Evolution and Development

PSC 113Jeff 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

– Conditions for the Evolution of Adaptations• Continuity• Quasi Independence

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 characters2. No modification of characters through generations3. Mendel’s first law: segregation occurs between factors4. 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:

• If altruism is favored then

• We know that the direct effect of altruistic behavior, Wd, on the altruist is negative

• So

• 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

• If we add a first cousin (r = 0.125, we getand an advantage for the altruist

Problems

• Altruism is most favored under conditions when r > 0.5, which can result in costs of inbreeding

• It is now known that there are many species of eusocial insects with multiple unrelated queens and workers kin selection is hard pressed to explain these species

Another Interpretation

• Inclusive fitness depends on the coefficient of relatedness, r• But, perhaps there is a more general mechanism that explains r?• Proximity: Individual that are in close proximity are more likely to

– Be related and/or– Exhibit similar behaviors

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 parent-offspring 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

Conditions for the Evolution of Adaptations

• If the constructivist view of adaptations is correct, then successful adaptations can occur only if there is

• Continuity: “...very small changes in a character result in very small changes in the ecological relations of the organism and therefore very small changes in reproductive fitness. So a very slight change in the shape of a fin or mammalian appendage to make it finlike cannot cause a dramatic change in the sexual recognition pattern, or make the organism attractive to a completely new set of predators.”

• Quasi Independence: “there exist a large variety of developmental paths by which a given character may change, and although some of these may give rise to countervailing changes in other organs and in other aspects of the ecological relations of the organism, a non-negligible proportion of these paths will not result in countervailing effects of sufficient magnitude to overcome the increase in fitness from the adaptation.”

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 in-pair mate’s investment in offspring) and can only reap the genetic benefit when fertile

Evidence for Olfactory Preferences1) Men with more symmetrical faces are perceived to be healthier

(Rhodes et al. 2001)2) More symmetrical men sexualize other women more and they invest

less time and emotional support in their primary partner (Gangestad & Thornhill 1997a);

3) (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);

4) 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

5) 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?