Lecture 2: Evolution by natural selection 2

Question 1

What is evolution

Answer 1

Evolution is the result of inaccurate and beneficial replication

Question 2

DNA

Answer 2

• The replication apparatus is DNA (Deoxyribonucleic acid), the structure of which was discovered by Watson and Crick in 1953
• DNA consists of a sequence of four nucleotide bases: adenine, guanine, cytosine and thymine (A,G,C,T) A fits to T and G fits to C, and they are arranged in a double helix

Question 3

Gene

Answer 3

• A gene is essentially a string of letters (like a word), with each letter containing information about how an organism should look and function

Question 4

When an organism replicates

Answer 4

The sequence of letters for an organism is huge – the genome for a simple bacteria like E.coli is a few million letters, whereas the human genome has over 3 billion – your DNA from one cell would stretch 2m

• When an organism replicates, the DNA ‘unzips’ and, because each letter only fits with its pair, spare molecules will bind to the stumps to form two strands where before there was only one. New cells can then form, and replication is complete

Question 5

Chromosone

Answer 5

A chromosome is a length of DNA (where the genome is stored), and essentially when sexual reproduction takes place the chromosomes from each parent unzip and recombine to create a new sequence of genes

Question 6

Problems with replication

Answer 6

sometimes a nucleotide base will change, or a genetic sequence will recombine differently – the consequence is a mutation

• With organisms having thousands of genes and millions of nucleotide bases, the opportunities for error are great – but too significant a mutation will result in the death of the organism. With too perfect replication the organism will not change and evolution will not take place – so the error rate has to be within certain boundaries
• The error rate is very low in higher organisms – less than 1 in 100,000,000 per replication, which have more genes (and are therefore more likely to die if a gene gets badly copied), while bacteria have a 1 in 1,000,000 error rate

Question 7

Mutation

Answer 7

Generally these mutations will have little effect or will be negative and the organism will reproduce less successfully than other organisms, so the mutation will eventually be lost

• But occasionally a mutation will provide a benefit and the organism will be a greater success than its peers, and reproduce more – the mutation will also be copied and so more and more individuals will pass on the mutation
• As different mutations occur over thousands or millions of years, eventually a species will become distinct (i.e. its genes cannot recombine successfully with individuals of what were once the same species)

Question 8

DNA molecule

Answer 8

we don’t know how DNA arose, or how its symbiosis with proteins developed, we do know that it has been the replication mechanism for life (along with RNA) for all known organisms and existed as far back as the earliest living thing we can be certain of (3.5 billion years)

• Every living thing on this planet is related to the first DNA molecule – and since all DNA does is replicate itself, essentially the whole history of life, and the sole purpose of an organism, is to replicate and produce more DNA –

from the DNA perspective, an organism is a useful machine for ensuring replication happens (because we can respond to our environment) – and we die to free up space for our offspring to exist and replicate

Question 9

Pangea

Answer 9

Since the continents formed they have been moving around, and at one time were very close

• The colonisation of the land began around 420 million years ago, and life rapidly spread across this new habitat
• At 200 million years ago Pangaea began to break apart due to plate tectonics, and separate islands

Question 10

Species of pangea

Answer 10

Early ecologists noted that the distributions of familiar species across the globe was impossible to explain relative to the distances involved.

• They then realised that the continents must have moved.
• Geologists disliked this theory until the processes involved in plate tectonics were discovered, and then global species biogeography began to make more sense.
• As evolution was taking place, the continents were separating, so that similar early forms of species were spread around the globe, and further evolution made them more and more distinct