DAY 3 1. Population Genetics and Evolution Darwin developed his theory of natural selection without...

DAY 3

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Population Genetics and Evolution Darwin developed his theory of natural

selection without knowing about genes. The principles of today’s modern theory

are rooted in population genetics and other related fields of study and are expressed in genetic terms.

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Populations, not individuals, evolve Can an organism respond to natural

selection by acquiring or losing characteristics?

Recall that genes determine most features Within its lifetime, an individual cannot evolve

to a new phenotype by natural selection in response to its environment

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Natural selection acts on the range of pheontypes in a population.

Each member of a population has the genes that characterize the traits of the species (as pairs of alleles).

All of the genes of the population’s individuals make up the population’s genes

Evolution occurs as a population’s genes and their frequencies change over time.

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How? Gene pool – picture all of the alleles of the

population’s genes as being together in a large pool.

The percentage of any specific allele in the gene pool is called the allelic frequency. Scientists calculate the allelic frequency of an allele in the same way that a baseball player calculates a batting average.

Genetic Equilibrium - a population in which the frequency of alleles remains the same over generations.

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Changes in Genetic Equilibrium A population that is in genetic equilibrium is not

evolving. If allelic frequencies remain the same, so do phenotypes.

Changes in genetic equilibrium result in evolution. Mutations Genetic drift – the alteration of allelic frequencies by

chance events. Can greatly affect small populations Life in the Galapagos Islands

Gene flow – the transport of genes by migrating individuals. When an individual leaves a population, its genes are lost

from the gene pool.

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Natural Selection acts on Variation Recall that some variations increase or

decrease an organism’s chance of survival in an environment. These variations can be inherited and are controlled by alleles. There are three different types of natural selection that act on variation: stabilizing, directional, and disruptive.

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Stabilizing Selection

favours individuals with an “average” value for a trait, and selects against those with extreme values. Human birth weight is an example. Until recent medical advance, infants that were too small tended not to survive and infants that were too large died during birth.

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Stabilizing selection10

Directional Selection

favours individuals possessing values for a trait at one extreme of the distribution, and selects against the average and other extreme.

The development of antibiotic resistant bacteria is an example of directional selection. Only those bacteria that can tolerate the presence of an antibiotic survive.

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Directional Selection12

Disruptive Selection

favours individuals at both ends of the distribution and selects against the average.

It is also known as diversifying selection.

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Disruptive Selection14

Natural vs. Artificial Selection Both are mechanisms of change in the

gene pool of a population In artificial selection - humans ensure

individuals with the more desirable traits are allowed to reproduce

In natural selection, those individuals who are best suited to their environment survive and reproduce.

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Natural Selection in a Candy Dish Yummy!

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