General Ecology: EEOB 404

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)
• Todays 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 SelectionDarwin (1859) Wallace (1859)
  Genetics???
• Particulate genetics inheritanceGregor Mendel
  (1856-1864)
• Mutations chromosomesHugo Devries others
  (1901)--sources of variation in populations
  rediscovery of Mendels 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 todays 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 Darwins 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 Darwins Finches. Princeton
  University Press.

8
What do we mean by genetic variation?

• Range (variance) of phenotypes, as in Darwins
  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
  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,
  howeverwhy?
• 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 dont mate, some alleles dont
  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