Evolution

Question 1

Define:

evolution

Answer 1

Evolution is the change in frequency of a population’s inherited traits over subsequent generations.

Evolution only occurs within populations; individual organisms do not change genetically over time to adjust to their environment.

Question 2

How are genetic differences between two species used to quantify their evolutionary relationship?

Answer 2

Random mutations occur at a predictable rate. Thus, the differences between the genetic sequences of two species can be used to estimate the time since they diverged from a common ancestor.

Natural selection can increase the rate at which a certain mutation predominates, so this measurement is only an estimate.

Question 3

In his famous theory, Jean-Baptiste Lamarck proposed that the “use or disuse” of a trait dictated its heritability. In a classic example, giraffes who often stretched their necks would have offspring with longer necks than average. Describe the problem with this hypothesis.

Answer 3

Traits that are acquired during an organism’s lifetime cannot be passed down to its offspring.

According to the “use and disuse” concept, organisms lose traits they do not need and develop traits that are useful to them. Since evolution occurs on the population level, and since genes, not traits, are inherited, this idea has been discredited.

Question 4

Define:

natural selection

Answer 4

Natural selection is the change in frequency of a trait within a population, based on the effect that trait has on reproductive success.

Traits that allow an organism to produce more viable offspring are more likely to be passed on to future generations.

Question 5

What qualities are exhibited by an organism with high fitness?

Answer 5

Organisms with high fitness are those that successfully produce a relatively large number of offspring. In general, fitness refers to the likelihood that an animal will pass down its genetic material.

On the MCAT, remember that fitness is determined by reproductive success alone. A stronger, more physically capable animal that cannot reproduce is less fit than a weaker animal that can reproduce.

Question 6

Members of a population who possess beneficial traits survive long enough to produce offspring, while members with harmful traits often die before reaching sexual maturity. Which evolutionary term describes this tendency?

Answer 6

Differential reproduction describes this variation in reproductive success.

This concept is a key component of natural selection. Through this mechanism, traits that affect survival are inherited differently, regardless of whether they directly relate to reproduction.

Question 7

What genetic value changes within a population as evolution occurs?

Answer 7

Evolution changes a population’s allele frequency.

Alleles are different forms of genes, and evolution is a change in the frequency of genetic material. Thus, evolution causes some alleles to become more prevalent while others become more rare.

Question 8

Which of the following is the least evolutionarily fit?

• A fish with underdeveloped fins
• A fish without functioning sex organs
• A fish without functioning eyes

Answer 8

The fish that lacks functional sex organs is the least fit.

Evolutionary fitness refers only to the ability to pass on genetic material. The other two fish certainly may have difficulty surviving, but they can still reproduce.

Question 9

Define:

group selection

Answer 9

Group selection refers to genetic changes that occur over time due to the fitness of a group rather than an individual.

Group selection has been used to explain altruism, or selfless behavior. Though it remains controversial among evolutionary biologists, this topic could certainly be mentioned on the MCAT.

Question 10

What term is often used to connect group selection to altruistic behavior?

Answer 10

Kin selection, or the evolutionary preference for behaviors that benefit relatives, is often used to explain altruism.

Not all species exhibit altruistic behavior. Within those that do, “selfless” actions that can harm an individual often benefit the surrounding members of the population. This indirectly passes on the genes of that individual by promoting the survival and reproduction of family members.

Question 11

Name the three types of natural selection.

Answer 11

Natural selection can either be directional, stabilizing, or disruptive.

Question 12

Which type of natural selection is shown in the image below?

Answer 12

Directional selection

Directional selection occurs when a single extreme trait provides a survival advantage. Over time, the frequency of individuals with that trait will rise.

Question 13

Which type of natural selection is shown in the image below?

Answer 13

Stabilizing selection

Stabilizing selection occurs when extreme phenotypes confer a disadvantage to an organism’s fitness. Over time, the frequency of moderate phenotypes will increase.

Question 14

Which type of natural selection is shown in the image below?

Answer 14

Disruptive selection

Disruptive selection occurs when extreme traits are advantageous while moderate traits are not. As a result, the frequencies of the extreme phenotypes increase over time.

Question 15

Which type of selection helps determine human birthweight?

Answer 15

Birthweight is influenced by stabilizing selection.

A baby born too small would have difficulty surviving on its own, but an abnormally large baby would be hard for the mother to deliver. Since birthweight is kept within a moderate range, it exemplifies stabilizing selection.

Question 16

A population of horses consists of many individuals with very long manes and many with short manes, but few in the middle. This is likely the result of which form of natural selection?

Answer 16

This population exemplifies disruptive selection.

Disruptive selection favors extreme variations of a trait over more moderate ones. A classic example is the London peppered moth, where black wings were easily camouflaged in industrial surroundings and white wings blended in within rural areas. Gray moths, which could be seen by predators in both areas, became less prevalent over time.

Question 17

Which type of selection best relates to antibiotic-resistant bacteria?

Answer 17

Antibiotic resistance is an example of directional selection.

Before selection, a bacterial colony contains members with varying degrees of resistance. After exposure to an antibiotic, only those individuals with high resistance remain, and they can propagate to form a new, resistant colony.

Question 18

What measurement determines the evolutionary success of an allele?

Answer 18

Evolutionary success is determined by the percent representation of an allele in the following generation.

If an allele becomes more prevalent in the gene pool, it is considered evolutionarily successful. However, factors other than natural selection can affect allele frequencies.

Question 19

Which of the following must increase in future generations for an allele to be evolutionarily successful?

• phenotypic ratio
• genotypic ratio
• both phenotypic ratio and genotypic ratio

Answer 19

Genotypic ratio must increase.

Technically, evolutionary success of an allele refers only to an increase in its frequency in the gene pool. Allele frequencies cannot change without altering genotype frequencies. Note that phenotype frequencies are likely to change as well, but this is not a necessary measure of success.

Question 20

What condition must be met for a group of individuals to be considered part of the same species?

Answer 20

They must be able to breed and produce viable, fertile offspring.

A species is the most specific category in which an organism can be classified.

Question 21

Horses and donkeys can mate and produce healthy offspring called mules. Why are horses and donkeys considered separate species?

Answer 21

Mules are sterile.

Within a species, the offspring of a breeding pair must be fertile as well as viable. Since mules cannot produce offspring, this condition is not met.

Question 22

What is the difference between adaptation and specialization?

Answer 22

Adaptation is the process of evolving a trait, through natural selection, that allows a species to better survive in its environment.

Specialization is a type of adaptation in which a species evolves to fill a specific niche.

Question 23

Viceroy butterflies mimic the appearance of monarch butterflies to repel predators. Is the evolution of this trait an example of adaptation or specialization?

Answer 23

This trait is an adaptation, not a specialization.

Through natural selection, this mimicry has evolved to increase survival. However, sharing the appearance of monarch butterflies does not fill a unique niche.

Question 24

Define:

ecological niche

Answer 24

An ecological niche is the way a species responds to the environment it lives in, usually by filling a unique role.

A species’ niche can include its method of obtaining resources, its competitive advantages, or its relationship with predators.

Question 25

What will occur between two different species of ant that occupy the same niche?

Answer 25

They will compete with each other for resources, such as food, water, and space.

Competition occurs because resources exist in a finite amount in a given environment. In the long term, a species can avoid competition by adapting to fill a different niche.

Question 26

How does intraspecies competition affect population growth?

Answer 26

Population growth slows in response to competition.

Members of the same species fill the same niche, so they compete with each other for resources. When resources are scarce, the environment cannot support more members of the population and growth is limited.

Question 27

Define:

symbiosis

Answer 27

Symbiosis occurs when two or more species live and closely interact with each other.

These interactions include those where both species benefit, where one benefits and the other is left unharmed, and where one benefits by harming the other.

Question 28

Name the three types of symbiotic relationships.

Answer 28

The three types of symbiosis are mutualism, commensalism, and parasitism.

Symbiosis always benefits at least one species; these categories are defined by its effect on the other.

Question 29

Define:

commensalism

Answer 29

Commensalism is a type of symbiosis in which one species benefits and the other is unaffected.

One example is the relationship between clownfish and anemones. The fish use the anemones for shelter and protection without impacting the anemones themselves.

Question 30

Define:

parasitism

Answer 30

Parasitism is a type of symbiosis in which one species benefits and the other suffers a varying degree of harm.

Tapeworms, which both feed off and sicken their human hosts, are examples of parasites.

Question 31

Define:

mutualism

Answer 31

Mutualism is a type of symbiosis in which both species benefit from the relationship.

Humans and intestinal bacteria mutually benefit from this type of symbiosis.

Question 32

Egrets (birds) often follow herds of cattle and eat insects the cattle stir up in the grass. Which symbiotic relationship does this exemplify?

Answer 32

This is commensalism.

The egrets benefit from easier exposure to a food source, but the cattle are unaffected by the presence of the birds.

Question 33

Bees collect and eat nectar from flowers, moving pollen to other flowers in the process. Which symbiotic relationship does this exemplify?

Answer 33

This is mutualism.

The bees benefit by having a source of food, while the flowers reproduce via the spreading of pollen.

Question 34

Define:

inbreeding

Answer 34

Inbreeding is the production of offspring from the mating of closely related individuals.

Human examples of inbreeding include some royal families in which cousins married each other for many generations.

Question 35

Define:

outbreeding

Answer 35

Outbreeding is the introduction of new genetic material through mating with unrelated individuals.

Many purebred species, such as dogs and cattle, have developed predispositions for genetic diseases as a result of inbreeding. When the diversity of the gene pool is increased, offspring are more likely to be healthy.

Question 36

What term describes a major reduction in the number of individuals in a population?

Answer 36

A significant reduction in population size is called a bottleneck.

Bottlenecks are generally caused by catastrophic events, whether environmental (such as a natural disaster) or human-related.

Question 37

At one point, overhunting caused the American bison population to drop to fewer than 1000 individuals. What term describes this situation?

Answer 37

In this example, the bison population experienced a bottleneck.

Bottlenecks can dramatically change the gene pool, since all subsequent members of the species descend from a small number of individuals.

Question 38

What kind of factors promote genetic drift?

Answer 38

Genetic drift is promoted by random chance. It relates to changes in allele frequency that occur separately from natural selection.

One example is a bottleneck, or a sharp reduction in population size. The gene pool of the individuals who happen to survive will then determine the characteristics of their offspring, regardless of whether these traits are favorable.

Question 39

What occurs when two groups undergo divergent evolution?

Answer 39

Divergent evolution causes related groups to become increasingly different. In other words, they evolve apart.

Over time, divergent evolution can lead to speciation, when groups differ to the point of being unable to interbreed.

Question 40

What occurs when two groups undergo parallel evolution?

Answer 40

Parallel evolution causes distinct lineages to evolve similar traits over time.

This differs from convergent evolution, in which two lineages become increasingly more similar to each other. In contrast, parallel evolution involves the independent development of like traits in organisms without an increase in overall resemblance.

Question 41

What occurs when two groups undergo convergent evolution?

Answer 41

Convergent evolution causes unrelated, originally dissimilar groups to become increasingly similar.

This often occurs when two species begin to fill the same ecological niche.

Question 42

What type of evolution is demonstrated by bats and owls?

Answer 42

Bats and owls developed through convergent evolution.

While both possess the ability to fly, bats are mammals while birds are avian species. Their anatomical structures did not come from a common ancestor, but developed separately and became more similar over time.

Question 43

What is the difference between ontogeny and phylogeny?

Answer 43

Ontogeny refers to the development of an individual organism throughout its life. Phylogeny describes the evolutionary history of a species or other set of organisms.

For example, human ontogeny relates to the transition from zygote to embryo to fetus, then adult. Human phylogeny deals with the descent of modern humans from early ancestors.

Question 44

Is the development of a fertilized egg into an adult chicken an example of ontogeny or phylogeny?

Answer 44

The development of an individual chicken is ontogeny.

Ontogeny refers to an organism’s development over its lifetime, while phylogeny refers to a species’s development over evolutionary time.

Question 45

Define:

homologous structures

Answer 45

Homologous structures are anatomical features of two species that developed from the same part of a common ancestor.

Species that possess homologous structures must be related. However, the modern versions of these structures do not have to be used for the same purpose.

Question 46

As mammals, both humans and whales developed from a common ancestor. What term describes the relationship between a human arm and a whale fin?

Answer 46

Human arms and whale fins are homologous structures.

Though used for very different purposes, the appendages are evolutionarily related; both derived from a similar structure in a common ancestor.

Question 47

Define:

analogous structures

Answer 47

Analogous structures are anatomical structures of two species that share a form or function, but did not develop from a common ancestor.

Species that possess analogous structures must be relatively unrelated. However, since their structures are similar in appearance or function, they may be mistaken for related species.

Question 48

Birds and insects evolved independently from very different ancestors. What term describes the relationship between a bird wing and a bee wing?

Answer 48

Bird wings and bee wings are analogous structures.

Since both structures serve a similar purpose, it can be tempting to think that they are related. However, since they developed independently from features of different ancestors, they are analogous.

Question 49

Which type of evolution can result in the existence of homologous structures?

Answer 49

Divergent evolution

By definition, homologous structures must have developed from a common ancestor. Thus, species with these structures are the eventual result of divergence from that ancestor.

Question 50

Which type of evolution can result in the existence of analogous structures?

Answer 50

Convergent evolution

When two unrelated species converge, they often evolve features with similar appearances or functions. Since these structures did not develop from a common ancestor, they are analogous.

Question 51

What benefit does a vestigial structure give a modern organism?

Answer 51

Vestigial structures provide little to no benefit to the organisms who possess them.

While these features were likely useful to ancestral organisms, they have evolved to lose most or all of their function.

Question 52

Evolutionary biologists would classify the human appendix as which kind of structure?

Answer 52

The appendix is generally classified as vestigial.

The appendix, an outcropping of the large intestine, has long been thought to provide no function to modern humans. Though recent research has challenged this idea, for the MCAT, continue to remember the appendix as a classic example of a vestigial structure.

Question 53

Define:

speciation

Answer 53

Speciation is the development of new species through evolution.

Generally, speciation occurs when groups of a single species undergo divergent evolution.

Question 54

With regard to evolution, what is an isolating mechanism?

Answer 54

An isolating mechanism is a feature that prevents two species from mating or producing fertile offspring. Such features prevent distinct species from combining.

Isolating mechanisms can be either prezygotic or postzygotic.

Question 55

What is the difference between prezygotic and postzygotic isolating mechanisms?

Answer 55

Prezygotic mechanisms prevent two species from mating. Postzygotic mechanisms allow mating but prevent the development of viable, fertile offspring.

If these terms are unfamiliar, their names can give away their meaning. “Prezygotic” relates to events before zygote formation, while “postzygotic” relates to events that occur afterward.

Question 56

What is hybrid inviability?

Answer 56

In cases of hybrid inviability, mating can occur and form a hybrid offspring, but that offspring will not develop into a healthy adult.

Hybrid inviability is an example of a postzygotic barrier between distinct species.

Question 57

What is hybrid sterility?

Answer 57

In cases of hybrid sterility, mating can occur and form a healthy hybrid offspring. However, the hybrid is sterile and cannot reproduce.

Mules are examples of hybrid sterility, which is a postzygotic barrier between distinct species.

Question 58

What is hybrid breakdown?

Answer 58

In cases of hybrid breakdown, mating can occur and form healthy, fertile hybrid offspring. However, later hybrid generations (F2 and onward) are unhealthy and sometimes cannot reproduce themselves.

Hybrid breakdown is the final post-zygotic barrier that separates distinct species.

Question 59

One species of moth mates in the fall, while a nearby species of moth only mates in the spring. Which mechanism keeps these two species distinct?

Answer 59

Temporal isolation, a type of prezygotic isolation, separates these two species.

Temporal isolation usually relates to differences in mating seasons or times, though it can also separate nocturnal from diurnal species.

Question 60

One species of fish mate with another to produce viable offspring. When these offspring interbreed, their progeny are viable but sterile. Which mechanism keeps these two species distinct?

Answer 60

Hybrid breakdown, a type of postyzygotic isolation, separates these two species.

Hybrid breakdown relates to a decrease in viability within progressive generations of hybrids.

Question 61

The Miller-Urey experiment used water, methane, ammonia, and hydrogen gas to simulate the conditions of early Earth. What molecules were produced by this experiment?

Answer 61

Amino acids were produced.

This result, along with those of similar experiments, implied that organic biomolecules could be synthesized from inorganic substances. This was a major breakthrough in the study of the origin of life.

Question 62

What biochemical evidence supports the idea that all life descends from a common ancestor?

Answer 62

All living organisms encode their genetic material as DNA. They also share the genetic code by which this material is converted into protein.

Specifically, DNA is transcribed into RNA, which is translated into protein.

Question 63

Which theory states that evolution occurs in short, rapid bursts within long periods of little change?

Answer 63

The Punctuated Equilibrium Theory

According to this theory, a species will usually exist in a static state where virtually no evolutionary change occurs. This state is interrupted by brief periods of evolution that are prompted by specific events.