Population Genetics Chapter 15

Red hair will eventually die out because it’s recessive... Brown eyes are dominant so they will take over... Little toes will go away because we don’t use them... Humans will get bigger brains because we need them... You may have heard that...

But is that how it works?

Genetics & Populations Evolution = Change in the gene frequencies of a population across generations. Red hair and blue eyes are inherited traits. If red hair or blue eyes are to “die out,” what would cause this kind of change in the genetics of a population?

Hardy & Weinberg Hardy & Weinberg asked the opposite question: Under what conditions would there be NO change in the genetics of a population? Conditions of NO change: large population, random mating, no migration, no mutations, no selection.

(a) Population size = 10,000(b) Population size = 4 generation frequency of allele A In the small population, one allele may become extinct in a few generations. In the large population, allele frequencies remain relatively constant. Population size Random events have a larger effect on smaller populations.

The gene pool of a population contains equal numbers of red, blue, yellow, and green alleles. After the population grows and returns to its original size, blue alleles predominate; red and green alleles have disappeared. By chance, the gene pool of the reduced population contains mostly blue and a few yellow alleles. A bottleneck event drastically reduces the size of the population. A drastic change in population size decreases genetic diversity in later generations.

Non-random mating Many living species have ways to compete for mates. In some populations, only a few individuals can mate. Mate preference is non- random. Even in the same species, many animals will prefer mates that resemble themselves.

Sexual selection, the result of non-random mating, selects for traits that secure mates — though some attractive traits are drawbacks in other ways.

Migration Migration can remove genes from populations, add genes to a population, or create a small population with different gene ratios than the population it derives from. “Founder effect” and polydactyly in the Amish.

Mutations Small mutations creep into populations over time. Some are helpful, some harmful, and many are neutral but may be beneficial or harmful later. Mutations increase genetic variation in a population. original DNA sequence nucleotide pair changed from A–T to T–A substitution

Selection Selection occurs when a trait or set of traits gives a survival advantage or a disadvantage. Selection in favor = trait helps individuals survive and have offspring. Selection against = trait interferes with survival and reproduction. Traits increase in a population if they cause individuals to have more offspring. Traits decrease in a population if they kill off individuals before they can reproduce.

Average phenotype does not change; phenotypic variability declines. BEFORE SELECTION AFTER SELECTION (c) DISRUPTIVE SELECTION (a) DIRECTIONAL SELECTION (b) STABILIZING SELECTION percent of population Larger-than-average sizes favored. Average sizes favored. Smaller-than-average and larger than- average sizes favored. range of a particular characteristic (size, color, etc.) Average phenotype shifts to larger size over time. Population divides into two phenotypic groups over time.

Is there any population on the planet where all Hardy-Weinberg conditions (large population, random mating, no migration, no mutations, no selection) holds true? What must be true of all populations of living things?

Red hair will eventually die out because it’s recessive. Brown eyes are dominant so they will take over. Little toes will go away because we don’t use them. Humans will get bigger brains because we need them. Using what you know about population genetics, reassess these claims: