilets 2000) Random drift of genes in each population can lead to incompatibilities So far, evidence suggests that speciation occurs more readily through selection on different populations than through genetic drift. Speciation genes : Speciation could arise with the breakdown in coadapted gene complexes for any functional gene that affects survival. However, mutations in any genes that affect reproductive compatibility (sperm, eggs) could lead to rapid speciation ( Speciation Genes ) 35 Evolutionary changes (mutations) at genes that affect reproduction (e.g. sperm motility, egg coat protein) could make speciation happen rapidly Sometimes divergence in single genes could cause speciation (often those involved in mating or reproduction) These genes could have diverged through selection or genetic drift Wu, C.-I and C.-T. Ting 2004 Genes and speciation. Nature Review Genetics 5: Title goes here 6

7 Speciation through Polyploidization Herron & Freeman, pp. 634, but with more mechanism presented here Example: Dandelion, Eurasian origin Diploid, triploid, tetraploid populations North American invasive populations are triploid and sterile, reproduce asexually Taraxacum officinale Polyploidy Polyploidy is the presence of extra sets of homologous due to accidents during cell division An autopolyploid is an individual with more than two chromosome sets, derived from one species An allopolyploid is a species with multiple sets of derived from different species Polyploidy Polyploidy is much more common in plants than in animals, and polyploidization is an important mechanism for speciation in plants Estimates suggest that 30 80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes (Rieseberg 2007) Polyploidization is estimated to be associated with 15% of speciation events in angiosperm (flowering plants), and 31% in ferns (Wood & Rieseberg 2009) Polyploidy Often, especially in the case of allopolyploidization, the hybrid can no longer cross with parental species, especially when the hybrid has an odd number of à cannot line up during meiosis à cannot make viable s, must reproduce asexually Such hybrids can no longer intercross with parental species, and become reproductively isolated à on a separate evolutionary trajectory à Instant new species in one generation! Polyploidization is probably the single most important mechanism for the evolution of major lineages and for speciation in plants Genome Duplication in Vertebrates 2R hypothesis (Ohno 1970): that there were two rounds of whole genome duplication in vertebrate ancestry: one immediately before, and one immediately after, the divergence of the lamprey lineage Multiple rounds of polyploidization might have occurred during the early evolution of vertebrates Vertebrates should have four copies of genes found in invertebrates, such as Drosophila But this depends on the extent of gene deletion after each round of duplication Title goes here 7

8 NEW SPECIES are produced instantaneously in a single generation Autopolyploid: Failure to reduce chromosome # during meiosis Allopolyploid: Hybridize between two species, and not reduce chromosome # Autopolyploid: Not reduce chromosome # within oneself (a mutation) during meiosis; Chromosome # ends up doubling Example: potato Autopolyploidization Autopolyploidization 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue (a mutation) 2n = 6 4n = 12 2n Gametes produced are diploid, instead of haploid. Autopolyploidization Allopolyploids Hybrids between different species where chromosome # not reduced (a type of mutation) Applies to many plants, most weeds 2n = 6 4n = 12 2n 4n Offspring with tetraploid karyotypes may be viable and fertile. Some are cannot reproduce sexually (often when odd number ), but can reproduce asexually Often results in fixed heterosis (fixed heterozygous advantage) Numerous examples of successful invaders in recently-derived allopolyploids (such as Barnyard grass, reed canary grass, Spartina anglica) 48 Title goes here 8

9 Two different species hybridize Allopolyploid: Hybridize and not reduce chromosome # Cannot reproduce sexually (odd # of, can t segregate during meiosis) Can reproduce sexually Allopolyploidization (e.g. canola, wheat, cotton, many invasive species) Allopolyploidization Allopolyploidization Cannot reproduce sexually (odd # of, can t segregate during meiosis) Allopolyploidization Can reproduce sexually Hybrids between different species where chromosome # not reduced (a type of mutation) Some are sterile (often when odd number ), but can reproduce asexually Species B 2n = 4 Species A 2n = 6 Meiotic error Unreduced with 4 (a mutation) Allopolyploidization Allopolyploidization Species B 2n = 4 Meiotic error Unreduced with 4 Hybrid with 7 Species B 2n = 4 Meiotic error Unreduced with 4 Hybrid with 7 Unreduced with 7 Species A 2n = 6 Species A 2n = 6 Title goes here 9

10 Allopolyploidization Allopolyploidization Species B 2n = 4 Meiotic error Unreduced with 4 Hybrid with 7 Unreduced with 7 Species B 2n = 4 Meiotic error Unreduced with 4 Hybrid with 7 Unreduced with 7 Species A 2n = 6 Sexually fertile hybrid (allopolyploid) 2n = 10 Species A 2n = 6 Sexually fertile hybrid (allopolyploid) 2n = 10 Allopolyploid hybridization could result in multiple different species If cannot segregate, some hybrids will be unable to reproduce sexually But many can reproduce asexually Allopolyploidization Allopolyploid hybridization could result in multiple species Species B 2n = 4 Meiotic error Unreduced with 4 Hybrid with 7 Unreduced with 7 Species A 2n = 6 Sexually fertile hybrid (allopolyploid) 2n = 10 These hybrids have more than two copies of homologous, and are polyploid The homologous are not identical, as they come from different species, but they do share many genes, and have a lot of redundancy If cannot segregate, some of the hybrids will be sterile, But many can reproduce asexually Many plants are allopolyploids Polyploidy is much more common in plants than in animals Polyploidization is an important mechanism for speciation in plants Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids 59 Many invasive plants are allopolyploids Many Allopolyploids are asexual and have rapid population growth rate Polyploids have the capacity for greater physiological buffering than their diploid progenitors because multiple copies of genes often leads to increase function... Such as increased enzyme activity due to enzyme multiplicity Polyploids often become diploidized over time, so that they could have sex and increase genotypic diversity, and respond to natural selection (odd# chromosome polyploids cannot undergo meiosis, and even# chromosome polyploids still have trouble with lining up the 60 during meiosis) Title goes here 10

11 Tradeoffs between polyploidy and diploidy Benefits of Polyploidy Grow faster Larger Rapid population growth rate in asexuals Often results in fixed heterosis (fixed heterozygous advantage among the multiple ) in asexuals gene redundancy, greater genetic robustness (if one gene is bad, have another Polyploids have the capacity for greater physiological buffering than their diploid progenitors due to enzyme multiplicity Increased enzyme activity, novel enzymes and metabolites, increased metabolic regulation Tradeoffs between polyploids and diploids Benefits of Polyploidy Majority of agricultural crop species appear to be polyploids (oats, cotton, potatoes, tobacco, and wheat) Polyploidy has conferred distinct advantages for the development of agronomically important traits. For example, polyploidization has been associated with increased size of harvested organs Many invasive species are recently-derived allopolyploids (such as Barnyard grass, reed canary grass, Spartina anglica, Dandelion) Rapid somatic growth and rapid population growth would enable invasive species to outcompete native species Tradeoffs between polyploids and diploids Costs of Polyploidy In asexual polyploids: No recombination with genomes of other individuals (no sex), no increase in genotypic diversity Problems during Mitosis and Meiosis: Polyploidy increases the complexity of the processes that are involved in managing and partitioning during cell division. Epigenetic Instability: Epigenetic remodeling occurs during polyploidization, which leads to both the activation and suppression of gene expression sometimes this leads to nonadditive changes in gene expression, some of which could be deleterious Diploidization of Polyploids tends to occur over time Almost all angiosperms are likely to be ancient polyploids that have undergone diploidization Ecological Implications Many species have arisen through allopolyploid hybridization Many new species are arising today through the hybridization of native and exotic species (or between exotic species) Big Concern: hybridization between weeds and genetically engineered crops! Three Main Species Concepts 1. Biological Species Concept 2. Phylogenetic Species Concept 3. Morphological (Phenetic) Species Concept Title goes here 11

12 1. Biological Species Concept (Ernst Mayr, 1942) 1. Biological Species Concept (Ernst Mayr, 1942) A group of interbreeding populations that are evolutionary independent of other populations Example: all human populations belong to the same biological species Biological Species Concept Strengths An unambiguous empirical criteria which is clearly linked to speciation (if populations can t intermate they can t belong to the same species) Using reproductive isolation as the criterion is meaningful as it confirms the lack of gene flow between groups Biological Species Concept PROBLEMS: Many species are asexual and do not intermate (viruses, bacteria, protists) Many highly divergent species can hybridize (plants) Only applicable to present (not fossil taxa) Ability to intermate sometimes drops off gradually ( ring species ) Ring Species 2. Phylogenetic Species Concept The smallest group that is monophyletic is called a species Title goes here 12

13 2. Phylogenetic Species Concept There are several monophyletic groups here Monophyletic group: A group with a shared derived (descendant) character A group that contains a common ancestor and all its descendents Phylogenetic Species Concept Typically, a phylogeny is constructed using DNA or other types of data (proteins, morphological traits) The phylogeny reveals hierarchical relationships among groups The smallest group that has a shared derived character and is monophyletic is called a species Phylogenetic Species Concept There is a derived character that is shared by the 4 populations A monophyletic clade consists of an ancestral taxa and all its descendants A A A B Group I B B C C C D D D E E Group II E F F F Group III The smallest monophyletic group is called a species Monophyly G G G (a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group 76 Phylogenetic Species Concept Strengths Easy to see evolutionary relationships on large and small taxonomic scales It can be used on any species (sexual, asexual) for which there is phylogenetic information (molecular, morphological, biochemical data) on extant or fossil species Title goes here 13

14 Phylogenetic Species Concept Problems: Need a good phylogeny time consuming and can be expensive Not recognize paraphyletic groups (a monophyletic group that does not include all the descendents; reptiles are paraphyletic, as they do not include birds, because birds emerged from within reptiles) A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species Examples of Paraphyletic Groups Paraphyly: a group which either does not include all its descendants or the ancestor. Phylogenetic Species Concept Phylogenetic Species Concept Monophyly Problems: A trivial trait (single mutation or trait) can make a group monophyletic, and may not warrant calling a group a new species The cut off for a species is often arbitrary. For example, 3% sequence divergence is often used for bacteria The smallest monophyletic group is a species Sometimes a trivial trait, like a single point mutation could make a group monophyletic, and a species according to the phylogenetic species concept 3. Morphological (Phenetic) Species Concept Identifying species using overall similarity (but not in a phylogenetic context no hierarchy no branching pattern, no ancestral-derived relationships) Most often morphological traits are used, but any phenotype could be used Morphological (Phenetic) Species Concept Strengths Most intuitive; the way we recognize species Easiest. Easier than constructing phylogeny or intermating Title goes here 14

15 Morphological (Phenetic) Species Concept Problems: Different species can look similar due to convergent evolution Populations that look distinct sometimes belong to the same species Speciation can occur without changes in morphology or other traits (cryptic species) Which species concept to use? When we discuss animals we often use the biological species concept (along with phylogenetic information) Plants: it depends, since very distant plants can hybridize phylogenetic species concept is often used (along with phenotypic traits). Bacteria: difficult problem. Bacteria do not interbreed ( Biological Species concept). In some cases massive exchange of genetic material (horizontal gene transfer) leads to phylogenetic confusion. Often a combination of the Phylogenetic and Phenetic Species Concepts (biochemical and morphological [like cell wall] traits) are used. Darwin s view: Species are arbitrary constructs of the human mind imposed on a continuum of variation Species are dynamic rather than static entities, with boundaries changing constantly Many groups are in the process of speciation However, concept of species is still useful: Species are considered the largest group with a common evolutionary fate Concepts Speciation Genetic Models & Genetic Drift & Natural Selection Polyploid Speciation Concepts Problems with the concept of Species Species Biological Phylogenetic Phenetic (Morphological) Monophyly Title goes here 15

16 1. How might microevolutionary mechanisms (genetic drift, selection, mutation) lead to the formation of new species? Which of the following is FALSE? (A) Disruptive selection could act to cause genetic differentiation between two populations (B) Hybrid breakdown between two populations can be a key factor (C) Mutations could accumulate in two populations and become fixed through selection or genetic drift, leading to hybrid sterility or inviability (D) Genetic drift is thought to be more effective at causing two populations to diverge and become separate species than natural selection (E) Geographic separation between two populations could start the process of speciation 2. You are a geneticist working for an agricultural research firm. Your job is to create new crop species of commercial value. You decide to hybridize two species that differ in chromosome number, and end up with hybrids with a chromosome number that differs from its parents. Which of the following is most FALSE? (A) You have created a new allopolyploid species. (B) Such species created through hybridization are always sterile, and can never reproduce without human assistance because the cannot line up during meiosis. (C) Such new species are often more vigorous (greater fitness, growth rate) than either parent species because of fixed heterosis (heterozygous across loci). (D) Such ability for very different plants species to hybridize poses a challenge to the biological species concept and is a major mechanism for speciation in plants. Optional, slides at end 3. Which of the following is TRUE regarding speciation (the formation of new species)? (a) Speciation requires geographic separation (b) Dobzhansky-Müller incompatibilities can result in more rapid speciation in asexual species (c) Dobzhansky-Müller incompatibilities require changes in "speciation genes," or genes that affect reproduction (sperm coat protein, sperm motility, etc.) (d) Speciation requires diversifying selection to act (e) None of the above 4. Many polyploids become diploids over time. Which of the following is NOT a contributing cause or mechanism of diploidization? (a) The key event is the switch from having four (or more) line up during meiosis to having two line up during meiosis (b) Diploidization occurs so that the organism could revert back to the more compact genome size and similar gene number prior to polyploidization (c) Some gene copies lose function due to random mutations (d) Some gene copies become novel genes due to random mutations (e) Some homologous gene copies become transposed to other parts of the genome and become new genes answers 1D 2B 3E 4B Concepts Geographic Models Allopatric Sympatric Reinforcement Problems with the concept of Species Species Biological Phylogenetic Phenetic (Morphological) Monophyly Title goes here 16

17 1. Which of the following is a species according to the biological species concept? (A) All hominin species (most are fossil species). (B) A population of bacteria for which 80% of their DNA sequences are identical. (C) All allopolyploid plants. (D) A group of beetles that can intermate and produce offspring for multiple generations. 2. Which of the following is NOT a reason that Species are difficult to define? (A) Many plants that are genetically divergent are able to mate (B) Many organisms that are morphologically similar are genetically distinct (C) Many organisms are asexual (D) Sometimes groups split off from within a monophyletic group (such as birds splitting off from the reptiles) (E) Sometimes sexual populations that are unable to interbreed could still be the same biological species Optional, based on book (don t have time to cover) 3. Which of the following is most likely to be a "species" according to the Phylogenetic Species Concept? (a) A population of bacteria that has a gene that allows glucose metabolism (b) Bird populations, which share a unique heritable feather structure (c) Spider populations that can interbreed and produce fertile offspring (d) Crustacean populations that form a clade (geneticallyrelated group), except for one population within the clade that colonized land and became insects (e) Populations of deer that share similar antler shape 4. Under which of the following scenarios is reinforcement most likely to evolve? (a) Different fish species, with each living in a separate pond (b) Two snail species, where each lives on opposite sides of a freeway (c) Different species of crickets living together in a park (d) Different insect species, each living on a different species of fruit in a forest (e) Different species of allopolyploid plants living in a field 5. Which of the following scenarios is likely to lead to the most rapid formation of new species? (a) Two populations become geographically separated, and there is continued migration between the populations (b) Two populations become geographically separated, and then new mutations arise in each population that become fixed due to genetic drift (c) Two populations that are in the same location diverge due to sexual selection for different traits in the two populations (d) Two populations become geographically separated, and then new mutations arise in each population that become fixed due to selection favoring different egg coat proteins in the different habitats (e) All of the above would on average lead to equivalent rates of speciation answers 1D 2E 3B 4C 5D Title goes here 17

18 Diploidization of Polyploids tends to occur over time Optional Slides on Diploidization While the overall gene content is sometimes reduced to one that is similar to the preduplicated ancestry, the overall architecture of the diploidized genome will be vastly different from the original pre-duplicated genome Almost all angiosperms are likely to be ancient polyploids that have undergone diploidization Diploidization of Polyploids tends to occur over time Diploidization: the evolutionary process whereby a polyploid species 'decays' to become a diploid (paleotetraploid) with twice as many distinct Diploidization For example, for a tetraploid, the key event is the switch from having four that form a quadrivalent at meiosis, to having two pairs of each of which forms a bivalent. In population-genetics terms, this is the switch from having four alleles at a single locus to having two alleles at each of two distinct loci This process of evolution probably happened in vertebrates (which includes humans) The key event is the switch from having four that form a quadrivalent at meiosis, to having two pairs of each of which forms a bivalent. Diploidization of Polyploids tends to occur over time Mechanisms of Diploidization: Differentiation or translocation so that the homologous loci are no longer homologous à end up with more genes Differential mutation of gene duplicates, leading to neofunctionalization or subfunctionalization Transposition events Chromosomal rearrangements, such as inversions in duplicated Mutations that lead to loss of function à gene loss, gene silencing In some gene families the duplicate copies will be retained Overall, end up with larger genome size after polyploidization followed by diploidization Diploidization Much of the process of diploidization is probably passive due to random mutations, drift causing differentiation And in some cases selection favoring a new gene, gene families, tranpositions, etc. Title goes here 18

19 Reproductive isolation could occur at many different levels Prezygotic (before the egg is fertilized) Genetic drift and divergence in bird song-won t mate Selection on coat color-don t recognize each other Postzygotic (after the egg is fertilized) DM incompatibilities cause embryo to not develop (enzymes don t work together) Prezygotic barriers Postzygotic barriers Gametic Isolation Reduced Hybrid ViabilityReduced Hybrid Fertility Hybrid Breakdown Fertilization Viable, fertile offspring Title goes here 19