genetics: genes in populations

Bio 106

Lecture 11

Genes in Populations

A. Population Genetics

B. Gene Frequencies and Equilibrium

1. Gene Frequencies

2. Gene Pool

3. Model System for Population Stability (Hardy – Weinberg Law)

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C. Changes in Gene Frequencies 1. Mutation 2. Selection

2.1 Relative Fitness 2.2 Selections and Variability 2.3 Selection and Mating

3. Systems 4. Migration 5. Genetic Drift

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D. Race and Species Formation

1. The Concept of Races

2. The Concept of Species

2.1 Reproductive Isolating Mechanisms

2.2 Rapid Speciation

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Population: a group of individuals of the same species that live in the same area and interbreed (interbreeding causes production of fertile offspring)

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POPULATION GENETICS

the study of genetic variation within populations

involves the examination and modelling of changes in the frequencies of genes and alleles in populations over space and time.

FOCUS: species or population

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Gene frequencies & equilibrium

GENE POOL: the collection of all

the alleles of all of the genes

found within a freely

interbreeding population

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GENE FREQUENCY (allele frequency)

The proportion of all alleles in all individuals in the group in question which are of a particular type.

Ex: 40 individuals which are AA 47 individuals which are Aa 13 individuals which are aa

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GENOTYPE

AA Aa aa TOTAL

# of individuals 40 47 13 100

# of A alleles 80 47 0 127

# of a alleles 0 47 26 73

Total # of alleles 200

Allele frequency of A: 127/200 = 0.635

pA=0.635 pa = 73/200 = 0.365 = 1- pA

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GENOTYPE FREQUENCY:

The proportion of individuals in a group with a particular genotype.

40 AA 47 Aa 13 aa = 100 Total individuals pAA = 40/100 = 0.4 pAa = 47/100 = 0.47 paa = 13/100 = 0.13

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Hardy-Weinberg Equation:

used to estimate frequency of alleles in a population

p = the frequency of the dominant allele (A) q = the frequency of the recessive allele (a)

For a population in genetic equilibrium: p + q = 1.0 (The sum of the frequencies of both alleles is 100%.)

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(p + q)2 = 1 so p2 + 2pq + q2 = 1

where

p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa

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Gene frequencies & equilibrium Assumptions of the HW model: 1. Organism is diploid.

2. Reproduction is sexual.

3. Generations are non-overlapping.

4. Mating occurs at random.

5. Population size is very large.

6. Migration is zero.

7. Mutation is zero.

8. Natural selection does not affect the gene in question.

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Changes in Gene frequencies

Factors that affect Gene Frequency: 1. Mutation 2. Natural selection 3. population size 4. genetic drift 5. environmental diversity 6. Migration 7. non-random mating patterns

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MUTATION is the primary source of new alleles in a gene pool.

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Changes in Gene

frequencies

Natural Selection: the differential reproduction of genotypes

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Relative Fitness: ability to survive in an environment long enough to reproduce

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Selections and Variability:

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selects against the average individual in a population.

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Favors the intermediate variants

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favors the intermediate variants

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Directional selection: an extreme phenotype is

favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.

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Changes in Gene frequencies Selection and Mating Sexual selection occurs when

individuals within one sex secure mates and produce offspring at the expense of other individuals within the same sex.

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Mating systems: RANDOM: mate choice is independent of

phenotype and genotype

POSITIVE ASSORTMENT: mate choice is dependent on similarity of phenotype

NEGATIVE ASSORTMENT: …..on dissimilarity of phenotype

INBREEDING: mating with relatives at a rate greater than expected by chance

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Mating Systems

Positive assortative mating

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Mating Systems

Negative assortative

mating

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Positive assortment:

- increases homozygosity (prevents HW equilibrium)

- does not affect allele frequency

- dominance dilutes its effect

- affects only those genes related to the phenotype by which mates are chosen

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Changes in Gene frequencies Negative Assortment/disassortative

mating:

- yields an excess of heterozygotes (compared to HW)

- does not affect allele frequencies *

- dominance dilutes its effect

- Increases the rate to equilibrium of alleles among loci (because linkage phases are disrupted by recombination in double homozygotes).

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Changes in Gene frequencies Inbreeding alone does

not change allele frequencies, but inbreeding does change genotype frequencies.

Inbreeding can affect allele frequencies, by changing how selection operates.

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Changes in Gene frequencies Inbreeding

- Can result to excess homozygotes

- Inbred individuals usually have lower fitness than outbred individuals. (inbreeding depression) - 2 possible reasons for inbreeding depression:

(1) deleterious recessive alleles

(2) overdominance

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Population Size:

Increase in population

causes increase in

gene frequencies.

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GENETIC DRIFT occurs as the result of random fluctuations in the transfer of alleles from one generation to the next, especially in small populations formed, as a result of bottleneck effect and founder effect

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Bottleneck effect: adverse environmental conditions

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Changes in Gene frequencies Founder effect: geographical separation

of a subset of the population

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Random genetic drift can continue until one allele is fixed or lost

As genetic drift progresses, - heterozygosity decreases.

- genetic variance within populations decreases.

- genetic variance among populations increases.

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Changes in Gene frequencies Migration: movement of individuals from

one population to another; translated as gene flow

Can equalize gene frequency. 37 cces2015


Gene flow

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Race and Species Formation

Race: geographically isolated breeding population that shares certain characteristics in higher frequencies than other populations of that species, but has not become reproductively isolated from other populations of the same species.

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Race & Species Formation

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Species: members of populations that actually or potentially interbreed in nature, not according to similarity of appearance

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Reproductive Isolating Mechanisms

prezygotic isolating mechanisms - prevent the formation of

viable zygotes postzygotic isolating mechanisms - prevent hybrids from passing on their

genes

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Race & Species Formation Reproductive Isolating Mechanisms

Prezygotic Isolating Mechanisms

Ecological Isolation: The geographic ranges of two species overlap, but their ecological needs or breeding requirements differ enough to cause reproductive isolation.

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Race

& S

pec

ies

For

matio

n Reproductive Isolating

Mechanisms

Prezygotic Isolating Mechanisms Temporal Isolation: two species whose ranges overlap have different periods of sexual activity or breeding seasons

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Prezygotic Isolating Mechanisms Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species

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Prezygotic Isolating Mechanisms Mechanical Isolation: Morphological differences prevent mating/pollination.

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Prezygotic Isolating Mechanisms Gametic Isolation: sperm and ova of the two species are chemically (genetically) incompatible, and will not fuse to form a zygote.

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Postzygotic Isolating Mechanisms Hybrid inviability: The hybrid offspring is either weaker than the parent species, or totally inviable. This could be caused by minor or major genetic defects, and even slightly reduced viability can cause big decreases in reproduction. 49 cces2015



Postzygotic Isolating Mechanisms Hybrid sterility: Viable hybrid is produced but is unable to reproduce due to meiotic problems

(mare) (jack)

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Hybrid sterility

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Postzygotic Isolating Mechanisms Hybrid breakdown: successive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population..

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Race and Species Formation

Speciation: Hybrids may actually be reproductively superior to

parent populations, and if they tend to breed with each other, this can result in what could be termed hybrid speciation.

Speciation can occur as a result of hybridization between two related species, if the hybrid • receives a genome that enables it to breed with other

such hybrids but not breed with either parental species; • can escape to a habitat where it does not have to compete

with either parent; • is adapted to live under those new conditions.

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genetics: genes in populations

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