Change Over Time:
Evolution
1. What is evolution? Is evolution a process or a result? Explain.
Evolution is the cumulative change in characteristics of populations over
successive generations. To evolve means to change. Evolution has occurred if
there is a change in the characteristics of a population over a number of
generations. Evolution is the result of change. The process that brings the change
about is ‘Natural Selection’.
2. Is evolution a theory or fact?
Evolution is a scientific theory. There is no such thing as a scientific fact! Some
theories are better than others. Many biologists believe there is much evidence
to support the evolution theory.
3. Is it always true to say that fossils are the remains of dead organisms?
Fossils provide evidence of once-living organisms. Fossils are not always the
dead organism’s remains. Fossil footprints or body impressions can also provide
evidence that an organism existed. Sometimes the actual remains may no longer
exist. A cast showing size, shape and texture of the outer surface may remain.
Fossils are not always the dead remains.
4. Under what environmental conditions are fossils formed? What conditions
reduce the chances of good fossils forming?
Any condition that reduces scavenging and decay by other organisms will assist
fossilisation.
- Quick burial. Fossils are most likely to occur in an environment where there is
rapid burial of the dead organism. Burial of marine organisms in sediments after
death is more likely than burial in a terrestrial environment. Quick burial of an
organism by volcanic ash provides excellent conditions for fossilisation.
- Cold temperatures. Frozen organisms are fossils. Low temperatures protect
the remains from decomposers, e.g. woolly mammoth remains!
- Sap. Sap also protects organisms from decay, e.g. insects in amber.
- Anoxic environments. The remains are protected as aerobic decomposers
cannot survive in anoxic environments.
Disturbance in organisms after death reduces chances of fossilisation. Exposure
to wind, rain, scavengers and bacteria also impede fossilisation. Earth
movements and erosion after burial tend to damage fossils. Erosion of
sedimentary rock, however, is the major way fossils are revealed allowing people
to find them.
5. What information can fossils provide about the lifestyle of once living
organisms?
Evidence of the once-living organism’s environment can be deduced by looking
at the sediments in which it is buried and other fossils present. An organism
whose fossil is found in sandstone probably lived in a marine environment
whereas the fossil of an organism living by a lake would probably be found in
mudstone. Fossilised pollen and seeds can indicate the types of plants growing in
an area in the past. The condition of a fossilised skeleton can tell a story.
Fossilised teeth can indicate diet and age of an organism before it died. The teeth
of carnivores and herbivores are very different. Tooth wear indicates the age of
an organism at death. For a toothed animal, the greater the teeth wear the older
the animal was at death.
6. Why is it difficult to identify different species using only fossil evidence?
- The remains are incomplete and so difficult to compare relationships with
living organisms.
- A major aspect of species identification relies on determining whether two
individuals can interbreed to produce fertile vigorous offspring. This cannot be
deduced through fossil evidence.
7. Describe the difference between absolute dating (radioactive isotope dating)
and relative dating (stratigraphy)?
Absolute dating gives an estimate of age in years. They are calculated using
known rates of decay of radioactive isotopes.
Relative dating relies on sedimentary layers (strata) being laid down in order. In
an environment undisturbed by earth movement the oldest fossils will be buried
more deeply than younger fossils. An actual date cannot be determined with
relative dating.
8. Different radioisotopes have different half-lives. What does this mean?
Radioisotope dating relies on paleontologists knowing the half-life of the isotope
with which they are dealing. Radioactive isotopes break down at known rates.
Carbon-14’s half-life is 5568 years. If you had 1 g of C-14 today, in 5568 years
you would have 0.5 g. After another 5568 years (a total of 11 136 years) you
would have 0.25 g of C-14. Radioactive rubidium isotopes have a half-life of 47
000 million years.
9. Different radioisotopic methods are used to date fossils of different ages. List
some commonly used isotopes and state their use. Is it always the fossil that is
dated?
Carbon-14: this method is used for dating organic matter up to 50 000 years in
age. Note: Carbon occurs in all organic molecules therefore the C-14 isotope is
useful for dating organic matter.
Potassium/Argon: Used to date volcanic rock from 10 000 – 100 million years
in age.
Uranium/Thorium: A method used to date bone and teeth of less than 350 000
years old.
Direct dating of fossils is not always possible. If sediment formation was at the
same time as the fossilisation, then dating the sediment dates the fossil.
10. What is the significance of fossils like Archaeopteryx?
Paleontologists believe Archaeopteryx represents an intermediate stage in the
evolution of birds from reptile-like ancestors. A problem for evolutionists is the
lack of a complete fossil record. The fossil record has many gaps. Excitement is
generated by fossils, like Archaeopteryx, because they fill gaps.
11. Apart from fossils, what other evidence supports evolution?
Evidence for evolution comes from a variety of other sources.
Homologous structures – For example all mammals have similar structures.
The skeleton is similar. The same bone pattern is present although the size and
shape of bones can vary.
Embryology – The developmental stages of related organisms are similar. For
example all chordates (including fish, amphibians, birds and mammals) have gill
slits at some stage in their life cycle. In most chordates the gill slits are not
present in the mature animal.
Biochemistry – The proteins and DNA of closely-related organisms are similar.
DNA from different species can be mixed and then heated to separate the DNA
strands. On cooling the DNA rejoins and hybrid DNA can form. The amount of
hybrid DNA depends on the similarity between different species’ DNA.
Hybridisation tests on chimpanzee and human DNA show there is similarity in
about 99% of the DNA. Similar experiments show that proteins of related species
are very similar.
Biogeography – For example, consider Darwin and his Galapagos finches. The
finches on the Galapagos islands are more similar to each other than finches
found in other places. This supports the idea that the Galapagos finches evolved
from a small group of ancestral finches. The large range of marsupials in
Australia provides further evidence. The Australian continent separated from the
continents before the rise of placental mammals. For this reason Australia was
populated by a wide range of marsupials that filled all ecological niches.
12. What are the major differences between the evolutionary theories of Charles
Darwin, Alfred Wallace, and Jean Baptiste Lamarck?
Darwin and Wallace independently concluded that natural selection is the
mechanism responsible for evolution. That is, in a population some individuals
survive longer and reproduce more often. These better-adapted individuals pass
their alleles on to the next generation.
Lamarck believed that what an organism does during its lifetime can
affect its offspring. Constant use of an organ would improve or strengthen it.
Alleles coding for the improved organ would be passed on to offspring. Not using
an organ might lead to its loss.
Most biologists believe that Darwin and Wallace’s theory is well
supported by evidence. Obviously in the last 100 years there have been
refinements made to evolutionary theory.
13. Darwin proposed the theory of ‘Evolution by Natural Selection’. Since then
developments in genetics have led to its modification. Summarise the modern
view of evolution by natural selection.
Genotypic variation
Phenotypic variation
Struggle to survive
Selection pressure
Favoured
Less favoured
Reproduce
Death
Offspring inherit
No offspring
Change in allele frequency
Within a species there is considerable variation in physical characteristics that
are genetically determined. Some individuals are more likely to survive than
others.
In nature large numbers of offspring are produced but not all will
survive. There is a struggle for survival due to competition for limited resources.
Only some individuals survive and reproduce.
Individuals with certain characteristics are more likely to survive to
reproduce in a particular environment (selection pressure) than other
individuals.
The individuals that survive pass on their characteristics to the next
generation.
Those individuals with different characteristics are less likely to survive
to reproduce. They are less likely to pass on their characteristics to the next
generation.
Over time there is an increase in the gene pool in the number of alleles
that determine the favoured characteristics.
14. What is the significance of geographical barriers in the evolution of new
species? Describe features that act as geographical barriers?
Geographical barriers can separate populations of one species to prevent
interbreeding. This is important in speciation. Some features that act as
geographical barriers are oceans, rivers, mountain ranges, deserts and deep
canyons.
15. Summarise the important steps that lead to the evolution of new species.
A single population of one species contains genetically determined variation.
A geographical barrier splits the original population into two. The isolated
populations are subjected to different environmental conditions. Different
phenotypes are selected in the two isolated populations. Over a number of
generations the populations become different. Mutations occur that affect
reproductive structures and behaviours. Of after removal of the geographical
barrier interbreeding is no longer possible then 2 species have formed.
16. How can you determine if you have one species or two distinct species in an
area?
To determine whether one or two species are present you need to look for
interbreeding. If individuals from the two groups can mate to produce vigorous
fertile offspring under natural conditions, they are members of one species. If
interbreeding or gene flow is not possible two species are present.
17. If two closely related species live in the same area, what are possible isolating
mechanisms that might prevent interbreeding?
Two types of isolating mechanisms exist. Pre-mating isolating mechanisms
prevent successful mating. Post-mating mechanisms prevent successful
production of vigorous and fertile offspring.
Pre-mating isolating mechanisms include habitat isolation, seasonal isolation,
differences in courtship, and mechanical isolation (reproductive organs are not
compatible).
Post-mating isolating mechanisms include gamete mortality (gametes do not
survive until fertilisation), zygote mortality (formed but not viable), and hybrid
sterility (for example mules).
18. Can individual organisms become extinct? Explain.
Individuals cannot become extinct. Individuals die. If all individuals of a species
are dead the species is extinct. A species that has living representatives is
extant.
19. Distinguish between convergent, parallel and divergent evolution.
Convergent evolution occurs when two unrelated species under similar selective
pressures develop similar structures and appearance. For example the placental mole
and the marsupial mole are similar in size and appearance. They have, however, evolved
from different ancestors. Convergent evolution occurs when unrelated species develop
the same solution to the same environmental problem.
Divergent evolution occurs when populations of one species are subjected to
different environmental conditions. Darwin’s finches provide an excellent example.
Colonisation of the Galapagos Islands was by one finch species. Populations on different
islands were subjected to different environmental pressures. There was selection of
different phenotypes on different islands.
Parallel evolution occurs when starting populations which are subjected
initially to divergent evolution, are then exposed to the same selective pressures. The
initial divergence is maintained but individuals of the two species develop in parallel.