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General Ecology: EEOB 404

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.

elijah-contreras
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General Ecology: EEOB 404

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  1. General Ecology: EEOB 404

  2. 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

  3. 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

  4. 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)!

  5. Main points of today’s class: • 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

  6. 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

  7. Example: introgression • 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.

  8. What do we mean by genetic variation? • 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)

  9. Starch gel electrophoresis

  10. Examples of Heterozygosity, Polymophism • 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%.

  11. Factors that erode genetic variation • 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

  12. Effects of genetic drift on population variation

  13. Inbreeding Depression in Captive Mammals

  14. Genetic variability depends on population size • 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)

  15. 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)

  16. 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

  17. Example: Population Size and Extinction Risk in Bighorn Sheep

  18. Conclusions: • 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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