Population Genetics

The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule

Populations A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits

Speciation Formation of new species One species may split into 2 or more species A species may evolve into a new species Requires very long periods of time

Modern Evolutionary Thought

Modern Synthesis Theory Combines Darwinian selection and Mendelian inheritance Population genetics - study of genetic variation within a population Emphasis on quantitative characters (height, size …)

Charles Darwin Collected specimens During journey on the HMS Beagle Wrote about conclusions in the Origin of the Species

Darwin’s Finches of the Galapagos

Wallace’s Line The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace that separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia.

Modern Synthesis Theory 1940s – comprehensive theory of evolution (Modern Synthesis Theory) Introduced by Fisher & Wright & Haldane Until then, many did not accept that Darwin’s theory of natural selection could drive evolution S. Wright Haldane

Sir Ronald A. Fisher Statistician combined Mendel’s work and natural selection --also created many biostatistics used today

Sewall Wright Geneticist used included selection in path analysis Founder of population genetics with Fisher and Haldane

Modern Synthesis Theory TODAY’S theory on evolution Recognizes that GENES are responsible for the inheritance of characteristics Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

Microevolution Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more VARIATION in individuals in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant)

Hardy-Weinberg Principle

The Hardy-Weinberg Principle Used to describe a non-evolving population. Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.   Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

The Hardy-Weinberg Principle Deviation from Hardy-Weinberg equilibrium usually results in evolution Understanding a non-evolving population, helps us to understand how evolution occurs                         .

5 Assumptions of the H-W Principle Large population size - violation: small populations have fluctuations in allele frequencies (e.g., fire, storm). No migration - violation: immigrants can change the frequency of an allele by bringing in new alleles to a population. No net mutations - violation: if alleles change from one to another, this will change the frequency of those alleles

5 Assumptions of the H-W Principle Random mating - Violation: if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. No natural selection - Violation: if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

Traits Selected = non-Random Mating

The Hardy-Weinberg Principle The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change)   The Hardy-Weinberg Equation:                 1.0 = p2 + 2pq + q2  Where: p2 = frequency of AA genotype 2pq = frequency of Aa q2 = frequency of aa genotype

The Hardy-Weinberg Principle Determining the Allele Frequency using Hardy-Weinberg:                 1.0 = p + q  Where: p = frequency of A allele q = frequency of a allele

Estimating allelic frequencies: If one or more alleles are recessive, can’t distinguish between heterozygous and homozygous dominant individuals. Use Hardy-Weinberg to calculate allele frequencies based on the number of homozygous recessive individuals. If q2 = 0.0043, then q = 0.065; p = 1 - q = 0.935 p2 = 0.8742, 2pq = 0.1216

Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

Allele and Genotype Frequencies Each diploid individual in the population has 2 copies of each gene. The allele frequency is the proportion of all the genes in the population that are a particular allele. The genotype frequency of the proportion of a population that is a particular genotype. For example: consider the MN blood group. In a certain population there are 60 MM individuals, 120 MN individuals, and 20 NN individuals, a total of 200 people. The genotype frequency of MM is 60/200 = 0.3. The genotype frequency of MN is 120/200 = 0.6 The genotype frequency of NN is 20/200 = 0.1

The genotype frequency of MM is 60/200 = 0. 3 The genotype frequency of MM is 60/200 = 0.3. The genotype frequency of MN is 120/200 = 0.6 The genotype frequency of NN is 20/200 = 0.1 The allele frequencies can be determined by adding the frequency of the homozygote to 1/2 the frequency of the heterozygote. The allele frequency of M is 0.3 (freq of MM) + 1/2 * 0.6 (freq of MN) = 0.6 The allele frequency of N is 0.1 + 1/2 * 0.6 = 0.4 Note that since there are only 2 alleles here, the frequency of N is 1 - freq(M).

Microevolution of Species

Causes of Microevolution Genetic Drift - the change in the gene pool of a small population due to chance Natural Selection - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) - Cause ADAPTATION of Populations Gene Flow -is genetic exchange due to the migration of fertile individuals or gametes between populations

Causes of Microevolution Mutation a change in an organism’s DNA Mutations can be transmitted in gametes to offspring Non-random mating - Mates are chosen on the basis of the best traits

Genetic Drift

Genetic Drift Genetic drift is the random changes in allele frequencies. Genetic drift occurs in all populations, but it has a major effect on small populations. For Darwin and the neo-Darwinians, selection was the only force that had a significant effect on evolution. More recently it has been recognized that random changes, genetic drift, can also significantly influence evolutionary change. It is thought that most major events occur in small isolated populations.

Factors that Cause Genetic Drift Bottleneck Effect a drastic reduction in population (volcanoes, earthquakes, landslides …) Reduced genetic variation Smaller population may not be able to adapt to new selection pressures Founder Effect occurs when a new colony is started by a few members of the original population May lead to speciation

Loss of Genetic Variation Cheetahs have little genetic variation in their gene pool This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age

Bottlenecks Example: Pingalop atoll is an island in the South Pacific. A typhoon in 1780 killed all but 30 people. One of survivors was a man who was heterozygous for the recessive genetic disease achromatopsia. This condition caused complete color blindness. Today the island has about 2000 people on it, nearly all descended from these 30 survivors. About 10% of the population is homozygous for achromatopsia This implies an allele frequency of about 0.26.

Founder’s Effect

Founder Effect Example Founder effect example: the Amish are a group descended from 30 Swiss founders who renounced technological progress. Most Amish mate within the group. One of the founders had Ellis-van Crevald syndrome, which causes short stature, extra fingers and toes, and heart defects. Today about 1 in 200 Amish are homozygous for this syndrome, which is very rare in the larger US population. Note the effect inbreeding has here: the problem comes from this recessive condition becoming homozygous due to the mating of closely related people.

Modes of Natural Selection

Modes of Natural Selection Directional Selection Favors individuals at one end of the phenotypic range Most common during times of environmental change or when moving to new habitats

Directional Selection

Disruptive selection Favors extremes over intermediate phenotypes Occurs when environmental change favors an extreme phenotype

Disruptive Selection

Modes of Natural Selection Stabilizing Selection Favors intermediate over extreme phenotypes Reduces variation and maintains the current average Example: Human birth weight

Variations in Populations

Geographic Variations Variation in a species due to climate or another geographical condition Populations live in different locations Example: Finches of Galapagos Islands & South America

Heterozygote Advantage Favors heterozygotes (Aa) Maintains both alleles (A,a) instead of removing less successful alleles from a population Sickle cell anemia > Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age. > Heterozygotes are less susceptible to malaria

Sickle Cell and Malaria

Other Sources of Variation Mutations In stable environments, mutations often result in little or no benefit to an organism, or are often harmful Mutations are more beneficial (rare) in changing environments  (Example:  HIV resistance to antiviral drugs) Genetic Recombination source of most genetic differences between individuals in a population

Co-evolution -- when changes in at least two species' genetic compositions reciprocally affect each other's evolution -- a change in the genetic composition of one species (or group) in response to a genetic change in another. -Often occurs between parasite & host and flowers & their pollinators, and predator and prey

Coevolution pollinators

Parasitic relationships