Lecture 7

Genetic consequences of small

population size

Chapter 4

• Small, isolated populations suffer accelerated inbreeding

• and loss of genetic diversity leading to reduced reproductive fitness (inbreeding depression)

• reduced ability to evolve in response to environmental change

• Some species only in captivity– Przewalski’s horse (reduced to 12)

– Reintroduced into the wild

• Inbreeding is unavoidable in small populations and leads to reductions in reproduction and survival

• Reduced population size also has acute effects resulting from inbreeding

• Loss of reproductive fitness as a consequence of inbreeding is referred to as inbreeding depression

• How do we measure this?• inbreeding coefficient (F ) • The inbreeding coefficient of an individual (F )

is the probability that it carries alleles at a locus that are identical by descent

Idealized population are:

• the number of breeding individuals is constant in all generations• generations are distinct and do not overlap there is no migration or gene flow• all individuals are potential breeders• all individuals are hermaphrodites• union of gametes is random, including the possibility of selfing• there is no selection at any stage of the life cycle• mutation is ignored• the number of offspring per adult averages 1, and has a variance of 1

Effective population size (Ne)

• Genetic processes in small populations depend on the effective population size rather than on the number of individuals

• The Ne of a population is a measure of its genetic behaviour, relative to that of an ideal population

• Effective population size can be estimated from demographic data on sex-ratio, variance in family sizes and fluctuations in population size over generations, or from genetic data

Elephant seals• If a harem has one male and 100 females, the

effective size is• Ne 4 ∼ Nef*Nem/(Nef + Nem)• = 4 × 100 × 1/100 + 1• = 3.96• Thus the effective size of the harem is 3.96,

approximately 4% of the actual size of 101.

• Calculate effective population size of a troop of gorillas (N = 15); 2 males, harem - 7 females; harem - 6 females

• Ne = ?

Fragmentation• The impacts of population fragmentation on

genetic diversity, differentiation, inbreeding and extinction risk depend on the level of gene flow among fragments

- number of population fragments- distribution of population sizes in the fragments geographic distribution of populations- distance among fragments- dispersal ability of the species- environment of the matrix among the fragments - and its impact on dispersal time since fragmentation- extinction and re-colonization rates across fragments

Measuring population fragmentation: F statistics

• The degree of differentiation among fragments can be described by partitioning the overall inbreeding into components within and among populations (F statistics)

• directly related to the inbreeding coefficients• Fit – inbreeding in total population• Fis – inbreeding relative to other populations• Fst – inbreeding due to differentiation

Resolving taxonomic uncertaintiesESU’s and MU’s

What is the importance?

• Many erroneous decisions may result if the taxonomic status is not correctly assigned– unrecognized endangered species may be allowed to become extinct– endangered species may be denied legal protection while populations– of common species, or hybrids between species, may be granted

protection– incorrectly diagnosed species may be hybridized with other species,

resulting in reduced reproductive fitness– resources may be wasted on abundant species, or hybrid populations– populations that could be used to improve the fitness of inbred

populations may be overlooked

• Results from inaccurate data!!! • Important !!! Read paper (Culver et al. 2000)

What is a species?

• The truth is there is no universal species concept• To date at least 22 different species concepts

exists– Ecology, morphology, genetics, evolutionary history– biological characteristics, phylogeny, even sexes!!– evolutionary units and geneflow

The Biological Species ConceptErnst Mayr 1942

• “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”

• It places the taxonomy of natural species within the conceptual scheme of population genetics

• For example, a community of interbreeding organisms is, in population genetic terms, a gene pool (total aggregate of genes in a population)

• does not deal adequately with asexual and habitually inbreeding forms, or hybridization and fossil specimens not covered

Allopatric speciation