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The forces of nature create a selective
pressure so that traits which are better
adapted in a particular environment
enable the organisms within a population
to survive to reproductive age.
The ability to reproduce is called fitness. Alfred Wallace
coined the term “survival of the fittest”.
• Natural Selection is a process based on the following:
• Organisms exhibit variations that can be passed
from one generation to the next (heritable
• Organisms compete for available resources.
• Individuals within a population differ in terms of
their reproductive success.
• Organisms become adapted to conditions as the
environment changes.
All species have such great potential fertility that their population size would
increase exponentially if all individuals that are born reproduced successfully.
Most populations are normally stable in size, except for seasonal fluctuations.
Natural resources are limited.
Inference #1: Production of more individuals than that environment can support leads to
struggle for existence among individuals of a population, with only a fraction of
offspring surviving each generation.
Individuals of a population vary extensively in their characteristics; no two
individuals are exactly alike.
Much of this variation is heritable.
Inference #2: Survival in the struggle for existence is not random, but depends in part
on the hereditary constitution of the surviving individuals. Those individuals whose
inherited characteristics fit them best to their environment are likely to leave more
offspring than less fit individuals.
Inference #3: This unequal ability of individuals to survive and reproduce will lead to a
gradual change in a population, with favorable characteristics accumulating over the
• There are 3 general types of natural
1. Stabilizing Selection
2. Directional Selection
3. Disruptive Selection
• This occurs when an intermediate phenotype provides
an advantage to the population. With this type of
selection, extreme phenotypes are selected against.
• Example of this is human birth rate. Based on hospital
data, infants born with an intermediate birth weight
have a better chance of survival than those at either
extreme. Those will a low birth rate may not be fully
functioning and those with high birth weights may
experience a difficult delivery.
• This type of selection occurs when an extreme phenotype
provides an advantage and therefore is selected for.
Over time, the average phenotype of the population
shifts. This type of selection generally happens when a
population adapts to a changing environment.
• Example of this is the peppered moth. In London around
the beginning of the 1800’s black moths became more
favorable because their coloring blended in better with
the black soot from coal being burned.
• This type of selection occurs when the extreme
phenotypes are selected for over the intermediate.
This leads to a wide range in phenotypes of a
• Example of this type is the British land snail.
The snails either have a light banded shell or a
dark shell depending on the habitat they live in,
the forested area or the grassland area.
• Adaptive changes in males and females that lead to
an increased ability to secure a mate.
• For males it may result in an increased ability to
compete and females select a male with the best
ability to produce surviving offspring, or fitness.
• Many consider sexual selection a form of natural
selection because of it affects fitness.
• Female Choice
• Females produce less eggs compared to males sperm
production, therefore their choice in mate is important.
• There are 2 hypothesis being tested regarding female choice:
1. Good Gene Hypothesis: females chose a mate based on
traits that improve survival
2. Runaway Hypothesis: Females chose a mate based on
traits that improve the males appearance.
In studies involving Satin Bowerbirds and Raggiana Bird of
Paradise neither hypothesis could be fully supported or
• Male Competition
• Cost-benefit analyses have been done to determine if the
benefit of access to mating is worth the cost of competing
with other males.
• Ex: Baboons compete with other males to gain dominance.
Dominant males have more access to females when they
are most fertile. The cost of dominance is requiring more
resources because of larger size and possibility of getting
• Diversity is maintained by a number of reasons:
• Mutation
• Sexual Reproduction recombines alleles
• Genetic Drift
• Natural Selection produces imperfect
• Evolution is constrained by the available diversity.
• The environment plays a role in the diversity and
evolution of a population.
• Heterozygote Advantage
• This occurs when the heterozygote is favored over
the homozygotes.
• This assists the maintenance of genetic diversity in
future generations.
• Examples: Sickle cell disease and Cystic Fibrosis