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Lecture 6

Lecture 6

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Western University
Biology 3466B
Yolanda Morbey

Handout 6: Speciation • Defining the indefinable… a species should o Reflect morphological distinctiveness o Reflect evolutionary independence o Be objective o Be testable o Be applicable to sexual and asexual forms, cryptic species, fossils, etc. o Make identification easy • 3 species concept o Morphospecies: populations that are designed as separate species based on morphological differences (structural form, or physical phenotype)  Species can be determined that are extinct or living and in species that reproduce sexually or asexually. However it is not applied carefully. o Biological species concept: identifying evolutionary independence is reproductive isolation. It confirms a lack of gene flow. However it is hard to apply. If a species is separated by environment it is hard to tell is they are reproductively isolated. o Phylogenetic species concept: focuses on a criterion for identifying species called monophyly. Species are identified by estimating the phylogeny of closely related populations and finding the smallest monophyletic groups. Populations must be evolutionary independent enough for the diagnostic traits to have evolved. • process of speciation o tree-stage process: initial step tha isolates the populations, second step tha results in divergence in traits such as mating or habitat, and a final step that produces reproductive iolation. o Secondary contact: when a species have fully diverged and become its on species and then come back into contact with the other old species that it once was. • Allopatric model: meaning different country or homeland. • Dispersal o Physical isolation creates an effective barrier to gene flow. Geographic isolation produces reproductive isolation, and thus genetic isolation. o The dispersal and colonization hypothesis makes 2 predictions  Closely related species should be found on adjacent islands  At least some sequences of branching events should correspond to the sequence in which islands were formed. • Vicariance o Encroaching physical feature such as a river, glacier, lava flow, or new habitat • Reproductive isolating barrier: genetic incompatibility o Changes in chromosome number isolates populations genetically • Prezygotic barriers: reproduction isolation between populations caused by difference in mate choice or timing of breeding, no hybrid zygotes are forms • Postzygotic barriers: reproductive isolation between populations caused by dysfunctional development or sterility in hybrid forms • Divergence by natural selection o Marked genetic differences have to emerge between closely related populations for speciation to occur. o Natural selection can lead to divergence if one of the populations occupies a novel environment or uses novel resources. o Apple and hawthorn flies have diverged because of natural selection on preferences fro a crucial resource: food 9/25/2012 o Apple fruits arrived and caused flies to parasitize in them. Are the flies that parasitize apple fruits and 9/25/2012 hawthorn fruits distinct populations? Natural selection based on food could have made them so. Or they are members of the same population. Hawthorn larvae experience cooler temps to the apple larvae. o The hawthorn flies were collected. The graph shows that the hawthorn race individuals that were exposed to a month of warm days a pupae, and had survived to adulthood, had enzyme allele frequency Divergence by sexual selection similar to those found in the apple fly. This shows that the hawthorn fly had alleles that enabled them to • selecselection exp. onrences among indsurvive in the warm temperatures
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