Population Changes

1. Population Changes Natural Selection

2. Natural Selection • Multiple observations that life has changed over generations. • What was missing was a mechanism of explaining HOW organisms changed. • Darwin’s contribution: A scientifically valid, testable theory accounting for the observed data and explaining why populations change over generations.

3. Theory of Natural Selection Specifics: • Organisms over-reproduce. • Genetic variation among young. • Competition among individuals • Individuals with traits (genes) that aid survival (reproduction) will have a better chance of producing offspring. • Over time, the population changes to include the a greater percentage of the traits (genes) of the more successful reproducers.

4. Natural selection Big Picture • Success varies in reproduction among individuals that vary in their heritable traits. • Differences emerge -- environment. • Natural selection can increase the adaptation over generations. • Environment changes or immigration, may result in adaptation to these new conditions, sometimes give rise to new species.

5. Significance of Variation • Variations that are heritable are the raw material for natural selection. • Unique combinations of genes • Recombination- Meiosis • Random Fertilization • Ultimate Source: Mutation

6. Natural Selection • Selection acts directly on: Phenotypes • By doing so, it impacts the frequency of certain genotypes.

7. Selection Modes • Natural Selection can alter the frequency distribution of heritable traits in 3 ways: • Directional • Stabilizing • Disruptive

8. Impact on a Population • As natural selection acts on a population, the frequency of alleles in the population changes (evolution occurs). • Certain alleles and traits become more prevalent, while others become less so. • If an environment changes over time, or individuals of a particular population move to a new environment, natural selection may result in adaptation to these new conditions, sometimes give rise to new species.

9. Impact on a Population • Genetic drift is fluctuation of allele frequencies in a population due to random events. • Consequence of small population size.

10. Impact on a Population • Bottleneck Effect- sudden change in the environment (ex. fire, flood) that results in a surviving population that is not reflective of the original population’s gene pool.

11. Impact on a Population • Founder Effect-occurs when a few individuals become isolated from a larger population with the result that the new population’s gene pool is not reflective of the original populations.

12. Impact on Populations • Gene flow between populations can introduce new alleles into a population. • Gene flow between populations tends to reduce differences, maintaining genetic similarity.

13. Speciation • With reproductive isolation (interrupted gene flow), populations will diverge as they become adapted to differing selective pressures and accumulate differing allele frequencies.

14. Reproductive Isolating Mechanisms • Reproductive Barriers • Prezygotic • Habitat • Temporal • Behavioral • Mechanical • Gametic • Reproductive Barriers • Postzygotic • Reduced Hybrid Viability • Reduced hybrid fertility • Hybrid breakdown

15. Reproductive Isolating Mechanisms Reproductive Isolation Postzygotic Prezygotic Habitat Temporal Behavioral Gametic Reduced Hybrid Viability Reduced hybrid fertility Hybrid breakdown Mechanical