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Natural selection

Content objective:

What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. This is where we build from.


How does natural selection work? Imagine a population of beetles:

(This section is from Understanding Evolution, Natural Selection, Evolution 101 website)

There is variation in traits. For example, some beetles are green and some are brown.


There is differential reproduction. Since the environment can’t support unlimited population growth, not all individuals get to reproduce to their full potential.

In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

beetles 2

There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis.

beetles 3

End result: The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population.

If this process continues, eventually, all individuals in the population will be brown. Eventually, the advantageous trait dominates

beetles 4

if you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. It is as simple as that.

Now learn about:

Natural selection at work

Fitness: how good a particular genotype is at leaving offspring, relative to how good other genotypes are at it.

Sexual selection is a special case of natural selection.

Sexual selection part 2

Adaptation: A feature of an organism that helps it fit in well with its environment

Misconceptions about natural selection


The degree of kinship between organisms or species can be estimated from the similarity of their DNA sequences; this similarity often closely matches organisms’ or species’ classification based on anatomical similarities.

The graphic above shows that:

All of these primates had a common ancestor, the ancestral primate.
The human and chimpanzee have the closest evolutionary relationship as their DNA is the most similar. Image from Regentsprep.org

Adaptation examples

Bats, dolphins, and many forms of mammals, all use adaptations of the same original five-fingered animal hand.

adaptive radiation from five fingered hand

Adaptation: Finches on the Galapagos Islands.

Here is a color graphic summarizing how one species of finch radiated over time to become many.

Finches color

Here is the same information, as presented on a Regent’s exam.

Finches BW regents


Are all of these changes successful?

Extinction is shown on a tree of life, when a line comes to an end, without any branches.

australo hominid primate cladogram extinction

Sample questions

Feb 2016 MCAS: The larvae of the common sulphur butterfly can be light green or bright yellow. Birds prey on the larvae, which are found on the green leaves of alfalfa plants. Based on the theory of natural selection, which of the following would scientists expect to observe in populations of common sulphur butterfly larvae?

A. All the green larvae develop yellow stripes before metamorphosis.
B. All the yellow larvae and none of the green larvae are eaten by birds.
C. The percentage of green larvae in the population is much greater than the percentage of yellow larvae.
D. The percentages of green larvae and yellow larvae in the population remain equal for many generations.


Feb 2016 MCAS: Populations of a European salamander, Proteus anguinus, live in underground caves that have a limited food supply. One of the salamander’s adaptations is the ability to significantly reduce its rate of metabolism when food is scarce. Which of the following statements best describes how the process of natural selection led to this adaptation?

A. Salamanders with the ability to slow their metabolism grew more slowly than other salamanders.
B. Salamanders with the ability to slow their metabolism were more likely to emigrate than other salamanders.
C. Salamanders with the ability to slow their metabolism underwent more rapid mutation than other salamanders.
D. Salamanders with the ability to slow their metabolism were more likely to survive and reproduce than other salamanders.


Feb 2016 MCAS .  Cowbirds are birds that lay their eggs in the nests of smaller birds such as warblers. The cowbird eggs develop quickly and usually hatch first. As a result, the larger cowbird chicks get most of the food and may push the smaller warbler chicks out of the nest. The warbler parent birds do not seem to know that the cowbird chicks are different from their own offspring. The original range of the cowbirds’ habitat was limited to the Midwest prairies. Today, cowbirds are found in all states in the continental United States.

Which of the following describes the most likely way that the expansion
of the cowbirds’ range has affected warblers?
A. Warbler population sizes have decreased.
B. Warbler eggs have become larger in size.
C. Warblers have increased their birth rates.
D. Warblers have become less protective of their nests.

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