Concepts of Evolution
Alfred Wallace
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From so simple a beginning, endless forms….
Natural Selection through Sexual Reproduction
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Phenotype: observable characteristics, e.g., beak size
Genotype: genes consisting of pairs of alleles that code for
the phenotype, e.g.,
B codes for big beak size.
b codes for small beak size.
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Dry weather, nuts are hard, B = 70%
Good weather, nuts are soft, B = 40%
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Many more generations later with the dry weather persisting:
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Industrial pollution in peppered moths in 1800
England and Bernard Kettlewell’s experiments of 1950
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A summary of the principles of natural selection
• Principle of Variation: Individuals within a species show variation in
their physical and behavioural traits.
• Principle of Inheritance: Some of this variation is heritable.
•
Principle of Adaptation: Individuals are in competition with one
another for scarce resources and some inherited variations will have
survival advantages.
• Principle of Selection: as a consequence of being better adapted to an
environment, some individuals will produce more offspring, who will
inherit the same advantages.
• Fitness is thus defined as reproductive success (RS) or the number of
surviving offspring produced.
• Evolution results in differential rate of reproduction and survival of
different genotypes in a population
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1. Variation of a trait
2. Differential reproduction of the trait
3. Heritability of the trait
4. End result: brown is more fit than green
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Where does variation come from? Mutation or copying errors.
Fitness = The relative number of surviving offspring, more specifically:
the extent to which copies of an individual’s genotype are present in
succeeding generations, relative to other genotypes; in other words,
reproductive success (RS)
Spencer’s quote, “survival of the fittest,” can be misleading.
Fitness is not related to survival except for survival to the reproductive age.
Fitness does not refer to physical well-being.
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Evolutionary Language
A trait or characteristic is selected for, is adaptive, is an
adaptation, is a fitness.
A phenotype is selected against because it is not
adaptive to the ecology.
Evolution is blind, random, unconscious, unintentional,
non-directional and non-progressive.
But just as a speaking habit, we often talk about
evolution as if it is a conscious process by referring to
phenotypes as “designs,” “design features,” “strategies,”
“solutions” or as something that is “useful,” “functional,” or
“successful.”
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Inclusive fitness: Extending Darwin’s
natural selection through sexual reproduction
Hamilton (1964) reformulated evolutionary theory by showing that the measure of
an individual’s direct reproductive success (RS) was too narrow a concept.
He introduced the term inclusive fitness to refers to the fitness or RS of the
individual plus the effects of a particular behavior or trait of the individual on RS
of the individual's relatives. C < B*r
Inclusive fitness = Direct fitness + Indirect fitness
= RS of Individual + RS of Relatives
Altruism: Decrease in direct fitness < than increase in indirect fitness
Altruism: Selfish restraint, e.g., not being too mean to relatives.
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William Hamilton
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Reciprocal Altruism (Trivers, Axelrod & Hamilton)
Why do organism help genetically unrelated?
C < B*r, r = reciprocating
What facilitate r or the probability of meeting each other again?
Stable residence
Small community
 characteristic of ancestral past
Long term memory
Still true today:
crimes higher in large cities than small towns
crimes more likely when identity is hidden than shown
we treat people well in part because we run into them again
We feel bad when we fail to return a favor from others
when others fail to return our favor
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Prisoner’s Dilemma
A
Cooperate
Defect
A:5
A:2.50
Cooperate
B:2.50
B:0
B
A:1
A:0
Defect
B:5
B:1
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Sexual Selection
Whereas natural selection increases the frequency
of a trait because its bearers are favored by nature,
sexual selection theory states that the frequency of
a trait increases because its bearers are favored by
the opposite sex.
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Who Chooses Whom?
Baterman’s Principle
The sex which invests the most in producing
offspring becomes a limiting resource over which the
other sex will compete.
For humans and many other animals, females are
the limiting sex because there are far fewer eggs than
there are sperms, the eggs carry far more nutritional
investment for the next generation than sperms, and
internal gestation limits reproductive capacity of females
compared to males.
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Sexual selection can take two forms
Intrasexual selection
Also known as Intrasex Competition and
Male-to-Male-Combat, in which members of
the less limited sex (typically males) compete
aggressively among themselves for access to the
limiting sex.
Intersexual selection
Also known as Intersex Competition or
Mate Choice or Female Choice, in which males
compete with each other to be chosen by
females.
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Weapons Intersex selection selects traits that give males the physical advantage to
fight against, to injur, and to kill other males. These include size, strength,
aggression, and violence and other related traits that serve as weapons in the male
to male combat.
Ornaments Intrasex selection selects traits which serve as brags to attract females.
The best known example is the peacock tail. These male ornaments originally
fascinated Charles Dawin and motivated his theory of sexual selection.
Zahavi’s Handicap Principle The elaborated ornaments that take much energy to
sustain and cut into one’s fitness serve as indicators for the overall wellbeing and
genetic quality of the individual carrying the extra cost of fitness. The showy males
are telling their females that they must have good genes because they can afford the
competition with a handicap.
Sex Dimorphism Intersex and intrasex selection lead to sex differences in these
“weapon” and “ornament” traits so that males are bigger, stronger, and more
aggressive and have brighter color, deeper voice or are more oratory or showy or
bragging than females.
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Parental Investment Theory: Extending Darwin’s
sexual selection theory
Robert Trivers (1972) argued that a
driving force behind sexual
selection is the degree of parental
investment each sex devotes to
their offspring. That is, parental
investment differences between the
two sexes are the engine driving
sexual selection and the evolution
of all related sex differences.
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Why do females choose?
Why do males compete?
What do females choose?
Female
Good genes
Males
Good providers
Good fathers
High parental investment
Low parental investment
Initial
Weapons
Postnatal
Ornaments
Excessive cheap sperms
Limited valuable eggs
Maternity Certainty
Paternity Uncertainty
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• The sex that invests most heavily in an offspring will be
more choosy about who they mate with (i.e females).
• The sex that invests least in an offspring should compete
more vigorously for access to the higher-investing sex (i.e.
males).
• Male competition leads to physical dimorphism (weapons
and ornaments), which correlates with male parental
investment and male reproductive success variation.
• Variations open ways for other reproductive strategies:
– Increased male parental investment
– Reduced sex dimorphism
– Reduced male reproductive variation
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• Where male parental
investment approaches
that of females,
competition will be
reduced as will sexual
dimorphism (e.g.
swans).
• Where male parental
investment is higher,
females will possess
typically male
characteristics (e.g.
seahorses, jacana).
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Mating Systems
Monogamy: pair bond for the breeding season or for life; parental
investment from both parents; low sex dimorphism.
Polygyny: males have multiple female partners; parental investment
from mothers but not fathers; high dimorphism
Resources based: males defend territories that sustain females.
Non-resources based: males gather at lek to compete for females.
Polyandry: females have multiple male partners; parental investment
from fathers but not mothers.
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Guppy; Sage grouse
Bowerbirds; Fiddler crab
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