The central ideas of evolution are that life has a history — it has changed over time — and that different species share common ancestors.

Mechanisms: the processes of evolution

Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life — but exactly how does it work?

Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:

Descent and the genetic differences that are heritable and passed on to the next generation; Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change; The importance of genetic variation; The random nature of genetic drift and the effects of a reduction in genetic variation; How variation, differential reproduction, and heredity result in evolution by natural selection; and How different species can affect each other's evolution through coevolution.

Lamarck’s Ideas• Theory of acquired characteristics.• Organisms can change during their lifetime

because they want to “better” themselves.• These changes acquired during their lifetime

can be passed onto the next generation.

Lamarck• Lamarck’s ideas don’t work.• Traits acquired during an organism’s life

cannot be passed onto the next generation.• Examples: missing limbs etc.

Scientists Contributions• Cuvier – fossil evidence

• Malthus – population growth

• Lyell – geological features

• Wallace – natural selection as a mechanism

Darwin’s Idea• 4 Main Ideas1.Variation within a species2. Organisms adapt to their environments3.Variation in characteristics were passed on

to the next generation: Inheritance4.There is competition to reproduce AND

there this competition to survive. Some organisms are more successful at both!

Natural Selection• Darwin realized that nature was selecting

the fittest individuals within a species.• These fit individuals were most likely to

survive and reproduce.• Darwin called this “survival of the fittest”.• Example: Peppered Moths

Summary of Darwin’s Theory

1. Organisms differ; variation is inherited2. Organisms produce more offspring survive3. Organisms compete for resources4. Organisms with advantages survive to pass those advantages to their offspring5. Species alive today have descended with modifications from common ancestors

Other Evidence for Evolution:

Fossil Record Homologous StructuresVestigial OrgansEmbryological Development

Sources of Variation• Where does variation

within a species come from?

1.Mutations: Physical changes in DNA. Most have negative effects, but some may actually increase an organism’s fitness.

More variation

1.Sexual reproduction: when meiosis occurs, homologous pairs of chromosomes are separated RANDOMLY.

• This results in many combinations of chromosomes.

• In humans there are 223 (8 million possible combinations).

Adaptive Radiation• Also known as divergent evolution.• Process by which 1 species gives rise to

many species.

Convergent Evolution• Different species come to look like one

another as they adapt to similar environments.

• Results in the formation of ANALOGOUS STRUCTURES look similar, but are of different origin

• Examples: wings of birds, bats and insects or penguin, whale and dolphin swimming structures and streamlining.

Speciation• The formation of new species.• In order for 2 species to form from one,

populations must be separated from one another.

• Over time, the populations will change such that they will no longer be able to reproduce.

Table 23.1a

Table 23.1b

Genetic Drift• Sometimes the genetic makeup of a

population can change due to chance rather than natural selection.

• Example: a natural disaster wipes out a random portion of a population.

• The survivors may not be the fittest.

Time will tell

1.Gradual change evolution occurs slowly and steadily.

2.Punctuated Equilibria there are long periods of equilibrium (no changes in populations) interrupted by brief periods of change. May be brought on by mass extinctions.