Theory of Evolution

Chapter 16 & 17

Evolution

• change over time

Acquired trait hypothesis

• Jean-Baptiste Lamarck proposed a theory that organisms pass on traits they acquire in their lifetime.

Thomas Malthus• In 1798, English economist Thomas

Malthus noted that humans were being born faster than people were dying, causing overcrowding.

• Malthus believed:– if the human population grew unchecked,

there wouldn’t be enough living space and food for everyone

– the forces working against population growth included war, famine & disease.

Natural Selection• Charles Darwin, a

naturalist, proposed a mechanism for explaining how evolution occurs

• During a voyage on the HMS Beagle, Darwin collected a vast amount of data, including the famous study of the finches on the Galapagos islands.

• Aided by some ideas from Malthus, Lamarck, and his own studies, Darwin began suggesting the mechanism of natural selection.

Evolution by Natural Selection

1. The Struggle for Existence:If more individuals are produced than can

survive, members of a population must compete to survive.

2. Variation & Adaptation:In a particular environment, some individuals of a population or species are better suited to survive

3. Survival of the FittestOver time, the traits that make certain individuals able to survive and reproduce tend to spread in that population

Evidence of Evolution1. Biogeography2. Fossils

– There is clear proof from fossils that living species evolved from older organisms

– homologous structures • Structures that are shared by related species and

that have been inherited from a common ancestor • Analogous Structures – share common function

but not structure• vestigial structures - structures that have no use

or are of little importance

3. DNA

• Differences in DNA sequences are less between species that are more closely related

• Very important DNA sequences are highly conserved between species.

• After over 25 years of research, Darwin published his ideas in his book The Origin of Species

• Darwin’s ideas are the basis for the latest insight on the evolution of species

Adaptation

• the changing of a species that results in its being better suited to its environment

• enough change within a species eventually leads to a new species

Speciation

• development of a new species

• Isolation can lead to species formation

• Isolation is the condition in which 2 populations of the same species cannot breed with one another; therefore they do not share gene pools and variations can develop separately in each population that prevent them from interbreeding later.

• Ex: the Kaibab (north rim) and Abert (south rim) squirrels of the Grand Canyon

Gradualism• speciation occurs

gradually

Punctuated Equilibrium• speciation occurs

rapidly between periods of little or no change

5 Principle Evolutionary Forces

1. Mutation-the source of all variation, won’t significantly change allele frequencies except over long periods of time.

2. Gene Flow-migration allows for genes to move into and out of a population

3. Nonrandom Mating-mates with others nearby or with similar phenotype; inbreeding a type of nonrandom mating.

4. Genetic Drift-a random event changes the frequency of an allele

5. Natural Selection-frequency of an allele will increase or decrease depending on allele’s effects on survival and reproduction

Trait Distribution

• Normal distribution- (bell curve) average value at the summit

• Directional selection-entire curve shifts in one direction

• Stabilizing selection-individuals become more and more alike

• Over long periods of time, events can occur that result in the species going extinct (disappear permanently)

Overview of natural selection:

• All species have genetic variation

• The environment presents many different challenges to an individual’s ability to reproduce

• Organisms tend to produce more offspring than a space can support, so individuals compete to survive

• Individuals that are better able to cope with the challenges of survival tend to leave more offspring than those who do not

• Traits of the individuals best suited to their environment tend to increase in the population over time