15.3: Patterns of Evolution:

• Microevolution

• Macroevolution

15.3: Microevolution

• When the relative frequencies of alleles in a population change over a number of generations, evolution is occurring on its smallest scale (microevolution)

• Genetic drift is a change in a gene pool due to chance (smaller populations)– Genetic drift can lead to the founder Effect and cause the

bottleneck effect

• Gene flow can change a gene pool due to the movement of genes into or out of a population

• Mutation changes alleles

• Nonrandom mating

• Natural selection leads to differential reproductive success

There are several potential causes of microevolution

Genetic Drift• Natural selection is not the only source of

evolutionary change.

• The smaller a population is, the farther the results may be from what the laws of probability predict. This kind of random change in allele frequency is called genetic drift.

• How does genetic drift take place? – In small populations, individuals that carry a

particular allele may leave more descendants than other individuals do, just by chance.

– Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Evolution

Founder Effect

Occurs when a small sample of a population settles in a location separated from the rest of the population

Alleles that were uncommon in the original population might be common in the new population.

15.3 Shaping Evolutionary Theory

Chapter 15

Evolution

BottleneckOccurs when a population declines to a

very low number and then rebounds

15.3 Shaping Evolutionary Theory

Chapter 15

Hardy-Weinberg Equilibrium:

• If a population’s gene pool remains constant, then the population will not evolve. (Hardy-Weinberg Equilibrium)

Hardy-Weinberg principle

• The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

• The situation in which allele frequencies remain constant is called genetic equilibrium (juh-net-ik ee-kwih-lib-ree-um).

• If the allele frequencies do not change, the population will not evolve.

Hardy-Weinberg Equation:

• Used to calculate the frequency of alleles

p2 + 2pq + q2 = 1

• Frequency of WW + Frequency of Ww + Frequency of ww = 1

• The combined frequencies of all alleles must be 100%

Five conditions are required for Hardy-Weinberg equilibrium

• The population is very large

• The population is isolated

• Mutations do not alter the gene pool

• Mating is random

• All individuals are equal in reproductive success

Evolution v/s Equilibrium

• Five conditions are required to maintain genetic equilibrium from generation to generation

• Public health scientists use the Hardy-Weinberg equation to estimate frequencies of disease-causing alleles in the human population

– Example: phenylketonuria (PKU)

The Hardy-Weinberg equation is useful in public health science

• Natural selection results in the accumulation of traits that adapt a population to its environment– If the environment should change, natural selection

would favor traits adapted to the new conditions

Adaptive change results when natural selection upsets genetic equilibrium

• Phenotypic variation may be environmental or genetic in origin– But only genetic changes result in evolutionary

adaptation

Variation is extensive in most populations

VARIATION AND NATURAL SELECTION

• Natural selection tends to reduce variability in populations– The diploid condition preserves variation by

“hiding” recessive alleles– Balanced polymorphism may result from the

heterozygote advantage

How natural selection affects variation

• The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria– Example:

Mycobacterium tuberculosis

– MRSA

The evolution of antibiotic resistance in bacteria is a serious public health

concern

Figure 13.22

Checkpoint Questions:

1. Describe how natural selection can affect traits controlled by single genes.

2. Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes?

3.  How does genetic drift lead to a change in a population’s gene pool?

4.  What is the Hardy-Weinberg principle?

5. How are directional selection and disruptive selection similar? How are they different?

15.3: Patterns of Evolution 

• Macroevolution refers to the large-scale evolutionary changes that take place over long periods of time.  

• Six important patterns of macroevolution

– mass extinctions– adaptive radiation– convergent evolution– Coevolution– punctuated equilibrium– changes in

developmental genes.

Mass Extinctions:

• New fossil studies show that those mass extinctions not only extinguished species but also wiped out whole ecological systems, disrupting energy flow throughout the biosphere and causing food webs to collapse.

• Many paleontologists think that most mass extinctions were caused by multiple factors.

• For the survivors, there was a new world of ecological opportunity.

• Often, the result was a burst of evolution that produced an abundance of new species.

Adaptive Radiation

• Studies of fossils or of living organisms can show that a single species or a small group of species has evolved into several different forms that live in different ways.

• This process is known as adaptive radiation. – Implies common descent

Convergent Evolution

• Unrelated organisms that come to resemble one another, is called convergent evolution.

• Natural selection may mold different body structures, such as arms and legs, into modified forms, such as wings or flippers. – EX: Streamlined body of penguin, shark,

dolphin

Coevolution

• The process by which two species evolve in response to changes in each other over time is called coevolution.

• An evolutionary change in one organism may also be followed by a corresponding change in another organism.

• EX: Many flowering plants, for example, can reproduce only if the shape, color, and odor of their flowers attract a specific type of pollinator.

Punctuated Equilibrium

• Evolution has often proceeded at different rates for different organisms at different times during the long history of life on Earth. (Rate of Evolution)

• Gradualism - slow, steady change in a particular line of descent.

• Punctuated equilibrium - long, stable periods interrupted by brief periods of more rapid change

Developmental Genes and Body Plans

• First, molecular studies show that homologous hox genes establish body plans in animals as different as insects and humans

• Second, major evolutionary changes—such as the different numbers of wings, legs, and body segments in insects—may be based on hox genes.

• Finally, geneticists are learning that even small changes in the timing of genetic control during embryonic development can make the difference between long legs and short ones

Changes in developmental genes are one major pattern of macroevolution.

• Fossil evidence shows that some ancient insects (top left) had no wings, but others (top right) had winglike structures on many body segments.

• In modern insects (bottom), genes may turn off wing development in all except one or two body segments.