Acts against the extremes. Favors the intermediates.
What is directional selection?
Favors one extreme. Normally seen during times of change, stress.
Ex: Industrial melanism, resistance against toxins, antibiots, pesticieds, increase in size of horses.
What is disruptive selection?
Favors both extremes selects against intermediates. Sometimes called "diversifying selection".
Disruptive selection can result in balanced polymorhpisms - 2 different morphologies, behaviors, etc. that occur at fairly stable levels in the population.
Ex: mimicry in Papillio butterflies
Homozygotes are negatively affected.
Ex: Sickle-cell anemia
Why is the allele so common?
What is sexual selection?
A special case of naturall selection that results in an increase in mating success.
Types of sexual selection:
Intrasexual selection: competition for mates among members of a sex.
Males often compete for breeding locations, fight for access to females, etc.
Types of sexual selection:
Intersexual selection- mate choice, One sex chooses a mate from the other. Females are usually the choosy sex.
A point on animal behavior
Instinct. Why do animals have instinct?
What is Anisogamy?
Gametes are not of same size.
Differntial parental investment. Time and energy.
Sperm- small and cheap. Egg- big and expensive.
What is fitness (w)?
is the ability to pass your genes along to the next generation. It requires succesful reproduction. There is risk to fitness in choosing a mate.
Female choice? How do they choose?
May relate to territory, may relate to male behavior (ie. song), may relate to other resources (male hanging fly provides nuptial gift),
What is sexual dimorphism?
A phenotypic difference between males and females of the same species. Female choice can have strange results.
What is runaway sexual selection?
Can result in extreme development of a trait not originally related to reproductive success. Requires a genetically determined female preference for some character.
Changing allele frequencies
Natural selection can change allele frequencies. Allele frequencies can be changed by other mechanisms.
Fraction of the parent population is isolated, or colonizes a new location. New population has a non-representative sample of the alleles in the parent population. Allele frequency has changed.
Population decreases to a low number, then recovers. At low number, not all alleles are represented in the individuals present. Genetic diversity is reduced. When population recovers, diversity remains low.
random change in allele frequency. Important in small populations.
Results in an increase in homozygosity. Mutations that occur in small populations have a higher probability of becoming fixed. Small, peripheral populations are considered important in evolution.
The origin of species
Allele frequencies change. This does not explain the diversity of species. Where do new species come from? Speciation - the evolutionary process by which new biological species arise.
What is a species?
Species can be defined based on visible characteristics. (structure, physiology, etc.)
This is the traditional view of a "species". Most species have been described this way.