Population Genetics and Evolution

Can individuals evolve? Can an organism respond to natural selection by acquiring or losing characteristics?

POPULATIONS, NOT INDIVIDUALS, EVOLVE!!!!!

How does Natural Selection work? Natural selection acts on the phenotypes of a population Evolution occurs as population’s genes and frequencies change over time

How can a populations genes change over time? Gene pool: *the combined genetic information of all the members of a particular population *contains two or more alleles Allelic Frequency: *percentage of any specific allele in the gene pool Genetic equilibrium: *a population in which the frequency of alleles remains the same over generations → population not evolving

5 conditions required to maintain genetic equilibrium Random mating Very large population No immigration or emigration No mutations No natural selection

What causes changes in genetic equilibrium? Mutations Gene Drift

Mutations Any change in a sequence of DNA Can occur due to mistakes in the replication of DNA or due to environmental factors. Occasionally mutations can lead to useful variations that can be added to the populations gene pool.

Genetic Drift A change in the allelic frequencies in a population that is due to chance In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes greater. Genetic drift may take place when a small group of individuals colonizes a new habitat.

The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. Genetic Drift Video Genetic Drift Video 2

Bottleneck and Founder Effects -Genetic drift can cause big losses of genetic variation for small populations. -Population bottlenecks occur when a population’s size is reduced for at least one generation. -A founder effect occurs when a new colony is started by a few members of the original population.

a population declines to a very low number and then rebounds

Knowledge Check What is the relationship between the terms natural selection and evolution? They mean the same thing. Evolution works against natural selection. C. Evolution explains how natural selection works. Natural selection explains how evolution works.

Speciation -A species is often defined as a group of individuals that actually or potentially interbreed in nature. -A species is the biggest gene pool possible under natural conditions. -Speciation is an event so dramatic that it produces two or more separate species.

Rate of Speciation Evolution proceeds in small, gradual steps according to a theory called gradualism. Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.

Causes of Speciation Isolation Reduction of Gene Flow

Types of Isolation Behavioral Isolation Geographic Isolation Temporal isolation

Behavioral Isolation A form of reproductive isolation in which two populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding

Geographic Isolation Organisms of the same species are separated (due to a river, canyon, mountain, etc) and slowly evolve along separate lines. Gene pools begin to show major differences, so reproduction with an organism on the other side of the physical barrier is no long possible

Temporal isolation Two or more species reproduce at different times. They cannot reproduce with each other Example: Orchids

Kinds of Speciation Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location. Allopatric speciation is just a fancy name for speciation by geographic isolation

Adaptive Radiation Can occur in a relatively short time when one species gives rise to many different species in response to the creation of new habitat or some other ecological opportunity

Mouse and Mango Video Clip

Knowledge Check What tempo of evolution does this model represent? gradual elevated sequential punctuated