BIOL 359: Mendelian Genetics in Populations II

Migration, in terms of Population Genetics?

the transfer of alleles from the gene pool of one population to the gene pool of another

the seasonal movement of individuals (departure from and return to a place by individuals) - round trip movement

What is migration also referred to as?

gene flow

What's Seasonal Migration Movement?

the long-distance movement of organisms from one habitat to another in response to seasonal changes

What species is an example of Seasonal Migration?

wildebeest

What is the driving force for season migration in wildebeests?

rainfall patterns, food availability and grass growth

What does gene flow/migration influence?

genetic diversity & allele frequencies across populations

How does HIGH gene flow affect populations?

populations stay similar, preventing speciation

How does LOW gene flow affect populations?

populations evolve separately, leading to new species

What does the "20 blue, 10 red & 20 red, 10 blue" diagram illustrate?

the fact that each population has different allele frequencies

What does the blue and red moving between the two populations in the "20 blue, 10 red & 20 red, 10 blue" diagram illustrate?

the gene flow is introducing new genetic material into the receiving population

How does the "migrating" group affect the population receiving this migration?

allele frequency shift

Receiving population: more blue/red individuals than before

Migrating Population: less blue/red individuals than before

What is the significance of Gene Flow?

1. Reduces Genetic Differences

2. Increases Genetic Diversity

3. Can Counteract Natural Selection or Genetic Drift

How does gene flow decrease genetic differences?

Gene flow makes populations more genetically similar, preventing them from diverging too much over time

How does gene flow increase genetic diversity?

If new alleles are introduced, gene flow can enhance genetic variation in a population

What is Genetic Drift?

A change in allele frequencies in population caused by random events

How does gene flow counteract genetic drift?

In small populations, gene flow can reintroduce alleles that might otherwise be lost due to genetic drift

What could gene flow possibly be caused by?

dispersal

What's Dispersal?

one-way movement of a juvenile (young) individual away from the place of their birth

Why does Gene flow depend on population size?

smaller populations (islands) are more affected by migration than larger ones (continents)

What is the impact of migration from a larger population to a smaller population?

weak impact

What is the impact of migration from a smaller population to a larger population?

stronger impact

What does LOW gene flow lead to?

genetic divergence and speciation

What does HIGH gene flow lead to?

maintain genetic similarity

What does gene flow from Large Continent to Small Island do?

weaker genetic impact

Since the target (island population) is small, only a limited number of migrants from the continent contribute to gene flow

The island has a smaller population, meaning fewer individuals receive the incoming genes

What does gene flow from Small Island to Large Continent do?

stronger genetic variation impact

The continent has a larger population, meaning more individuals can receive and incorporate the incoming genes from island migrants

the island can introduce a significant genetic impact on the larger continent

Fixed allele frequency

when a population exhibits only one allele at a particular gene (100%)

What is Evolution?

change in allele frequencies in a population over time

How is Gene flow an effective mechanism of Evolution?

homogenizes (make the same) allele frequencies among populations

What does gene flow's mechanism to homogenize populations prevent?

evolutionary divergence of populations

How does gene flow decrease natural selection?

Natural selection favours alleles that improve survival and reproduction in a specific environment.

However, gene flow can introduce maladaptive alleles from another population where different traits are favoured. This can slow down or counteract the effects of selection.

What does gene flow reduce?

the probability of evolutionary divergence

they conceptualize the idea of "homogenize", to make all populations (new and old) the same

What animal is an example of Gene Flow?

Lake Erie Water Snakes

Ectothermia

cold blooded animals that rely on environmental heat sources for warmth

ex. Lake Erie Water Snakes

Endothermia

metabolism providing our own heat

Which Lake Erie Water Snake has a higher level of predation?

Non-banded

Which Lake Erie Water Snake camouflages best?

Banded snake

How are Lake Erie Water Snakes an example of Gene Flow?

the Lake Erie water snakes traveling between the mainland and islands

increases genetic variation in island populations

natural selection favors light-colored snakes on some islands, while gene flow from the mainland maintains the banded form

As a result, the frequency of color patterns remains stable over time, demonstrating how gene flow and natural selection interact to influence genetic diversity

What will happen over time when individuals from different populations exchange migrants at an equal rate?

the allele frequencies in each population will gradually converge toward an equilibrium

this equilibrium is the average allele frequencies

What's the fixation index (Fst)?

a measure of population differentiation that quantifies the genetic variability among populations

compares the differences in allele frequencies at a specific locus between populations

What does a fixation index of 0 represent?

all populations have identical allele frequencies (no differentiation)

What does a fixation index of 1 represent?

the populations do not share any alleles (max differentiation)

How does gene flow affect fixation index (Fst)?

reduces Fst

increases the genetic similarity between populations by introducing new alleles into each population

Is genetic drift in H-W equilibrium random or non-random?

random (unpredictable changes in allele frequencies from one generation to the next)

What is a word to describe H-W equilibrium Genetic Drift?

"blind-luck"

What concept conceptualizes the genetic drift in H-W equilibrium?

Think: each generation is a random sample of the gametes produced by the previous generation

How to describe genetic drift across generations?

sampling error (it is a random sample of gametes)

What does H-W Equilibrium hypothesize about population size & allele frequencies?

population size is infinite, thus allele frequencies never change (since there are enough individuals that random chance alone doesn't significantly alter allele frequencies)

What is the reality about population size & allele frequencies?

population size is finite and allele frequencies change

What sized populations experience more genetic drift (small or large)? Why?

small

random events have a bigger impact on allele frequencies when there are fewer individuals

What can we say about the rate and the likelihood of genetic drift as population size decreases?

incidence (likelihood) and rate (speed) of genetic drift increases

What's Law of Probabilities?

uses probability to predict how allele frequencies change in genetic drift

Does genetic drift lead to adaptation (change to survive in their environment)?

NO, it does not necessarily favour beneficial traits

natural selection does though

What is the difference between natural selection and genetic drift in terms of adaptation?

Natural Selection: favours beneficial traits, which increases an organism's fitness, leading to adaptation

Genetic Drift: does not necessarily favour beneficial traits, so it does not lead to adaptation.

What is an example of genetic drift?

a storm

wipes out most of a population, the alleles that remain are just the ones that happened to survive—not necessarily the "fittest" ones

If we have a large population, would the next generation have a close or far allele frequency than the first?

close

If we have a small population, would the next generation have a close or far allele frequency than the first?

far

What are the 2 things that Genetic Drift Typically leads to?

1. Fixation or Loss of Alleles

2. Decline in Heterozygosity

What's Fixation?

every copy of that gene expresses one allele (no heterozygosity, only homozygotes)

Why does genetic drift lead to a Decline in Heterozygosity?

while one allele increases (100%), the other decreases (0%)

lower ratio of 50/50 alleles, start becoming either one or the other allele

Describe the 2 concepts of genetic drift in Drosophila Melanogaster

Over generations, random events cause one allele to gradually increase in frequency while the other decreases

Eventually, one allele reaches 100% frequency (fixation) while the other disappears (loss).

As genetic drift progresses, more individuals become homozygous (AA or aa) purely by chance

Heterozygosity declines over time as populations become more genetically uniform.

Why does the theoretical predication for a population of 9 are closely align with the actual heterozygosity trend than the theoretical prediction for a population of 16 in the Drosophila melanogaster example?

When using a larger population size (N = 16) in the model, it assumes genetic drift is weaker, leading to a slower decline in heterozygosity than what actually happens in the real, smaller population. This results in an overestimation of genetic diversity compared to the observed data.

In reality, not all individuals contribute equally to the next generation, so the actual number of breeding individuals is often much smaller than the total population.

What is Effective Population Size (Ne)?

the idealized population size that would lose heterozygosity at the same rate as the actual (census) population

Why is the Effective Population Size (Ne)​ almost always smaller than the Census population?

not all individuals contribute to reproduction (reducing the effective population size)

What are factors that effect the Effective Population Size​ being smaller than the Census population?

- Some individuals die before reproducing

- Some individuals don't mate or pass on alleles

- Unequal contribution to the next generation

What is the overall takeaway from Effective Population Size being Smaller than Census Population Size?

genetic drift happens faster than expected

What is the probability of fixation of one allele or another?

it depends on the starting allele frequency

(ex. The probability that allele A of frequency of A=0.4 will fix is 0.4)

What are the Demographic events that can cause genetic drift?

1. Founder Event

2. Bottleneck

What is Founder Event?

when a small group of individuals from a larger population starts a new population

This small group carries only a subset of the genetic diversity from the original population, leading to limited genetic variation in the new population.

Over time, genetic drift can cause the new population to differ from the original one.

What species was used as an example of a Founder event?

Silvereye

What do large population sizes usually imply in other factors?

a lot of mutations & genetic variation

How does a founder event lead to genetic drift?

A founder event leads to genetic drift because the new population starts with only a small group of individuals.

With fewer individuals, the genetic variation is limited, and over generations, random changes in allele frequencies (genetic drift) can lead to differences between the new population and the original population.

How is the silvereye an example of a founder event?

The silvereye bird species provides an example of a founder event when a small group of birds from mainland Australia flew to Tasmania and started a new population.

This small group had limited genetic diversity compared to the larger mainland population, and over time, genetic drift caused the Tasmanian silvereyes to become genetically different from the mainland group.

What is the effect of genetic drift on the silvereye population in Tasmania?

Genetic drift in the Tasmanian silvereye population, starting from a small group of founders, caused random changes in allele frequencies.

Over time, these changes led to the Tasmanian population becoming genetically different from the mainland population.

Allelic diversity decreased.

What is a bottleneck in terms of genetic drift?

A bottleneck occurs when a population experiences a sharp decline in size due to external factors like natural disasters, disease, or habitat destruction. This reduction leads to a loss of genetic diversity, and the surviving population may not represent the full genetic diversity of the original population.

How does a bottleneck cause genetic drift?

After a bottleneck, the surviving population is small, and genetic drift (random changes in allele frequencies) becomes more pronounced.

The gene pool is influenced by the random alleles carried by the survivors, leading to reduced genetic diversity and the potential for the population to evolve in ways that are not necessarily adaptive.

What is sampling error in the context of genetic drift and bottlenecks?

the random fluctuations in allele frequencies that occur in small populations

(the small number of surviving individuals may not represent the full genetic diversity of the original population, leading to changes in the gene pool purely by chance)

How does a population recover after a bottleneck?

After a bottleneck, a population can recover as the surviving individuals reproduce and the population size increases again.

However, the recovery population may have reduced genetic diversity due to the limited number of original genetic variants in the surviving individuals.

What does the Hardy -Weinberg principle say about mating?

individuals mate at random (panmictic population)

What are the 4 types of non-random mating?

1. Inbreeding

2. Outbreeding

3. Positive Assortative Mating

4. Negative Assortative Mating

What is Inbreeding Non-Random Mating?

mating along related individuals

What is Outbreeding Non-Random Mating?

mating along unrelated individuals

What is Positive Assortative Non-Random Mating?

between individuals with similar phenotypes (ex.large and large individuals)

What is Negative Assortative Non-Random Mating?

between individuals with dissimilar phenotypes (ex.large and small individuals)

Does Non-Random Mating change allele frequencies?

NO, it does not change allele frequencies -- it reshuffles alleles into different genotypic frequencies

Changes genotypic frequencies

How does inbreeding affect genotype frequencies?

Inbreeding increases the frequency of homozygotes (AA and aa) and decreases the frequency of heterozygotes (Aa)

Why doesn't inbreeding change allele frequencies?

Inbreeding only redistributes existing alleles into different genotype combinations but does not create or remove alleles, keeping total allele frequencies the same.

What is inbreeding depression?

a reduction in the fitness of inbred individuals relative to non-inbred offspring

What are two possible mechanisms that cause inbreeding depression?

1. In populations with recessive deleterious alleles, these alleles are more likely to occur in the homozygous condition in inbred offspring.

2. Loss of genome wide heterozygosity (loss of heterozygote advantage)

What genes are responsible for a higher fitness the larger of the genes you have?

heterzygotes

How can inbreeding contribute to evolution?

While inbreeding itself does not cause evolution, it changes phenotype frequencies, which can alter the pattern of natural selection. Over time, this interaction can lead to evolutionary changes in a population.

What are the Agents of Evolution?

Selection

Mutation

Gene Flow

Genetic Drift

Non-Random Mating

What's Selection?

change allele and genotype frequencies

What's Mutation?

weak evolutionary force but provides the raw variation for evolution

What's Gene Flow (Migration)?

when source and recipient populations differ in allele frequencies

Gene Flow/Migration can cause recipient population to evolve (usually homogenizes allele frequencies among populatoions)

What's genetic drift?

random shift in allele and genotype frequencies from generation to generation

What's non-random mating?

allele frequencies does not change

genotype frequencies change (can cause evolution with natural selection)

What's the Extinction Vortex?

a self-reinforcing cycle that can lead to the extinction of a population

a model that describes how small populations can become increasingly likely to go extinct

What is the cycle of the Extinction Vortex?

1. Small Population Size → Leads to increased inbreeding and genetic drift.

2. Loss of Genetic Variation → Reduces adaptability to environmental changes.

3. Inbreeding Depression → Increases the expression of harmful recessive traits, reducing fitness.

4. Lower Survival & Reproduction Rates → Further reduces population size.

5. Cycle Repeats → The population shrinks further, making extinction more likely

How can a change in genotype frequency affect evolution?

genotypes > phenotypes > natural selection > evolution

What would happen to the genotypes if offspring is produced from inbreeding?

genes are homozygous

causes inbreeding depression (mortality rate of offspring increase from parents who inbred and were related)