Population Genetics:Population Genetics:How do Genes Move through Time

and Space?How do Genes Move through Time

and Space?

• Variation can be smooth or discontinuous.– Two views of biology

• “Naturalists”–Supported Darwin’s ideas.

• “Experimentalists”–-Supported Mendelian Ideas.

How Do We Characterize Variation?

Naturalists’ Viewpoint

• Saw most traits in a population exhibited a continuum of forms.

• Believed the ability to survive and reproduce might depend on having traits that fall within some range of a spectrum.

• Believe that traits within populations change or evolve as features of environmental change.

Experimentalists’ Viewpoint

• Rejected naturalists’ view of evolution.

• Viewed variation as a sudden change due to mutations.

• Maintained evolution progressed by leap and bounds by sudden random mutations.

• Evolution could not be a gradual process

Stalemate Broken

• Herman Nilsson-Ehle (1909)– Using wheat

kernels proved that traits that appear in populations as a continuous spectrum of forms with a genetic basis.

Stalemate Broken

• Nilsson-Ehle showed a cross between – true-breeding red- and

white-kerneled plants– produced all light red-

kerneled plants

• Cross between light red-kerneled plants yielded 7 categories of color.

Frequency Diagrams Illustrate Variation

• Useful graphing tool for illustrating variation in population

• X-axis:range of different forms that a trait can exhibit

• Y-axis number of individuals in population that exhibit each form of the trait

Frequency Diagram of Human Height

• Graphing human height creates a bell-shaped curve.

• So many different forms that the categories blend.

Frequency Diagram of Wheat Kernels

• Frequency diagrams of Nilsson-Ehle F2 variation.– Plants grown in

controlled laboratory vs. those grown in the wild

• Proved that genes can be responsible for seven different forms.

Continuous Variation is Determined by Two or More Genes

• Polygenic (quantitative) traits:– Influenced by two or

more genes residing at different loci on the same or on different chromosomes.

How Do Populations Differ?

• Brachydactyly– Human trait in which the terminal bones of the fingers

and toes do not grow their normal length

Populations are Collections of Alleles

• Populations:– Group of interbreeding organisms of the same species

that exist together in both time and space.

• Gene pools:– All of the alleles found in the population.

• Think of a beanbag– Beans are analogous to alleles and the entire bag of

beans is the population’s gene pool.

Alleles Occur at Certain Frequencies

• Example: gene pool for sickle cell anemia– Possible alleles humans could have: HBA or HBS

– If we let p = HBA and q = HBS, the sum should equal 100 % of the alleles in the gene pool.

– This could be rewritten

p + q = 1

Hardy Weinberg Principle

• Makes it possible to calculate allele frequencies (p, q) based on phenotypes.

• Can calculate the sum of the genotypes:

p2 + 2pq + q2 = 1

p2 = frequency of homozygous dominant genotype

2pq = frequency of heterozygous genotype

q2 = frequency of homozygous recessive genotype

Hardy Weinberg Principle

• States allele frequencies for a population will remain the same from generation to generation as long as specific conditions are met.

• Populations in which p and q do not change are said to be in a genetic equilibrium.

Hardy Weinberg Principle

• Required conditions for genetic equilibrium:– 1. Populations are large.– 2. Individuals mate randomly.– 3. Populations do not gain or lose individuals.– 4. Natural selection is not occurring in the

population.– 5. Mutation is not occurring at a high enough rate to

influence genetic variation.

Hardy Weinberg Principle

• Power of this principle:– Allows us

• To calculate what would happen if natural selection were not occurring

• To compare what does happen in the real world

• Also allows us to calculate the proportions of individuals in the population that have each of the three possible genotypes.

Microevolution

• Definition: Change in allele frequencies in a gene pool over time

• Factors that contribute to microevolution:– Natural selection– Genetic Drift

• Founder effect

• Bottleneck effect

– Mutation– Gene flow

Natural Selection

• Example of natural selection: industrial melanism– Rapid shift in the color

of peppered moth populations during the 19th century in England

Natural Selection

• Color of moth due to pair of alleles:– Carbonaria = M

– Speckled = m

• Before industrialism:– Speckled moths had

advantage because their coloring served to camouflage them

Natural Selection

• After industrialism:– Lichens on the trees

died.

– Made speckled moths visible.

– Darker moths were more likely to survive.

– Resulted in change in the allele frequency of the population.

Natural Selection

• Heterozygote advantage:– Tendency of red blood

cells to sickle makes these cell resistant to penetration by the parasite that causes malaria.

– Heterozygotes can survive disease and have immunity against malaria.

Types of Selection

• Directional selection– Selection that acts on

one extreme of the range of variation for a particular characteristic.

– Example: Frog tongue length.

Types of Selection

• Stabilizing selection– Selection that operates

against the extremes in the distribution of a particular trait in a population.

– Example: human birth weight

Types of Selection

• Disruptive selection

– Selection that favors the extremes and disfavors the middle range of particular traits in a population.

– Example: bird beak size

Some Changes in Allelic Frequency Are Random

• Genetic Drift:– Random change in allelic frequencies as a

result of chance alone.– Seen in small populations – Two types:

• Founder effect• Bottlenecks

– Often referred to as neutral selection• Occurs independent of natural selection

Founder effect

• Eventual genetic difference between an isolated offshoot population and the original population from which it came.– Example:

Pennsylvania Amish, settlers of Tristan da Cunha

Bottlenecks

• A drastic decrease in the size of a population with a resulting decrease in the genetic variability within a population.– Usually due to a

catastrophe (drought, hunting, flood etc.)

Mutation

• A permanent change in the genetic material of a cell or organism.– Can be inherited from generation to generation.

• Introduces new alleles into the population

• Effects can be lethal, neutral or advantageous in a population.

Gene Flow

• A shift in the allelic frequencies within a population and between populations resulting from migration.– Either immigration or

emigration– Example: DDT and

mosquitoes