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

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

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: 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

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 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?

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

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

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

Effects of genetic drift on population variation

Inbreeding Depression in Captive Mammals

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)

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

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: 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