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Chapter 2
Mechanisms of Evolutionary Change
Selection in Darwin’s Theory of
Evolution
• Artificial selection
• Natural selection
– No ultimate goal
– Current environmental parameters
– Traits that increase likelihood of survival
• Sexual selection
– Traits that increase likelihood of mating
– May actually decrease individual’s survival
Differential Reproductive
Success
• Not just how many offspring you produce
• Number of offspring that survive to
reproduce themselves
Mendelian Genetics
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Phenotype, genotype
Gene, allele, locus
Homozygous, heterozygous
Dominant, recessive
Particulate, not blended, inheritance
Born with the genes you pass on
Mendelian Genetics
phenotype
x
SS
ss
S
s
x
gametes
male
genotype
female
gametes
Punnett Square
Ss
Ss
S and s
S and s
S
s
S
SS
Ss
s
Ss
ss
phenotype: 3 big, 1 small
genotype: SS, 2Ss, ss
Beyond Mendel
• Not everything in genetics follows Mendel’s basic
principles
• Linked genes
• Chromosomal crossing over during meiosis
– Recombination, or “chromosomal mutation”
• Autosomal and sex chromosomes
• Gene mutation
– Neutral, detrimental, beneficial
– Only one source of genetic variability
Accounting for Variability
• Sexual reproduction (“Mendelian
variation”)
• Recombination (aka “chromosome
mutation”)
• Gene mutation
Polygenetic Effects
• Not all genes fully dominant/recessive
• Pleiotropy: genes with multiple phenotypic
effects
• Turning a gene “on” or “off”
– “Modifier” genes
– Gene-gene interaction
– Environment
Genomic Imprinting
• Usually, parental contributor of an allele is
irrelevant for gene expression
• With imprinted genes, parental contributor matters
– <1% of genes
– About 80 discovered so far in humans
– Mostly involved in embryonic and placental
development
• Parental Conflict Hypothesis
– Moore & Haig (1991)
Igf2 Gene
• Paternally expressed
– Gene only activates if it is contributed by the father
(i.e., from the sperm)
• Influences embryo growth
– Insulin-like growth factor --> bigger embryo
• Male
– “Wants” largest embryo possible
– Cost to mother doesn’t affect father’s future
reproductive output
• Female
– “Wants” large embryo, but not too large
– False-receptor imprinted gene; blocks additional growth
hormone
DNA
• Deoxyribonucleic acid
• Double helix
• Nucleotide
– Backbone
– Base pairs: adenine-thymine, cytosine-guanine
• Amino acids coded for by triplets of base
pairs (a codon)
• The genome
Book Metaphore of Genome
• There are 23 chapters, called CHROMOSOMES
• Each chapter contains several thousand stories,
called GENES
• Each story is made up of paragraphs, called
EXONS, which are interrupted by advertisements,
called INTRONS
• Each paragraph is made up of words, called
CODONS
• Each word is written in letters called BASES
What Genes Do
• Amino acids, when strung together, code for
polypeptide production
• Proteins formed from multiple polypeptides
linked together (“transcription”)
• How this translates to physical and/or
behavioural traits is highly interactive,
depending on environment
– E.g., sugars in cell can affect polypeptide
folding, altering proteins that can be made
30,000
• Human Genome Project (2001)
• Mapped loci on all chromosomes
• Don’t actually know what all 30,000 genes
do
• Don’t forget polygenetic and other
environmental effects
Heritability
• Degree to which phenotypic variance in
population is due to genotypic variance
• Genetic rather than environmental
influences
• Greater a trait’s heritability, the greater the
genetic control over it (i.e., the less
environmental influence)
Source of Control
• Even high heritability traits can still be affected by
environmental factors
• Compare phenotypic variation in people of
differing degrees of relationship (e.g.,
monozygotic, dizygotic twins)
• Correlations
– Zero to one, positive or negative
• Even high heritability traits only show correlations
in the 0.5-0.7 range
Variation in the Population
• Consider from perspective of an
individual’s offspring
• Proximate level interpretation
– Sexual reproduction, chromosomal and gene
mutations
• Ultimate level interpretation
– Change in environment less likely to eliminate
all offspring
– Asexual vs. sexual reproduction
– Disease and parasitism
Evolution is Generational
• Shifts in gene frequencies based on generations
– ~300,000 generations b/t common ancestors of modern
chimpanzees and humans, (~5 million years)
– Same number of bacteria generations in ~25 years
• Environment can change rapidly
– Time scale issues
– Short-term shifts, long-term stability
• Under certain conditions, gene frequencies can
change “moderately” rapidly
• Variability in population as well as learning ability
allows for adaptation to changes
Non Natural Selection Evolution
• Not all evolution due to natural (or sexual)
selection
• Gene flow
– Animal moves from one population to another
– If its genotype confers an advantage, this can
alter the gene frequencies in the local
population “rapidly”
• Genetic drift
– Changes in population due to chance, because
traits not selected for/against (i.e., neutral)
– Generally only significant in small populations;
each mating act has larger influence on gene
frequencies of population
• Founder effect
– Subset of genetic drift
– New population is established by one or a few
individuals; biases initial gene frequencies
Other Factors
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Disease
Climate change
Volcanism
Asteroid impacts
Human intervention on environment, species, etc.
Generally, sudden large scale changes that operate
too rapidly for natural and sexual selection to
adapt organism (i.e., change gene frequencies)
Levels of Selection: Group or
Individual?
• Group
• Species, population, community, etc.
• Initially seemed to explain altruism
– Altruism: sacrificing your own fitness to
improve the fitness of others
• “Greater goodism”
Group Selection
• Evolution of traits benefiting population, not
individual
• Sacrifice/loss for individual
• Williams (1966)
– Group selection can seldom counter force of individual
selection
• Don’t necessarily need to invoke group selection
Individual as the Level of
Selection
• Individual generally seen as the level on
which evolution operates
• Individuals do/don’t pass their genes on
– Gene frequencies in population shift over time
• Direct (“classical” or “individual”) fitness
– Genes passed on specifically by individual
reproducing
Inclusive Fitness
• William D. Hamilton (1964)
• Share genes with close relatives (kin)
• Can increase fitness by assisting relatives’
reproductive success
• Sum of
– Individual’s reproductive success (direct fitness)
– Individual’s success in promoting the successful
reproduction of relatives
• Can explain many cases of altruism
0
Cousins
Half siblings
Uncle & niece
Grandparents & grandchildren
Full siblings
Parents & children
Identical twins
• Probability that
two individuals
share the same
gene because
they inherited it
from a common
ancestor
Genetic relatedness, r
r, Coefficient of Relatedness
1
Theory of Kin Selection
• Altruism: sacrifice for the benefit of another
• “Hamilton’s Rule”
• rB > C
– Where B = benefit to recipient
– C = cost to yourself
– r = coefficient of relatedness
• EP explains altruism via kin selection
(inclusive fitness)
Reciprocal Altruism
• Altruistic acts between non-relatives
• A low cost to the donor and a high benefit to the
recipient
• In future, the debt is repaid (i.e., “reciprocated”)
• Both donor and benefit benefit
• Requires stable social group, ability to identify
individuals, longer lifespans
• Not basic altruism
Error on p. 53 Text
• “Imagine, for example, that we are living on the plains of the
Serengeti in Africa and that I have just killed a wildebeest.
There is more meat than I could possibly eat before it either
goes off in the heat… or is stolen by a clan of hungry hyenas.
You may be starving but I can save your life at very little cost
by giving you meat that is left over when I’ve had my fill.”
• Bad wording in the example
• Actually “tolerated theft”
• No real cost to donor here: “…more meat than I could possibly
eat…”
• Must be some cost to donor for reciprocal altruism
• Example would work if donor gave some meat that he could
still have eaten
Competitive Altruism
• Sacrifice, or appearance of sacrifice, to gain
future personal benefit
• Attention, possible mating opportunities,
resources, status
• “Donor” gains more than he/she sacrifices
• Not basic altruism
Selfish Gene
• Richard Dawkins
• Gene’s-eye view
– Metaphor
• Replicators and vehicles
• Genes are eternal, individuals are ephemeral
• Can gene be actual level of selection?
– 30,000 genes in a human
– Individual genes canceling out each others’ attempts to
behave selfishly?
– Behaviour occurs at the level of the individual, not the
gene
Multi-Level Selection
• Elliot Sober and David Sloan Wilson
• Group, individual, and gene selection
• Group cooperation, for example, increases
reproduction of individuals in group
– But, often group members are genetic relatives,
so…
• Levels of causation seem to be an issue here
• Recent, and actively debated theory