Population genetics

meiosis creates variation among individuals

consequences at the population level

The gene pool

Technically speaking: sum total of genes in a population at a given time (all alleles at all loci)

Practically speaking: usually limited to particular gene(s) of interest

Describe population-level variation: phenotype, genotype, and allele frequencies

Phenotypes

Describe population-level variation: phenotype, genotype, and allele frequencies

Genotypes

"red" (A2) and "white" (A1) alleles: which is dominant?

Describe population-level variation: phenotype, genotype, and allele frequencies

Alleles

Make absolutely sure you understand the differences, and the relationships, among these three parameters!

Allele frequencies: your turn

brown (B) vs. blue eyes (b)

2000 people

1280 homozygous dominant (brown)

640 heterozygous (brown)

80 homozygous recessive (blue)

how many copies of eye color alleles are there?

how many of these copies are B?

how many copies are b?

what are the allele frequencies?

Genotype frequencies

2000 people (1280 BB, 640 Bb, and 80 bb)

Calculating genotype frequencies is easy

BB: 1280/2000 = 64%

Bb: 640/2000 = 32%

bb: 80/2000 = 4%

Hardy-Weinberg (fig. 23.3)

How will this population's gene pool change through time?

Hardy-Weinberg theorem provides an answer

Genotype frequencies:

RR = 0.64; Rr = 0.32; rr = 0.04

Frequency of different types of gametes:

R: 0.64 + .16 = 0.80

r: 0.16 + 0.04 = 0.20

Hardy-Weinberg

If gametes come together at random, what are the genotypes in the next generation?

RR: 0.80 * 0.80 = 0.64 (64% will be homozygous dominant)

Rr: 0.80 * 0.20 plus 0.20 * 0.80 = 0.16 + 0.16 = 0.32 (32% heterozygous)

rr: 0.20 * 0.20 = 0.04 (4% homozygous recessive)

NO CHANGE!

populations will not evolve without some cause (i.e., allele frequencies do not change on their own)

Hardy-Weinberg: next generation

p and q are the allele frequencies; 0.8 and 0.2

(note if only two alleles: p + q = 1)

p2 + 2pq + q2 = 1

p2 gives the genotype frequency for RR

2pq gives the genotype frequency for Rr

q2 gives the genotype frequency for rr

Calculating allele, phenotype, and genotype frequencies

you have got to be clear on what each of these refers to!

Remember what p and q mean

Learn to move between these two equations

p + q = 1

p2 + 2pq + q2 = 1

Always start by looking for homozygous recessives (why?)

Assumptions of Hardy-Weinberg

very large population size (otherwise chance begins to play a role)

closed population (i.e., no exchange with other populations)

no mutation (mutations instantly alter the gene pool)

random mating (individuals with certain alleles don't seek out or avoid mates with those same alleles)

no selection (selection favors certain alleles)

Genetic drift

genetic drift is the chance change in allele frequencies due to small population size

How do we visualize this effect?

Flip a coin 1 million times

you expect about 50/50 heads vs. tails

you'd be suspicious if you got 700,000 heads and 300,000 tails

Now flip it 10 times.

still expect, on average, a 50/50 ratio

but you would not be surprised if you got 7 heads and 3 tails (a chance deviation from p=q=0.5)

Better yet, demo time!

In small populations, allele frequencies can change by chance or accident

Bottleneck effect

populations are reduced to small size and subsequently recover

reduced population ("a bottleneck") unlikely to be perfectly representative of original population

thus, allele frequencies change (i.e., the population evolves)

Founder effect

similar, but occurs when a new population is founded by a small group of colonists

Can result in rapid evolution

responsible for prominence of certain diseases

IMPORTANT: genetic drift is nonadaptive, random change

Bottleneck effects: some classic examples (all with unusually low genetic variability)

Cheetahs

so little genetic variation, skin grafts from one to another do not trigger immune response

Bottleneck effects: some classic examples (all with unusually low genetic variability)

Elephant seals

from N=30 (1890's) to tens of thousands today

Bottleneck effects: some classic examples (all with unusually low genetic variability)

Wisent (European Bison)

all alive today descend from population of 12

more prone to various diseases than related species

increasing rates of reproductive failure in males

Gene flow (migration)

Migration of individuals between populations

potentially introduces new alleles into a population or alters existing allele frequencies

e.g., human populations

tends to be nonadaptive

reduces genetic differences among populations; thus retarding genetic differentiation

Gene flow example

Great horned owl >>>> deer mouse

Body size

Mobility

Average distance young disperses from nest

Average home range

Great horned owl <<< deer mouse

Genetic variation across range

Mutation

Rare (about one mutation per locus per one million gametes)

On the other hand, each individual has thousands of genes

Populations can have thousands to billions of individuals

Generation times can be very short (so more gametes are produced per unit time)

Evolutionary time is very long

Thus, tends to have a small effect on populations in the short term, but profound effect on long term evolutionary trends

Nonadaptive

This is important: a mutation won't happen just because an organism could use it!

What will typically happen to a new allele?

Nonrandom mating

Inbreeding

individuals tend to mate with nearby individuals (especially in plants); nearby individuals are more likely to be related

Nonrandom mating

Assortative mating

individuals tend to select mates that are similar to themselves (positive assortative mating)

Nonrandom mating

Assortative mating

Body size or coloration most common similarities

Goldenrod leaf beetle (Trirhabda canadensis) field data

Nonrandom mating

Sexual selection

Some individuals are more successful than average at acquiring mating opportunities

Nonrandom mating tends to increase homozygosity; may or may not affect allele frequencies; may or may not be adaptive

Natural selection

Darwin's greatest contribution; he proposed a testable mechanism for evolution

Based on simple observations, some of which we have already made:

Populations have tremendous growth potential

Populations tend to remain stable

Resources are limited

Thus, there is a "struggle for existence" in which most offspring produced die

Individuals vary

This variation is heritable

The best adapted (most "fit" for the environment) organisms survive and pass their traits to their offspring

Thus, the population will change over time

Adaptive, nonrandom

Assumptions of Hardy-Weinberg

very large population size (otherwise chance begins to play a role)

closed population (i.e., no exchange with other populations)

no mutation (mutations instantly alter the gene pool)

random mating (individuals with certain alleles don't seek out or avoid mates with those same alleles)

no selection (selection favors certain alleles)

Natural selection

Darwin's greatest contribution; he proposed a testable mechanism for evolution

Based on simple observations that we have already made:

Populations have tremendous growth potential

Populations tend to remain stable

Resources are limited

Thus, there is a "struggle for existence" in which most offspring produced die

Individuals vary

This variation is heritable

The best adapted (most "fit" for the environment) organisms survive and pass their traits to their offspring

key terms here: fitness and adaptation

Thus, the population will change over time

Adaptive, nonrandom

Selection operates on phenotypes, not genotypes (why?)

The peppered moth: a classic example

Pre-industrial England; birches had white bark

Industrial England; birch bark coated with soot

When light bark predominated, so did white moth morphs

Pollution controls in England; birches had white bark, white morphs again predominated

What's going on here?

Same phenomenon reported for dozens of species in both Europe and North America

Natural selection requires variability, heritability, and differential fitness of traits

Traits with no variation provide selection with nothing to select among

Nonheritable traits, and thus the advantage they may provide, cannot be passed on from one generation to the next

Two different, heritable characters that provide equal levels of fitness benefits should be equally successful (i.e., selection cannot distinguish between them)

Natural selection results in evolution of adaptations

adaptation: trait that enhances an organism's survival and reproduction (e.g., fig. 22.10)

Adaptations: variations on a theme

Praying mantis: a modified cockroach!

Adaptations: variations on a theme

Adaptations, homologies, and analogies

Evolution diversifies homologous structures to perform different functions

Example

Within any group of flying or floating organisms

Large, broad "wings" used by soarers, slow, not manueverable

Narrow, long "wings" fast (in all directions!), not manueverable

Short, broad "wings" maneuverable, not especially fast or buoyant

BBC vid pt. 1

BBC vid pt. 2

Adaptations, homologies, and analogies

Can also shape non-homologous structures to perform similar function (analogies)

Example

Within any group of flying or floating organisms (including birds, bats, dragonflies, even tree seeds!)

Large, broad "wings" used by soarers, slow, not manueverable

Narrow, long "wings" fast (in all directions!), not manueverable

Short, broad "wings" maneuverable, not especially fast or buoyant

Adaptations, homologies, and analogies

Can also shape non-homologous structures to perform similar function (analogies)

Example

Within any group of flying or floating organisms (including birds, bats<, insects, even tree seeds!)

Large, broad "wings" used by soarers, slow, not manueverable

Narrow, long "wings" fast (in all directions!), not manueverable

Short, broad "wings" maneuverable, not especially fast or buoyant

Adaptations, homologies, and analogies

Can also shape non-homologous structures to perform similar function (analogies)

Example

Within any group of flying or floating organisms (including birds, bats, dragonflies, even tree seeds!)

Large, broad "wings" used by soarers, slow, not manueverable

Narrow, long "wings" fast (in all directions!), not manueverable

Short, broad "wings" maneuverable, not especially fast or buoyant

Natural vs. artificial selection

Natural selection:

Selection: differential survival and reproduction

Natural selection: favored traits improve the fit of organisms to their environment

Artificial selection: favored traits are those that for one reason or another are preferred by humans

Types of selection (fig. 23.12)

Natural selection can affect traits in three distinct ways:

directional

stabilizing

disruptive (diversifying)

Directional selection

individuals with more extreme values for a trait have greater fitness

result: directional change in the trait

e.g., Darwin's Finches (fig. 23-13)

Stabilizing selection

Most common (average) phenotype is most fit

stabilizing because it eliminates extremes

example: human birth weight

Disruptive (diversifying)

occurs when extreme individuals are more fit (Fig. 23.12, 23.14)

diversifies the population; often associated with frequency-dependent selection

Selection and variation

Why doesn't selection "use up" variation?

Continued selection should eventually make every individual look alike; several things prevent this:

diploidy - heterozygotes hide recessive alleles

balanced polymorphisms

heterozygote advantage

frequency-dependent selection

fluctuating selection - direction of selection varies

(jump to 21)

Fitness components

Fitness = relative number of offspring that survive to sexual maturity =

p(your surviving to sexual maturity) *

duration of reproductive lifespan *

frequency of successful matings *

number of offspring/mating *

proportion of offspring that survive to sexual maturity

(jump to 26)

Sexual selection

Darwin realized that not all traits could be explained by natural selection

some traits enhance mating success, even at the expense of survival

definition

sexual selection favors traits that improve the mating success of the individual

in most animals, operates most intensely among males

sperm is cheap!

there are a few exceptions

Sexual selection vs. "survival of the fittest"

Natural selection favors those traits that lead to improved representation of genes in next generation

Jim's genes allow him to live to be 100, but make him, er, unappealing to prospective mates

Joe's genes make him attractive to the ladies, but at a cost: he's not likely to live past 65

Who has the higher evolutionary fitness?

That is, who's genes are going to be found in subsequent generations?

Intrasexual selection

Favors traits that deal with conflict between individuals of the same sex

Sexual selection and intrasexual competition

When males compete for access to females, bigger usually better

Hermit crabs: in a direct conflict between a bigger and smaller male, who gets the girl?

Clibanarius digueti: Bigger 12, smaller 0

Pagurus hirsutiusculus: Bigger 15, smaller 0

Diogenes nitidimanus: Bigger 9, smaller 0

result? Males at least twice as large as females in these species

Secondary results: evolution of "weapons" to improve chances in combat, followed by ritualization to reduce risks

Intersexual selection (mate choice)

Favors traits that enhance attractiveness to the opposite sex

result:ornaments (plumes, wattles, bright color, songs, symmetry, etc.) - apparently these "attractive" features are preferred by the opposite sex; why?

Why do females prefer particular traits?

good genes - (sexually selected traits are indicators of good genes), or...

Runaway selection

Preference is for an arbitrary trait, or a trait good at moderate levels but eventually negative at high levels

Limits to selection

historical constraints

compromise with other needs (i.e., biological tradeoffs)

variation is not limitless

chance (i.e., selection cannot create variation)

Historical constraints and compromises

Advantages of bipedalism vs. inconveniences of late stage pregnancy

Playing the game: key challenges to living organisms

find appropriate habitat & environmental conditions

obtain necessary resources

avoid predators

find a mate (or mates)

produce as many successful offspring as possible

Selection can act on any trait that affects any of these challenges

Biological tradeoffs

Problem is,

time spent on one activity can't be used for others

Biological tradeoffs

Problem is,

resources invested in one activity can't be used for others

Biological tradeoffs

Problem is,

solutions to one essential problem may interfere with solutions to others

being able to deal with many situations may prevent you from dealing with any one situation very well (Jack of all trades)

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