BIO372 : Biological & Ecological Implications of Sex : Benefits & Trade-offs : Lesson

I. Organisms can be composed of one cell or of many

1. Cell - the smallest unit of life
   • Nucleus
     • Genes are located on (homologous) chromosomes
     • Chromatin: the genetic material of the chromosomes
     • Ploidy: haploid, diploid, polyploid (approximately 1/3 of flowering plants)
   • Cytoplasm
     • Builds proteins
     • Cell cycle

II. Asexual Reproduction

Mitosis ("my-toe'-sis") A single cell division Two daughter cells receive exactly the same genetic material that the parent cell had Mitosis with animation Blue chromosomes from the dad, red from the mom Two strands in each chromosome, strands are exact copies of each other One strand of each chromosome goes to one daughter cell, the other strand to the second daughter cell

2. Advantages
   • Speed: rapid production of offspring
   • Fidelity to parents: well-adapted parents produce equally well-adapted offspring
   • Available resources are quickly monopolized
   • Population well adapted to a narrow niche does well under special environmental conditions, e.g., polluted areas

3. Disadvantages
   • Limited flexibility to meet a changing environment: a particular strain of cells is well-adapted to a relatively narrow niche
   • Low diversity: all cells are identical
   • Efficient parasites wipe out an entire population (cf., the Red Queen Hypothesis)

III. Sexual Reproduction

Meiosis ("my-oh'-sis") Two cell divisions Produces four daughter cells called gametes Each gamete contains only half the chromsomes as the parent cell Recombination (=crossing over): each gamete may contain genetic material which is different from each other and from the parent cell In "higher organisms" the fusion of two gametes produces a new individual

2. Advantages
   • Diversity of offspring: in a changing environment, there is a high probability that at least some offspring will be well adapted
   • Novel combinations of genes potentially produce new adaptations (through recombination)
   • Favorable mutations in different individuals can come together in a single offspring
   • Offspring with harmful mutations die, resulting in removing the harmful mutations from the population
   • New combinations of genes lead to exploitation of new niches: evolution
   • Preadaptation: when the environment changes, the population already contains individuals which are adapted to the new conditions
     • Example: when colonists brought measles to the New World, some Native American individuals were resistant although most were not
     • Example: some few individuals resistant to HIV have been found

3. Disadvantages
   • Slow: it takes two cell divisions
   • Complicated: crossing over
   • Higher probability of errors
   • Genetic load: reassortment of genes can bring deleterious alleles from different parents together in a single offspring
   • Cost of sex: need to produce males, but this is not a disadvantage if the male contributes to the survival and success of the offsping
   • Cost of recombination: breaks up favorable combinations of genes, both on the same chromosome and in the same individual

When you have completed this lesson, go on to Review Questions