Natural selection

1. Natural selection

2. Natural Selection How does it work?

3. What is going to happen here?

4. Natural selection • Definition: differential reproductive success • Individuals who have best fit traits for the environment tend to have a better chance of surviving and if they survive then they are more likely to find a mate and have offspring.As a result they pass down certain traits that help them survive. The populations then shifts to having certain/specific traits.

5. Natural selection • Def: differential reproductive success • In other words: some individuals have the best fit trait while others in the population do not= a difference in each individuals ability to reproduce/ have offspring successfully.

6. Natural selection questions • What does natural selection select for genotypes or phenotypes? • NS selects for phenotypes • What does natural selection do to the variation of a population? • Natural selection will decrease the variation of a population.

7. Natural Selection • Depends on reproductive success • Operates on a population, NOT on an individual • Dynamic between genetic change and the changing environment • Environment exerts Selective Pressure: environmental conditions that select for certain individuals and select against others • Natural Selection can take time or can happen quickly • eg: Bacteria vs. Finches

8. Natural selection criteria • In order for a population to undergo the process of natural selection they have to meet all of the following criteria : • Variation • Heritable • Differential (difference in survival and reproduction success) • High rate of population growth. 

9. 1. Variation Within a Species

10. Variation with in a species • Variation: Individuals in a population have to different traits from each other. * Variation is the raw material of evolution = allows for specific traits to be “Best fit” for an environment (chosen) , creating a higher frequency of them = a change in the frequency of traits found in a population.

11. 2. Heritable • Best fit traits have to be inheritable/ pass down to offspring.

12. 3. Differential survival and reproductive success Individuals in a population compete for resources (food, water, protection against prey, a niche (home), and mates). Some individuals are better at acquiring those traits over others.

13. 4. High Rate of population growth Creates two conditions: 1. a high # of offspring will result in a struggle for resources (food, water mates) which leads to difference in survival and reproductive success. 2. greater chance for the best fit trait to appear in high frequency in the population

14. Peppered moth and natural selection

15. Types of Natural Selection

16. Think of different sized bird beaks

17. Directional Selection Original distribution of beak sizes How distribution of beak sizes change due to a changing environment

18. Directional Selection • Imagine that the environment changes and the supply of small and medium-sized seeds runs low. Which beak size will be favored? • Those birds with the larger beak will be able to survive on the large seeds! • In “directional selection,” natural selection favors a characteristic on the extreme of the phenotypic variation. • How does this affect variation? • Variation of beak size shifts in ONE direction.

19. Stabilizing Selection

20. Stabilizing Selection • Imagine that the environment changes and the supply of small and large-sized seeds runs low. Which beak size will be favored now? • Those birds with the medium beak will be able to survive on the medium seeds! • In “stabilizing selection,” natural selection favors characteristics near the middle of the phenotypic distribution. • How does this affect variation? • Variation of beak size shifts in towards the middle, decreasing genetic variation at the extremes.

21. Disruptive Selection How distribution of beak sizes change due to a changing environment Original distribution of beak sizes

22. Disruptive Selection • Imagine that the environment changes and only the supply of medium-sized seeds runs low. Which beak size will be favored now? • Those birds with BOTH the smaller and larger beak will be able to survive! • In “disruptive selection,” natural selection favors characteristics at BOTH extremes of the phenotypic variation. • How does this affect variation? • Variation shifts to favor two subgroups of birds specialized in eating different sized seeds.

23. Sexual Selection Males normally compete for females, behavior or physical features that females look for in a suitable male are the basis for sexual selection. Ex. Peacock tails

24. The Process of Speciation Question: Knowing that natural selection occurs, how does this process result in entirely new species??

25. The Process of Speciation • In order to create a “new species,” the gene pools for two populations must become separated (or isolated). • Both populations continue to change (through mutations or meiosis). • Once they can no longer breed with one another, a new species has formed. This is called reproductive isolation.

26. Reproductive Isolation • Mating Behavior (called behavioral isolation) when members of two populations have different mating or courtship rituals (i.e. mating call, attractive coloration)

27. Reproductive Isolation • Mating Time (called temporal isolation) when members of two populations breedat different times of the day, different seasons, or different years. (i.e. release of pollen in orchids)

28. Reproductive Isolation • Location (called geographic isolation) when members of two populations are separated by geographic barriers (i.e. rivers, mountains or bodies of water, islands, disaster, etc.)

29. Geographic Isolation Populations separate geographic barrier Populations meet up again; interbreeding does not occur. Speciation has occurred. Populations meet up again; interbreeding occurs. Speciation has not occurred.

30. Speciation in Darwin’s Finches (p. 408-409) • founders arrive from mainland • geographic isolation • changes in gene pool occur over time • reproductive isolation • competition • continued evolution