1. Genetic consequences of small population size Chapter 4
Lecture 7
Genetic consequences of small
population size
Chapter 4

2. 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

3. inbreeding coefficient (F )
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

4. 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

5. 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

6. Elephant seals
If a harem has one male and 100 females, the effective size is
Ne ∼ 4 Nef*Nem/(Nef + Nem)
= 4 × 100 × 1/
= 3.96
Thus the effective size of the harem is 3.96, approximately 4% of the actual size of 101.

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

8. 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

10. 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

11. Resolving taxonomic uncertainties ESU’s and MU’s

12. 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)
The taxonomic status of many species are unknown, this includes plants animals of lower order and higher order

13. 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
Now we have been talking a lot about species, their evolution, genetic variation, how they go extinct and how they behave in populations etc,
yet what do you guys know what a species is exactly?
What exactly does people use to define a new species? Or at least what criteria do they use?
Well most species we see around is based on morphological features, how many of this colour of feathers , length, etc....

14. The Biological Species Concept Ernst 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
Amidst all the confusion one person gave a possible answer, Do you know who the man is?
This friendly old chap is Ernst Mayr and in 1942 he proposed the Biological species concept.
It reads...

15. Allopatric speciation