Transcript Evolution and Development

Evolution and Development
PSC 113
Jeff Schank
Outline
• Evolution
– Change
– Species
– Phylogeny
• Evolution by Natural Selection
– Influences on Darwin: Lyell, The Voyage, Malthus, Artificial Selection
– Three Principles of Natural Selection: Variation, Heritability, Fitness
•
The Modern Synthesis
– Blending Inheritance
– Mendelian Genetics
– The Synthetic View
•
Units of Selection
– The Selfish Gene
– Kin Selection and Inclusive Fitness
Outline continued
• Adaptations
– Adaptationist View: Ecological Niche (3 views)
• Ecological Niches are fixed targets of Natural Selection
• Red Queen Hypothesis (Leigh Van Valen)
• Constructivism (Richard Lewontin)
– Problems with Adaptationism
• Not all Characters and Behaviors are Adaptive
• Exaptations
• Constraints on Adaptations
Evolutionary Psychology
Change
• Evolution means change but but what kind of
change?
– Change in gene frequencies in a population?
– Change in phenotypic characters and behaviors
over generations?
– Change in species?
Species
• Fixity of Species
– Species at the time of the Origin, were viewed as fixed forms or
kinds of things
• Darwin argued that species are not fixed but can change
• This is now the modern view in which species are a level of
organization consisting of organisms that are reproductively
isolated from other species
• Reproductive isolation does not mean that members of
different species cannot inter breed, but only that there is a
strong tendency not to (e.g., wolfs, coyotes, domestic
dogs).
• Therefore, species are not kinds of things but rather
individuals composed of organisms that persist over many
generations and are reproductively isolated from other
species
Phylogeny
• Before Darwin, species were viewed as fixed
and ordered by a great chain of being
• In the great chain of being, species had a fixed
and strict linear order with the “lowest” forms
of life at the top and the “highest” forms at
the top
• Darwin’s idea of evolution by descent was a
radical departure
Great Chain of Being
From Darwin’s Notebook B (1937)
Tangled Phylogenetic Trees
Evolution by Natural Selection: Key
Influences on Darwin
• Sir Charles Lyell’s Principles of geology
– View at the time: Geological change was catastrophic—motivated by
the belief in a Biblical flood
– It was critical for Darwin to see that natural processes could change
gradually but significantly
– Darwin concluded that if evolutionary change occurred gradually, this
would require a lot of time
• The Voyage
– Darwin took away the idea that species could be related by descent
– There is variation within species
• Thomas Robert Malthus
– Populations grow exponentially but resources are limited
– Thus, not all individuals can survive
– Which individuals are likely to survive to reproduce?
Evolution by Natural Selection: Key
Influences on Darwin
• Artificial Selection
– Dog breeds
– Dogs and wolves
– Origins of dogs, Central Asian domestication origin
– Are dog breeds different species?
Three Principles of Natural Selection
• Variation
– There must variation among phenotypic characters and
behaviors
• Heritability
– Characters and behaviors must be heritable
• Fitness
– Characters and behaviors must be beneficial in the
struggle for existence in competition with others or in the
struggle against the environment
• Evolution by Natural Selection occurs when there is (1)
heritable (2) variation in (3) phenotypic fitness
Darwin’s Theory of Inheritance
• Blending Inheritance
– Darwin proposed pangensis
– Each part of the body throws off particles
(pangens), which are collected in the reproductive
organs
– Sexual reproduction blends these particles
together in the offspring.
– Why is blending inheritance incompatible with
evolution by natural selection?
Mendelian Genetics
• Mendelian Genetics
– Mendel, a monk published his
paper on genetics in 1866
– He showed that there are
hereditary factor affecting the
phenotypic characters
– He discovered 7 pea
characteristics:
Characters and their Varieties
F1 Generation
Mendel’s Conclusions
• Inheritance follows simple rules.
1. No blending of characters
2. No modification of characters through generations
3. Mendel’s first law: segregation occurs between
factors
4. Mendel’s second law: Between pairs of factors
assortment is independent
• Mendel did send a copy of his paper to Darwin,
though there is no evidence that he read it
• Do you think it would have affected Darwin’s view
of inheritance?
The Synthetic View
• From 1866 to 1900, Mendel’s paper went essentially
unnoticed.
• There were several reasons for this:
– Mendel was not connected to the biological community of the
time.
– It was thought that Mendel’s results were special to peas
• Hugo de Vries, Carl Correns, and Erik von Tshcermak
“rediscovered” Mendel’s laws in their own experiments
• But, Mendelism was associated with sports (large
mutational changes in phenotypes)
• Ronald A. Fisher, Sewall Wright, and J. B. S. Haldane
demonstrated mathematically that Mendelism was
compatible with gradual evolutionary change when
multiple genes affected a character
Kin Selection and Inclusive Fitness
• How does social behavior evolve?
• Clearly some social behavior comes at costs to
individual fitness of individuals
– Eusocial insects are the paradigm
• J. B. S. Haldane stated the essence of kin
selection with his statement (paraphrased):
– I would gladly give up my life for two sibs, or 8 first
cousins, etc. -----J. B. S. Haldane
• W. D. Hamilton captured the essence of kin
selection in a simple equation that describes the
concept of inclusive fitness
Hamilton’s Equation
The fitness of a behavior is determined by the direct affect of
the behavior on the fitness of the altruist and the indirect
fitness effects for all kin that are affected by the behavior.
Hamilton’s Equation: Analysis
• Let’s set W = 0:
n
0 = Wd + å rWi
i=1
n
0 < Wd + å rWi
• If altruism is favored then
i=1
• We know that the direct effect of altruistic
behavior, Wd, on the altruist is negative
• So
n
0 < Wd + å rWi
i=1

n
Wd < 0 < å rWi
i=1
• Thus, as long as the costs to altruist < benefits to
kin, the behavior will be favored by kin selection
Example
• if an altruist were to sacrifice its life, its fitness
would be Wd = –1
• If the altruist saves two sibs (n = 2) and their
relatedness is r = 0.5, then the indirect effect
of the behavior would be W = -1+ å 0.5(1) = 0
• If we add a first cousin (r = 0.125, we get
and an advantage for the altruist
n=2
i=1
W = -1+ 0.5(2)+ 0.125(1) = 0.125
When does it work?
• The problem for kin selection to work is that the
altruist must actually help kin
• Hamilton pointed out that it could work in viscous
populations
– That is, populations are viscous when related
individuals tend to stay close to where they were born
– Thus, in viscous populations if an altruist helps a
nearby neighbor, it is likely to be helping kin
• Kin selection will also work when individuals can
recognize kin
Problems
• Altruism is most favored under conditions
when r > 0.5, which can result in costs of
inbreeding
• There are many species of eusocial insects in
which colonies consist of multiple unrelated
queens
– In this case, an altruist helping a nearby neighbor
has a low probability of helping kin
Another Interpretation
• Perhaps the most important component of
the evolution of altruism is population
viscosity
• If that is the case, then the transmission of the
fittest phenotypes is the essential component
and not how they are transmitted
• Let’s look an alternative mode of phenotypic
transmission
Another Interpretation
• • •
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Another Interpretation
• • •
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• • • •
Adaptations
• Strong Adaptationist view
– Almost all behaviors
and characters of an organism
are adaptations
– Adaptations are produced
gradually by natural selection
– Organisms adapt to “problems”
in their ecological niche
– Typically, assume genes are the units of selection
Adaptations
• Weak adaptationist view
– Not all characters and behaviors are adaptations
– Adaptations are produced gradually by natural
selection
– Organisms adapt to “problems” (for example finding
food, avoiding predators, moving) in ecological niches
– However, “problems” are not fixed because niches
change in two important ways
• First, other organisms, which are part of an organism’s niche
evolve and change niches
• Second, organisms actively change their environment
Example of Strong Adaptationist View
• Sociobiology (Robert Trivers): Takes a strong
adaptationist view
– Parents take care of young because they have a genetic
investment in them
– They further invest resources into their young until the
costs in fitness to the parents outweighs the benefits they
receive from their genetic investment in their offspring
• E.g., for unrelated sexually reproducing parents, the expected
genetic investment is 50%
– Thus, at some point there must be a conflict between
parent and offspring
– Offspring want to get all the resources they can and
parents want to limited it to match their genetic
investment
Problems
• Research does not necessarily support parentoffspring conflict, at least in all cases
– Consider the example of 15 and 20 day-old rat
pups
• Conflict is costly
– There is a cost in wasted energy and cost of
conflict
– Thus, natural selection should favor non-conflict
resolutions to weaning
Ecological Niche: Fixed Targets
• Ecological Niches are fixed targets of Natural Selection
– Definition: An ecological niche of an organism are all those features of
an environment that affect the organism’s fitness
– These features include the resources it can access, any predators,
other organisms it is competing with
– Not every aspect of the environment is part of an organism’s niche
– Anything the organism does not interact with or that does not affect
an organism’s fitness is not part of its niche, For example
• For many birds, the upper parts of trees are part of their niche.
• While for many ground dwelling animals (e.g., many species of snakes or
mice), the shade of trees are part of their niche.
– The classical view is that there are niches in the world towards which
organisms are gradually adapting
– Somewhat analogous to gradually filing a key to fit a lock
Ecological Niche: The Red Queen
Hypothesis
Ecological Niche: The Red Queen
Hypothesis
• Characters and behaviors chase ever changing niches
• Leigh Van Valen, recognized that niches are not static
“problems” of the environment
• According to the definition of ecological niche given
above, niches must change!
• Part of an organism’s niche are other organisms that
are also evolving
• Thus, niches are dynamic and not static
• The Red Queen Hypothesis asserts that organisms are
in a perpetual state of adaptation, chasing ever
changing niches
Ecological Niches: Constructivism
• Richard Lewontin has argued that niches are even more
complicated
• Organisms actively:
– Assemble: Organisms partially assemble their environments,
• e.g. many birds build nests, social insects build nests, spiders weaving webs.
– Alter: In interacting with their environments they alter them.
• This is not necessarily the same as assembling
• For example, masses of locusts devouring vegetative matter in their path or
elephants pulling down trees to feed on the leafy branches
– Transduce: The take in energy and perceive stimuli and produce
physical products and behavioral interactions
• Consider a female mammal nursing her young
• She takes in energy in the form of food and transforms it into milk.
– Modulate: They regulate their environments
• Even Army ants can control the temperature and climate of their nests.
Problems with Strong Adaptationism
• Not all Characters and Behaviors are Adaptive, e.g.,
Scull sutures
• Skull sutures in humans and other mammals are not
adaptations for birth because they are present in
reptiles and birds
Exaptation
• Behaviors or characteristics that have evolved
by natural selection for one function
• Ritualized preening in birds may have been an
exaptation for mating rituals
• The evolution for the human thumb, not
selected for writing
Constraints on Adaptations
• Genetic constraints: Not all characteristics and behavior are
influenced by genetics
– Thus, characters and behaviors are not indefinitely malleable by
natural selection
• Allometry: different growth rates of body parts during
development
– Animals aren't simply linearly scaled up versions of themselves during
development
– Look at a baby, and their heads and eyes seem large relative to the
rest of their body
– As a baby develops there is a shift in proportions over time, with the
body growing faster than the head so that by adulthood they have
adult proportions
– These morphological changes are not necessarily adaptations, but
rather differences due to rates of growth
Example
Another Example
Evolutionary Psychology
• Behavioral and Psychological Phenomena are
traits
• Behaviors and psychological phenomena have
effects on the organism’s environment and thus
have fitness
• There are constraints on evolution of adaptations
• Modularity: Different behaviors and
Psychological phenomena can evolve relatively
independently of each other
– Different genes affect different behaviors and
psychological phenomena
Identifying Psychological Phenomena
as Traits
• Evolution by natural selection and cultural evolution
occur on different time scales
• In general, it takes much longer for a trait to evolve in a
population than traits to change as cultures evolve
– For example, food preferences
• However, if certain preferences develop, even when
they do not fit current cultural preferences or
knowledge, then evolutionary psychologists suggest
that they may have evolved by natural selection
• Our “sweet tooth” is viewed as one example
A More Detailed Example
• Gangestad and Thornhill (1998; Thornhill &
Gangestad 1999) suggest that
• The shift in female olfactory preferences toward
the scent of symmetrical men when fertile
• May be an adaptation for seeking genetic
benefits for offspring in the context of extra-pair
sex
• That is, women may pay a cost (e.g., loss of an inpair mate’s investment in offspring) and can only
reap the genetic benefit when fertile
Evidence for Olfactory Preferences
1)
2)
3)
4)
5)
Men with more symmetrical faces are perceived to be healthier
(Rhodes et al. 2001)
More symmetrical men sexualize other women more and they
invest less time and emotional support in their primary partner
(Gangestad & Thornhill 1997a);
(More symmetrical men are more likely to have extra-pair sex
partners and are more likely to be chosen as extra-pair sex
partners (Gangestad & Thornhill 1997b);
Women are more likely to have extra-pair sex mid-cycle, a pattern
not observed for sex with a primary partner (Bellis & Baker 1990);
and
Women report greater feelings of sexual attraction to and fantasy
about men other than a current primary partner when fertile, a
pattern not observed for feelings about in-pair partners
(Gangestad et al. 2001).
Problems with Evolutionary
Psychology
• If evolutionary psychology as an adaptationist
program is going to yield a number of important
results, then a number of behavioral and
cognitive processes must be modular, i.e. have
relatively little influence on each other
– There is little evidence for this assumption
– In psychology, the opposite assumption is usually
made
• Where are all the genes?