Origin of Sex and Multicellular Animals (Wellman)

Question 1

Mitosis

Answer 1

Involved in natural cell division (asexual reproduction) in unicellular organisms and natural cell division in multicellular organisms.

Question 2

Meiosis

Answer 2

• Sexual reproduction in unicellular multicellular organisms.
Crossover of genetic material.

Question 3

Variation

Answer 3

• In asexually reproducing organisms the only variation is due to mutation.
• In sexually reproducing organisms, variation is due to mutation and crossing over.

Question 4

Problem with sex

Answer 4

Wasteful process: When pregnant, the organism cannot get pregnant again until their offspring is born.
-Population is normally 50:50, for reproduction to be as efficient as possible, a higher % of females is required.

There must be an advantage to sex as it has evolved and ~95% of eukaryotes carry it out.

Question 5

Advantages of sex

Answer 5

Two parents of different sexes are needed to produce one offspring, but in an asexual species, each individual has one parent.

• The offspring is not identical to the parent
• Harmful recessive mutations are often reduced, as it is more likely that one of two parents will have the harmless dominant allele, than only parent.

Question 6

Advantages of sex - mutation

Answer 6

• AS: The fate of a new mutation when a population of a constant size is reproducing by asexual fission. Eventually, the whole population carries the mutation.
• S: The fate of a new mutation is much more confusing, but in the same number of generations, only one member has the mutation with a few organisms being carriers, but the amount suffering from the mutation is considerably reduced.

(Beneficial mutations)In the sexual species, two new mutations can combine and may spread rapidly through the population.
In the asexual species, the two mutations are in competition and one or the other will be eliminated.

Question 7

Theories of the evolution of sex

Answer 7

1. A method of adapting to change by evolving faster
   - There is much more variation, due to crossing over, instead of producing a genetically identical offspring (excluding mutations)
2. Historical accident
   - Sex evolved by swapping genes accidentally.
3. A method of fending off disease/parasites
   - Used to keep ahead of disease/parasites
   - A mutation to protect which is passed on through sexual reproduction.
   - Dutch Elm Disease
4. A method of repairing genes
   - Chromosomes line up, they cannot pair up if they’re damages, so will repair DNA before passing on.

Question 8

When did sex evolve?

Answer 8

• Probably 1,200Ma
• The basal-most eukaryotes did not have sex, they just divide(d)
• No amoebae and flagellate organisms.

Therefore, sex must’ve evolved with eukaryotes, not before.

Question 9

Advantages of Sexual reproduction

Answer 9

• Helps natural selection remove harmful mutations

- Offspring are much different than parents

Question 10

Disadvantages of Sexual reproduction

Answer 10

• Reproduction rate is slower

- Reproduction is not always guaranteed.

Question 11

Origin of multicellular life

Answer 11

Multicellular life evolved because it is much more efficient for carrying out division of labour.

• Multicellularity evolved several times independently in different biological groups (various protists, plants, animals and fungi).
• It probably evolved via experimentation with colony formation.
• Forming groups of cells forming casual relationships to obligate.
• It has the advantage of division of labour.

Fungi, animals and algae/plants all became multicellular, independently of each other.