First Class Lecture notes Concepts in Evolution

EVOLUTION BY NATURAL SELECTION

• Natural selection and its analogues (sexual selection, kin selection etc) produce the
evolutionary change we observe

– Keeping in mind the possibility of ‘neutrality’

• Here we will explore some major concepts in natural selection to understand the
process of evolution by natural selection

– Focus on adaptive evolution

DARWINIAN FITNESS:

 Fitness is the key/fundamental concept

• Generally equated with the number of offspring an individual contributes to the next
generation – survival can be a major player, but only when it alters the number of
offspring produced by an individual

RELATIVE FITNESS:

 What really matters is relative fitness – fitness of an individual measured relative to
everyone else
 High fitness means higher than the average for that population – individual fitness
divided by average fitness
 Why? – It measures differential success – are individuals more/less successful than the
average for the population

wi
wi ( R ) 
 w
 Fitness of an individual = (absolute fitness) / (average fitness of population)

FITNESS AND DEMOGRAPHY:

 The correct measure of fitness can be complicated…
- Demography can matter (a lot) – timing at which individuals have offspring
- e.g. in a growing population with overlapping generations, the timing of reproduction can
be a critical determinant of true fitness
- ‘Malthusian fitness’ – fitness that is weighted by when individuals have offspring
 Why?
- individual that reproduces early leaves genes to offspring that are already maturing
and having their own offspring while the late-producing individual is still waiting to have
their own offspring

MALTHUSIAN PARAMETER:

 Logistic growth – different lineages increase at some rate (r)
 Doubling time of a lineage is directly proportional to this rate (Td ~ 70/r)
 r = Malthusian parameter, predicts rate of increase of a lineage, which impacts on
fitness

, 
 The small advantage in growth rate gets compounded over lineages

IMPLICATIONS:

 With overlapping generations, early reproduction is worth more than late:
- Offspring start having offspring = faster growth of lineage
- Difference can be evolutionarily important
- e.g. might help explain senescence (physiological ageing) – selection stronger earlier in
life (selection shaping individuals to reproduce early, even if that shortens their
lifespan)

FECUNDITY VS. LONGEVITY?

 Trade off: produce lots of offspring early but die young, vs. produce fewer offspring
early, but survive longer
- Same total number of offspring
- BUT early reproduction can be ‘more valuable’ than later reproduction. So selection can
favour alleles conferring higher early reproduction

SELECTION VS. EVOLUTION

 Darwin described ‘evolution by means of natural selection’ as a process
 Today we tend to separate ‘selection’ from the response to selection (i.e. evolution)
 Selection = the processes where differential fitness is associated with some trait
differences (so there is selection on that trait)
 Evolution = the cross generational change in traits or allele frequencies in response to
selection

ADAPTATION

 The outcomes of the process of natural selection can be viewed as ‘adaptations’
- Some inherited trait increases fitness
- Arises from natural selection for its primary function
 ‘Exaptation’ – appear as adaptations now, but evolved for some other reason originally

EXAPTATION

 A trait may be adaptive as it is currently ‘used’, but many traits did not originally evolve
for their current use
 They have been co-opted by natural selection to play a role that differs from why they
originally evolved
 E.g. Skull sutures (plates in the skull which don’t fuse fully until after birth – allow
subsequent brain growth not constrained by live birth) in mammals, feathers in birds