1. Natural Selection in Populations
Biology 7(C)

2. Natural Selection in Populations
Learning Objectives
Analyze and evaluate how natural selection produces change in populations, not individuals
After this lesson you will be able to analyze and evaluate how natural selection produces change in populations, not individuals.

3. Natural Selection in Populations
Population – group of organisms of the same species living together in a given region
Natural Selection – process whereby organisms with favorable variations survive and produce more offspring than less well-adapted organisms
Proposed by Darwin
Key mechanism of evolution
A population is defined as a group of organisms of the same species living together in a given region.
Natural selection is the process whereby organisms with favorable variations survive and produce more offspring than less well-adapted organisms. This theory was first proposed by Charles Darwin. Natural selection is the key mechanism of evolution.

4. Lamarck’s Theory Lamarck’s theory of evolution
Inheritance of acquired characteristics – the characteristics an organism develops over time can be passed on to offspring
Mouse gets its tail cut off, offspring should have no tails
Giraffe stretches its neck out to reach tall leaves, offspring should have longer necks
Incorrect, not supported by scientific evidence
Stimulated thought about evolution
Jean Baptiste de Lamarck was a French biologist who proposed one of the first theories about evolution. His theory is referred to as the inheritance of acquired characteristics. He had the idea that the characteristics an organism develops over time can be passed on to offspring. For example, if a mouse gets its tail cut off, this mouse will produce offspring with no tails. A proposed example is that if a giraffe stretches its neck out to reach tall leaves, it passes on this stretched, longer neck trait to offspring.
We know this theory is incorrect because it is not supported by any scientific evidence. It is now known that genetic traits result from genes encoded in the DNA of organisms. Traits that are acquired throughout life due to environmental conditions are not passed on to future offspring.
Even though Lamarck’s theory is incorrect, it did begin to stimulate thought about evolution and laid the groundwork for Darwin.

5. Natural Selection in Populations
No individual is capable of evolving
Natural selection produces changes in populations, not individual organisms
Organism may be born with a favorable mutation
Acquired traits are not passed on to offspring
No individual organism is capable of evolving. Natural selection produces changes in populations, not individual organisms.
An organism may be born with a favorable mutation that increases its chance of survival. Natural selection does not begin to take place until that favorable mutation is actually “selected” for. The organism with the favorable genetic trait must survive, reproduce, and pass on that favorable mutation to its offspring.
Acquired traits are not passed on to offspring. Remember that Lamarck’s theory was incorrect.

6. (Own work) [CC-BY-SA-3.0]
Natural Selection in Populations
How does natural selection produce changes in populations?
No two individuals in a population are exactly alike
Organisms vary in size, coloration, structural characteristics, and behavior
Variations important in populations
Individuals with most adaptive variations survive and pass on traits
Over time, population shifts towards adaptive variations
How does natural selection produce changes in populations? Due to their genetic makeup, no two individuals in a population are exactly alike. Organisms vary in size, coloration, structural characteristics, and behavior. This picture of the Northern leopard frog shows extensive color variations in these members of the same species.
These variations are important in populations and the key to natural selection. Individuals with the most adaptive variations survive and pass on their traits to offspring. For example, the leopard frogs with the most adaptive coloring allowing them to blend into their habitat will have a better chance of survival. The frogs which survive will reproduce and pass on their coloring to future offspring.
Overtime, the population will shift. There will be more frogs with adaptive coloring than frogs without.
Image by Loba Wolf
(Own work) [CC-BY-SA-3.0]

7. Natural Selection in Moth Population
Random mutations - color variations in moth population
Enables the moths to blend into tree trunks
Harder for birds to locate “camouflaged” moths
Natural selection occurs
Moths with mutation survive and reproduce
Favorable coloring is passed on to offspring
Imagine a population of brown moths. Random mutations causes color variations in the population, and a few moths are born with a speckled gray and white color. This coloring enables the moths to blend into the tree trunks of their habitat. Birds which prey on this moth population have more difficulty locating the “camouflaged” moths.
Remember that in order for natural selection to occur, organisms must reproduce and pass on this favorable mutation to offspring. Due to the ability to camouflage themselves, moths with the color mutation survive and reproduce. The favorable coloring trait is passed on to future offspring, and the percentage of camouflaged moths increases in the population.
Natural selection occurred in the moth population, not an individual moth.
Image by Gilles San Martin
(Own work) [CC-BY-SA-3.0]

8. Natural Selection in Giraffe Population
Evolution of the modern day giraffe
Giraffe population had short necks and ate grass
Some had longer necks than others
Could eat lower leaves of trees
Could eat when grass was scarce
More likely to survive
Offspring would inherit favorable variation of a longer neck
Over generations, average neck length in population increases
Natural selection in the giraffe population resulted in evolution of the modern day giraffe. The original giraffe population had short necks and ate grass. Variations resulted in some giraffes having longer necks than others. Those with longer necks could eat the lower leaves of trees as well as grass. When grass was scarce, the longer-necked giraffes would be more likely to survive and reproduce. Their offspring would inherit the favorable variation of a longer neck.
Over generations, the average neck length in the population would increase.
Natural selection occurred in the giraffe population, not individual giraffes.
Image by Yann Fauché and Alma Mulalic
(Own work) [CC-BY-SA-3.0]

9. Natural Selection in Populations
Genetic Equilibrium – the genetic makeup of a population will remain relatively stable unless something happens to make it change
Populations in genetic equilibrium do not change or evolve
Natural selection upsets genetic equilibrium and causes changes in populations
Natural selection directly affects the genetic equilibrium of populations. The genetic makeup of a population will remain relatively stable unless something happens to make it change. Populations in a state of genetic equilibrium do not change or evolve. All the variations in the population remain stable. There is no increase or decrease in the number of variations among the population.
For evolution to take place something must upset the genetic equilibrium of a population. Natural selection upsets genetic equilibrium and causes changes in populations.

10. Natural Selection in Populations
Learning Objectives
Analyze and evaluate how natural selection produces change in populations, not individuals
After this lesson you should now be able to analyze and evaluate how natural selection produces change in populations, not individuals.