Homework: �True or false?�Human beings, as we know them,�developed from earlier species of�animals.�

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• 10 random samples from 公館捷運站
• True %
• False %
• Not sure %

Evolutionary Analysis

Chapter 3

Darwinian Natural Selection

Scott Freeman • Jon C. Herron

African wild dogs

Impala

Using plasticine models painted to mimic natural variation in coat color, Sacha Vignieri and colleagues (2010) found that conspicuous mice suffer more predator attacks.

3.1 Artificial Selection: Domestic Animals and Plants

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• To increase the frequency of desirable traits in their stocks, plant and animal breeders employ artificial selection.

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The small fruit allele, fw2.2 gene encodes a protein that represses fruit growth

3.2 Evolution by Natural Selection

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• Darwin and Wallace realized that a process similar to artificial selection happens automatically in nature.

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Darwin’s 4 postulates—claims about the nature of populations �

• 1. The individuals within a population differ from one another.
• 2. The differences are, at least in part, passed from parents to offspring.
• 3. Some individuals are more successful at surviving and reproducing than others.
• 4. The successful individuals are not merely lucky; instead, they succeed because of the variant traits they have inherited and will pass to their offspring.

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Darwin’s Theory of Evolution

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• Natural selection is a process that results in descent with modification, or evolution.

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Darwinian Fitness

• Darwin described individuals who are better at surviving and reproducing, and whose offspring make up a greater percentage of the population in the next generation, as more fit.
• He thus gave the everyday English words fit and fitness new meanings. Darwinian fitness is an individual’s ability to survive and reproduce.

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Adaptation

• Note that fitness is relative. It refers to how well an individual survives and reproduces compared to other individuals of its species.
• A trait that increases an organism’s fitness relative to individuals lacking it, is called an adaptation. Such a trait is also said to be adaptive.

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3.3 The Evolution of Flower Color in an Experimental Snapdragon Population

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• Postulate 1: Individuals Differ from One Another

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• Postulate 2: The Variation Is Inherited

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Flower Color in an Experimental Snapdragon Population

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• Testing Postulate 3: Do Individuals Vary in Their Success at Surviving or Reproducing?

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• Testing Postulate 4: Is Reproduction Nonrandom?

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Testing Darwin’s Prediction: Did the Population Evolve? �

• The theory of evolution by natural selection is testable.
• When researchers set up a plant population in which postulates 1 and 2 were true, they found that postulates 3 and 4 also were true—as was Darwin’s prediction that the population would evolve as a result.

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3.4 The Evolution of Beak Shape in Galápagos Finches

Diversity in Darwin's finches

The medium ground finch, Geospiza fortis

(top) An adult male; (bottom) an adult female.

The Galápagos Archipelago and Isla Daphne Major

Isla Daphne Major

A map of Daphne Major

Evolution of Beak Shape in Galapagos Finches

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• Testing Postulate 1: Is the Finch Population Variable?

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Variation in beak depth in medium ground finches

Some Geospiza fortis have beaks that are only half as deep as those of other individuals.

Evolution of Beak Shape in Galapagos Finches

• Testing Postulate 2: Is Some of the Variation among Individuals Heritable?

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Heritability

• The heritability of a trait is defined as the fraction of the variation in a population that is due to differences in genes.

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• It can take any value between 0 and 1.

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Class exercise: Body height – Parent-Offspring Regression

Evolution of Beak Shape in Galapagos Finches

• In finches, the beak depths of parents and offspring are similar.
• This suggests that some alleles tend to produce shallow beaks, while other alleles tend to produce deeper beaks.

Bone morphogenic protein 4 and beak depth in Darwin’s ground finches

Beak depth versus mesenchymal Bmp4 expression.

Evolution of Beak Shape in Galapagos Finches

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• Testing Postulate 3: Do Individuals Vary in Their Success at Surviving and Reproducing?

the drought

Decline of ground finch population and available seeds during the 1977 drought

the drought

the drought

Evolution of Beak Shape in Galapagos Finches

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• Testing Postulate 4: Are Survival and Reproduction Nonrandom?

Evolution of Beak Shape in Galapagos Finches

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• During a terrible drought, finches with larger, deeper beaks had an advantage in feeding, and thus in surviving.

Beak depth before & after natural selection

During a terrible drought,

finches with larger, deeper

beaks had an advantage in

feeding, and thus in surviving.

Beak depth in the finches hatched the year before the drought versus the year after the drought

Evolution of Beak Shape in Galapagos Finches

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• Testing Darwin’s Prediction: Did the Population Evolve?

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Evolution of Beak Shape in Galapagos Finches

• Because of the drought, the finch population evolved.

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• Selection occurs within generations; Evolution occurs between generations.

Thirty years of evolution

3.5 The Nature of Natural Selection

Natural Selection

• Natural Selection Acts on Individuals, but its Consequences Occur in Populations

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• Natural selection does not change the characteristics of individuals. It changes the characteristics of populations.

Natural selection happens to individuals,

but what changes is populations

Natural Selection

• Natural Selection Acts on Phenotypes, but Evolution Consists of Changes in Allele Frequencies

Populations evolve only if traits are heritable

Natural Selection

• Natural Selection is Not Forward Looking

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• Natural selection adapts populations to conditions that prevails in the past, not conditions that might occur in the future.

Natural Selection

• New Traits Can Evolve, Even Through Natural Selection Acts on Existing Traits

Persistent long-term selection can result in dramatic changes in traits

The panda's thumb

a modified wrist bone

Natural Selection

• Exaptation (Gregory 2008) – A trait that is used in a novel way and is eventually elaborated by selection into a completely new structure, like the radial sesamoid of the ancestral panda.

Natural Selection

• Natural Selection Is Not “Perfect”

Figure 3.21 No guy is perfect

These males sport gonopodia

that attract mates but hinder

escape. The lower male is from a high-predation population. From Langerhans et al. (2005).

Natural Selection

• Natural Selection is Nonrandom, But It Is Not Progressive.

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• There is no such thing as a higher or lower plant or animal.

Natural Selection

• Fitness is Not Circular

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• Critique: “Of course individuals with favorable variations are the ones that survive and reproduce, because the theory defines favorable as the ability to survive and reproduce.”

Natural Selection

• Selections Acts on Individuals, Not the Good of the Species

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• Individuals do not do things for the good of their species. They behave in a way that maximizes their genetic contribution to future generations (fitness).

3.6 The Evolution of Evolutionary Biology

Variation

• Because Darwin knew nothing about mutation, he has no idea how variability was generated in populations.

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• Thomas Hunt Morgan, early 1900s. Experiment with fruit flies.

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Inheritance

• Mendel’s work
• Before Mendel’s laws became known, many biologists thought inheritance worked like pigments in paint.
• Advocates of this hypothesis, called blending in-heritance, argued that favorable variants would merge into existing traits and be lost.

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Time

• The discovery of radioactive isotopes early in the 20th century changed all that.

The Modern Synthesis

• Variability, inheritance, and time posed such difficult problems that the first 70 years of evolutionary biology were fraught with turmoil.
• But between 1932 and 1953, a series of landmark books integrated genetics with Darwin’s four postulates and led to a reformulation of the theory of evolution.

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Modern synthesis

• Two propositions:
• 1. Gradual evolution results from small genetic changes that rise and fall in frequency under natural selection.
• 2. The origin of species and higher taxa, or macroevolution, can be explained in terms of natural selection acting on individuals, or microevolution.

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Darwin’s original 4 postulates restated �

• 1. Individuals vary as a result of mutation creating new alleles, and segregation and independent assortment shuffling alleles into new combinations.
• 2. Individuals pass their alleles on to their offspring intact.

Darwin’s original 4 postulates restated

• 3. In every generation, some individuals are more successful at surviving and reproducing than others.
• 4. The individuals most successful at surviving and reproducing are those with the alleles and allelic combinations that best adapt them to their environment.

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3.7 Intelligent Design Creationism

• End