Transcript General Ecology: EEOB 404

General Ecology:
EEOB 404
Genetic Diversity and
the Diversity of Life
Topics for this class:
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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
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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
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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:
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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
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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
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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?
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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)
Starch gel electrophoresis
Examples of Heterozygosity,
Polymophism
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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
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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
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Inbreeding = breeding by genetically related
individuals
Effects of genetic drift on
population variation
Inbreeding Depression in
Captive Mammals
Genetic variability depends
on population size
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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)
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Large populations favor maintenance & spread of
genetic variability (see factors that maintain
variation)
Reproductive
problems in
greater prairie
chickens
alleviated by
translocation
of new (nonIllinois)
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
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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
Example: Population Size and
Extinction Risk in Bighorn Sheep
Conclusions:
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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
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Conservation Biology is the relatively recent, and
applied field that uses these insights (among others)
to help protect threatened, small (and isolated)
populations