Lecture Outline: The Origin Of Species

1. Introduction to Speciation (Origin of Species)
   1. Speciation is the process of a species being created
   2. Speciation is represented by a node (branch point) on a phylogenetic tree, where one lineage splits into two new lineages
2. Species Concepts
   1. There is not a single universal definition of species; multiple concepts are used simultaneously
   2. Problems with defining species based purely on appearance:
      1. Similarity between different species (separate species may look alike)
      2. Diversity within a species (e.g., humans look different but are one species)
   3. Biological Species Concept
      1. Definition: A group of individuals that reproduce with each other and produce fertile offspring
      2. Shortcoming: This concept cannot be applied universally because it only works for sexually reproducing organisms, and not for asexual organisms (like prokaryotes and some eukaryotes)
   4. Other Species Concepts
      1. Morphological Species Concept: Based on how organisms look (shape or form, or morphology)
      2. Phylogenetic Species Concept
         1. Definition: A species is a terminal lineage on a phylogenetic tree (the smallest group that shares a common ancestor)
         2. This concept is inclusive of asexually reproducing species
      3. Ecological Species Concept: Defines a species based on its niche (the set of conditions under which it lives)
3. Reproductive Barriers: Mechanisms that Define Separate Species
   1. Barriers according to the Biological Species Concept violate the requirements of successful sexual reproduction
      1. A zygote is the single cell produced when two gametes fuse (fertilization)
      2. Barriers are categorized as pre-zygotic (before fertilization) or post-zygotic (after fertilization)
   2. Pre-zygotic Barriers (Prevent successful fertilization or zygote production)
      1. Habitat Isolation: Separation in space (individuals live in different habitats and never meet)
      2. Temporal Isolation: Separation in time (individuals live in the same space but have different reproductive seasons)
      3. Behavioral Isolation: Individuals live together and are active at the same time but choose not to mate (e.g., based on different behaviors)
      4. Mechanical Isolation: Physical incompatibility (reproductive structures, or genitals, do not match up, e.g., certain snails)
      5. Gametic Isolation: Gametes (sex cells like sperm and eggs) are chemically incompatible and reject fusion (e.g., sea urchins)
   3. Post-zygotic Barriers (Zygote is produced, but fertile offspring are not)
      1. Reduced Hybrid Viability: Hybrids (offspring of two different species) are produced but rarely survive long enough to reproduce (most die early)
      2. Reduced Hybrid Fertility: Hybrids survive but are sterile (e.g., mules, which are the offspring of a horse and a donkey)
      3. Hybrid Breakdown: Hybrids may be viable and fertile initially, but only for a few generations, after which fertility breaks down
   4. Hybrid Example: Growler Bear
      1. Hybrid offspring of polar and grizzly bears (two separate species)
      2. The existence of the hybrid demonstrates a shortcoming of the limited biological species concept
4. Speciation Mechanisms
   1. Allopatric Speciation
      1. Definition: Speciation that occurs when there is spatial isolation (allo means different)
      2. This is the more common way speciation occurs in nature
      3. Geographical separation leads to different evolutionary paths because natural selection operates differently in the two separated subgroups
      4. Examples of Allopatric Speciation
         1. Fish populations separated by dropping water levels (high predation favors sprinters; low predation favors marathoners)
         2. Snapping shrimp separated by the growth of the Isthmus of Panama, creating 15 sister species pairs separated into Pacific and Atlantic Oceans
         3. Experimental Speciation in Fruit Flies (Drosophila)
            1. Initial population was separated and raised on different media (starch versus maltose) for 40 generations
            2. When reunited, starch-fed flies preferred mating with other starch-fed flies, and maltose-fed flies preferred maltose-fed flies, demonstrating reproductive isolation due to separation and differential selection
   2. Sympatric Speciation
      1. Definition: Speciation that occurs even though the new species are still living together (sym means same)
      2. Sympatric speciation is rarer than allopatric speciation
      3. Mechanisms of Sympatric Speciation
         1. Polyploidy
            1. Happens much more easily and frequently in plants than animals
            2. Ploidy refers to the number of each kind of chromosome (e.g., haploid, diploid)
            3. Involves meiotic nondisjunction (failure of homologous chromosomes to properly split apart)
            4. Autopolyploidy: New species created immediately in one generation from a single parent producing abnormal, yet compatible, gametes
            5. Allopolyploidy: New species created from the fusion of abnormal, compatible gametes from two different parents
         2. Habitat Differentiation
            1. Subsets of the original group specialize on different parts of the same habitat
            2. Example: Apple maggot flies originally utilized Hawthorne fruit, but when apples were introduced, the apples matured earlier in the year
            3. Flies specializing on apples evolved to reproduce earlier, leading to temporal isolation even though they lived in the same geographic area
         3. Sexual Selection: Individuals select mates based on appearance or behavior
5. Hybrid Zones and Outcomes
   1. A hybrid zone is a geographic area where the ranges of two species overlap and where hybrids are likely to be found
      1. Example: Yellow-bellied toad and fire-bellied toad ranges overlap, leading to a zone where both parental alleles appear
   2. Possible Outcomes when previously separated populations encounter each other (Secondary Contact)
      1. Reinforcement: The hybrid zone diminishes; sexual incompatibility is reinforced, leading to continued divergence of the two species
      2. Fusion: Reproductive barriers break down, and the two groups fuse back into one species (e.g., fish in Lake Victoria due to pollution-caused light interference affecting sexual selection)
      3. Stability: The hybrid zone is stable; some hybrids are always made, but the two species remain separate without further divergence
6. Tempo of Speciation
   1. Punctuated Model: Divergence happens rapidly in a short amount of time, followed by long periods where traits remain relatively static
   2. Gradual Model: Divergence happens gradually and steadily over a long period of time
   3. Both models can occur in nature
7. Coevolution and Speciation
   1. Evolution and speciation are affected by interactions with all other surrounding species; nothing evolves in isolation
   2. Experimental Example: Flower Color in two species of the genus Louisi (Louisi and Cardinalis)
      1. These are insect-pollinated flowers, meaning their evolution is strongly affected by animal pollinators
      2. Genetic manipulation to transfer color genes between species resulted in a shift in pollinator preference