Conservation Genetics: Lessons from

Population & Evolutionary Genetics

I. Definition

Conservation Genetics:

The science of understanding how genetic issues affect the conservation and restoration of populations and species.

II. Major Issues (from Frankham 1995)

-Inbreeding depression-Accumulation of deleterious alleles-Loss of genetic variance in small populations-Genetic adaptation to captivity and effect on reintroduction success-Fragmentation of populations-Taxonomic uncertainty (unique?, novel?, hybrid?, hybridize for successful reintroduction?)

smallpopulationsize

III. Taxonomic UncertaintyExample: Dusky Sea Side Sparrow (Ammodramus maritimus nigrescens)

Avise and Nelson 1989

IV. Small Population Size

-Most threatened/endangered species exist in Small Isolated Populations

Must focus on consequences of small population size

Gaston et al. 1997 (ECOGRAPHY)

Newton 1997 (ECOGRAPHY)

Genetic Consequences of Small Population Size:

-Loss of Genetic Variation-Inbreeding Depression-Accumulation of Mutations

All as a result of Drift and Fragmentation

V. Drift

History: Natural historians, including Darwin, noted that some variation among individuals would not result in differences in survivorship and reproduction

e.g., Gulick, Hawaiian land snails exhibited great diversity

of shell color patterns

Changes in pattern across generations arises by chance

Drift (population genetic translation- Wright):

Evolutionary process by which allele frequencies changeby accidents of sampling

VI. Origin of Accidents of Sampling

Assume diploid population with 2 alleles at a locus

A with frequency pa with frequency q

Zygote = union of 2 independent gametes or union of 2 independent events

Thus genotype frequencies represent binomial probabilitydistribution:

(p + q)2 or AA= p2, Aa = 2pq, aa = q2

Assume: finite population size (N)

Zygotes are a sample of gametes:

A or a with frequency p and q

Thus random sampling process will introduce variationof allele frequencies across gernation of

Variance of binomial: pq/N

Diploid organisms: pq/2N

Loss of Heterozygosity is proportional to 1/2N or 1/2Ne

(Population Geneticists use Ne because loss of

heterozygosity is often greater than the census number)

Effect of sampling variation after many generations

Change in allele frequencey of Drosophila melanogaster populations

VII. Consequences of Drift:

-allele frequencies fluctuate randomly-populations vary by chance-increase variation among populations-decreased heterozygosity in populations-increased homozygosity in populations-increased genetic relatedness in population-SELECTION NOT AS EFFICIENT

NeS < ¼ then deleterious alleles and new deleterious

mutations will become fixed by drift (more later)

VII. Consequences of Fragmentation

A. Wahlund Effect:

All of the same consequences as Drift

decreases heterozygosity within populationsincreases homozygosity within populationsincreases genetic relatedness within populations

Natural History Examples of Fragmentation(From Hamrick and Godt)

# of P Gst species (within population) (among pop)

pollen dispersal animal 164 36 0.2 wind 102 50 0.1

seed dispersal gravity 199 30 0.3 wind 105 43 0.1

P = % of loci with > 2 allelesGst = proportion of genetic variation distributed among pop.

FRAGMENTATION LOSS OF GENETIC DIVERSITY WITHIN POPULATIONS

B. Further consequences of Fragmentation

Allee Effect: As density decreases, ability to find mates also decreases

e.g. Oostemeiger, Arnica montana, Netherlands

Visitation rates in small and large populations:

Small Large Large High Density Low Density

IX. Consequences of Inbreeding

A. Inbreeding depression

Low High Heterozygosity

Lo

w

H

igh

Ext

inct

ion

Rat

e

B. Loss of Genetic Variation

Lakeside Daisey (hymenoxys acaulis var. glabra)

Last remaining population in IllinoisLakeside Daisey is Self Incompatible

M. Demauro, 1994

Number of Mating Groups

Selection of D. melanogaster for resistance to ethanol fumes in Large vs. Small populations

Generation

Res

ista

nce

(m

inu

tes)

Weber, 1992

L = LargeS = Small

Consider response to global climate change!

C. Mutation Accumulation NeS < ¼

1. Fixation of ancestral mutations (From Lynch and Burger, 1995)

2. Introduction of new mutations

3. Extinction Risks Due to Mutational Meltdown

R = Reproductive Rate; K = Carrying Capacity

Consequences of Mutations for Small PopulationsCritically Depend on:

Mutation Rate

Distribution of Mutation Effects (all deleterious?)

X. Genetic Manipulation to Counteract Small Population Size

A. Purging of “bad” mutations

Natural History Examples:

Husband and Schemske, 1996

Drift led to both thefixation and extinctionof deleterious alleles

Purging critically depends on genetic basis ofinbreeding depression:

Inbreeding depression: expression of recessive deleterious alleles in homozygous condition

Dudash and Carr, 1998

Inbreeding depression due to recessive alleles

B. Crossing Programs to Restore Genetic Variability

Case Study: Fenster and Colleagues

Chamaecrista fasciculata

XI. Conclusion

Small population size may lead to lower genetic fitnessthrough fixation of deleterious alleles

XII. Future Directions

We Need:

-Better estimates of mutation rates and effects-Field based experiments to determine if a population can be purged of deleterious mutations-Studies to quantify effect of adaptation to captivity-Better understanding of the genetic basis of adaptive differentiation