Chapter 16 Evolution by natural selection

Question 1

Compare and contrast Aristotle’s scala naturae to Carolus Linnaeus’ classification scheme.

Answer 1

• Aristotle (384–322 A.D.) also viewed species as fixed and unchanging.
• All living forms could be arranged on a ladder of increasing complexity = scala naturae
• Perfect, permanent species on every rung, with humans at the top.
• These ideas were consistent with the Old Testimant account of creation.
• Scientists viewed the remarkable match of organisms to their environment as evidence that the Creator had designed each species for a specific purpose.

Carolus Linnaeus (1707 – 1778)
• Swedish physician and botanist.
• Developed a binomial system for naming species according to genus and species, and for classifying species into a hierarchy of increasingly complex categories.
• In contrast to the linear hierarchy of Aristotle’s scala naturae, it was a nested classification system, grouping
similar species into increasingly general categories.
• Similarity between species implied the pattern of their creation (and not evolutionary relationships).

Question 2

Explain the mechanism for evolutionary change proposed by Jean-Baptiste de Lamarck. Explain why modern biology has rejected Lamarck’s theories.

Answer 2

• During the 18th century, several naturalists began to suggest that life evolves as environments change.
• In 1809, French biologist Jean-Baptiste de Lamarck published a theory of evolution based on his observations of fossil invertebrates.
• Body parts that are used extensively become larger
and stronger, while those that are not used deteriorate.
• Modifications acquired during the life of an
organism can be passed onto offspring.

Why rejected? b/c cant change their genetics, cant pass on to offspring. Stretching of necks wont change the genes.

Women that gets breast implants doesn’t mean her daughter is going to have big BOOBS

Question 3

Describe the observations Darwin made during the voyage of the HMS Beagle and how these led him to formulate and support his theory of evolution

Answer 3

• The plants and animals of South America were very different from those of Europe.
• Organisms from temperate regions of South America more closely resembled organisms from the tropics of South America than those from temperate regions of Europe.
• South American fossils, though different from modern species, more closely resembled modern species from South America than those from Europe.
• The characteristics of similar species varied somewhat from place to place.
• Most of the animal species on the Galápagos lived nowhere else; however, they resembled species living on the South American mainland.
—> Darwin hypothesized that the islands had been colonized by plants and animals from the mainland that had subsequently diversified on the different islands.

Darwin observed many adaptations (recall: = characteristics of organisms that enhance their survival and reproduction in specific environments).
• Darwin began to perceive that the origin of new species and the adaptation of species to their environment were closely related processes.
• Proposed that new species arise from pre-existing species by the gradual accumulation of adaptations to a different environment.
• Explained that adaptations arise by NATURAL SELECTION
• A process in which individuals with certain inherited characteristics leave more offspring than individuals with other characteristics.

Question 4

. Explain how Darwin’s explanation for the diversity and unity of life on Earth challenged the prevailing views at the time about Earth and its life.

Answer 4

Views at the time said Species are fixed and incapable of change –and they have remained unchanged since their creation.

Darwin suggested that species adapt and give rise to new species

Question 5

Compare and contrast Darwin’s and Lamarck’s views on evolution. What did they have in common and what was the primary difference between them?

Answer 5

They both saw that organisms evolved over time but Lamarck beleived that Modifications acquired during the life of an organism can be passed onto offspring. Where as Darwin Explained that adaptations arise by NATURAL SELECTION A process in which individuals with certain inherited characteristics leave more offspring than individuals with other characteristics.

Question 6

Explain what Darwin meant by “descent with modification.”

Answer 6

• All organisms are related through descent from a common ancestor that lived in the past.
• Over evolutionary time, the descendants of that common ancestor have accumulated diverse modifications (adaptations) that allow them to survive and reproduce in specific habitats.
• Over long periods of time, descent with modification has led to the rich diversityof life we see toda

Question 7

State the pattern and process components of the theory of evolution

Answer 7

• The PATTERN COMPONENT makes two claims about the nature of species:
1. They change through time.
2. They are related by common ancestry.
• The PROCESS COMPONENT of the theory proposes natural selection as the mechanism that explains the observed patterns of descent with modification.

Question 8

Explain the 4 postulates that Darwin used to propose natural selection as a mechanism for evolutionary change

Answer 8

1. The individuals that make up a population vary in the traits they possess, such as size and shape.
2. Some of that trait difference is heritable.
3. In each generation, many more offspring are produced than can possibly survive. Thus, only some individuals in the population survive long enough to produce offspring.
   • Among the individuals that do produce offspring, some will produce more than others.
4. The subset of individuals that survive and that produce more offspring is not a random sample of the population. Instead, individuals with certain heritable traits are more likely to survive and reproduce than others.
   —> Unequal ability of individuals to survive + reproduce
   –>accumulation of favorable traits in population over time.

Question 9

Define “adaptation” and “evolution”.

Answer 9

Evolution:
is the change in genetic characteristics (i.e. allele frequencies) of a population over time.

Adaptions:
characteristics of organisms that enhance their survival and reproduction in specific environments

Question 10

Define “Darwinian fitness”.

Answer 10

• Darwinian fitness: the ability of an individual to produce
offspring relative to the ability of other individuals in the
population to produce offspring.
• Natural selection thus favors traits that increase fitness in the current, local environment.
• NOTE: What is adaptive in one situation is not necessarily adaptive in another!

Question 11

Explain why variation in a population is necessary for evolution to occur

Answer 11

• Natural selection is an “editing mechanism”, not a
creative force. It can only act on existing variation in
the population.
• Natural selection cannot create favorable traits, it selects for favorable traits that are already present in the population.
• This means that variation between individuals in a population is essential if evolution is to occur.

Question 12

Explain why an individual organism cannot evolve.

Answer 12

While natural selection acts on individuals, individuals do
not evolve – populations do!
Natural selection occurs through interactions between individual organisms and their environments. There is little evidence that acquired traits can be inherited by their offspring. Rather, it is the heritable traits that are either amplified or diminished via natural selection

Question 13

Describe evidence from the fossil record that supports Darwin’s theory of evolution by natural selection.

Answer 13

1) Transitional forms:
• Fossil species with traits that are intermediate between those of older and younger species.
• Physical evidence of successional evolutionary changes.

2 • Some fossils found = unlike any organisms that currently exist.
• Scientists concluded that extinction was a common occurrence in the history of life.
• Records also indicate that species have gone extinct continuously throughout Earth’s history.
• Fossils found in a certain geographic region frequently closely resembled species currently living in that area.
–> Strong evidence that species change through time and that the extinct forms and living forms are related and
represent ancestors and descendants.

Question 14

Explain how the existence of vestigial structures can be explained by Darwin’s theory of evolution by natural selection and why these features are inconsistent with the theory of special creation.

Answer 14

• Vestigial trait: a reduced or incompletely developed structure that has no function or reduced function, but that is clearly similar to functioning organs or structures in closely related species.
• What are some examples?
• Tiny hip and leg bones in bowhead whales.
• Reduced wings in ostriches and kiwis.
• Eye sockets in blind, cave dwelling fish.
• The appendix and coccyx in humans.
• Goosebumps.
• The existence of vestiges is inconsistent with the theory of special creation – if species were perfectly designed by a Creator, then why would species have these vestiges?
• Instead, vestiges are evidence that species are not static - their characteristics have changed through time.

Question 15

Explain how the existence of homologous structures can be explained by Darwin’s theory of evolution by natural selection and why these features are inconsistent with the theory of special creation.

Answer 15

• When biologists first began to study the anatomy of organisms, they noticed a remarkable similarity between their skeletons, muscles, and other traits, even if these traits served very different functions.
• Under the theory of special creation, biologists could not explain these patterns, nor could they explain why these similarities existed between some organisms but not others.
• Homology: a similarity that exists in species descended from a common ancestor.
• Under descent with modification, related species can have characteristics that have underlying similarity, but
different functions

Question 16

Define “homology” and differentiate between homologous and analogous features.

Answer 16

Homology: a similarity that exists in species descended from a common ancestor

• Analogous features share a common function, but not a common ancestor.
• Homologous features share a common ancestor, but not necessarily a common function.

Question 17

Explain how Darwin’s observations of mockingbirds on various islands of the Galapagos could be
explained in the context of evolution by natural selection.

Answer 17

• Mockingbirds collected on different islands in the Galapagos had different colorations and beak sizes and shapes.
• Why would species on neighboring islands be so similar, but yet have such distinctly different features as well?
• Because they had descended from the same ancestor, rather then being created independently.
• Populations that colonized different areas had changed through time and formed new species as they adapted to their environment.

Question 18

Describe three experiments or case studies in which evolution by natural selection was observed. Your description should clearly illustrate the role of heritable variation and differential survival and reproductive success in the process of evolution.

Answer 18

Example 1: Guppy experiment
• In guppies, a trade-off exists between attracting mates and avoiding predators.
• Females are attracted to brightly colored males and are more likely to choose them as mates.
• Predators more likely to feed on prey that do not avoid detection.
• In pools with few predators, male guppies tended to be bright.
• In pools with many predators, male guppies were drab.
• Hypothesis: More intense predation caused natural selection to favor the drab coloration in males.
• Experiment:
a. Transplanted brightly colored guppies to pool with predators.Overtime population became more drab.
b.Transplanted drab guppies to pool with few predators
Overtime, population became more brightly colored

Example 2: Medium Finches on Daphne Major
• Research by Peter and Rosemary Grant + students.
• Examined beak size, shape, and body size of medium ground finches native to Galapagos for 30 years.
• In 1977, a major drought occurred.
• Few plants were able to produce seeds - those that did
produced large, tough seeds.
• 84% of the medium ground finch population disappeared.
• Missing birds had died of starvation.
• Average beak depth in the population
increased!
• WHY?
• Birds with larger, deeper beaks were able to crack and eat the larger seeds  survived drought and produced offspring in following breeding season.
• Since beak depth is heritable, survivors passed the large beak trait alleles onto their offspring
• In 1983, environment changed again.
• Severe El Nino  VERY wet season; >1300mm rain
• Plant growth flourished in years that followed.
small soft seeds produced in abundance.
• Result?
 Shift towards individuals with smaller, pointed beaks.
• WHY?
• These individuals could more easily handle the small seeds  had more food available than individuals with
larger beaks  could eat more and thus raise more offspring.

Question 19

Describe three experiments or case studies in which evolution by natural selection was observed. Your description should clearly illustrate the role of heritable variation and differential survival and reproductive success in the process of evolution. continued…

Answer 19

Example 3: Case study of tuberculosis antibiotic
resistance
• Young man admitted to hospital – diagnosed with TB.
• Given antibiotics over 6 weeks, followed by rifampin 2X per month for 33 more weeks.
• After 10 months of therapy – infection had cleared.
• Two months later, same patient re-admitted with symptoms of TB.
• Given antibiotics, but died 10 days later.
• Rifampin works by binding to RNA polymerase –> interferes with transcription.
• Research team analysed DNA from bacteria cultures of original infection and second infection.
• Found only ONE difference: a point mutation in a single gene.
• Changed a cytosine to a thymine; TCG  TTG; leucine instead of serine at 153rd amino acid in polypeptide of RNA polymerase.
• RESULT:
• Substitution of just one a.a. prevented rifampin from effectively binding to RNA polymerase!
• Bacteria with mutation could still reproduce even when rifampin present
• Evolution by natural selection occurred in this population because:
• There was variation in the population - i.e. cells with and without the mutation.
• This variation was heritable.
• Under rifampin therapy, there was variation in reproductive success favoring the survival of the mutant individuals.
• The resulting population had a higher frequency of the mutant allele than the original population.

Question 20

Explain why Evolution is not goal orientated; it does not give organisms what they want or need.

Answer 20

• Natural selection is not goal oriented!
• Adaptations do not occur because organisms “want ” or “need” them.
• Natural selection simply acts on the genetic variation that is present in a population. This genetic variation is generated by random mutations – unaffected by what organism “needs”!
• The mutation that caused the change in RNA polymerase in TB bacteria occurred by chance, due to
an error in DNA synthesis, and this change happened to be advantageous.
• Likewise, organisms cannot “try” to adapt.

Question 21

Explain why Evolution is not progressive.

Answer 21

• Evolution is not progressive and does not always result in more complex or “better” organisms.
• We have seen many examples of where evolution led to the loss of complex traits – can you name one? (sharks loss of mineralized skeleton)
• Under evolution by natural selection, there is no such thing as “better ” organisms – organisms do not progress over time.
• Green algae may be more “ancient” (i.e. evolved earlier) than flowering plants, but neither group is more advanced than the other. Green algae simply have a different set of adaptations than flowering plants, allowing
each group to thrive in particular environments.
• Evolution simply results in organisms that are adapted to living in a given environment at a given time.
• What may be beneficial in one environment may be poor for survival in another environment or when the environment changes.

Question 22

Explain why Evolution does not lead to perfect organisms.

Answer 22

• Evolution by natural selection does not lead to perfection because of genetic and historical constraints and trade-offs.

Question 23

Explain why Not all traits are adaptive.

Answer 23

• Many of the traits that any individual organism has do not necessarily increase its fitness, and are therefore not
adaptive.
• Examples:
• Vestigial organs serve little to no physiological purpose.
• Some traits exist in adults as holdovers from structures that appear early in development (ever wonder why men have nipples?).

Question 24

Explain why Natural selection does not act for the good of the species.

Answer 24

• Natural selection favors the reproductive success of
individuals – it does not act at the level of the species.
• In evolutionary biology, an organism is said to act
altruistically when it behaves in a manner that benefits
another individual, at a cost to itself.

Question 25

Explain why Under the theory of evolution by natural selection, altruistic behaviors should not exist in nature, but under what circumstances they may evolve

Answer 25

• Under the theory of evolution by natural selection, why should altruistic behaviors not exist in nature?

• –> Natural selection favors behaviors that increase an individual’s own chances of survival and reproduction, not those of others.
• –> By behaving altruistically, an animal reduces its own fitness, so it would be at a selective disadvantage

• How can we explain altruistic behaviors? What is the
evolutionary explanation for altruism?
• If altruistic acts are “repaid” at other times.
• If altruistic behavior increases the survival and reproduction of an individual’s relatives.
–>Would be favored by natural selection!

Question 26

Explain why Evolution is not “just a theory”…in the English sense.

Answer 26

In science, a theory is a broad explanation for a wide range of phenomena that is strongly supported by many lines of evidence.

Question 27

Explain why Natural selection is not about the “survival of the fittest”.

Answer 27

• Natural selection is all about “survival of the fit-enough”
individuals.
• In most populations, organisms with many genetic variations survive + reproduce.
• It is NOT simply the fittest individuals that pass on their genes!

Question 28

Explain why Selection acts on individuals, but populations evolve.

Answer 28

• Natural selection acts on individuals because it is the individual that survives or dies and reproduces or not.
• BUT it is the population that evolves – not individuals.
• Tuberculosis example:
• A mutation occurred and individuals either survived or died, or produced fewer or more offspring.
• B/c individuals with mutation survived and produced more offspring under rifampin therapy, the mutant allele became more common in population.
• Finch example:
• During the drought, the beaks of the finches did not get deeper.
• Over time, the average beak size of the population increased. This occurred because individuals with deeper beaks survived and produced more offspring .
• Allele frequencies change in populations –> populations evolve.