Biology Unit 7: Evolution | Notes and practice

Notes

Natural selection

Evolution is change in a population's inherited traits over generations.
Darwin's ideas: variation, overproduction, competition, and selection of individuals best suited to the environment.
Fitness = reproductive success. Individuals do not evolve; populations do.
Artificial selection: humans choose which organisms breed.

Evidence

Evidence	What it shows
Fossils	Life has changed over time
Homologous structures	Common ancestry
Vestigial structures	Leftovers from ancestors
Embryology	Similar early development
DNA and proteins	Fewer differences = closer relatives

Mechanisms that change allele frequencies

Natural selection, mutation, gene flow, genetic drift (bottleneck and founder effects), and non-random mating. A population with none of these is in Hardy-Weinberg equilibrium.

Patterns and speciation

Directional, stabilizing, disruptive and sexual selection.
New species form through reproductive isolation: geographic, behavioral or temporal.
Adaptive radiation produces many species from one ancestor.

Topics in this unit

Each topic has its own short notes and practice.

Practice

A population of beetles has green and brown individuals. Birds eat green beetles more easily. After many generations, what will most likely happen?

Which is the best evidence that whales and hippos share a recent common ancestor?

A bird wing and a butterfly wing are both used for flying but have very different structures. They are:

A few birds are blown to a new island and start a population there. By chance, many carry a rare allele, so it becomes common on the island. This is:

What must happen for one species to split into two?

Darwin found that finches on different Galápagos islands had beaks suited to different foods, and all descended from one mainland ancestor. This is an example of:

Free response

Write your answer first, then open the scoring guide.

Free-response question

A scientist studies a population of mice living on light-colored sand. Owls hunt the mice. In the population, 80% of mice have light fur and 20% have dark fur. A volcano erupts and covers the area with dark lava rock.

(a) Predict how the percentage of dark-furred mice will change over many generations. Explain using natural selection.

Show the answer and scoring guide

The percentage of dark mice will increase. On dark rock, dark mice are camouflaged and light mice are easy for owls to see. Dark mice survive and reproduce more, so they pass on the dark-fur allele, and it becomes more common.

Check off the points you earned:

1 point: predicts an increase in dark-furred mice
1 point: explains differential survival and reproduction passing on the allele

(b) Explain why this change could not happen if fur color were not inherited.

Show the answer and scoring guide

Natural selection only changes a population if the trait can be passed to offspring. If fur color were not inherited, surviving dark mice would not produce more dark offspring, so the population would not change over generations.

Check off the points you earned:

1 point: states the trait must be heritable
1 point: explains offspring would not inherit the advantage

(c) Name the type of selection shown and identify one other mechanism that could change allele frequencies in this population.

Show the answer and scoring guide

This is directional selection, because one extreme (dark fur) is favored. Another mechanism is genetic drift (random change, especially if the eruption shrank the population), gene flow from mice moving in, or mutation.

Check off the points you earned:

1 point: identifies directional selection
1 point: names a valid other mechanism (genetic drift, gene flow or mutation)