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Transcript
EVOLUTIONARY ECOLOGY
SOME USEFUL DEFINITIONS
Lecture #2
1. “Nothing in biology makes sense
except in the light of evolution”
Theodosius Dobzhansky 1973
2. The book is one written by G.E.
Hutchinson 1965 “The ecological theater
and the evolutionary play”
The quote is self-explanatory
and the intent of the books
title is to state that the process
of evolution takes place in an
ecological context
- evolutionary events occur because of the
relationships between an organismic
unit and their environment
To develop the tie between
ecology and evolution we will
begin with a discussion of
Natural Selection
1. The theory of natural selection is a truly fundamental
unifying theory of life
2. A thorough appreciation of it is is essential background
for understanding evolutionary ecology
3. Natural Selection comes as close to being a “fact” as
anything in biology
Many think that Natural
Selection is synonomous with
Evolution; however, it’s not!
- Evolution refers to temporal changes,
whereas natural selection specifies one
particular way in which these changes
are brought about
- Evolution more specifically refers to a
change in gene frequencies or descent
with modification
Darwins’ Definition of Natural
Selection
1. In any population, more offspring tend
to be produced than can survive to
reproductive age.
2. Members of the population vary in
form and behavior. Much of the
variation is heritable.
3. Some varieties of heritable traits are
more adaptive than others
Darwins’ definition
4. Because bearers of adaptive traits have a greater
chance of reproducing, their offspring tend to make
up an increasingly greater proportion of the
reproductive base for each new generation. This
tendency is called differential reproduction.
5. “Differential reproduction” is natural selection.
Adaptive forms of traits show up (are selected for)
with increased frequency in a population, because
their bearers contribute proportionately more
offspring to succeeding generations
Endler’s (1986) definition of
Natural Selection
a. variation in some attribute or trait
b. a consistent relationship between that trait and
mating ability, fertilizing ability, fecundity and/or
survivorship: fitness differences
c. a consistent relationship, for that trait, between
parents and their offspring, which is at least
partially independent of common environmental
effects: i.e., there is a heritable component
Then:
1. There will be a within-generation effect;
individuals of a given age will differ
predictably from the individuals which do
not survive to that age
2. There will also be a between-generation
effect; the offspring generation will differ
predictably from their parental generation
The modern statement of the
theory of natural selection is in
terms of genes; i.e., what changes
during evolution is the relative
frequency of genes
Modern restatement in genetic
terms
1. All organisms have genes which code for protein
synthesis. These proteins regulate the
development of the nervous system, muscles and
structure of the individual.
2. Within a population many genes are present in
two or more alternative forms, or alleles, which
code for slightly different forms of the same
protein. These will cause differences in
development and so there will be variation within
a population.
Modern restatement
3. Any allele that can make more surviving
copies of itself than its alternative will
eventually replace the alternative form in the
population.
Therefore, natural selection is the differential
survival of alternative alleles
Modern restatement
- genes will be selected through a complex interaction
with their environment, including their genetic
environment (other genes) and ecological
environment (climate, competitors, herbivores, etc.)
- the individual can be regarded as a temporary
vehicle by which genes survive and are replicated
- the most successful genes will be those which
promote an individuals survival and reproductive
success
Selection
The three most common include
– Stabilizing
– Directional
– Disruptive
Stabilizing Selection - reducing
the variance of extremes
Fitness
Freq
Before
Freq
After
Phenotype
Disruptive Selection - can
increase the variance in a trait
Fitness
Freq
Before
Freq
After
or
Phenotype
Phenotype
Directional Selection - affects the
mean value of the trait; the
variance may also change
Fitness
Freq
Before
Freq
After
Phenotype
Other types of selection
encountered:
- Frequency-dependent
- Density-dependent and density-independent
- Hard and soft
- Artificial
- Sexual
- Mortality
- Phenotypic
- Correlational
Frequency Dependent Selection
- occurs when the fitness of a phenotype
depends upon the frequency of that
phenotype in the population. Selection
of this sort could favor either rare or
common phenotypes
- leads to oscillations in the mean value of
a trait
Density-dependent & Densityindependent Selection
- density-dependent selection, selection
may only emerge when competition
between individuals becomes
pronounced, i.e., when density is high
- by contrast, the effects of extreme
weather may be density-independent,
so the survival or death of an organism
occurs regardless of density
Hard & Soft Selection
Parallels density-dependent & densityindependent
- Soft selection occurs when individuals
are in competition - only a certain
proportion will be able to survive
- Hard selection - the survival or death of
one individual is not conditional on the
suvival or death of another individual
Artificial Selection
Selective breeding; fitness differences
imposed by humans - goal oriented
By contrast, natural selection is blind and
operates without a goal; humans can
certainly impose natural selection
Sexual Selection
Traits that seem more a hinderance than a
help
Selection that assisted individuals in
gaining mates
– competition for mates
– preference for particular traits in the other
sex
Mortality Selection
Consistent phenotypic-specific mortality
A single component of fitness variation
and therefore an incomplete definition
Phenotypic Selection
Phenotypic selection requires conditions
a and b of Endler’s definition - trait
variation & fitness differences
Does not include genetic change
Correlational Selection
- certain combinations of traits or alleles
are favored
- this will result in patterns of gametic
phase (“linkage”) disequilibrium for
suites of traits
Example of Correlational
Selection
Brodie, E.D. 1989. Genetic
corelations between
morphology and antipredator
behavior in natural populations
of the garter snake Thamnophis
ordinoides. Nature.
Examples
Gibbs and Grant. 1987. Oscillating selection
on Darwin’s finches. Nature
Directional Selection - Darwin’s
Finches
Hori, M. 1993. Frequency-dependent
natural selection in the handedness of
scale-eating cichlid fish. Science
Frequency-Dependent Selection
in Cichlid Fish
Disruptive Selection in Salmon
Fitness (Dawkins 1982)
1. Historical
2. Genotypic
3. Individual (Classic)
4. Inclusive
5. Personal
Historical Fitness
Definition used by Wallace, Spencer and
Darwin
- The fittest individuals would be those
with the keenest eyes, the strongest
muscles, the sharpest ears, the swiftest
reflexes - a definition close to everyday
usage, physical fitness
Genotypic Fitness
Used by population geneticists
- May be regarded as a measure of the
number of offspring that a typical
individual of genotype Aa is expected
to bring up to reproductive age
Individual Fitness (Classic
Fitness)
Individual reproductive success
- Fitness is a property of an individual
organism, often expressed as the
product of survival and fecundity
Inclusive Fitness
Calculated from an individuals own
reproductive success plus his effects on
the reproductive success of his
relatives, each one weighed by the
appropriate coefficient of relatedness
- IF = Wind +
(Wrel ) X (r)i
Inclusive Fitness = Direct Component + Indirect Component
Personal Fitness
Backwards way of looking at inclusive
fitness
- Personal fitness focuses on the effects
that the individual’s relatives have on
his or her fitness
Natural Selection as a Tautology
Natural selection is defined as “survival
of the fittest” therefore the fittest
survive is circular or an inherently true
statement
There are at least two reasons
why this is misleading
1. Fitness can be directly related to biological
properties and the design of organisms.
2. It is heritable variation in the biological properties
of organisms which gives rise to fitness differences
and the process of natural selection - From heritable
variation in some attribute or trait and a correlation
between that trait and fitness we can deduce the
effects - evolutionary change
Therefore, natural selection is a
syllogism (2 premises leading to a
conclusion) rather than a tautology
(or circular reasoning)
New insights can be gained from an
understanding of the process
How did we get into this
tautology mess?
Herbert Spencer & Alfred Russel
Wallace were responsible for the
phrase
“Survival of the fittest”