Organic Evolution Exam 3 Study Guide

Lectures 13 - 23

How do organisms reproduce: asexual reproduction

• mode of reproduction that doesn’t require union of gametes

• asexual lineages are overwhelmingly of recent origin

how do organisms reproduce: sexual reproduction

• fusion of male and female gametes

cyclical parthenogenesis

• some organisms do both asexual and sexual reproduction

• a form of asexual reproduction in which the embryo develops directly from an egg without the need for fertilization

• some organisms alter between asexual and sexual reproduction using this method

• found in gall wasps and aphids

• parthonogenesis has evolved independently hundreds of times, yet it’s still very rare among haplodiploid arthropods

what is the two-fold disadvantage of sex?

• while sexual reproduction is generally favored, there is a cost associated with it

• asexual females always give rise to females

• sexual females give rise to both males and females

• sex reproduces the net reproductive rate because males cannot reproduce and producing them instead of females impedes further reproduction

• this is known as the demographic cost of males

• generally, selection should favor evolution of asexual reproduction

why is there still sexual reproduction despite the fitness costs?

chromosome reshuffling and recombination allows:

• faster reassembly of beneficial mutations

• reducing load of harmful alleles (see mueller’s rachet)

• adaptation to varying environments

  • faster tracking of phenotypic optimum (red queen hypothesis)

mueller’s rachet

• related to evolution of sex as opposed to asexual reproduction

• associated with load reduction of harmful allelesf

• asexual lineages accumulate multiple mutations across tens of thousands of loci

• genetic load of deleterious mutations keeps ratcheting up

  • this is the cost of asexuality

• recombination unlinks harmful mutations from beneficial alleles exposing them to purifying selection

red queen hypothesis

• sex is maintained because it allows host species to counter evolve to parasites and pathogens

• you must run as fast as you can just to stay in place

• co-evolutionary arms race

• recombination allows for novel allele combinations that may provide an advantage against parasites and/or hosts

sexual selection

• refers to differential reproduction success resulting from competition for fertilization

• can occur through competing individuals of the same sex

• OR it can occur through attraction to the opposite sex

• sexual selection acts differently on each sex

Why is sexual selection asymmetric

• parental investment differs between the sexes (eggs are more expensive than sperm)

  • this is Bateman’s principle

• reproductive success in the heavily investing sex is limited by time and energy

  • low variation in fitness, weak sexual selection

• reproductive selection in the lightly investing sex is limited by the number of mates

  • high variation in fitness, strong sexual selection

intrasexual consequences of sexual selection

• relates to overt combat over access to females

• larger body size

• weaponry

• armor

• tactical cleverness

intersexual consequences of sexual selection

• attraction to the opposite sex

• colorful phenotype

• courtship song

• courtship dance

why don’t we see infinite optimization of these traits that are sexually selected?

• there are tradeoffs between survival and reproduction

• reduced fitness in male phenotypes that stand out

• higher predation rates among males due to bright coloration

• increased mortality with intense fighting

why do females prefer certain traits that are under sexual selection? In other words, why be choosy?

• direct fitness benefits

  • enhance females’s survivability or fecundity

  • protection

  • territories/nests

  • food

  • help with parental care

  • reduce risk from contagious diseases

• indirect fitness benefits

  • enhance survivability or attractiveness of a female’s offspring

    • male traits as indicators of “good genes”

• females may also have pre-existing biases which evolved in response to other selective pressures (foraging, predator avoidance, etc)

  • pre-existing biases may select male displays even if they originally have no relation to mating or fitness

  • male coloration is a byproduct of natural selection

  • example in fishes: females feed on orange food and can see this color really well, and males that happen to have this color have better fitness outcomes

sexual conflict

• evolution of phenotypic characteristics that confer a fitness benefit to one sex but a fitness cost to another

• male adaptations to intrasexual selection can be costly to females

  • extraneous copulations can lead to a potential loss of foraging opportunities, physical harm, and sexually transmitted diseases

what is social behavior?

• interactions with other members of the same species

• can be competition/aggression or cooperation/altruism

why does altruism exist?

• natural selection favors the survival of the fittest, so why would individuals sacrifice their precious energy and time to help others?

• to answer this, we must look at kin selection (Hamilton’s rule)

Hamilton’s rule

• inclusive fitness = direct fitness (personal reproduction) + indirect fitness (reproduction by relatives)

• fitness benefit to actor must outweigh the fitness cost

• prediction: individuals should behave altruistically towards kin more than towards unrelated individuals

eusociality

• extreme cooperative breeding

• relies on reproductive division of labor: reproductive and sterile (worker) castes

• cooperative brood care: workers care for queen’s offspring

• overlapping generations: queens and adult offspring exist in same colony

• in haplo-diploid societies, females share have their genes with daughters and sons

  • males share half with daughters

  • sisters share ¾ of their DNA

• workers are more genetically releated to their sisters than to their own offspring, and so by foregoing their own reproduction to hel sisters that will be queens, the helper genes get spread

game theory

• deals with interactive decision making, where the outcome for each participant or player depends on the actions of all

• say you have two prisoners:

  • if both prisoners remain silent, they each get two years

  • if either one confesses, their sentence is reduced to one year while the other’s is increased to 8 years

  • however, if they both confess, they both get 5 years in prison

  • it’s best strategy to confess

• evolutionary game theory is a way of thinking about evolution at the phenotypic level when the fitness of particular phenotypes depend on their frequencies in the population

• relates to hawks and doves game (See card)

Hawk and Dove game

• hawk = aggressive strategy

• dove = cooperative strategy

• V = benefit of cooperation

• C = cost of escalation, the more intense the aggresion, the higher the cost

• relies on a payoff matrix

• this payoff matrix allows us to calculate the absolute fitness, W, of each strategy

• When C>V, the hawk genotype cannot go to fixation

• as the more competitive gentoype increases in frequency, its fitness gain through inflicting aggression on others is countered by the damage it receives from an increasing number of aggressors

  • genes for aggression can create social nevironment that decreases its expected fitness

  • in other words, fitness is frequency-dependent

types of species interactions:

interactions between species:

• competition

• mutualism

• predation

• commensalism

coevolution

evolutionary change in one species may evoke a reciprical change in another species

relates to reciprical selection (See card)

reciprocal selection

• selection that occurs in two species due to their interactions with one another

• critical prerequisite of coevolution

coevolutionary arms race

• reciprocal selection on predator and prey species

• elude and evade traits are selected for in prey species

• hunt and capture traits are selected for in predator species

diffuse coevolution

• unlike evolutionary arms race which is a 1:1 evolution, diffuse evolution is a multiple:multiple species interaction and pattern

• describes the evolution of similar traits in several species in response to a set of selective pressures imposed by one or more interacting species

• ex: shrubby plant species often evolve thorns and unpalatable leaves to protect them from grazing herbivores of various species

escape and radiate coevolution

• mode of coevolution where one of several prey or host species evolves a major new defense

• this new defense allows it to escape typical association with predator or parasite and allows it to diversify further

• later, a different predator or parasite adapts to the host clade and diversifies

concordant phylogenies

• between host and specialized parasite/pollinator

• direct 1:1 linkage between two interacting species

discordant phylogenies

• between host and specialized parasites/pollinators

• much more common in nature

• multiple:one relationships, often multiple parasites to one host

• most common reason for discordant phylogenies is host shift: organisms shift to start feeding on different host

geographic mosaic theory

based on three observations:

• species are often collections of genetically distinct populations

• interacting species often differ in their geographic ranges

• interactions among species differ between environments in their ecological outcomes

geographic and environmental variation can exert selection on pair-wise species interactions leading to reciprical local adaptation in traits of both species

three main predictions of geographic mosaic theory:

• natural selection on species pairs varies across landscapes and environments

• variation occurs because genes are expressed in different ways in different environments (GxE interactions)

  • how one species affects the evo fitness of another species depends on the environment where the interaction is occuring

• coevolutionary hotspots

  • intensity of reciprocal selection differs among environments

  • trait remixing: mismatched coevolved traits will occur due to gene flow, extinction of local populations, genetic drift, or mutations

typological species concept

• linnaeus

• individuals conform to morphological type

• limitation: cannot address convergent evolution

phylogenetic species concept

• joel craft

• a cluster of individuals recognizably different from other such clusters, descending from a common ancestor

• populations within a species (e.g. B1-B3) are more closely related to each other than any other species

• has monophyletic groups (see card)

monophyletic group

• relates to phylogenetic species concept

• a species name is given to a collection of populations which operates under the assumption that they all descend from a common ancestor

• includes ancestor and all of its descendants

what are the limitations of the phylogenetic species concept

• requires well supported and thorough phylogenies

• it’s difficult to decide how similar the populations within a clade must be to gain species designation

biological species concept

• E. Mayr

• species defined as group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups

• this concept groups organisms based on reproductive isolation

  • this works because species rarely or do not hybridize in nature

• it’s a biologically meaningful definition because it confirms lack of gene flow

• defines a species as a discrete unit of evolution, which forms a boundary for the spread of alleles

• difficult, however, to determine whether or not species are reproductively isolated

  • like in asexual species or in extinct species

• also difficult to apply the concept to species that are geographically separated

What are the broad mechanisms of reproductive isoation?

reproductive isolation: pre-mating barriers

• prezygotic

• isolating barriers that impede gene flow before sperm or pollen can be transferred to other species

• ecological isolation

  • habitat

  • temporal

  • pollinator

• behavioral isolation

• mechanical isolation

  • genital incompatibilities that arise through genital coevolution

reproductive isolation: post-mating, pre-zygotic isolation

• isolating barriers that impede gene flow after sperm or pollen transfer, but before fertilization

• copulatory behavioral isolation

• gametic isolation

  • sperm and egg are able to meet, but they’re incompatible and cannot form a zygote

  • example in sea urchins which which all release sperm and egg into sea water

reproductive isolation: post-zygotic isolation

• isolating barriers that act after a zygote begins to develop

• isolation can either be intrinsic (independent of the environment) or extrinsic (fitness of hybrid depends on the environment)

post-zygotic intrinsic isolation

• intrinsic isolation

  • hybrid inviability—> hybrids can form, but embryos often have lethal morphology

  • hybrid sterility—> mule (sterile hybrid of horse and donkey)

post-zygotic extrinsic isolation

• ecological inviability

  • hybrids have half of the genes of each parent, producing intermediate phenotype that's maladapted to either parental environment

• behavioral sterility

  • same concept as ecological inviability, but with mating instead

  • hybrids have difficult time attracting mates because of their intermediate phenotype that is unattractive to both parental species

how do reproductive barriers evolve?

• necessary to understand evolutionary forces causing reproductive isolation

allopatric speciation

• isolation of populations, typically via geographic barrier

  • gene flow eliminated

• divergence through selection and drift

  • reproductive isolation arises as a correlated trait or by chance

• if the barrier is removed and they make contact again:

  • positive selection for mating behaviors that reinforce reproductive isolation

    • for this to occur, there must be a cost of interspecific mating, like sexual isolation of hybrids

  • can also result in fusion (incipient speciation reversal) or hybrid speciation

what is the mechanism for reproductive isolation in allopatric species once the geographic barrier is removed?

• in absence of selection

• dobzhansky-muller incompatibilities

• epistatic interactions between mutated genes creates less-fit hybrids

non-allopatric speciation

• new species appear to arise without physical separation

• can happen when a new host is introduced, new feeding niche promotes speciation

• example in the hawthorne flies, which started feeding on apples that have different phenologies causing temporal isolation between the populations’

• relates to ecological speciation (see card)

ecological speciation

• reproductive isolation as a byproduct of adaptation

  • evolving between populations as a result of ecologically-based divergent selection

• selection is strong enough to promote trait divergence even in face of gene flow

• adaptive changes may inadvertently impact reproductive isolation as a byproduct

  • potential partners:

    • don’t meet

    • don't recognize each other

    • aren’t compatible during mating

    • produce unfit offspring

micro vs macroevolution

• microevolution: populations and population changes

• macroevolution: evolutionary patterns in different speices and how species are related to each other through evolutionary processes

the boundary between the two fields is the species:

• if you talk about sub-species differences, this is microevolution

• differences between species is macroevolution

what are the three main patterns in macroevolution

• origin of major new forms of life

• gradualism and saltation

• evolution of novelty

patterns in macroevolution: origins of new forms of life

• example: how did mammals evolve from other forms of life?

patterns in macroevolution: gradualism and saltation

• evolution can occur either in incremental steps (gradualism) or large leaps (saltation)

• this debate is shown in the evolution of cetatians (marine mammals)

  • the closest extant relative to a whale is a hippo, and there’s obviously a big morphological gap between those two species, suggesting saltation

  • however, macroevolutionary studies looking at extinct ancestors shows that there was a gradual change between land mammals and marine mammals

• this concept can also be applied within one group of organisms, like turtles with the evolution of the carapice

  • turtles are an example of gradual evolution again, with a gradual movement of the shoulder blade within the ribcage

patterns of macroevolution: evolution of novelties

• novelty: trait or character that is completely new to a group of organisms, oftentimes with a new function

• example: evolution of eyes giving rise to vision

conflicts between micro and macroevolution: evolutionary rates

• microevolution rates are muchhhh higher than macroevolutionary rates which creates a conflict

• one potential explanation is due to the fact that macroevolutionary studies are limited by data

• concepts that can connect the two fields:

  • paradox of stasis: living fossils

  • phylogenetic niche conservatism (habitat tracking)

  • stabilizing selection

  • genetic constraints like gene pleiotropy and genetic lines of least resistance

phylogenetic niche conservatism (habitat tracking)

• some organisms can move to track their optimal niche instead of adapting to a changing environment

• gives rise to low rates of change

genetic constraints with micro and macroevolution

• gene pleiotropy—> one gene is responsible for multiple traits (one gene controlling hair and eye color)

  • evolutionary force working on one trait will inadvertently work on the other connected traits vis pleiotropy

• genetic lines of least resistance

  • constrain evolutionary pathway of a species

punctuated equilibrium

• long periods of no change interrupted by periods of sharp change creating a more diagonal, sharp line

phyletic gradualism

• organisms undergo very gradual change creating a smooth trait curve over time

macroevolution: passive trends

• trend: describes persistent directional change of a trait over time

• passive trends have larger variance

• changes in character value are random but there is a lower or upper limit on the trait value

• the average trait value might not change much, but the variance changes a lot, increasing as traits deviate from the mean

macroevolution: active trends

• character state is persistently changing on one direction, changing the mean value of the trait, and not necessarily the variance

Cope’s rule

• the tendency for organisms in evolving lineages to increase in size over time

• not much empirical support for this rule

• often thought to be an active trend because:

  • Larger size can confer advantages: predation resistance, greater reproductive success, thermoregulation, competitive ability.

  • In some groups (e.g., mammals, dinosaurs), many lineages do independently show size increases over time

What is a human

• homo sapiens

• obligate bipedalism

• large brains

• complex, symbolic behavioral capabilities

key trends in hominin evolution

• bipedalism

• dentition

• diet

• tools

• brian

• genetics

bipedalism

three main types:

• habitual bipedalism—> regularly bipedal but uses other locomotion categories too

• obligate bipedalism—> only bipedal; doesn’t use other locomotor capabilities

• occasional bipedalism—> generally uses other locomotor categories but may occasionally use bipedalism

how can you tell if something is bipedal?

• foramen magnum position (big hole)

  • where the spine enters the skull

• spine: thoracic and lumbar curvatures maintain center of gravity over the pelvis

• pelvis shape

• femur angle

• foot (do they have opposable toes)

adaptive explanations for the origin of bipedalism

• social factors

  • carrying tools, infants, food

  • provisions for the “Family”

• ecological factors

  • moving more efficiently on the ground

  • thermoregulation

  • finding food and spotting predators over tall grass

diet in hominins

• teeth interact with the food we eat as individuals

• diet can be inferred from tooth size and shape, tooth microwear, and isotypes

• microscopic scratches and pits on the teeth occur from food being chewed

evolution of tool use

• allows access to resources that may have previously been inaccessibe

• paleoarchaeological record

  • stone tools

  • generally organic materials to not preserve,

brain evolution

• modern humans have large brains relative to body size

• increase in brain size throughout human evolution

genetic evolution in hominins

• tracked through both nucleic DNA and mitochondrial DNA

• 1-4% of the genome of eurasian people comes from neanderthals

what are the four main topics of evolutionary medicine?

1. why do we age and die

2. maladaptation and diseases of civilization

3. epidemiology and managing pathogen evolution

4. antibiotics and evolving superbugs

rate of living theory

• aging as a consequence of accumulation of irreparable damage to cells and tissues

• natural selection has already pushed lifespan to the maximum

• maximum lifespans observed by different species today are bound by intrinsic constraints that are impossible for natural selection overcome

• see card for main preidctions

predictions of rate of living theory

1. the higher the metabolic rate, the lower the lifespan

   1. because cell and tissue damage are caused in part by the by-products of metabolism, the aging rate should be correlated with metabolic rate

2. selections on longevity should not lead to longer lifespans

   1. in other words, lifespan is not evolvable

evolutionary theories of aging

• under the evolutionary theory, aging is not caused by cell and tissue damage

• it is instead caused by failure of organisms to repair such damage

• failure to repair caused by:

  • mutation accumulation hypothesis

  • antagonistic pleiotropy hypothesis

mutation accumulation hypothesis

• deleterious mutations that cause failure to repair accumulate due to lack of selection

antagonistic pleiotropy hypothesis

• mutations can create tradeoffs between repair and other fitness related functions

• some genes have beneficial effects early in life (things like increasing reproductive success buy can be harmful later in life (like contributing to aging or disease

• simply put, this is known as the fitness cost of longevity

ecology and the evolution of aging

• evolution of aging shaped by the rate of extrinsic mortality:

  • the likelihood that external factors like predation, disease, resource limitation, or exposure to environmental stress can kill an organism

• two major strategies:

  • reproduce fast and die young

  • reproduce fast and die old

how is aging built into our genome?

• deleterious mutations have accumulated in our genomes for millions of years

• many have likely only become visible in recent times as human lifespans have increased

  • ex: aging related diseases

• many have and will be sticking around because carriers actually have higher fitness