Remember! Evolution is a genetic change in a population.

1. Remember! Evolution is a genetic change in a population. In order for that genetic change to occur, there has to be some genetic variation within the population.

2. Can we see evolution occurring in a population just by observing the phenotypes?

3. Evolution & Selection of Traits in Populations

4. Selection on a Single-Gene Trait • Remember! A single-gene trait is a trait that is controlled by just 1 gene. • A single-gene trait with 2 alleles will show only 2 phenotypes. • So, a change in frequency (evolution) is easy to see in a population. There is only 1 gene that determines whether you have a widow’s peak or not!

5. Selection on a Polygenic Trait • A polygenic trait is controlled by more than 1 gene. • Human height is NOT determined by just 1 gene. Scientists estimate that there are between 7-20 genes that influence our height! • We can inherit any combination of those genes. This is why there’s NOT just short & tall; there’s a large range of heights!

6. Selection on a Polygenic Trait • If you were to graph the frequencies of the phenotypes, you would get a bell shaped curve. This shows that the majority of people have the average phenotype (average height), while fewer people have extreme phenotypes (are very tall or very short).

7. These students & faculty have sorted themselves into columns by their height.

8. Selection on a Polygenic Trait • To see if the frequency of a polygenic trait is changing in a population, we have to examine the bell shaped curve & how it changes over time.

9. Selection on a Polygenic Trait • Directional Selection: occurs when individuals at 1 end of the curve (with phenotypes at 1 end of the spectrum) are advantaged, & selection against the other end occurs If food becomes scarce, & only large beaks are favored, what will happen?

10. The individuals with the higher fitness will succeed & over time the population will shift in its phenotypes in 1 direction.The birds with the larger beaks will survive & over time the population will have a higher frequency of large beaks.

11. Peccaries are pigs that enjoy feeding on desert cacti. Naturally, peccaries choose the cacti with fewer spines. If a population of peccaries was introduced into an area with lots of cacti, how would you expect the population of cacti to change over time?

12. When a bell shaped curve shifts in 1 direction, that’s directional selection!

13. Frequency vs. Running Speed of Rabbits • Which rabbits would you predict have the greatest fitness? • If we graphed running speed several generations later, what differences would we see?

14. Selection on a Polygenic Trait • Stabilizing Selection: when individuals in the middle of the curve are more advantaged & have a higher fitness • This causes the frequency of the mid-phenotypes to increase & the ends to decrease. Birth weight in humans undergoes stabilizing selection.

15. A population of desert mice come in a variety of fur colors. The medium fur color is more beneficial, because the mice blend in with the rocks & can hide from predators. What do you think the population will look like in a few generations?

16. Selection on a Polygenic Trait • Disruptive Selection: when individuals at the ends are more advantaged • Less common • A single curve will appear to split into 2. When only large & small seeds are common, the birds with large & small beaks will eat!

17. When the bell curve seems to split into 2, that represents disruptive selection.

18. Disruptive Selection In a species of African butterfly, Papiliodardanus, the colorations range from brown to orange. In both cases, these extremes of color, from different ends of the spectrum, look like (mimic) other species of butterflies that are not palatable (they don’t get eaten). So, selection favors the extremes in coloration within the Papiliodardanuspopulation. Those butterflies that are moderate in coloration are eaten in far greater numbers than those at the extremes of the color spectrum. The butterflies with extreme coloration survive & pass on those genes for coloration to the next generation. What type of selection does this illustrate? Mimics another unpalatable species Mimics an unpalatable species Doesn’t mimic – gets eaten

20. Directional Selection The greyhound dog breed was originally used to hunt the fastest game, fox & deer. The breed dates back to 2900 BC in Egypt. Early breeders were interested in dog with the greatest speed. They only allowed the fastest dogs to mate. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran the fastest, they gradually produced a dog who could run up to 40mph. What type of selection does this illustrate?

21. # of Dogs Running Speed of Dog

22. Stabilizing Selection The Siberian Husky is a dog bred for working in the snow. It’s a medium dog, weighing 35-60lbs. These dogs have strong pectoral & leg muscles, allowing it to move through dense snow. The Siberian Husky is well designed for working in the snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink & get stuck. Yet if the husky had lighter muscles, it would not be strong enough to pull sleds & equipment, so it would have little value as a working dog. What type of selection does this illustrate?

23. Too Light Too Heavy # of Dogs Weight of Muscles