Evolution and natural selection: flashcards, quiz and summary

Flashcards

1. 1What is evolution in genetic terms?

   Evolution is a change in the inherited traits of a population over generations. Genetically, it is a change in allele frequencies within the population.

2. 2What are the five steps of natural selection?

   Variation among individuals, heritability of some differences, overproduction of offspring that compete for limited resources, differential survival and reproduction, and over many generations the helpful traits becoming more common.

3. 3Why does natural selection depend on the environment?

   A trait is only helpful if it improves survival or reproduction in a particular setting. Dark peppered moths became common when soot darkened tree bark in industrial England, then declined when the air got cleaner.

4. 4How does Darwin's work on the Galapagos connect to his theory?

   Darwin saw that finches and tortoises differed from island to island. This variation among related species supported his idea that populations change over time and adapt to local conditions.

5. 5How do homologous and analogous structures differ as evidence?

   Homologous structures share a basic bone layout used for different jobs, which points to a common ancestor. Analogous structures have the same function but different structure and origin, which reflects convergent evolution rather than shared ancestry.

6. 6What do vestigial structures show about evolution?

   Vestigial structures are reduced parts with little or no function, such as whale hip bones or the human tailbone. They are left over from ancestors that used those structures, which supports the idea that organisms change over time.

7. 7How does DNA evidence support common ancestry?

   Closer relatives have more similar DNA and protein sequences. Humans and chimpanzees share about 98 to 99 percent of their DNA, and nearly all life uses the same genetic code.

8. 8What is the difference between the bottleneck effect and the founder effect?

   The bottleneck effect happens when a disaster or overhunting shrinks a population, and the survivors carry only some alleles by chance. The founder effect happens when a small group starts a new population with only part of the original gene pool.

9. 9How do populations become separate species?

   New species form when populations become reproductively isolated, so they can no longer interbreed in nature to produce fertile offspring. Isolation can come from a physical barrier or from barriers within the same area.

10. 10What is the difference between allopatric and sympatric speciation?

    In allopatric speciation, a physical barrier splits a population, and the groups evolve separately until they cannot interbreed. In sympatric speciation, a new species forms within the same area, as through polyploidy in plants or shifted mating times.

11. 11Why is survival of the fittest a misleading phrase?

    In biology, fittest means best at reproducing in a given environment, not strongest or most dominant. Fitness is measured by how many offspring an individual leaves compared with others.

Practice quiz

Pick an answer to check it.

1. 1Which statement best describes evolution at the population level?

   1. A change in allele frequencies in a population over generations
   2. A permanent change in one individual's traits during its lifetime
   3. Individual organisms change their genes when they try hard enough
   4. The replacement of all alleles with new ones in a single generation
   Show the answer

   A. A change in allele frequencies in a population over generations Evolution is a change in the inherited traits of a population over generations, which genetically means a change in allele frequencies. Individuals do not evolve, and traits acquired during a lifetime are not inherited.

2. 2In natural selection, which condition must be true for a trait to become more common over many generations?

   1. The trait must arise only after the environment changes
   2. The trait must appear in every individual in the population
   3. The trait must be caused by the organism's own effort
   4. The trait must be heritable and help individuals survive and reproduce
   Show the answer

   D. The trait must be heritable and help individuals survive and reproduce Only heritable differences are passed on, and only those that improve survival or reproduction are favored. A trait that appears in every individual is not required, and effort or later changes in the environment do not create the trait.

3. 3Bat wings and human arms share the same basic bone layout but do different jobs. What does this pattern most likely indicate?

   1. That bats evolved from modern humans
   2. A common ancestor with the same bone structure
   3. Convergent evolution from similar environments
   4. That the bones were copied from one species to another
   Show the answer

   B. A common ancestor with the same bone structure Homologous structures share a basic layout inherited from a common ancestor, even when the function differs. Convergent evolution explains analogous structures, such as bird and insect wings, not this pattern.

4. 4A small group of birds is cut off on a remote island and starts a new population with only a few of the original alleles. Which process is this an example of?

   1. Founder effect
   2. Gene flow
   3. Bottleneck effect
   4. Sympatric speciation
   Show the answer

   A. Founder effect The founder effect occurs when a small group starts a new population and carries only part of the original gene pool. A bottleneck involves a population that shrinks in size, and gene flow involves migration between existing populations.

5. 5Two populations of the same species are separated by a new river that they cannot cross, and over time they can no longer interbreed. Which type of speciation is this?

   1. Sympatric speciation
   2. Allopatric speciation
   3. Gene flow
   4. Polyploidy in plants
   Show the answer

   B. Allopatric speciation Allopatric speciation happens when a physical barrier, such as a river, splits a population and the groups evolve separately. Sympatric speciation occurs within the same area, and gene flow would make the populations more alike.

6. 6Which pair is an example of analogous structures?

   1. A whale flipper and a human arm
   2. The human tailbone and a whale hip bone
   3. A human arm and a bat wing
   4. Bird wings and insect wings
   Show the answer

   D. Bird wings and insect wings Analogous structures have the same function but different structure and origin, as with bird and insect wings. Human arms, bat wings and whale flippers are homologous, and the tailbone and whale hip bone are vestigial structures.

7. 7A student says that selection creates a helpful trait whenever an environment needs one. Which response is most accurate?

   1. The student is correct because environments cause helpful traits to appear
   2. The student is wrong because selection never affects traits in any species
   3. The student is wrong because selection only acts on variation that already exists in the population
   4. The student is correct only for plants, which can change their traits on demand
   Show the answer

   C. The student is wrong because selection only acts on variation that already exists in the population Natural selection acts on existing heritable variation and does not create traits on demand. Antibiotic resistance is an example: resistant bacteria already existed and survive while susceptible ones are killed.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing evolution of populations with change in individuals. Students often write that an individual evolves or changes its genes by trying, but only populations evolve through changes in allele frequencies.
• Mixing up homologous and analogous structures. Bat wings and bird wings look similar in function, but the bat wing is homologous to the human arm, while bird and insect wings are analogous.
• Saying selection creates helpful traits when needed. Teachers expect you to say selection acts on existing heritable variation, using the antibiotic example.
• Using fitness to mean strength. In exam answers, fitness means reproductive success in a given environment, not how strong or healthy an organism is.
• Mixing up the bottleneck and founder effects. The bottleneck effect is a population that shrinks, while the founder effect is a small group starting a new population.
• Confusing allopatric and sympatric speciation. Check whether a physical barrier separates the populations before deciding which type is being described.

Summary

• Evolution is a change in the inherited traits of a population over generations, and genetically it means a change in allele frequencies.
• Natural selection works when variation exists, some of that variation is heritable, more offspring are born than can survive, and individuals with helpful traits survive and reproduce more.
• An adaptation is an inherited trait that improves survival or reproduction in a particular environment, and adaptations depend on the setting.
• Fitness means reproductive success, measured by how many offspring an individual leaves compared with others.
• Fossils, homologous structures, vestigial structures, DNA and protein similarity, and embryology all provide evidence that species share common ancestors.
• Analogous structures come from convergent evolution, not shared ancestry, so similar appearance alone does not show close relatedness.
• Mutation supplies new alleles, while genetic drift, gene flow, and natural selection change allele frequencies in populations.
• Speciation happens when populations become reproductively isolated, either by a physical barrier (allopatric) or within the same area (sympatric).

Key terms

Evolution

A change in the inherited traits of a population over generations, genetically a change in allele frequencies.

Natural selection

The main process that adapts populations to their environments: individuals with heritable traits that help them survive and reproduce pass those traits on more often.

Adaptation

An inherited trait that improves survival or reproduction in a given environment, such as camouflage or antibiotic resistance in bacteria.

Fitness

Reproductive success, or how many offspring an individual leaves compared with others in the population.

Homologous structures

Structures with the same basic bone layout used for different jobs, such as a human arm, bat wing and whale flipper. They point to a common ancestor.

Analogous structures

Structures with the same function but different structure and origin, such as bird and insect wings. They reflect convergent evolution, not shared ancestry.

Vestigial structure

A reduced body part with little or no function, left over from ancestors, such as whale hip bones or the human tailbone.

Mutation

A random change in DNA and the original source of new alleles. Mutations do not happen because they are needed.

Genetic drift

Random changes in allele frequencies, strongest in small populations. Bottleneck and founder effects are two examples.

Gene flow

The movement of alleles between populations through migration, which makes the populations more alike genetically.

Allopatric speciation

Speciation in which a physical barrier, such as a river or mountain range, splits a population and the groups evolve separately until they cannot interbreed.

Sympatric speciation

Speciation in which a new species forms within the same area, for example through polyploidy in plants or shifted mating times.

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