Psych 119 Week 2

Evolution: The Basics

• Evolution: The change in the genetic composition of

populations over time

• Individuals within a species differ from one another, in

part due to genetic, or heritable, difference

yes

The process of natural selection predicts that:

§ If a heritable trait confers an advantage, it will

become more pervasive in the population over

time

§ If a trait is disadvantageous, it will become less

pervasive in the population ov

yes

§ “Survival of the fittest

= ability to pass down genes to the next generation

Charles darwin 

Noticed variation between species 

The human offspring will appear different but tend to resemble the mother or father

He wanted to study heritability: parents passed down traits to offspring will they get an adaptive advantage or not? If it's advantageous, become better and passed down 

Natural selection = 

Evolution is a theory → 

yes

Evolution: The Basics

• Heritable traits are encoded by genes, which are

linear stretches of DNA

§ DNA is contained in chromosomes within the

nucleus of each cell in an organism

• The human genome contains an estimated 20,000–

25,000 genes

• Information contained in genes, interacting with the

environment over time, results in evolution

• Gregor Mendel - worked with pea plants, father of molecular genetics 

• Thanks to him, we know heritability 

• There is no direct correlation between genes and ?? the complexity 

• The genes themselves cannot act on themselves; genes are codes that need to be turned on by the environment 

• GENE X ENVIORMENT 

• If a gene is advantageous depends of it interacts with the environment

• Interaction between nature and nurture 

• Genes interact with our experiences to develop an organism 

• The current organism and current situation can influence current behavior 

• Many cycles of this will lead to evolution 

• INTERACTION IS THE KEY

• Identical twins have a few differences, because of the ineraction witht he environment

yessss

Diverse Methods of Reproduction Have Evolved

• The earliest known organisms to use DNA as a

genetic repository were microscopic, single-celled

organisms that reproduced by splitting in two (binary

fission). In these early organisms, DNA was

exchanged between individuals

• Today, bacteria swap DNA by conjugation. Bacteria

are connected by a hairlike tube (pilus), and a small,

circular piece of DNA (plasmid) passes through the

tube into the recipient bacterium’s genome

Diverse Methods of Reproduction Have Evolved

• Organisms that contain a nucleus in their cells are

eukaryotes

§ DNA contained within nuclei

§ Eukaryotes can be single-celled or multicellular

• Diversification of eukaryotic animals occurred in the

Cambrian explosion 545 million years ago

• The nuclei are eukaryotes 

• 2.5 billion years ago 

• Until 545 million years ago

• The Cambrian explosion  happened fast; diversity happened

Diverse Methods of Reproduction Have Evolved

• Eukaryotic cells replicate through mitosis

• Nearly all eukaryotic organisms can reproduce

sexually. This results in the mixing of DNA from each

parent through two specialized cells called gametes

• Gametes are usually haploid (possessing one-half the

number of chromosomes contained in typical, or

diploid, cells)

• E.g., Ovum & sperm

• Haploid gametes are produced by specialized cell

division known as meiosis, which takes place in the

Gonads

yes

Sexual Reproduction: Why?

• Most multicellular organisms are capable of sexual

reproduction, and most vertebrate species reproduce

sexually

§ Implies that this strategy must be more adaptive

than asexual reproduction to some degree

• But why is sexual reproduction adaptive when there

are considerable costs involved?

§ Risks of mortality (or just time & $)

§ “He’s half the man [or woman] he [or she] used to

Be

• Another cost Many male animals compete for female mates; they fight each other to death 

• They risk their lives for sex 

• For humans → male spends all the time and money buying flowers, chocolate, or other things 

• Seeking and maintaining mates is a cost 

• Another cost: loss of genetic legacy 

•  Fitness in the dominant sense is ability for you to pass your genes on to the next generation.

Theories about Sexual Reproduction

• Elimination of harmful

mutations

§ Mutations are

random and

irreversible

§ Mutations can affect

an organism’s ability

to survive and

reproduce

§ Deleterious

mutations removed

by “shuffling

yes

• Mistake for mitosis or a environment harm (radiation) mutations 

• Mutations can be non-harmful too 

• Mutations can be harmful 

• Father's copy with harmful mutation and mother's good, 

• Sexual reproduction does not produce all genetically perfect offsprings, its random 

• Super lucky ones get good copies of mom and dad 

• On avg, the good combinations will outnumber the bad combinations 

1. Generation of novel genotypes

• Genotype: An organism’s genetic makeup.

• Always room for improvement: New combinations of genes may give offspring advantages over their parents.

• Changing environments: If the environment changes, some new genetic combinations may help organisms survive, resist disease, or reproduce more successfully.

2. Sexual vs. asexual reproduction

• Sexual reproduction: Combines genetic material from two parents, creating genetically varied offspring.

• Asexual reproduction: Usually involves one parent and can produce many offspring quickly, often with very similar genes

Stable environment

• Aphids can reproduce asexually.

• They produce many offspring quickly.

• This helps their population grow rapidly.

Changing or stressful environment

• Some aphid species switch to sexual reproduction.

• Genetic mixing creates new gene combinations.

• Some offspring may be better suited to new conditions

The Red Queen Hypothesis

suggests that sexual reproduction is beneficial because it creates genetic variation, helping organisms continually defend themselves against rapidly evolving parasites

Theories about Sexual Reproduction •

 Which theory is correct? §

 Evidence for all exists 

§ Research continues 

§ Naturalistic observation of host-parasite interactions

 § Computer simulation

• Lakes → study gut parasites and lake, find that red queen hypothesis 

• Computer programs: we virally eliminate any confounding variables and just press fast forward through generations; happens genotype 

Why Are There Two Reproductive Sexes?

• “Male”: Individuals that possess small, nutrient-poor

Gametes→ sperm 

• “Female”: Individuals that possess large, nutrient-rich

Gametes– ovaum 

yes

Why Not “Sex Without Sexes”?

Main idea: Scientists explain the evolution of two sexes through differences in the size and investment of reproductive cells, called gametes.

yes

What is Disruptive Selection?

Disruptive selection happens when natural selection favors individuals with extreme traits on both ends and selects against individuals with average traits.

Example: Gamete size (sperm and eggs)

Imagine reproductive cells can be small, medium, or large.

• Small gametes: Can be produced in large numbers.

• Large gametes: Provide more nutrients and resources.

yes

• Medium gametes: May not be as effective as either small or large gametes.

Over time, natural selection may favor the two extremes: small gametes and large gametes

Disruptive Selection (Evolution)

Disruptive selection is a type of natural selection in which individuals with extreme traits on both ends are favored, while individuals with average or intermediate traits are less likely to survive or reproduce.

Example: Bird Beak Sizes

Imagine a population of birds with different beak sizes:

• Small beaks: Good at eating small, soft seeds.

• Medium beaks: Not very effective at eating either type of seed.

• Large beaks: Good at cracking large, hard seeds.

If only small and large seeds are available, birds with small or large beaks may survive better than birds with medium-sized beaks.

Why Are There Two Reproductive Sexes?

• No known species have more than two sexes

§ Some species have more than two mating types (e.g., fungi)

• Why are there about equal numbers of males and females?

§ Evolution results in a compromise between the conflicting

interests of individuals (e.g., langur monkeys)

§ Even with uneven numbers of both sexes, the sex ratio

will always tend to approach equilibrium

yes

The Paradox of Sexless Species

• There are very few species that are sexless

§ E.g., Whiptail lizard (Cnemidophorus uniparens)

§ Only females exist; reproduce parthenogenetically

yes

The Paradox of Sexless Species

• There are very few species that are sexless

§ E.g., Whiptail lizard (Cnemidophorus uniparens)

§ Only females exist; reproduce parthenogenetically

yes

• Reproduce without sex 

• Reptiles in reproduce asexually 

• Only females exsist YES lol, they have large gametes 

• Thye reproduce there on, 

• Adult female prpoduces more daughters 

• Females need to be sexually stimuated still- they do it themselves 

• They rub each other- trigger hormonal things inside and no sex  

• All lizards are idenival, thye need to still evoluve due to envtioemntal changes

The Paradox of Sexless Species

• Another example: Bdelloid rotifers (Rotaria tardigradia)

§ Existed much longer than whiptail lizards (~40 million years)

§ Only females exist; reproduce parthenogenetically

§ How did they avoid/survive the “deleterious genes” problem?

§ Ability to “absorb” genes

§ Zombies

• How do they avoid the red queen problem 

• They can survive by absorb the genes or they can become zombies 

• They have parasite they get dehydrate and then wouthotu the moisture, the parasite die they come back to life 

• They genes have no sexual condbiatio - not sex

• Sex reprodction =  good iis parasite protection and downsodes are competition, investiemnt  

• Red queen hypotheiss - species need sex to eovlive fast enoiugh to keep up with parasites and pathiogen 

yes

• Experiemnt - on Bdelloid Rtifers  

• They inject a parasit to the bdelloid , they dry out and come back to life, allows for survival 

• 5 weeks of dehydration killed pathogesn 

yes

Best of both worlds: Hermaphrodites

• Hermaphrodites: Organisms that combine both male and female

reproductive functions

§ E.g., Snails, slugs, earthworms, conifers, maize (corn) are

simultaneous hermaphrodites

• Do not self-fertilize

§ Would undo the advantages of sexual reproduction!

§ Generally, mechanisms to avoid self-fertilization exis

yes

2. Why don't they self-fertilize?

Many hermaphrodites reproduce with another individual instead of fertilizing themselves.

• Sexual reproduction combines genetic material from two individuals, creating new gene combinations.

• Self-fertilization uses an organism's own reproductive cells, which reduces genetic mixing.

• If organisms only self-fertilized, they could lose some of the genetic variation that makes sexual reproduction beneficial.

yes

Best of both worlds: Simultaneous Hermaphrodites

• “Penis fencing” in marine flatworms - “Tag - you’re it!”

They decide who plays which role; they have female and male gametes 

• Flatworms get in battles to see who can put in 

• Losing mother absorbs the sperm, now works longer, and father stays alive 

• The father flows away, and the mother stays looking for food 

• Penis fencing 

Evolution Has Led to Diverse Methods of Sex Determination

• In some reptilian species, sex is determined by the

temperature at which eggs are incubated

Offspring will be male or female

Low and high temperatures determine their sex

Examples of Temperature determines sex

• E.g.: Red-eared slider (Trachemys scripta)

§ Low temperature: Male; High temperature:

Female

• E.g.: American alligator (Alligator

mississippiensis)- the opposite 

§ Low temperature: Female; High

temperature: Male

• E.g.: Snapping turtle (Chelydra serpentina)

§ Low & High: Female; Intermediate: Male 

Temperature Experiment: on lizards 

• Using hromal manupulation control the sex of the hatchlings. So using hormones, they determine that this will have just male or female just using hormones.

• Manipulate - temperature at which they incubate their eggs 

• Right temperature = he sex that they use hormones to manipulate them to match the temperature that would produce that sex.

  • Tempatrie and sex match 

- Wrong temperature = incubator embryos where there was a mismatch between temperature and sex.

- temperature and sex not match 

Results 

Right temperature embryos: they hatched earlier and grew faster and developed better, reproductive successfully in adult life 

• Wrong temperature - mismatch with hatch slower and grow slow, less reproductively successful

Evolution Has Led to Diverse Methods of Sex Determination

• Many species of fish are sequential

hermaphrodites

• E.g.: Kobudai (a.k.a. Asian

sheepshead wrasse) are

protogynous (Greek for “female first”)

§ Males are larger than females

§ Once females grow to a certain

size range, they turn into males

§ Size advantage hypothesis: One

dominant male controls a harem of

females, so it is advantageous to

become male later in life when

Larger

• Sequential - they start off producing one and switch to the other one 

• One example  asian sheepshead are protogunpus female first where they make eggs, large gametes, and these species are sexually dimorphic; males are larger and have a bump, and females do not; females can grow and then turn into males; they will stop making eggs and making sperm 

female becomes large enough to become male to mass produce gametes, mass production of gametes , stop making eggs and start making sperm 

Males rule a harem of females, but females large enought to become a male, then becomes advtanges later in life, mass production gametes

• Many species of fish are sequential

hermaphrodites

• E.g.: Some clownfish are protandrous

(Greek for “male first”), depending on

conditions

§ Females are larger than males

§ Live within sea anemones, with one

breeding male and female pair among

subordinate non-breeding males with

undeveloped gonads

§ When dominant female dies, the male

turns into a female; largest non-

breeding male sexually matures and

becomes breeding male

§ Large females are more fertile than

smaller ones; spreading sperm when

small and gestating eggs when larger

• Clown fish  

• Male gets to to be the female is determined by size, the bigger male will become a female 

• Not all of them will change into female, only when the dominate llarge female dies, the male that's the largest in the group will become a female. 

• Its advantage so that they can reproduce

• The ultimate sex-changers: bidirectional hermaphrodites (or serial

hermaphrodites)

• Can change back and forth between sexes, depending on circumstances

• E.g.: Coral gobies

§ Live within crevices inside coral reefs, move very little

§ Limited opportunity to find mates

§ Any two fish can find each other and become a male-female mating pair,

no matter what sexes they are when they meet – very advantageous

• Live inside coral reerfs 

• They stay and rarely move around 

• They tend to move only when the coral dies 

• If you are male and male, but one male can change into female so there is a male and female to reproduce 

• They can change back to female or male dependin on their  coral environment 

• The homeless fish changes sex

Evolution Has Led to Diverse Methods of Sex Determination

• In mammals, sex is determined by sex chromosomes (XX =

female, XY = male); any individual possessing at least one Y

chromosome will be male if that chromosome encodes for a sex-

determining gene SRY

yes

• Does environment play a role in sex determination in mammals?

• Red deer mothers produce more sons than daughters when

they are in good condition, and more daughters than sons

when they are in poor condition

• “Condition” = Body size, parasitic load, social dominance

ranking, access to food and nutrients, etc environment

• Deer, mothers produce more sons in good conditions and more daughters in poor conditions 

• Trivers-Willard Hypothesis (1973): Natural selection should

favor ability of females to vary offspring sex ratios in response to

their maternal conditions

• Polygynous species in good conditions: More males than

females; vice-versa for poor conditions

How does it work?

• Mothers in good condition: May benefit more from producing sons in species where healthy, high-status males can father many offspring.

• Mothers in poor condition: May benefit more from producing daughters because daughters may have more predictable reproductive success in those species.

Example

Imagine a species where strong, healthy males can mate with many females, while weaker males may not reproduce at all.

• A mother in excellent condition may produce sons who could grow up strong and father many offspring.

• A mother in poor condition may benefit more from producing daughters, who may still reproduce even if they are not the strongest individuals.

• Larson et al. (2001): Key could lie in the amount of circulating

glucose in bloodstream

• Cow study melisa 2001 

• Lab they saw blastous higher levels of glucose favor males surival than females 

• Blood glucuse levels may serve as a proxy for materal 

• high level of circulating glucose in the mother's bloodstream, um, may favor the survival of male blastocyst, favors male survial 

•  The blastocyst they were exposed to higher levels of glucose, favored the survival of male blastocyst over females.

• Blood gluclose levels are correlated to high quality food, good conditions , 

• glucose  functions are one biochemical mechanism behind this triggered affect

• What about humans? Is there an environmental factor to sex determination

in humans?

• Evidence suggests: “You are what your mother eats” (Mathews,

Johnson, & Neil, 2008 - Cereal

• Study of pregnant women unaware of their baby's sex 

• Survey- asked how many cereals you intake, the number of bowls per cereal 

• Low group - one bowl or less per week 

• Med group - 2-6 bowls per week 

• High group - one or more per day 

• Correlated results = number of bowls of cereal increase the percentage of males born to responders also increase a little bit.

Replicated studies with calories 

Results: correlation between energy intake and the percentage of meals that were born of those pregnancies in that,

snvestment in male offspring when resources are plentifu

• The population Over 40 years ,

•  decline of male births gently with diets of young women 

• The mean energy intake has been decline , more emphasis sof being skinny in society, avoid excessive eating, possible reason why 

• Also over past years, skipping breakfast is a reason 

• Observe in humans

yes

Sexual Selection Produces Anatomical and Behavioral

Differences between Males and Females

• Sexual selection is the evolution of sex differences due to

competition related to mating and reproduction. It leads to

differences between males and females in their appearance and

behavior (Sexual dimorphism)

• Males and females follow different reproductive strategies and

are exposed to different reproductive risks

• Most commonly: Males often compete for access to females,

while females often choose among males

• Different reproductive risks between males and females

• Males: Waste of life!- if you cannot pass the genes on 

• Females: Waste of time and energy! -

Two main types of sexual selection

1. Intrasexual selection (competition): Members of the same sex compete with one another for access to mates.

Example: Male deer fight using their antlers to gain access to females.

2. Intersexual selection (mate choice): One sex chooses mates based on certain traits.

Example: Female peacocks may prefer males with large, Sexual Selection and Sexual Dimorphism

yes

Sexual dimorphism refers to differences in physical appearance, anatomy, or sometimes behavior between males and females of the same species.

yes

The Male Strategy: Competing for Access to Females

• Physical competition

• Obvious advantageous traits: Size, strength, & aggression

• Growling, screaming at each other, fight to death,

yes

The Male Strategy: Competing for Access to Females

• Ability to “detect” receptive females

• Through pheromones

• E.g., Cecropia moths (Hyalophora cecropia)

• Humans - “The scent of a woman”

What are pheromones?

• Pheromones are chemicals released by an organism that can influence the behavior of other members of the same species.

• Some female animals release pheromones to signal that they are ready to mate.

• Males detect these chemicals and may use them to locate females.

• Female cecropia moths release sex pheromones.

• Males have large, feathery antennae that can detect tiny amounts of these chemicals.

• This helps males locate females from a distance.

§ Miller & Maner, 2010:

» Women told to wear t-shirts for 3 days over various

stages of their menstrual cycles

» Highest testosterone in bloodstream found in men

who smelled shirts of women during ovulation but not

other phase

yes

Miller & Maner (2010): Pheromones and Ovulation

This study relates to how men may respond to chemical signals associated with female fertility.

What did the researchers do?

Women wore T-shirts for three days during different stages of their menstrual cycles.

Men were asked to smell the T-shirts worn by these women.

Researchers measured the men's testosterone levels in their bloodstream.

What did they find?

Men who smelled T-shirts worn by women during ovulation showed higher testosterone levels than men who smelled T-shirts worn during other phases of the menstrual cycle.

• Results, the highest testosterone, smell shirts of women who ere more ovulation part of their cycle and the lowest testosterone levels of smelling shirts of women who listening lowest ovulation part of their cycle 

• Survey for men, highest rating was for women in their highest part of their cycle 

• Unconscious ability for humans to know 

yes

The Male Strategy: Competing for Access to Females

• Early sexual maturation - beat the crowd!

• Early in life or early in mating season

• The earlier you mature, the more advantages you have 

yes

• Sexual endurance

• Ability to remain reproductively active

for a longer time

§ E.g., male pectoral sandpipers

(Calidris melanotos) can mate

continuously for nearly 3 weeks

• The most extreme example 

• These birds spend breeding season in high 

• During mating season, all birds mate, the male can continue to have sex for 3 weeks 

• They do the mating; they rarely mate with the same female twice, very advantageous 

• The mating season in summer,

The Female Strategy: Making the Best Choice

• Choice based on appearance: Bigger, Brighter, Stronger!

• Often, the appearance of males give indication about their fitnes

• These advangtegous allow to give u about good genes, size (larger size advantageous)  color (peacocks flashy and i can escape them) and strength (use this to fight iover males)  

• Males have long tails, female more attracted to males with longer tails 

• The longer the tail , the sexier they are to females

The Female Strategy: Making the Best Choice

• Choice based on appearance: Bigger, Brighter, Stronger! - ask the graph in office hours 

• Often, appearance of males give indication about their fitness

• Runaway selection: Supernormal stimuli

• Herring gull chicks peck at a red spot on their parents's bill to induce their parents to regurgitate food. Chicks will also peck at a model consisting of a red spot against a yellow

 Avoid it thooo

Runaway Selection

Runaway selection is when a trait becomes more extreme over generations because the trait makes an individual more attractive to mates.

And even though these are sexy traits, uh, you know, they they cannot get super exaggerated because of, you know, the cost outweighing the benefits.

Um, they could still go as far as they can, as long as they don't run so far to the point where the costs outweigh the benefits.

The Female Strategy: Making the Best Choice

• Choice based on appearance: Symmetry

• Fluctuating asymmetry (FA): Small random deviations

from perfect symmetry in bilaterally paired structures

§ High FA correlated with chromosomal defects,

infections, parasites, exposure to toxins, &

environmental stress

§ Low FA correlated with high quality sperm in males

§ Males selected for symmetry; females selected for

ability to evaluate symmetry

yes

§ Low FA associated with increased genetic, physical,

and mental health, including cognitive skills and IQ

§ Men with low FA tend to be rated as being more

muscular, vigorous, and more facially attractive

(Gangestad & Thornhill, 1997; Baker 199

• Good genes and environment 

• Symmetrical good indicator of good sperm in males 

• Perfer to mate with makes with symatrical mates, 

• Females are selected for symmetry, they do not have to show much symmetry since they have a more larger gamete 

• Experiments where stick on feather on one side or cut feather, 

• Humans 

• Lower FA, the better male for repropducitve successful, 

The Female Strategy: Making the Best Choice

• Choice based on scent: Symmetry

• Gangestad & Thornhill, 1998:

 Humans appear to have the ability to detect low FA through odors, especially

in females

§ Experiment: Women evaluated scents of t-shirts worn by men for 2 nights

§ Women near the peak fertility of their cycle tended to prefer the scent of

shirts worn by symmetrical men (low FA) 

• Female for the more fertile part of the cycle favored  more the men with low FA 

• They were better at detecting males with low FA

• Low women fertile and contraceptives they showed no preferescne in symeptical or non symetrical l male scent 

yes

The Female Strategy: Making the Best Choice

• Choice based on scent: Genes affecting immune system

• Major histocompatibility complex (MHC): A highly polymorphic gene complex that encodes

for cell-surface receptors that play a role in the initiation of most immune responses

§ By binding and presenting peptides derived from either endogenous or foreign proteins,

the MHC plays a central role in the discrimination of self from non-self, thereby regulating

recognition of infectious diseases.

yes

What are MHC genes?

• MHC stands for Major Histocompatibility Complex.

• These genes help the immune system recognize pathogens, such as viruses and bacteria.

• MHC differences can influence body odor.

§ Mouse and human experiments: MHC genes influence body odor, mate choice based on

body odor attractiveness

§ Humans: Normally ovulating women preferred the scent of men who had MHC

phenotypes dissimilar to their own (Wedekind, Seebeck, Bettens, & Paepke, 1995)

• Women who were normally ovulating smelled T-shirts worn by different men.

• They tended to prefer the scent of men whose MHC genes were different from their own.

• One proposed explanation is that choosing genetically dissimilar partners could increase immune-system diversity in offspring.