BIL 104 - Lecture 18

EVOLUTIONARY GENETICS

Mendelian Genetics (a.k.a. "transmission" genetics) is concerned with genetic events at the level of the individual organism.

Molecular genetics is concerned with genetic events at the level of the cell.

POPULATION GENETICS is concerned with genetic events at the level of the population, and hence, genetics as it pertains to evolution.

REMEMBER: Individuals adapt. Populations evolve.

EVOLUTION: change over time

ORGANIC EVOLUTION: change in living organisms over time.

MICROEVOLUTION: changes in allele frequency within a population

MACROEVOLUTION: speciation (reproductive isolation)

A few definitions:

POPULATION: all individuals of the same species (recall the definition of a species) living in a defined geographic area. (anything from your eyebrows to the Himalayas).

GENE POOL: all the genes at all loci in every member of an interbreeding population.

DEME - partially isolated subset of a population.

WHAT MAKES A SPECIES A SPECIES? REPRODUCTIVE ISOLATION FROM OTHER SPECIES.
And this, too, comes in various "flavors"...

REPRODUCTIVE ISOLATING MECHANISMS

When one species splits and gives rise to two new species (cladogenesis), what is it that determines whether or not the members of the two species can mate and produce fertile, viable offspring?

We can help define separate species by considering the mechanisms that prevent successful reproduction between them.

PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes.

environmental/spatial isolation - the geographic ranges of two species overlap, but their microhabitats or breeding conditions differ enough to cause reproductive isolation.
e.g. - Rana aurora (Red-legged frog) breeds in fast-moving, ephemeral streams, whereas its relative Rana catesbiana (Bullfrog) breeds in permanent ponds. (The metamorphosis times of the tadpoles are correspondingly different.)
temporal isolation - two species whose ranges overlap have different periods of sexual activity (or breeding season)
Rana aurora - breeds January - March
Rana boylei - breeds late March - May
another example -
Drosophila persimilis - breeds in early morning
Drosophila pseudoobscura - breeds in the afternoon
behavioral isolation - In species with courtship rituals (breeding calls, mating dances, etc.), there is usually a complex, give-and-take "ritual" before actual mating takes place. This prevents "wasted effort" with a partner who will not produce fertile offspring with you!
mechanical isolation - genitalia don't fit together.
gametic isolation - sperm and ova are chemically incompatible, and will not fuse.

POSTZYGOTIC ISOLATING MECHANISMS prevent hybrids from passing on their genes.

hybrid inviability - zygote forms, but dies after a few series of cell divisions (the genetic information from male and female parent were insufficient to carry the organism through morphogenesis)

hybrid sterility - viable hybrid is produced (often physically more vigorous than either parent), but is unable to reproduce due to meiotic problems.

hybrid breakdown - successive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population.