Chapter 8: Changes in Species Over Time

Question 1

Outline the timeline of life on earth

Answer 1

• Prokaryotes
• Photosynthetic bacteria
• Aerobic microbs
• Eukaryotes (unicellular and then multicellular)
• Vertebrates (jawless fish)
• Insects
• Land plants
• Amphibians
• Ferns
• Reptiles
• Conifers
• Dinosaurs
• Birds
• Mammals
• Flowering plants
• Humans

TIP TO REMEMBER: Pretty Elephants Are Really Dangerous, Majestic and Hairy.

Question 2

Define mass extinction

Answer 2

• Mass extinction ccurs when a larger than normal number of groups become extinct (at a regional/global scale)
• It is an evolutionary opportunity for other species to thrive and diversify
• Are followed by a period of rapid divergent evolution

Question 3

Define fossilation

Answer 3

• Fossilation is the process of preserving parts of organisms that lived in the geological past

Question 4

Describe the process of fossilisation

Answer 4

• An organism dies and is rapidly buried in sediment
• It must be protected from scavengers and decomposition (by low oxygen levels and low temperatures)
• The organism soaks in ground water for a long time
• Sustained deposits of sediment bury the organism more and more deeply
• Over time, hard sections (e.g. the skeleton) of the organism are replaced by minerals from the ground water
• The weight of overlaying sediments compress the original layer of sediment, turning it into rock
• Eventually, the rock is eroded, uplifted through the movement of tectonic plates or is excavated therefore revealing the fossil

Question 5

Identify the conditions that favour fossilation

Answer 5

• Rapid burial
• Low oxygen levels and low temperatures to prevent decomposition
• Protection from scavengers

Question 6

Name and describe the 3 fossil categories

Answer 6

• Physical fossils are remains of structures
• Trace fossils are evidence of activities
• Biosignatures are inferred evidence

Question 7

Describe and provide an example of mineralised/petrified fossils

Answer 7

• Form when organic materials or structures are replaced by minerals
• E.g. Dinosaur eggs

Question 8

Describe and provide an example of mold/cast fossils

Answer 8

• Form when a mineralised/petrified fossil dissolves and leaves an impression of the original
• The impressions can then be filled in to make cast fossils
• E.g. An ammonite mold and cast fossil

Question 9

Describe and provide an example of trace fossils

Answer 9

• Form when traces of activity are buried before they are erased and turn into rock
• E.g. Footprints, burial remains and teeth marks

Question 10

State the difference between mineralised fossils and fossil impressions

Answer 10

• Mineralised fossils are formed when the organic material of a dead organism is replaced by minerals
• Fossil impressions are formed when the organic matter is broken down leaving an imprint on sediments

Question 11

State the purpose of the fossil record

Answer 11

• The fossil record reveals that over time changes have occurred in the types of living organisms on Earth

Question 12

Explain faunal succession

Answer 12

• Faunal succession is the principle that fossilised fauna and flora in sedimentary rock strata are arranged vertically in a specific order
• Fossils can be used to identify rocks of the same age
• Different organisms do not occur randomly in the fossil record but are found in rocks of particular ages and appear in a consistent order
• E.g. The first amphibians appear in the fossil record before the first reptiles

Question 13

Explain index fossils

Answer 13

• Index fossils are fossils of geologically short-lived species that are widely distributed but are found in a restricted depth of rock strata
• Using them for dating only provides relative age (limitation)

Question 14

Describe the characteristics of a good index fossil

Answer 14

• Wide geographic distribution (abundant)
• Distinctive (easy to identify)
• Short-lived in order to give accurate data about the strata in which they are found
  
  • Reappearing after their extinction may lead to inaccurate conclusions

Question 15

List common examples index fossils

Answer 15

• Ammonites, trilobites and graptolites
• Index fossils are commonly aquatic organisms because they are universal to the ocean and can be easily covered by marine sediment

Question 16

Explain relative dating

Answer 16

• Relative dating is used to determine the comparative age of a fossil (whether a fossil or strata is newer or older compared to another)
• Sedimentary rocks form in layers (strata)
• New layers are at the top while older layers are at the bottom
• Does not provide a precise numerical age (limitation)

Question 17

Explain absolute dating

Answer 17

• Used to determine the numerical age of a fossil
• It measures the relative decay of radioactive materials and their products
• Radioactive isotopes (parents) break down over time to form stable products (daughter)
• Compares the amount of a radioisotope remaining in a fossil (or rock surrounding a fossil) to the amount that was originally present in the sample
• Using the isotope’s half-life, the age of the fossil can be determined

Question 18

Explain carbon dating

Answer 18

• Carbon 14 (in organisms) decays into nitrogen 14 with a half life of 5700 years
  
  • Organisms take up C14 when they eat and stop when they die
• By comparing the amount of C14 to N14, the time since death can be determined
• Samples older than 60 000 years are difficult to date due to limited amounts of carbon 14 left in the organic matter

NOTE: Potassium-Argon dating can be used when dating a fossil over 500 000 years old.

Question 19

Define species and speciation

Answer 19

• Species are organisms that can breed and produce fertile and viable offspring
• Speciation is the process in which a new species is formed

Question 20

Outline the process of speciation

Answer 20

• Variation of characteristics is present in a population
• The breeding population becomes isolated
• Different characteristics arise through genetic drift, mutation and environmental pressures, leading to genetic divergence
• Environment changes resulting in the survival of better suited characteristics
• Survivors reproduce and pass on their favourable genes and features to offspring
• The frequency at which the genes for the new characteristics appear increases
• The isolated population is now different, producing a new species

Question 21

Define genetic divergence

Answer 21

• Genetic divergence is when two or more populations accumulate genetic changes, leading them to become reproductively isolated

Question 22

Describe the process of allopatric speciation

Answer 22

• A single population is divided due to a geographical barrier (no gene flow between populations resulting in genetic divergence)
• Isolated populations are subjected to different environmental selection pressures and, thus, different phenotypes are favoured
• When the two populations come together again, they can no longer interbreed leaving two seperate species with two different gene pools

Question 23

Describe how the Galapagos finches diverged into 13 seperate species

Answer 23

• After arriving on the islands, the finches became geographically isolated due to the surrounding ocean
• There are different environmental selection pressures on each island due to food availability
• Differences in food sources caused changes in beak shape
• Overtime, allopatric speciation occurred

Question 24

Describe the process of sympatric speciation

Answer 24

• A population is divided due to a non-geographical barrier (e.g. diurnal and nocturnal species)
• The two groups are subject to different selection pressures and, thus, different phenotypes are favoured
• Over generations, genetic divergence occurs (mutations accumulate)
• When the two populations come together again, they can no longer interbreed leaving two seperate species with two different gene pools

Question 25

Summarise the events that led to different species of Howea palms in the same area

Answer 25

• A population of Howea palms was divided due to the soil they grew in
  
  • Volcanic soil is higher in nutrients, thus, supporting plant growth
  • Calcareous soil is sandy with lower levels of nutrients
• Differences in the soil’s nutrients caused differences in the palms’ flowering times (non-geographic isolation)
  
  • Contributed to a lack of gene flow and therefore reproductive isolation
  • Reduced chance for mating and therefore genetic divergence is promoted
• Over time, sympatric speciation occurred

Question 26

Explain why sympatric speciation is rare

Answer 26

• Gene flow can still occur which prevents reproductive isolation
• Due to the lack of physical isolation, genetic drift has little impact on populations
• Natural selection would need to be strong to favour specific traits allowing for divergence

Question 27

Describe the difference between pre-zygotic and post-zygotic isolation mechanisms (include examples)

Answer 27

• Pre-zygotic isolation mechanisms are barriers that prevent an organism from finding and securing a mate
  
  • Temporal isolation → nocturnal and diurnal species are unlikely to meet
  • Geographical isolation → species who live on mountain tops are unlikely to meet those in valleys
• Post-zygotic isolation mechanisms are barriers that prevent fertile offspring from developing after mating
  
  • Incompatibility of gametes → sperm cannot succesfully penetrate the egg, thus, fertilisation does not occur
  • Zygote mortality → fertilisation occurs but the zygote (fertilised egg) fails to develop

Question 28

Explain how mass extinction allow for the rapid evolution of other species

Answer 28

• Reduced competition for resources
• Environmental changes due to mass extinction may confer a selective advantage for species
• Surviving species may have the opportunity to develop new traits, behaviour and adaptations for rapid speciation