NATURAL SELECTION***READ 8.1***

http://www.youtube.com/watch?v=nS1tEnfkk6M

Types of SelectionDirectional Selection: when selection

favourindividuals with a more extreme variation

of a trait;selection that favours an increase or

decrease in thevalue of a trait from the current population

average.

Types of SelectionStabilizing Selection: occurs when the

averagephenotype within a population is favoured

by theenvironment; selection against individuals

exhibitingtraits that deviate from the current

populationaverage.

Types of Selection

Types of SelectionDisruptive Selection: selection that favours

two ormore variations of a trait that differ from

the currentpopulation average; favours individuals

withvariations at opposite extremes of a trait

overindividuals with intermediate variations.

Types of Selection

Types of SelectionSexual Selection: differential reproductive

success caused by variation in the ability to obtain

mates;results in sexual dimorphism, and mating

and courtshipbehaviours.

Types of Selection (Sexual) Favours any trait that enhances the

mating success of an individual Ie. Evolving appearances and behaviours

that are quite different in males and females

Ex. Male-male competition These traits might be a disadvantage

when it comes to predators (bright colours, distinctive song)

Also with plants: attract pollinators

Altruism At times, animals behave in ways that

reduce their individual fitness and increase the fitness of others

“helping others” There are two types of Altruism:

1. Kin Selection2. Group Selection

Kin Selection Helping relatives to survive and reproduce even

if it means forgoing breeding yourself You share 50% of your genes with each parents

and 50% with a sibling If an individual forgoes breeding and instead

helps raise a close relative, it is helping pass on a portion of its own genes to the next generation

The more closely related an individual is to another, the more likely it is to forego a breeding attempt

Group Selection “for the good of the group” In social populations, small groups will compete

with other small groups for resources Any adaptation that would improve a group’s

survival over another group would also improve the individuals survival even if it doesn’t directly benefit the individual

By cooperating with each other they ensure each other’s survival

Evolutionary Change without Selection

Genetic Drift: changes to allele frequency as a result of chance; such

changes are much more pronounced in small populations Random shifting of the genetic makeup of the

next generation When individuals produce offspring, the chances

of passing on a particular allele is subject to random chance

In small populations Can result in a particular allele can be very common

or disappear entirely When it disappears, it reduces genetic diversity

1. BOTTLENECK EFFECTBottleneck: a dramatic, often temporary

reduction in population size, usually resulting in significant genetic drift

Loss in genetic diversity after an extreme reduction in population size

Can have adverse consequences

2. FOUNDER EFFECTFounder effect: genetic drift that results when

a small number of individuals separate from their original population and establish a new population

Initial population would have a different mix of alleles, by chance alone

Populations from bottleneck and founder effects are subject to the effects genetic drift Further increases the chances that their gene pool

will differ from the original population

Big Horn Sheep1975: 20 sheep1999: ~650 sheep

HARDY-WEINBERG PRINCIPLE

The Hardy-Weinberg Principle: in large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation

Any factor that causes allele frequencies to chance leads to evolutionary change

HARDY-WEINBERG PRINCIPLE

These conditions lead to evolution Natural selection: favours the passing of some

alleles over others Small population size: increases the likelihood

of genetic drift Mutation: introduced new alleles to a

population Immigration/emigration: introduces or

removes alleles in a population Horizontal gene transfer: the gaining of new

alleles from a different species