Lecture 2: Evolution by natural selection 4

Question 1

The great dying

Answer 1

Late Permian event (252 mya) the largest known extinction event to have occurred

• Several papers have suggested that an asteroid or comet impact may have caused this (for example a 300-mile wide impact crater has been found under Antarctica), but the most likely cause is a combination of extreme volcanic activity linked to the breakup of Pangaea, and global climate change – though we are not really sure yet
• We do know that this period was associated with 1) an extreme period of global warming, which led to methane being released from permafrost creating a positive feedback, and 2) large parts of the ocean becoming anoxic and currents circulating more slowly

Question 2

Consequences of mass extinction

Answer 2

Global climate change leading to changes in species distributions as their ranges expand/contract – this favours generalist species
• Opening up of many ecological niches which survivors can exploit (e.g. mammals after the K/T impact)

Question 3

Consequences of mass extinction- Liliput effect

Answer 3

surviving species temporarily become much smaller for thousands or millions of years following the extinction event, possibly due to a decrease in productivity, food shortage or energy efficiency

Question 4

Ant-plant mutualism

Answer 4

Some plant species produce sugars to encourage ants to nest

The ants then protect the plant from herbivores

Gardener ants poison any plant that grows close and that may compete with their home plant

Question 5

The K/T impact

Answer 5

Mass extinction event 65 mya – removed the dinosaurs, pterosaurs, large marine reptiles, ammonoids and many insects – estimates have been put as high as 75% of all species and 25% of all families became extinct

• Allowed for the diversification and dominance of the mammals
• The K/T (Cretaceous/Tertiary) boundary refers to an abundance of rare- earth minerals (found in meteorites) in a layer of clay laid down throughout the world – above this layer no more evidence of the extinct species is found
• Chicxulub crater in Mexico made 64.5 million years ago – the meteor was 10 km in diameter and struck at 32 000 kph

Question 6

More K/T impact

Answer 6

Fireball and ejecta = forest fires (up to 90% of the world’s forest) = pyrotoxins in the atmosphere

• Nitric acid = acid rain, reduction in ozone = more ultra-violet radiation
• Between 1000-4000 km3 thrown up by the impact, along with CO2, SO2 and H2O, having an effect on the weather (sulphuric acid aerosol) – combined with smoke = 6 months of dark skies = global cooling by 10oC for several years, followed by 3-10oC increase due to increased CO2 from limestone particles

Question 7

Being critical

Answer 7

Fossil records are limited – the chances of being fossilised are very small, and remember that animals and plants can change their location

• Mass extinction events occurred a long time ago, and much evidence has been lost – e.g. the K/T minerals can only be seen because it was recent – most of the Permian crust would have been subducted by now
• Taxonomy is difficult, and extrapolating figures from limited data will cause error