Biology 106: Lecture # 5 EVOLUTION OF SEX

I. Important questions

A. Why is sex ubiquitous?

B. Adaptive significance?

II. Definition

A. sex = process whereby genes inherited from different parents (gametes) are brought together (syngamy) in a single offspring

B. For diploid organisms process is meiosis

C. Why Isogamy ----> Anisogamy

D. Sex in viruses:

E. Result: offspring are often a genetic mixture of two or more individuals

III. Asexual Reproduction - ubiquitous ( freq. much rarer & w/o fertilization)

A. Defn. = production of offspring which are identical to parents

B. Types of Asex/Apomictic Repro.

1. Fission- mitosis in unicellular organism

2. Vegetative Reproduction - offspring sprout from body and then separate; ex's fungi, plants, animals (starfish)

3.Parthenogenesis = unfertilized egg develops into either a diploid or haploid individual; ex's invert's, esp. arthropods, bees

4. Agamospermy = unfertilized ovules develop into diploid seeds; ex. Dandelion

IV. Disadvantages of Sex (Cost of Sex)

1. SEX: Cost of producing daughters (pass on only 1/2 of its genes)

a. 2 fold cost may explain why asexual reproduction has evolved repeatedly in the plant and animal kingdom

b. Parental care may reduce cost of sex by increasing probability of young surviving

2. Meiosis takes more time than mitosis

3. Cost of attracting mates

4. Breakdown of favorable gene combinations:

V. Advantages of Sex - Evolutionary rationale

A. Get rid of disadvantageous mutations Muller's Ratchet = Deleterious mutations will increase in a population without recombination

B. Bring together favorable gene combinations

C. Recombination quicker than mutation!

D. DNA repair mechanism via meiosis

VI. Take Home Message

A. Sex is a trait: major role in evolution of eukaryotes

B. Asexuality is sometimes adaptive

1. cost of daughters is avoided

2. mitosis quicker and less error prone than meiosis

3. cost of attraction is avoided

4. maintain favorable gene combinations

5. Limited response to selection

C. Sex is often adaptive

1. get rid of disadvantageous mutations

2. recombination rapidly brings together favorable gene combinations

3. DNA repair mechanism

4. More genetic diversity

5. Greater response to selection