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Ryerson University
Biological Sciences
Mark Fitzpatrick

BIOA01H3 – Lecture 32 nd Chapter 19 19.2 Definition of Species  According to Stanford Encyclopedia of Philosophy, “…The Biological Species Concept defines a species as a group of organisms that can successfully interbreed and produce fertile offspring. The Phylogenetic Species Concept (which itself has multiple versions) defines a species as a group of organisms bound by a unique ancestry. The Ecological Species Concept defines a species as a group of organisms that share a distinct ecological niche…”  Several problems w/ biological definition; one major problem is that many species reproduce asexually; patterns of reproduction can blur definition of species  Androdioecy (andro = male; dioecioy = male and female) and gynogenetic (gyno = female) species are two variations in patterns in reproduction  Androdioecous organisms exist as natural populations of functional males and hermaphrodites but include no true females i.e. mangrove rivulus and clam shrimps  Gynogenetic species have only females i.e. amazon molly. Use only internal fertilization, eggs require mechanical stimulation by sperm to initiate development; must seduce & mate w/ males of other species to obtain sperm  Definition doesn’t apply to hybridization: when two species interbreed and produce fertile offspring i.e. 10% of ~8000 bird species hybridize naturally and produce fertile offspring 19.3 One Size Does Not Fit All  Our concepts of species appear to be applicable to static situations  Diversity of species & lifestyles partly reflects mechanisms underlying processes of speciation  Biological species concept applies to population genetics and evolutionary theory in static world  Definition alludes to genetic cohesiveness of species  Populations of same species said to experience gene flow, mixing genetic material and could be the “glue” that holds species together  Second part of concept emphasizes genetic distinctness of each species; b/c populations of diff. species are reproductively isolated, cannot exchange genetic info. Process of speciation frequently defined as evolution of reproductive isolation between populations  Biological species concept could explain similarities in appearance of individuals of a species; if phenotypes reflect genotypes, members of same gene pool should share same genetic traits (genotype) that determines phenotype 1  Biologists often use similarities or differences in morphology as markers of genetic similarity or reproductive isolation; describe new species and use visible morphological traits to distinguish new species from those previously known  This approach based on morphological species concept  idea that all individuals of a species share measurable traits that distinguish them from individuals of other species 19.4 Gene Flow: Four Examples  Ecology, habitat availability, and behavior can affect distribution patterns and genetic differences 19.5 Geographic Variation Subspecies: geographically separated populations of a species exhibiting dramatic, easily recognized phenotypic variation. o Local variants of a species  Individuals from different subspecies usually interbreed where geographic distributions meet, and offspring often exhibit intermediate phenotypes  Zoologists sometimes use race as shorthand term for subspecies  Botanists use variants or, for domesticated stock, cultivars  Various patterns of geographic variation have provided insight into speciation process: i.e. ring species and clinal variation 19.5a Ring Species: Genes Flow between Some Populations  Some plant/animal species have ring-shaped geographic distribution that surrounds uninhabitable terrain  Adjacent populations of these ring species can exchange genetic material directly, but gene flow between distant populations occurs only through intermediary populations  i.e. lungless salamander Ensantina eschscholtzi, widely distributed in coastal mountains and Sierra Nevada of California but cannot survive in hot, dry Central Valley 19.5b Clinal Variation Is Change along a Gradient  when species distributed over large, environmentally diverse area, some traits may exhibit a cline  pattern of smooth variation along geographic gradient  clinal variation usually results from gene flow between adjacent populations that are each adapting to slightly different conditions o i.e. many species of birds and mammals in northern hemisphere show clinal variation in body size and relative length of appendages 2  when cline extends over large geographic gradient, populations at extremes may be very different  most species are morphologically or behaviourally distinct from other species but local and geographic variation can mask interspecific (between species) differences 19.6 Reproductive Isolation Reproductive isolating mechanism: a biological characteristic that prevents the gene pools of two species from mixing even when they’re sympatric (occupying the same spaces at the same time).  can be achieved in two basic ways: prezygotic isolating mechanism and postzygotic isolating mechanisms Prezygotic isolating mechanisms: exert their effects before the production of a zygote (fertilized egg). Postzygotic isolating mechanisms: operating after zygote formation.  These two isolating mechanisms are not mutually exclusive, can operate simultaneously 19.6a Prezygotic Isolating Mechanisms Isolate before Fertilization  At least five mechanisms that prevent interspecific mating or fertilizations & the production of hybrids offspring; they’re the following: ecological, temporal, behavioural, mechanical, and gametic isolation  Species living in same geographic areas but different habitats may experience ecological isolation o i.e. lions and tigers both common in India until mass extermination of lions; since lions live in open grasslands and tigers in dense forests, two species don’t encounter one another and didn’t interbreed  species living in same habitat but mate a different times of day or years can experience temporal isolation o i.e. fruit flies Drosophila persimilis and D. pseudoobscura overall extensively geographically but b/c D. persimilis mates in the mornings and D. pseudoobscura in the afternoon, they don’t interbreed  many species rely on specific signals, which differs dramatically between species, to identify potential mates  behavioral isolation occurs when signals used by one species aren’t recognized by another. o i.e. female songbirds often rely on the song, colour, and displays of males to identify members of their own species  mechanical isolation results when differences in structure of copulatory organs prevent successful mating between individuals of different species o i.e. many plants have anatomical features that allow only certain pollinators to collect and distribute pollen 3  even if individuals of different species do mate gametic isolation may prevent fertilization o i.e. many marine invertebrates release gametes into the environment for external fertilization o sperm and eggs recognize one another’s complementary surface proteins 19.6b Postzygotic Isolating Mechanisms Are Barriers after Fertilization  despite existence of prezygotic isolating mechanisms, sperm from one species sometimes may fertilize an egg of another species  in such cases, two species are reproductively isolated if their offspring, called interspecific hybrids, have lower fitness than those produced by intraspecific matings  three postzygotic isolating mechanisms can reduce fitness of hybrid individuals: hybrid inviability, hybrid sterility, and hybrid break-down  Hybrid inviability can occur b/c many genes govern the complex processes that transform a zygote into a mature organism o have two sets of developmental instructions, one from each parent, which mayn’t interact properly for successful completion of embryonic development  hybrid organisms frequently die before reaching term  some hybrids develop into healthy and vigorous adults, they mayn’t produce functional gametes  hybrid sterility o results when parent species differ in number of structure of chromosomes, which cannot pair properly dur
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