Sources of variation

Evolution depends on genetic variation within and among populations.

Individuals vary

Populations vary

Genetics: the scientific study of variation and heredity

Chromosomes and DNA

Genetic information (genome) encoded in long strands of DNA

Eukaryote genome subdivided into smaller fragments (chromosomes)

Inheritance: chromosomes passed properly from one cell to the next

Cell division

mitosis

growth

meiosis

sexual reproduction

Click here for an animation of mitosis

Meiosis and sexual life cycles

basic terminology

somatic cell vs. gamete

diploid vs. haploid

homologous chromosomes vs. non-homologous chromosomes

autosome vs. sex chromosome

The sexual life cycle

a sexual life cycle involves: (Figs. 13.5, 13.6)

formation of haploid gametes (meiosis)

fertilization

growth and development (mitosis)

repeat

How meiosis works

How does meiosis reduce chromosome number and result in haploid gametes?

Basic steps: (Figs. 13.7, 13.8)

Diploid cell

Replication

each chromosome consists of sister chromatids held together by centromere

Synapsis: homologous chromosomes line up alongside each other

Meiosis I: homologous chromosomes separate into two daughter cells

haploid; each chromosome consisting of two chromatids

Meiosis II: the chromosomes in each cell then line up, and sister chromatids separate

results in four haploid daughter cells

Click here for an animation of meiosis

Key differences between meiosis and mitosis (Fig. 13.9)

mitosis

Two daughter cells

genetically identical to parent cell

two full sets of chromosomes (diploid)

makes bigger organism

meiosis

Four daughter cells

genetically different from parent cell (and each other)

one full sets of chromosomes (haploid)

helps makes another organism (sexual reproduction)

Meiosis creates variation

(more generally, sexual reproduction is a source of variation)

how?

independent assortment (Fig 13.10)

homologous chromosomes have different origins (one from mom, one from dad)

whether a gamete gets a maternal chromosome or paternal chromosome is like a coin flip

results in many possible combinations (e.g., 8 million for humans)

Meiosis creates variation

crossing over (Fig 13.11)

creates new combinations of genes on any given chromosome

during synapsis, nonsister chromatids cross (chiasma), break, and reattach (called recombination)

each chromosome mixture of maternal and paternal genes.

random fertilization: millions of possible genetic combinations egg X millions of possibilities sperm = trillions of possible diploid combinations

Why sex?