Natural Selection and Evolution

1. Natural Selection and Evolution

2. Organisms produce more offspring than can survive and they compete for limited resources. Random variations in traits occur among individuals of a population. Variations are heritable (may be passed on to the next generation through reproduction). Individuals with helpful variations are more likely to survive and pass on their traits. Over time, offspring with these helpful variations make up more and more of the population. Five Principles of Natural Selection

3. Organisms produce more offspring than can survive. • All organisms produce more offspring than can survive. • Survivors continue to pass their traits on to future generations.

4. Random variations occur among individuals of a population. • Organisms have various traits. • Among the members of a population, individuals have differences in their traits. • These differences are called • variations.

5. Variations Continued • Measure any trait in a population of organisms, and random differences will be noted. • Variations may be helpful to survival, harmful, and neutral. It is all a matter of random chance.

6. Variations are Heritable • Heritable means they may be passed on to the next generation from parent to offspring. • Variations that cannot be inherited do not contribute to evolution (i.e. Lamark).

7. Individuals with helpful variations are more likely to survive and pass on their traits. • Helpful variation such as stronger muscles, long legs, or good camouflage help an individual survive. • Survivors have a better chance at reproduction. • Unhelpful variations…

8. Over time, offspring with helpful variations make up more and more of the population. • Less useful variations place an individual at a disadvantage. • These individuals are more likely to succumb to disease, starvation, or predators. • “Beneficial” variations lead to better chances and are passed on to offspring.

9. Variations & Natural Selection • Given a trait such as leg length in zebras, variations will occur among individuals. • Some zebras will have longer legs, others average, others still will have shorter than average. • Assuming leg length is related to speed, individuals with longer legs will be better equipped to outrun predators and survive. • Their offspring will inherit longer legs and survive better than short legged zebras!

10. So what is Natural Selection? The tendency of individual with helpful variation to survive and reproduce more often that individuals with less helpful or harmful variations.

11. Adaptation • An adaptation is any variation that makes an organism better suited to its environment. • It is a noun not a verb! • A species has various adaptations, but individuals cannot adapt by changing their traits.

12. How do adaptations arise? • Natural selection works on variations within each generation. Consider bears… • Long ago, some bears living in the north had thicker coats than average (and some had thinner) • Thick coat bears had a survival advantage and produce more surviving offspring than thin coat bears. • Over many generations, thicker coats become more common until they are a trait that is fixed in the species. • Thick coats are now an adaptation – polar bears have them!

13. What does it mean to be well adapted? • When the traits of a species are well matched to the environment it is said to be well adapted. • This organism is fit, and has a high degree of fitness. • Fitness is a measure of an how well an organism is adapted to its environment, and its chances of surviving and reproducing successfully.

14. Fitness

15. The Big IdeasGenes, Mutations Variations, and Heritability • Genes – segments of DNA that control traits. • Mutations – mistakes made when DNA is copied before meiosis or mitosis. • Mutations change genes and thus change traits. • Changed traits are really just variations. • Since variations are from DNA – they’re heritable. • Natural Selection favors helpful variations that were created by lucky mutations – EVOLUTION!

16. Mutations Lead to Variations in Traits

17. Geographic Isolation • For a new species to arise, a group of individuals must be reproductively isolated from its parent population. • Often this happens due to geographic isolation. • Isolation allows natural selection to slowly change the traits of one population while the other population does not.

18. Geographic Isolation & The Grand Canyon Kaibab Squirrel (North Rim) Albert’s Squirrel (South Rim)

19. What if isolation does not occur? • Without isolation, individuals of both populations will interbreed – swapping genes in the process. • This will keep the traits of the two groups more or less together • A new species cannot forms, since all of the individuals exchange genes (and traits) amongst each other.

20. Sometimes evolution may occur due to minute changes over many millions of years. This is called gradualism. Camel & whale evolution are two good examples. These organisms changed over millions of years leaving many intermediate forms. Other times evolution may occur in much shorter periods of time with little change before or after that period. This is Punctuated Equilibrium. When a sudden change in the environment causes heavy natural selection pressure – new traits can arise quickly. Antibiotic resistance is a good example. The speed of evolution…