1. Variation in Populations Tennessee SPI Objective:
Notes
Science
Tennessee SPI Objective:
Analyze data on levels of variation within a population to make predictions about survival under particular environmental conditions.
Essential Question
1) What is the link between genetic variations within a
population, environmental conditions, and the survival rate of sub-groups within that population?
Success Criteria
1) I can explain the link between genetic variations, the
environment, and survival rates.
2) I can analyze data relating to survival rates and variations within a population of organisms.

2. Variation in Populations
Variation refers to a variety or diversity of traits
(different kinds) within any given population.
Three causes of variation
1) Diversity of genetic traits from sexual reproduction
2) Diversity of genetic traits that result from mutations
3) Changing environmental conditions
Advantage of Variation in a Population
Environments always change, sooner or later.
Organisms must adapt to these changes or die.
If too many individuals die, then the species goes extinct.
When a significant environmental change occurs, having a variety of traits within a population provides the species a better chance that at least some small portion will have traits that are suited to the new environmental condition.

3. Let’s examine variation in a population of bears
From Brown Bear to Polar Bear:
The Origin of Polar Bears
Species Name:
Ursus Arctos
Species Name:
Ursus Maritimus

4. Our story begins with two Brown Bears living in the Northern Canadian wilderness about 70,000 years ago. f
…one male
…and one female
Let's call him "Jack"
Let's call her "Jill"

5. Jack and Jill fell in love one spring and mated.
About 7 months later, Jill gave birth to four male cubs.
Jack
Jill
Dad
Mom
Moe
Larry
Curly
Shemp
Light Brown
Dark Brown
Dark Brown
Light Tan

6. Genes have pairs of alleles which determine physical traits.
In this case the alleles determine the color of the bear's fur.
Dominant allele B will result in brown fur.
Recessive allele b will result in light-tan fur.
B b
3 cubs will have brown fur B b BB Bb
1 cub will have
tan fur
(Brown)
(Brown) Bb bb
(Brown)
(Tan)
Punnett Square
This is variation in this bear population

7. In their local habitat there was a population of 50 bears.
(25 males and 25 females)
Each mating pair had 4 cubs.
There were 100 cubs born (25 mating pairs x 4 Cubs=100)
Three cubs were brown, and one cub was tan.
There were 75 brown cubs
There were 25 tan cubs
This is variation in the population
of these bears

8. Yet environments ALWAYS change over time.
For thousands of years brown fur provided the best camouflage (structural adaptation) in the Canadian forests.
Yet environments ALWAYS change over time.
Winter started arriving sooner and lasted longer, and the glaciers began to move southward.
It was the start of a new Ice Age

9. In the new snow-white environment, brown fur was a disadvantage.
Light colored fur became an advantage.
Many of the brown fur bears died.
Yet the light colored bears thrived and multiplied.
Originally 75% were brown and 25% were light colored.
Over time only light colored bears remained because they had better camouflage to blend in while hunting prey.
Brown Fur
Light Tan Fur

10. Occasionally, due to the genetic diversity inherent in sexual reproduction, cubs were born whose fur was almost white.
These almost-white cubs, blended in even better than the light-tan bears. Therefore they were better able to hunt prey and thrive
Since the near-white bears were better adapted to the icy/snowy environment, they dominated while the other bears died off.

11. The lighter colored bears had a significant survival
advantage in this new Ice Age environment.
Over time lighter colored fur became the norm.

12. Review Questions

13. Review Questions

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