4 How does Natural Selection work? Natural selection acts on the phenotypes of a populationEvolution occurs as population’s genes and frequencies change over time
5 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 allelesAllelic Frequency:*percentage of any specific allele in the gene poolGenetic equilibrium:*a population in which the frequency of alleles remains the same over generations → population not evolving
6 5 conditions required to maintain genetic equilibrium Random matingVery large populationNo immigration or emigrationNo mutationsNo natural selection
7 What causes changes in genetic equilibrium? MutationsGene Drift
8 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.
9 Genetic DriftA change in the allelic frequencies in a population that is due to chanceIn 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.
10 The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.Genetic Drift VideoGenetic Drift Video 2
11 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.
12 a population declines to a very low number and then rebounds
13 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 selectionworks.Natural selection explains how evolutionworks.
14 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.
15 Rate of SpeciationEvolution proceeds in small, gradual steps according to a theory called gradualism.Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly.
18 Causes of SpeciationIsolationReduction of Gene Flow
19 Types of Isolation Behavioral Isolation Geographic Isolation Temporal isolation
20 Behavioral IsolationA form of reproductive isolation in which two populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding
21 Geographic IsolationOrganisms 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
23 Temporal isolation Two or more species reproduce at different times. They cannot reproduce with each otherExample: Orchids
24 Kinds of SpeciationSympatric 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
25 Adaptive RadiationCan 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
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