The Genetic Basis for Evolution

Lecture 2

Genotype

The information stored in the genes of an individual. DNA transcribed via RNA to protein.

DNA

Deoxyribonucleic acid. Most is organised within chromosomes within the cells. Genomes are very variable but little is known about the reason why..

Chromosome variation example

Muntjacus reevsii (64 chr) and Muntjacus muntjac (6 or 8 chr) are the same genus.

DNA variation

within an individual DNA is similar in most cells but gene expression differs greatly depending on the tissue. Some DNA regulates this rather than coding protein.

Introns

Other DNA is apparently useless and repetitive (non-coding DNA, satellite DNA and helps with DNA fingerprinting).

Introns example

Nobrega et al (2004) deleted 1243 sequences of mouse non-coding DNA without measurable effects.

Alleles

… are alternative genes which occupy a locus within the genome.

Loci

Locations of the alleles and are often multiallelic.

Genes interact

Makes it hard to track down a specific gene which controls what you are monitoring so we assume they play a role but don’t look at exactly what role that is.

Polygenic

Most traits are controlled by many genes

Pleiotrophic

Some genes influence many traits.

Epistasis

One allele may influence the expression of another. E.g. sex related traits.

Epistasis example

Plumage colouration (females can pass on this gene even though they won’t express it).

Phenotype

The observable properties of an organism. Connection between genes and phenotype is not always clear and environmental factors play a part too.

Nature VS Nurture

Phenotypic variation has genetic and environmental components.

Nature VS nurture equation

Phenotypic variation = Genetic variation + Environmental variation

Genetic variation can also be broken down by…

Additive genetic variance (resemblance to parents), effects due to dominance and gene interactions.

Genetic variation equation

= Additive genetic variation + Dominance affect + gene interactions

Quantifying heritability

Is not easy to measure and compares similarities between offspring and parents and it squared varies between 0 (no heritable variation) and 1 (all variation is heritable).

Quantifying heritability equation

h (squared) = Additive genetic variation / Phenotypic variation

Fisher’s fundamental theorem

Traits which influence reproductive success are under great selection pressure to maximise it, anything other than maximisation is selected out over generations.

Fisher’s fundamental theorem leads to

… no variability for the trait - and so all variability observed is due to components other than heritable ones (e.g. environment)

Evidence supporting Fisher’s fundamental theorem

Collared Flycatchers - Lars Gustafson (1986). It was a long term study near Sweden.

Collared Flycatchers - Lars Gustafson (1986) evidence

Lifetime reproductive success - h (squared) = 0.0083

Number of fledged young - h(squared) = -0.0052

Height of forehead patch - h(squared) = 0.0431

All these are important for reproduction (the females select for forehead patch).

Lars Gustafson (1986) - Traits showing the greatest correlation with reproductive success were…

least heritable

Evidence from artificial selection experiments

Dudley and Lambert (1992) - selection for oil yield in corn/maize.

Dudley and Lambert experiment method

They took the top and bottom 15% oil yielders and breed the lowest with each other and did the same with the greatest for 90 generations.

Dudley and Lambert experiment result for the heritability of oil content

1-9 generations - 0.32 (high line), 0.5 (low line).

59-90 - 0.12 (high line), 0.14 (low line).

Heritability drops for both lines as selection goes on.

Problems with Fisher’s fundamental theorem

While the heritability of those traits is low it is still significant and this is not explained by this theory as variation must be arising from somewhere.

Variations come from

Mutations (small random changes)

Environments are not constant (especially pathogens/parasites).

Heritability

It is the proportion of variation in trait explained by inherited genetic variations. It estimates how well we could predict a trait from genetics and measures how important genetics is to a trait.

Heritability is a property of the…

population not the individual.

Heritability is a property of the population not the individual

When then heritability of a trait is described, it reflects how much variability in the population is a consequence of genetic factors and it does not ‘explain’ why an individual has a disease.

Traits that are harder to measure

These will be less heritable due to them having more random measurement error (as this error is not genetic).

Heritability is specific to whom the trait was measured in

Since the heritability involves the total variation of the trait in the population, it matters in what population you’re comparing the genetic effects with.

The importance of heritability

Gives geneticists some indication of what traits would be good to study.

Provides insight on how much family history predicts outcomes.