Natural Selection• Natural selection is the

process by which those heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations.

• It is a key mechanism of evolution.

-Genetic variation is essential for natural selection. -The natural genetic variation within a population of organisms means that some individuals will survive and reproduce more successfully than others in their current environment.

-Natural selection acts on the phenotype, or the observable characteristics of an organism, but the genetic (heritable) basis of any phenotype which gives a reproductive advantage will become more common in a population. -Over time, this process can result in adaptations that specialize organisms for particular environment.

The idea that evolution takes place at one point in time, followed by a long period without change is ______.

From: Test bank 07-08

A gradualismB punctuated equilibriumC artificial selectionD succession

Charles Darwin proposed his theory of evolution based on observations of nature. Which observation that contributed to his theory is illustrated by this population of beetles?A.Species produce more offspring than can survive.B.Populations remain stable over time.C. Individuals within a population may vary widely.D. Environmental resources are limited.

-Genetic drift, along with natural selection, mutation, and migration, is one of the basic mechanisms of evolution.

-In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.

Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of

evolution, it doesn’t work to produce adaptations.

Bottleneck and Founder Effects

-Genetic drift can cause big losses of genetic variation for small populations.-Population bottlenecks occur when a population’s size is reduced for at least one generation.-A founder effect occurs when a new colony is started by a few members of the original population.

Sample Question 3A small population of chimpanzees lives in a

habitat that undergoes no changes for a long period. How will genetic drift probably affect this population?

A It will accelerate the appearance of new traits. B It will promote the survival of chimpanzees with

beneficial traits. C It will increase the number of alleles for specific

traits. D It will decrease genetic variation.

Sample Question 4Genetic drift is a ______________

process. A. directedB. selection-drivenC. randomD. co-evolutionary

-A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.-Speciation is a lineage-splitting event that produces two or more separate species.

Causes of Speciation1. Geographic Isolation2. Reduction of Gene Flow

Kinds of Speciation

Allopatric speciation is just a fancy name for

speciation by geographic

isolation

Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

This diagram illustrates what kind of speciation?A Sympatric B AllopatricC Geographic D Catastrophic

River

CladogramA cladogram is a

diagram used to show ancestral relations between organisms, to represent the evolutionary tree of life.

These are two identical cladograms.

Analyzing a cladogram…1.Which species are most closely related?2.Which is the common ancestor of flies and bees?3.Which insects share the most recent ancestor?

1

23

Here’s a more complicated one…

What conclusions

can you draw from this ‘tree of

life’?

According to the cladogram above, which two species are most closely-related?A clubmosses and spikemossesB spikemosses and fernsC ferns and flowering plantsD flowering plants and clubmosss

Fossil RecordFossils are the preserved remains or traces of

animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record.

• Fossil evidence indicates that over time organisms of increasing complexity appeared on Earth. Bacteria and blue-green bacteria are the first fossils that were preserved from the Precambrian era.

• The fossil record contains many examples that could be interpreted to mean that species evolved from more ancient organisms.

Sample Question 7In a section of the Grand Canyon, scientists have found the fossil remains of several different groups of organisms. The diagram on the left represents the number and age of the fossils the scientists found. The width of each shaded area in the diagram below indicates the relative number of fossils found.

Which of the following statements is supported by the fossil record?A Group C is the only group that no longer exists for it’s now extinct.B Group B was the most numerous organism 10 million years ago.C Group D has been in existence the longest.D Group A is the most recent organism to come into existence.

Sample Question 8Which of the following is usually most helpful

in determining the age of the fossils found in rock strata?

A the size of the fossilsB the color of the fossilsC the water content of the rock layer where the

fossils were locatedD the depth of the rock layer where the fossils

were found

Sample Question 9Which of the following best explains how the

fossil record provides evidence that evolution has occurred?

A It indicates the exact cause of structural and behavioral adaptations of organisms.

B It shows that the form and structure of groups of organisms have changed over time.

C It shows how the embryos of many different vertebrate species are very similar.

D It indicates that forms of life existed on Earth at least 3.5 billion years ago.

Sample Question 10Orchids were studied to determine if the

amount of humidity affected the flowering of these plants. Which of these was the independent variable in this study?

A The percentage of humidityB The length of time required for floweringC The amount wateredD The number of flowers on each plant

Remember…

INDEPENDENT VARIABLE

-is the manipulated

variable in the experiment.

DEPENDENT VARIABLE

-is the variable that is

measured at the end of the experiment.