Lecture 5 - Evolution of Life Histories Flashcards
What are life histories?
Life history is the age-related pattern of reproduction & survival. It contains many heritable characteristics and looks at factors such as:
size at birth growth rate age and size at maturity frequency of reproduction number of offspring lifespan
What are the two types of variation?
Discontinuous (Mendelian)
Continuous (Biometrics)
What did RA Fisher propose in 1918?
That many small genes have small individual additive effects.
These combine to give a normal distribution.
Continuous variation is thus polygenic in origin.
Interactions amongst genes is known as…
Epistasis
What does VP stand for?
VP= total phenotypic variance
What does VG stand for?
VG=total genetic variance
What does VE stand for?
VE=total environmental variance
What does VGxE stand for?
VGxE=total variance due to the interaction between genotypes and environment.
What is heritability?
The heritability (h2) of a trait is a measure of the degree of resemblance between relatives. h2 is the proportion of variation that is inherited genetically
What range does heritability fall in?
Heritability ranges from 0 to 1
Traits with no genetic variation have a heritability of 0
What is the equation for heritability?
h2 = VA / VP
That is the ‘additive genetic variance’ / ‘phenotypic variance’
What is heritability influenced by?
Since heritability is a function of the environment (VE), it is a context dependent measure.
It is influenced by both the environment that organisms are raised in and the environment that they are measured in.
If h2 = 1, what does this mean?
This means that all phenotypic variation is purely genetic.
If h2 = 0, what does this mean?
This means that all variation is environmental, no evolution is possible as there is no inheritance.
Why should h2 be measured?
h2 measures the ability of a population to respond to selection. Selection provides a response up to a limit, any remaining variation must then be environmental.
What is Fisher’s paradox?
Characters or traits with the highest fitness should have the lowest h2 because they are strongly selected i.e. they have the least potential to evolve further
There is directional selection for increased fitness, for h2 => 0.
What is the Lek paradox?
The Lek paradox is the question of how is additive genetic variation maintained in the face of consistent female preference. Persistent female choice for particular male traits should erode genetic variations and remove the benefit of choice, as females should always cause strong directional selection for sexually selected characters but choice still persists.
There are two conditions under which the Lek paradox arises. The first is that males contribute only genes and the second is that female preference does not affect fecundity.
Female selection should lead to directional selection, which would result in a greater prevalence for that trait. Stronger selection should lead to impaired survival because there will be a decrease in genetic variance since more offspring will have similar traits, this is known as runaway selection.
Explain how h2 has dropped in racehorses?
In race horses there is strong directional selection which leads to a drop in h2.
Most thoroughbreds are from 31 ancestors, so there has been a limited gene pool for 200 years
How does the ‘mutation-selection balance theory’ explain how genetic variation for fitness is maintained?
Selection removes variation, but mutations put it back again. These processes come to an equilibrium. However the rates are not enough to explain the observed amount of variation in natural populations.
How does the ‘fluctuating selection’ theory explain how genetic variation for fitness it maintained?
This theory says that genetic variation is maintained because the direction of selection changes.
eg. In the case of the Copepod Crustacean
Winter: eggs develop immediately
Spring: eggs experience a summer diapause; they sink onto sediment and are resistant to digestion, they therefore avoid intense fish predation in the summer.
The crustacean detects the photoperiod, which is highly heritable (h2 = 0.5).
If they switch too early → lose out in competition
Switch too late → eaten by fish
The selection varies between years depending on the fish density. The change in timing of the switch = the force of selection.
High fish density → Early switch, Low fish density → Late switch.
Genetic variation is maintained because of the switch in direction of selection.
Describe how ‘the existence of genetically-based tradeoffs’ theory can explain what maintains genetic variation for fitness.
Prior to selection individuals are distributed between life history characters for example reproduction and survival. Their positions are set by genes only.
After selection there is little change to the individuals fitness but there is lots in the components of fitness.
Under long term stasis, even small fitness differences are selected for, in constant conditions.
If the environment changes, gene expressions alter (GxE interaction) and therefore all positions are redistributed introducing genetic variation.
Explain how dyslexia can show GxE interactions.
PET scans of the brains of dyslexia readers were taken. Dyslexia is the same everywhere, affects the same parts of the brain, but Italian dyslexics are better at word reading than English or French.
This is because Italian is an easier language, with close matches between spelling and pronunciation. Therefore a simple genetic problem (dyslexia) is expressed differently due to the environment (language), a GxE interaction.
What technique can make reading for dyslexics easier?
Spacing out the text is shown to improve reader ability in dyslexics regardless of the presentation order.
Why is h2 low for life history traits?
Because they are strongly selected there is little room for them to evolve further.
