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General Ecology: EEOB 404. Genetic Diversity and the Diversity of Life. Topics for this class: Introduction to Evolutionary Ecology Factors that create and erode genetic variability Importance of population size to genetic diversity Practical importance of genetic diversity to conservation.

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Genetic diversity and the diversity of life
Genetic Diversity and the Diversity of Life

Topics for this class:

  • Introduction to Evolutionary Ecology

  • Factors that create and erode genetic variability

  • Importance of population size to genetic diversity

  • Practical importance of genetic diversity to conservation


Intro to evolutionary ecology
Intro. To evolutionary ecology

  • Major question in Ecology: What determines distribution & abundance of species?

  • Two classes of answers

    • Contemporary, local factors (domain of traditional Ecology); e.g., physical factors (water depth) limiting hackberry more than bald cypress trees in bottomland hardwoods

    • Historical factors (= evolutionary ones)

      • These can be important: E.g., marsupial mammals like kangaroos limited to Australia because placental mammals mostly never made it there (plate tectonics)

      • Today’s class looks at some evolutionary factors influencing population genetics, and thus abundance--this is a relatively young, and vigorous field


Brief history of integration of genetics into ecological studies
Brief history of integration of Genetics into Ecological studies

  • Natural Selection—Darwin (1859) & Wallace (1859): Genetics???

  • Particulate genetics & inheritance—Gregor Mendel (1856-1864)

  • Mutations & chromosomes—Hugo Devries & others (1901)--sources of variation in populations; rediscovery of Mendel’s work

  • “The Modern Synthesis” (Dobzhansky, Wright, Fisher, Haldane, Mayr, Simpson--1930s & 1940s)

    • Integration Natural Selection & mutation; genetic drift; migration

    • Appreciation of genetic variation within populations in nature

  • DNA structure/importance elucidated by Watson & Crick (1953)

  • Much molecular variation in natural populations (Harris; Lewontin & Hubby 1966)--using starch gel electrophoresis

  • Synthesis of Ecology with Genetics --> Evolutionary Ecology & Conservation Biology (starting in 1970s)!


Main points of today s class
Main points of today’s class: studies

  • Success of a population or species over time is proportional to its genetic variation = genetic diversity

  • Net population genetic diversity is a function of the forces that create new variation, and those that erode it

  • Genetic diversity is closely tied to population size

  • These assertions (above) are hypotheses, well supported at present, but not “laws”, because exceptions, & complications are numerous


Factors that enhance or maintain genetic variation within a population
Factors that enhance or maintain genetic variation within a population

  • Mutation

  • Chromosomal rearrangements (e.g., deletion, duplication, inversion, translocation)

  • Introgression & migration (= gene flow)

  • Diversifying natural selection (selection against the mean phenotype)

  • Natural selection acting on a population in heterogeneous environments-->ecotypic variation

  • Natural selection favoring heterozygote (= heterozygote superiority); e.g., sickle-cell anemia

  • Thus, large populations, spread over different environments tend to be genetically diverse


Example introgression
Example: introgression population

  • Bill depth variability of Isla Daphne Major Geospiza fortis Darwin’s finches is increased

  • Cause is introgression of G. fuliginosa genes, via hybridization of immigrant G. fuliginosa birds from Santa Cruz mating on Daphne Major with G. fortis population there

  • Data from P.R. Grant, 1986. Ecology and Evolution of Darwin’s Finches. Princeton University Press.


What do we mean by genetic variation
What do we mean by genetic variation? population

  • Range (variance) of phenotypes, as in Darwin’s Finch example on previous slide

  • Different chromosomal arrangements (cytogenetics)

  • DNA sequence differences among individuals

  • Electrophoresis--> electromorphs = allozymes

  • Indices of within-population variability

    • Heterozygosity = proportion of individuals that are heterozygotes, averaged across all genetic loci

    • Polymorphism = proportion of loci within a population that are polymorphic (with two or more alleles, and most frequent is <95% of total alleles)



Examples of heterozygosity polymophism
Examples of Heterozygosity, Polymophism population

  • In the starch gel on previous slide, 8 of 20 individuals at this particular locus (i.e., one enzyme or protein gene product, at one locus) are heterozygotes. Thus heterozygosity = 8/20 =40%. This is a poor estimate for the population, however…why?

  • In text, 30 percent of loci in Drosophila fruit flies and humans are variable (more than one allele). Thus polymorphism = 30%.


Factors that erode genetic variation
Factors that erode genetic variation population

  • Stabilizing, directional natural selection

  • Random (chance) loss of alleles, increasingly in small populations

    • Founder effect--> genetic bottleneck (one or a few generations)

    • Genetic drift, over multiple generations, leads to chance loss or fixation of alleles because some individuals don’t mate, some alleles don’t make it into successful gametes

  • Inbreeding = breeding by genetically related individuals




Genetic variability depends on population size
Genetic variability depends on population size population

  • Genetic drift erodes variability--in small populations

  • Inbreeding depression (i.e., reduced reproductive success in inbred populations) worst in small populations

    • E.g., captive-bred mammals

    • Dim-wittedness, & other genetic defects in reproductively isolated human populations

    • Greater prairie chicken example (below)

  • Large populations favor maintenance & spread of genetic variability (see factors that maintain variation)


Reproductiveproblems in greater prairie chickens alleviated by translocation of new (non-Illinois) individuals into inbred Illinois population in 1992 (from Westemeier et al. 1998. Tracing the long-term decline and recovery of an isolated population. Science 282: 1695-1698)


Practical application of these findings conservation biology
Practical application of these findings: Conservation Biology

  • Smaller population sizes tend to be most at risk, thus to go extinct (e.g., desert big-horned sheep)

  • “50/500” rule-of-thumb in conservation biology:

    • At least 50 individuals needed in population to avoid inbreeding problems

    • At least 500 individuals needed to avoid problems of genetic drift

    • Endangered species generally exhibit low genetic variability

  • Low level of migration (or deliberate translocation--> outbreeding) can mitigate genetic problems (e.g., greater prairie chicken; see also Fig. 2.11, text)

  • Low genetic variability also tends to inhibit evolutionary response to changing environments-->increased extinction risk



Conclusions
Conclusions: Sheep

  • Ecological questions (e.g., reproductive success, survival, population size, population persistence) are addressed by evolutionary and genetic approaches

  • Ecological success is related to genetic variability

    • Genetic variability tends to be lost in small populations

    • Viability reduced in small populations

  • Conservation Biology is the relatively recent, and applied field that uses these insights (among others) to help protect threatened, small (and isolated) populations


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