Download Natural selection

October 5, 2009
Bioe 109
Fall 2009
Lecture 5
Natural selection - theory and definitions
Darwin’s formulation of the principle of natural selection
- Darwin developed his theory of natural selection on the basis of five “facts” and three
“inferences” that result from the acceptance of these facts.
- the formulation of the theory of natural selection is an example of what is called a “syllogism”.
- a syllogism may be defined as “a form of reasoning in which a conclusion is drawn from
several given or assumed premises”.
- none of the five “facts” were actually discovered by Darwin but two of the inferences are.
Fact 1. Natural populations have large excess fecundity.
Fact 2. Population sizes generally remain stable.
Fact 3. Resources are limiting.
Inference 1. A severe struggle for existence must occur.
Fact 4. An abundance of variation exists among individuals of a species.
Fact 5. Some of this variation is heritable.
Inference 2. Genetically superior individuals outsurvive and outreproduce others.
Inference 3. Over many generations, evolutionary change must occur in the population.
We can now define evolution by natural selection as:
“changes in the relative frequencies of different genotypes (genes) in a population because
of differences in the survivorship and/or reproduction of their phenotypes”.
- this definition leads to an interesting philosophical issue.
- what persists over evolutionary time are genes, not organisms.
- individual organisms are merely the vehicles by which genes propagate themselves through
time.
- in a sense, organisms are merely “gene transport machines”.
- natural selection favors more “efficient” gene transport machines.
- why? individuals possessing genes that do not confer this “desire” to efficiently transport genes
into subsequent generations will quickly be displaced by individuals possessing genes that do.
Some important principles of natural selection
1. Natural selection acts at the level of individuals, not populations.
- organisms may be decomposed into two components - the genotype and the phenotype.
- genotype is the hereditary material, or set of genetic instructions, that determine an organism’s
structural, physiological, and behavioral characteristics.
- the phenotype represents the physical expression of a particular genotype.
- it results from an interaction between genotype and environment.
- a genotype may thus produce a number of different phenotypes depending on the environmental
conditions.
- selection acts directly at the level of the phenotype and indirectly at the level of the genotype.
2. Populations, not individuals, evolve.
- in sexually reproducing species, the evolutionary process takes place as the genetic consequence
of selection favoring some genotypes over others.
- the composition of the population will thus change over time.
3. Natural selection is retrospective and cannot predict the future.
- the “backward looking” nature of selection causes every generation to reflect the effects of
previous generations.
- in the first lecture, we learned that an AIDS patient taking AZT creates an environment for the
HIV virus that strongly selects drug-resistant genotypes.
- the viral population thus evolves to become dominated by drug-resistant genotypes.
- if the patient stops taking AZT, the evolutionary process will actually reverse and AZT
susceptible genotypes will again predominate.
- why?
- because in the new AZT-free environment, genotypes that replicate well in the presence of AZT
are out competed by genotypes that have a pol gene that lack the mutation at the active site
producing resistance.
4. Natural selection is not necessarily progressive.
- natural selection is a strongly deterministic process but chance plays an important role in
determining the path of evolution.
- evolution has no way of always increasing complexity - there is no “orthogenesis” driving the
process.
- the random way in which environments - and thus selection pressures - are expected to fluctuate
over evolutionary time periods means that a species will “meander about” responding to
immediate selective pressures and not necessarily always result in some net “improvement”.
Natural selection and the concept of fitness
- what is fitness?
- there are two definitions of fitness in the evolutionary literature - Darwinian fitness and relative
fitness.
Darwinian fitness: the number of gene copies a phenotype places into the next generation.
Relative fitness: a phenotype’s Darwinian fitness relative to other phenotypes.
- relative fitness is the form of fitness most relevant to understanding the process of natural
selection.
- this is because it doesn’t matter what a genotype’s Darwinian fitness is - what matters more is
how well it does (on average) compared to all other genotypes in the population.
- for example, genotype A may do “well” leaving 10 offspring.
- however, if genotype B leaves 15 offspring then over time, genotype A will quickly be
eliminated from the population.
- therefore, what matters is how well a genotype does relative to the rest of the population.
- this illustrates that understanding the concept of fitness is fundamental to understanding the
process of natural selection.
What is fitness? How do we measure it?
- considerable confusion exists among scientists and the general public over the term “fitness”.
- many evolutionary biologists still argue over what “fitness” actually is.
- is common to hear complaints among both biologists and non-biologists that natural selection is
a tautology, or something that is true by definition, and thus of no meaning or value.
- the typical argument goes like this:
- one can ask the question: What is evolution by natural selection?
- a common answer would be: natural selection is the “survival of the fittest”!
- OK, its the survival of the fittest - who are the most fit individuals?
- why, those that survive!
- In other words, fitness is thought to “explain” why some individuals survive and reproduce and
others do not.
- this is not what fitness is at all.
1. Fitness is a description not an explanation.
- fitness is simply a summary measure that describes the relative reproductive success of different
genotypes.
- fitness quantifies the biological differences among individuals that cause differential survival
and reproduction.
- it does not explain how these differences come about.
- as a consequence, the concept of fitness is not circular.
2. Fitness is an average property.
- fitness is an average property of individuals that possess a certain genotype.
- for any one individual, the probability of survival to a certain age is either 0 or 1.
- not all genotypes with the highest fitness will necessarily outsurvive all genotypes with a lower
fitnesses - some will perish by accidents alone.
- however, all else being equal, genotypes with higher relative fitness will outsurvive and
reproduce those with lower fitness.
- the average probability of survival of a genotypic class is an measure of its fitness.
- it is a summary measure - not a predictor of reproductive success. In other words, it does not
explain why an given genotype has a reproductive advantage - it simply measures what this
advantage is.
3. Fitness is “relative”.
- the fitness of a given genotype or individual is measured relative to other genotypes or
individuals.
- by convention, the most fit genotype at a locus is given a fitness of 1.
- other genotypes are assigned fitnesses that are reduce by an amount termed the “selection
coefficient”.
Example:
Genotype:
Frequency:
Fitness (w):
AA
p2
w11
Aa
2pq
w12
aa
q2
w22
4. Total fitness is comprised of several individual components
- an individual’s total fitness is a measure of its ability to transmit genes to the next generation
through the production of progeny.
- included are components due to differential viability, longevity, fecundity.
- it is common for evolutionary biologists to estimate “fitness components” instead of total
fitness (because it is easier).
- Darwin viewed natural selection as being mediated through differential survivorship, or
viability.
- today, we realize that viability is obviously important but other components of total fitness may
be equally, if not more, important - particularly fecundity.
Forms of natural selection
1. Purifying selection
- purifying selection is selection acting against deleterious (harmful) alleles.
- the majority of deleterious mutations are recessive or nearly so.
- therefore, individuals heterozygous for such mutations will be nearly identical to homozygotes
for the unmutated allele.
- what this means is that selection will be ineffective against such alleles in heterozygous state.
- however, when in homozygous state such mutations have usually drastic effects and here
natural selection can effectively act on them.
- it cannot entirely eliminate such mutations entirely from the population because for
homozygotes to be produced heterozygotes must exist at a certain low frequency.
- eventually a balance is established between the continuous introduction of these alleles into the
population by mutation and their removal by selection (in homozygous state).
- this equilibrium is called mutation-selection balance.
rate of introduction = rate of removal
by mutation
by selection
- purifying selection acts to prevent harmful alleles from becoming common in natural
populations.
- it thus acts to prevent polymorphism.
2. Directional selection
- the opposite of purifying selection is called directional selection.
- directional, or positive, selection is the process by which a selectively favored allele is
introduced into a population, sweeps through the population to become fixed (i.e., reach a
frequency of 1.0).
Example:
Genotype:
Fitness (w):
AA
w11
1.0
Aa
aa
w12
w22
1.005 1.010
- in the above example, the small a allele if introduced into the population at a low frequency will
eventually reach a frequency of 1.0.
- this is a strongly deterministic process - theory predicts that with a selection coefficient of s =
0.01 it would take about 3,000 generations for a to reach fixation.
- if s = 0.001, it would take about 100,000 generations.
- this may seem like a long time, but is fast on an evolutionary time scale.
- directional selection of this form will not typically lead to variation.
- this is because the time taken for the selectively favored allele to become fixed is short relative
to the time it would take for such a strongly favored allele to arise by mutation.
3. Balancing selection
- in contrast, balancing selection is a term given to forms of natural selection that lead to the
active maintenance of genetic variation in natural populations.
- the alleles are said to be “balanced” because a stable equilibrium state is reached.
- at this equilibrium state, the alleles are maintained at certain frequencies, determined by the
relative selection acting on the various genotypes, and if the frequencies are perturbed from this
equilibrium point, selection will act to return it to this point.
- what kinds of balancing selection exist?
1. Overdominance.
- this arises when the heterozygote is more fit than either alternate homozygote.
- suppose the fitness of three genotypes are as follows:
Genotype:
Fitness (w):
AA
w11
0.88
Aa
w12
1
aa
w22
0.14
- a stable polymorphic equilibrium is established in the population by virtue of the fact the
heterozygote enjoys a higher fitness than either homozygote.
- one the best known examples of overdominance is sickle cell hemoglobin in humans.
- the HbA allele is the normal allele, HbS is the sickle cell allele.
- individuals who are homozygous for the HbA allele are susceptible to malaria in West-central
Africa.
- homozygotes for the HbS allele suffer from a severe anemia.
- HbAHbS heterozygotes enjoy resistance to malaria but do not suffer from anemia.
- the fitness of the three genotypes AA, AS, and SS have been estimated at 0.88, 1, and 0.14,
giving equilibrium allele frequencies of HbA = 0.89 and HbS = 0.11.
- please note that the polymorphism is stable only in malarial environments - in areas that do not
have malaria, the S allele is strongly selected against.
- another classic example of single-locus overdominance is warfarin resistance in Norway rats,
Rattus norwegicus, in Wales
- the poison warfarin is an anticoagulant that has been used to kill rats for many decades in this
area.
- resistance to warfarin was developed in rats that is attributed to a single locus.
- at this locus there is a dominant resistance allele, “R”.
- homozygotes for the normal allele “S” are killed by warfarin.
- homozygotes for the R allele are less fit than heterozygotes because they suffer from vitamin K
deficiency.
- heterozygotes for both R and S alleles are resistant to warfarin poisoning but do not suffer from
vitamin K deficiency.
- the strong advantage of heterozygotes at this locus can lead to marked departures from H-W
equilibrium.
(2) Frequency-dependent selection.
- with overdominance, the fitnesses of genotypes are assumed to be constant, i.e., they remain
unchanged irrespective of the genotypic composition of the population.
- this may not be realistic - genotypes may use resources differently, such that the fitness of a
genotype is highly dependent on what other genotypes happen to be present in the population.
- if a genotype utilizes a unique resource, then it is likely to have an advantage when it is rare
because it will experience little, if any, competition for that resource.
- as it increases in frequency, it will have a lower fitness because other of competition with other
individuals.
- the simplest type of frequency dependent selection can be modeled by assigning genotypes
fitnesses that incorporate their own frequencies.
- for example:
Genotype
Fitness
AA
w11
1-p2
Aa
aa
w12
w22
1-2pq 1-q2
- this leads to a stable equilibrium state at p = q = 0.50. At this point, the fitness of the
heterozygote is less than either homozygote.
- an excellent example of frequency-dependent selection involves self incompatibility loci in
plants.
- self-incompatibility (S) loci act to prevent inbreeding in many sexually reproducing plants.
- S alleles loci prevent pollen from growing on stigma if they happen to share the same allele.
- for example, pollen with an S1 allele will be able to fertilize a plant with a genotype of S6S10,
or S4S20, but not S1S4, or S1S28.
- at the S locus, the fitness of a genotype is thus a function of how frequent in the population the
two alleles it possesses are.
- if an allele is rare, then it will enjoy a higher reproductive success than if it is common.
- this system evolves to a state where large numbers of alleles are maintained in populations by
frequency-dependent selection.
- plant species having self-incompatibility loci typically have between 30 and 50 alleles present
at quite uniform frequencies.
(3) Heterogeneous environments.
- the third major type of balancing selection is produced by variable environments - some
genotypes are more fit than others in some habitats, or under some environmental conditions,
than others.
Environment A
Genotype
Fitness
Environment B
AA
Aa
aa
Genotype
AA
Aa
aa
w11
w12
w22
Fitness
w11
w12
w22
1-s1 1
1
1
1-s2 1-s3
- environments may vary on two different scales. There may be spatial variation and there may be
temporal variation.
- an excellent example of spatially-varying selection is provided by an enzyme polymorphism in
the blue mussel, Mytilus edulis.
- the enzyme is called leucine aminopeptidase, or Lap, which catalyzes the cleavage of n-terminal
amino acids from di-, tri- and tetrapeptides.
- the enzyme has two distinct functions.
- one is to serve as a digestive enzyme - it is abundantly expressed in the gut lumen.
- the other function is osmoregulatory.
- marine bivalves are osmoconformers - the osmolarity of their tissues is identical to that of
surrounding sea water. This osmoconformation is achieved by modifying intracellular levels of
free amino acids - notably proline, glycine and alanine.
- as salinity goes up, small peptides are cleaved and the a.a. pool increases.
- as salinity falls, these a.a.’s are removed from the pool to reform small peptides.
- some amino acids are exported to the haemolymph some are ultimately excreted. This results in
a net loss of nitrogen which may affect the animals energy budget.
- Lap functions in this capacity.
- a sharp cline exists in the frequency of the Lap94 allele at the entrance to Long Island Sound.
- the frequency of the 94 allele declines sharply from 0.55 in full oceanic salinity environments to
0.12 over a 50 km area.
- what is the cause of this cline?
- is there any environmental factor responsible for producing this cline?
- Yes - at the entrance to Long Island Sound there is a drop in salinity from oceanic levels (33-35
ppt) to estuarine levels (25-30 ppt).
- the Lap-194 allele appears to be optimized for functioning in a high salinity environment.
- at the biochemical level it has been found to have a higher catalytic efficiency - about 20%
greater than the other alleles (96 or 98).
- find that genotypes possessing the 94 allele have higher catalytic activity.
- in the brackish water environment of Long Island Sound, the Lap-194 allele is at a disadvantage
- individuals possessing this allele suffer a higher loss of nitrogen and experience higher levels of
mortality than genotypes lacking the Lap-194 allele.
- the form of balancing selection acting to maintain the Lap-1 polymorphism is thus
environmental heterogeneity in salinity.