Population and Genomes Exam 3

What is gene flow?

The movement of alleles between populations due to migration

What is migration in population genetics?

The movement of individuals from one population to another, carrying their alleles with them

What does gene flow cause between populations?

It makes populations more genetically similar by equalizing allele frequencies

How does gene flow affect genetic diversity within a population?

It usually increases genetic diversity by introducing new alleles

What is the “Island Model” of gene flow?

A model with discrete populations connected by uniform, bidirectional migration

What is the migration rate (m)?

The proportion of individuals in a population that originate from other populations each generation

How does migration rate affect homogenization?

Higher migration rates cause populations to converge and homogenize faster

What happens when m = 0 in the island model?

Populations do not converge and retain their original allele frequencies

How does gene flow interact with genetic drift?

Gene flow counteracts drift by reintroducing alleles and preventing random fixation

What is divergence?

The accumulation of genetic differences between populations due to drift or selection

How does gene flow reduce divergence between populations?

By continually exchanging alleles, preventing populations from becoming genetically distinct

How much gene flow is needed to prevent divergence?

About one migrant per generation (“OMPG rule”) keeps FST from exceeding ~0.2

Why does a smaller m produce the same FST in larger populations?

Larger populations drift more slowly, so fewer migrants are needed to balance drift

How does smaller Ne versus larger Ne affect gene flow’s impact?

• Small Ne: drift is strong, migrants have a big effect on allele frequencies

• Large Ne: drift is weak, migrants have a smaller impact

Why does absolute number of migrants (Nm) matter more than the proportion m?

Because drift and migration scale differently with Ne; Nm determines the balance

What is the “Stepping Stone” model?

A model where gene flow only occurs between adjacent populations

How do we detect isolation by distance (IBD)?

By plotting FST against geographic distance—FST increases with greater distance

What is isolation by distance (IBD)?

A pattern where geographically distant populations are more genetically different

How does dispersal ability affect IBD?

Fast-dispersing organisms show weaker IBD; slow dispersers show stronger IBD

How do geographic barriers affect gene flow?

Barriers like mountains, oceans, or deserts reduce or block gene flow

How can gene flow facilitate adaptation?

Beneficial alleles can spread to new populations (adaptive introgression), helping them adapt without new mutations

What is “assisted gene flow”?

Human-managed movement of individuals or alleles to help populations adapt to climate change

What is heterozygosity (He), and how does gene flow affect it?

He measures genetic diversity; gene flow increases it by reintroducing lost alleles

What is FST?

A statistic measuring population divergence; higher FST means populations are more different genetically

How does gene flow influence FST?

Increased gene flow lowers FST by reducing differentiation

What is speciation?

The evolutionary process leading to the formation of new species through the development of reproductive barriers

What does the Biological Species Concept (BSC) define as a species?

Groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups

What is reproductive isolation (RI)?

The inability of different populations to interbreed and produce viable, fertile offspring

What are reproductive barriers?

Traits or mechanisms that prevent gene flow between species by blocking mating or reducing hybrid viability or fertility

What are prezygotic barriers?

Reproductive barriers that act before fertilization to prevent mating or fertilization

What are postzygotic barriers?

Barriers that act after fertilization, reducing viability or fertility of hybrid offspring

Name some types of prezygotic barriers

Geographic, ecological, mechanical, temporal, behavioral, and gametic barriers

How does the BSC apply to asexual species?

It does not apply well; other concepts like the genotypic cluster species concept (based on genetic similarity) are used instead

How can you measure when speciation is "complete"?

When hybrids cannot form or are completely inviable or sterile, indicating no gene flow

What is allopatric speciation?

Speciation that occurs due to geographic isolation preventing gene flow

What is sympatric speciation?

Speciation occurring without geographic barriers, often requiring special ecological or genetic conditions

What is the Bateson-Dobzhansky-Muller (BDM) model?

A genetic model explaining how reproductive isolation evolves through negative interactions between alleles at different loci after populations diverge

Define fitness epistasis.

When the fitness effect of an allele at one gene depends on the alleles present at another gene

What are BDM incompatibilities (BDMIs)?

Pairs of alleles from different loci that cause reduced hybrid fitness when combined, leading to reproductive isolation

Why does the BDM model require allopatry?

Because incompatible alleles must fix independently in separate populations to avoid immediate fitness costs

What is the “Snowball Effect” in speciation?

The exponential increase in the number of BDM incompatibilities over time as populations diverge

How fast can speciation occur?

Usually over long periods (100,000+ generations), but sometimes rapidly (e.g., 10,000 years in sticklebacks or instantly via polyploidy)

What role does polyploidy play in speciation?

Changes in chromosome number can create new species instantly, especially common in plants

Give an example of a postzygotic barrier in animals

Sterile hybrids like mules, which result from a horse and donkey mating

What happens if hybrids have low fitness due to ecological mismatches?

Even if viable and fertile, hybrids may have reduced survival or reproduction, reinforcing reproductive isolation

What is reproductive isolation (RI)?

Reproductive isolation is when two populations can no longer exchange genes, preventing gene flow and allowing them to evolve independently

What does the BDM model explain?

The BDM model is a genetic model explaining how reproductive isolation evolves through negative fitness interactions between alleles at different loci after populations diverge

What evolutionary force does the BDM model require to fix BDMIs?

The BDM model does not specify any particular force; drift, selection, or both can fix different alleles in different populations

What is divergent natural selection?

It is selection that favors different alleles or phenotypes in different environments, causing genetic and phenotypic divergence

What is ecological speciation?

Ecological speciation occurs when divergent natural selection on ecological traits causes reproductive isolation as an indirect byproduct

How does low fitness of migrants reduce gene flow?

Migrants adapted to one environment perform poorly in another, reducing their survival or reproduction and limiting gene flow

What is immigrant inviability?

A reproductive barrier where non-local genotypes have low fitness in a new environment, preventing successful immigration between populations

Why do hybrids often have low fitness?

Because they may be ecologically maladapted to both parental environments or possess harmful genetic incompatibilities

What is reinforcement?

Reinforcement is natural selection that strengthens prezygotic barriers because hybrids are low fitness, reducing heterospecific mating

Why do genotypes with a preference for own-type matings have higher long-term fitness?

Because they avoid producing low-fitness hybrids, increasing the survival and reproductive success of their lineage

Can speciation via natural selection occur without allopatry?

Yes. Strong divergent natural selection can produce reproductive isolation even in sympatry

What is secondary contact?

When two species or populations that evolved reproductive isolation in isolation come back into contact

What happens if RI is weak when species re-establish contact?

They may interbreed extensively, sometimes resulting in a hybrid swarm.

What is a hybrid swarm?

A genetically mixed population formed when species boundaries collapse due to widespread hybridization and weak RI

What happens if RI is of intermediate strength during secondary contact?

A stable hybrid zone can form where hybrids are produced but the species remain partially distinct

What is a hybrid zone?

A stable geographic region where two species ranges overlap and hybrids are regularly formed

How many mutations are needed to cause speciation?

It varies widely—from one large change (e.g., polyploidy) to two interacting alleles (BDM incompatibilities) to many ecological adaptation mutations

What does reinforcement predict about sympatric vs allopatric populations?

Populations in sympatry should evolve stronger mating preferences and mating signals than allopatric populations

Why is biological variation not perfectly continuous?

Populations become separated in space or ecology, accumulate differences through drift or selection, and eventually evolve RI, producing discrete species

Why is speciation considered a byproduct of regular evolutionary forces?

Because mutation, drift, and natural selection naturally generate divergence and reproductive barriers over time without special processes

What is the overall level of genetic diversity in humans?

Humans have exceptionally low genetic diversity—much lower than other great apes

Why do human populations vary continuously rather than forming discrete genetic groups?

Because of shared evolutionary history and constant gene flow across populations

What evolutionary force explains most genetic differences between human populations?

Genetic drift

Does “race” have a biological or genetic definition?

No. “Race” is a social construct that does not map onto real genetic patterns

When did primates first arise?

About 65 million years ago, from rodent-like ancestors with arboreal lifestyles

When did modern humans appear and where?

Around 200,000 years ago in East or Southern Africa

What caused low genetic diversity in humans compared to other great apes?

Severe population bottlenecks (e.g., ~900,000 years ago when effective population size dropped to ~1,000)

What proportion of human genetic variation occurs within populations?

About 85–90% (only ~10–15% is between populations)

What is the global human FST value and what does it indicate?

FST ≈ 0.12 → very low differentiation among populations

What genetic pattern is produced by migration across space?

Isolation by distance—the farther apart populations are, the slightly more genetically different they become

What does PCA (Principal Component Analysis) do in population genetics?

It compresses multi-locus genotype data into a 2D plot that highlights major axes of variation

Why does PCA make human groups look more separated than they are biologically?

PCA maximizes visible variation, exaggerating differences that are actually very small

Did “pure” or isolated human populations ever exist?

No. Gene flow and admixture have happened continuously throughout human history

How do consumer DNA ancestry tests create the illusion of discrete groups?

They use culturally defined categories and focus on a tiny set of highly differentiated loci

What percentage of the human genome shows signs of local adaptation?

Less than 1%

What traits represent rare examples of local adaptation in humans?

Skin pigmentation and lactase persistence

What environmental factor primarily shaped the evolution of skin color?

UV radiation levels across Earth’s surface, leading to trade-offs in vitamin D production and folate protection

How many genes explain most of the genetic (non-environmental) variation in human skin color?

About four genes (e.g., SLC24A5)

How many people worldwide are lactase persistent as adults?

Only about one-third of humans

What caused the evolution of lactase persistence?

Pastoral cultures consuming cow or goat milk over thousands of years → strong local positive selection at the LCT locus

What does the genome look like at most loci regarding selection?

99.99% of loci show patterns consistent with neutral drift, not selection

Why is “race” not genetically meaningful?

Because genetic variation is continuous, not discrete, and does not align with socially defined racial categories

If race isn’t genetic, why does it still matter for health?

Because social experiences—including discrimination, structural inequality, and economic conditions—affect biological outcomes

How did ancient societies typically classify humans?

Mostly by culture, language, or customs—not by biological “race” (e.g., Greeks, Zhou Dynasty China)

Why has the idea of race been historically harmful?

It has been used to justify oppression, hierarchy, and the false belief in biological superiority