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 1. Random mating 2. Very large population 3. No immigration or emigration 4. No mutations 5. No natural selection
What causes changes in genetic equilibrium? 1. Mutations 2. 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
D. Natural selection explains how evolution works. A. They mean the same thing. B. Evolution works against natural selection. What is the relationship between the terms natural selection and evolution? C. Evolution explains how natural selection works. Knowledge Check
-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 1. Isolation 2. 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: OrchidsOrchids
Kinds of Speciation Allopatric speciation is just a fancy name for speciation by geographic isolation Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.
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