Variation

Phenotypes, Chromosomes, Proteins, DNA

Variation is essential for adaptive evolutionary change

Natural selection cant operate without phenotypic differences

Most studies either measure and compare variation or try to link variation to fitness (reproductive success and survivorship)

Ways variation has been measured

Phenotypes

Mendel: 7 traits with discrete variation (yellow vs. green) rather than continuous (height)
Do phenotypic differences have a genetic basis?
Use crosses and frequencies of phenotypes to determine genetic basis
Doesn’t really account for the environment - can be attributed to genetics when you control the environment

Behavior (considered a phenotypic trait)

Genetically based differences in behavior
- local adaptation
- adaptation to captivity
Ex. migratory behavior in Blackcaps
- tendency to migrate and direction to migrate have a genetic basis

Morphology

individuals vary in size, shape, number of body parts
problem using morphological traits to study patterns of genetic variation
- variation caused by genetic and environmental differences
- need to partition total phenotypic variation caused by genetic and environmental differences

Differences among populations

is difference due to genetic variation or environment?
common garden experiment: eliminate the effect of environment by raising individuals in identical environments
ex. altitudinal forms of yarrow
- genetic basis to differences

Chromosomes and Proteins

genetic basis to phenotypic variation
- examine differences among individuals using chromosomes and proteins
- reflects historical sequence of study
- these are old tools

Chromosomes

little emphasis on chromosome variation
can reduce fertility/survivorship is you cross individuals with a different complement of chromosomes
some captive breeding programs hybridized individuals that were morphologically similar but had a different number of chromosomes
- e.g. orangutans, gazelles, dik-dik
may also be a problem in translocations and reintroductions if individuals are translocated among chromosomally distinct groups
e.g. graceful tarplant
- morphologically similar and live in similar habitat
- size and shap of 4 chromosomes differenced among pops
- experimentals crossings: failed to produce F1 or F1 was sterile

Quanitfying chromosome variation

number
size: largest to smallest
centromere location (middle or towards the end)
banding pattern - some regions of DNA condense more than others forming dark bands

Number of chromosomes

evolves slowly in some taxa and quickly in others
- ~100 cetacean species all with 2n = 42 or 44
- Equus: 2n varies between 32 and 66
polyploidy
- most animals are 2n (except eggs and sperm = 1n)
- some spp polyploid: more than 2 sets of chromosomes, usually fish, amphibs, lizards, plants (3n to 6n+)

Gray Tree frogs

Proteins

another method to measure variation
- major advance
- direction relationship between genes (DNA base pair sequence) and proteins (amino acid sequence)
- Allozymes = variant enzymes produced by different alleles at a locus (enzymes are proteins)
Variation in proteins
- electrophoresis: separates proteins (enzymes) based on net change and molecular weight
  - proteins negatively charged
  - Gel: bands are revealed by straining the gel with the substrate of the protein being screened. if the protein reacts with the substrate, the band appears on the gel
- disadvantages:
  - proteins can be specific to tissue
  - proteins degrade is not stored properly
  - needs a lot of tissue
  - most tissue taken invasively
  - often reveals little variation
- Advantages:
  - not technically difficult - no markers needed
  - only game in town before DNA techniques

Measuring Variation

Types of DNA

Nuclear - biparental inheritance (one copy/cell)
Organelle - uniparental inheritance
- Mitochondrial (mtDNA) - 1000s of copies/cell
- Chloroplast (cpDNA) - 1000s of copies/cell

mtDNA

First studies of DNA genetic variation used mtDNA in animals
characteristics:
- small size, conserved arrangement of genes
- variable: less stringent repair of errors during replication so mutations accumulate
- no recombination and maternal inheritance in animals so inherited as a single haplotype: lineages can be tracked over time and space easily - often used in phylogenetic studies
- b/c haploid and uniparentally inherited it represents ¼ the pop size of diploid nuclear DNA - sensitive to bottlenecks

Chloroplast DNA

mtDNA not very variable in plants so cpDNA generally used instead when haploid markers are required
inherited uniparentally: maternal in flowering plants, paternal in conifers/cycads
slower mutation rate compared to plant nuclear genes

Uniparental DNA: pros

mtDNA, cpDNA, and Y chromosome genes
can follow transition of maternal/paternal genotypes thru time
can look at differences in disperal
- distribution of mtDNA (maternally inherited) reflects patterns of seed dispersal but not pollen dispersal
- can identify hybrids

Dispersal in black spruce

grow in areas covered by ice 6000 years ago
new pops have little mtDNA variation, lots of nuclear variation
mtDNA dispersed by seeds, dont travel far: only a few established new pops
- nuclear genes can disperse widely bc pollen wind-dispersed
- pollen that blew to the new pops likely originated from several places so high nuclear genetic variation
- if you looked at just mtDNA or just nuclear DNA, would get an incomplete or misleading picture of dispersal

Hybrids

contain a mix of alleles from both parental species
hybrids often have cytoplasmic markers (mt or cp) from one species or pop and nuclear markers from both
known as cytonuclear disequilibrium

Uniparental DNA: cons

behave as single inherited units so effectively a single locus: may not reflect the true history of a species
not representative of pops as a whole, e.g. dispersal and mtDNA
- if males disperse and females dont, the mtDNA haplotypes would be pop specific and you may conclude that individuals never move among pops
better to use a mix of markers