9.1: Mechanisms of Evolution and Their Effect on Populations
pg. 350 - 359
Key Terms: gene flow, non-random mating, genetic drift, founder
effect, bottleneck effect, stabilizing selection, directional selection
sexual selection.
Genetic variation of individuals within a population makes
evolution possible.
Sexual reproduction leads to variation, as each new individual
obtains a new combination of alleles from their parents.
Mutations also randomly occur entering the generation, providing
the potential for new traits to develop.
Natural selection acts upon the variation in a non-random way,
where an individual with genes that help them survive will
reproduce, allowing them to pass along those genes to their
offspring and increasing the proportion of that gene in the
population.
Individuals do not evolve, populations do.
The gene pool of a population consists of all the alleles of all genes
of each individual in the population. (p + q = 1.00)
Factors That Change Allele Frequencies in Populations
Changing percentages, or frequencies, of alleles within populations
are the small events that lead to evolution within a population, or
microevolution. When the frequency of an allele in a population
changes; microevolution has occurred.
There are five common factors that lead to evolutionary change;
Mutation: changes in DNA, leads to the introduction of new
alleles into the population.
An inheritable mutation has the potential to affect an entire gene
pool. The more genetic variation within the population, the greater
the diversity of the population, and the greater the chance of a
selective advantage to some individuals in a changing environment.
e.g.: Norway rat and Warfarin rat poison.
Gene Flow: occurs when individuals from two different
interbreeding populations that have different allele frequencies,
migrate to the other population and reproduce.
Gene flow describes the movement of alleles from one population
to another by migration of individuals. Having a greater diversity
may help the population survive by natural selection.
Figure 9.2: Gene flow between grey wolf populations is quite common. Individuals travel
long distances and may join or otherwise interact with members of other populations.
Gene flow – is the net movement of alleles from one population to
another due to the migration of individuals.
Figure 9.1: Through gene flow, modelled here, genetic information is exchanged between
individuals of different populations.
Non-random Mating: occurs when mates are specifically selected,
often selected on the basis of phenotype. Non-random mating
determines which male will mate with which female leading to
preferred phenotypes, selective breeding.
Random mating is unable to predict which males will mate with
which females.
Non-random mating – is mating among individuals on the basis of
mate selection for particular phenotype or due to inbreeding.
Figure 9.3: The male caribou spar with each other to be able to mate with a female in the
herd.
Learning Check: questions 1 – 6
Genetic Drift
pg. 352
Genetic Drift – is the change in frequencies of alleles due to
chance events in a breeding population.
In nature the size of the population is a factor in allele frequency
changes.
The smaller the population is, the greater the opportunity for a
change by chance. The larger the population is, the less likely a
chance event will change the allele frequency.
Most natural populations are large enough that the effects of
genetic drift are small. There are two situations that can lead to
significant genetic drift, The Founder Effect and Bottleneck
Effect.
Genetic Drift: The Founder Effect
Founder effect – is a change in a gene pool that occurs when a few
individuals start a new isolated population.
These individuals will carry some alleles from the original
population gene pool, but not all. Therefore when they repopulate a
new region the gene pool is not the same as the original population.
Genetic Drift: The Bottleneck Effect
Bottleneck effect – are changes in gene distribution that results
from a rapid decrease in population size.
A natural disaster can quickly reduce the size of a large population.
The survivors only represent a small sample of the original gene
pool of the population. Once they repopulate the region, it is less
likely that it would be the same as the original gene pool.
The bottleneck effect is often seen in species driven to the edge of
extinction.
Figure 9.8: In this model, the parent population has equal numbers of green and yellow
individuals and a few red. A chance catastrophe leaves mostly green survivors with a few
yellow and no red. The next generation is mostly green, with a few yellow.
Learning Check: questions 7 – 12
pg. 356
Natural Selection
Natural Selection: is the result of environmental factors selecting
individuals in the population with certain traits that make them
better suited to survive and reproduce.
Population has a wide variety of phenotypes and genotypes, and
some individuals produce more offspring than the environment can
support (carrying capacity). Selective forces, such as; predation
and competition, will lead to selecting individuals with traits best
suited to survive.
If there is a greater chance of an individual with the slightly
favourable allele surviving, reproducing, and passing this allele to
offspring, then the trait will appear more often within the
population and its gene pool, which can lead to an evolutionary
change.
There are three types of natural selection graphs;
Stabilizing Selection
Stabilizing selection – is natural selection that favours intermediate
phenotypes and acts against extreme variants.
Figure 9.9: The population before natural selection (top) and after (bottom). The brown
area shows the general population.
Directional Selection
Directional selection – is natural selection that favours the
phenotypes at one extreme over another, resulting in the
distribution curve of phenotypes shifting in the direction of that
extreme.
Figure 9.9: The population before natural selection (top) and after (bottom). The brown
area shows the general population
Disruptive Selection
Disruptive selection – is natural selection that favours the extremes
of a range of phenotypes rather than intermediate phenotypes; this
type of selection can result in the elimination of intermediate
phenotypes.
Figure 9.9: The population before natural selection (top) and after (bottom). The brown
area shows the general population
Sexual Selection
Sexual selection - is natural selection for mating based, in general,
on competition between males and choices made by females.
Sexual selection also involves the choices made females when
selecting their mates. Males and females of many animal species
often have different physical characteristics, such as; male mallard
ducks colourful plumage compared to the female (sexual
dimorphism), and courtship displays help in selecting a mate.
Figure 9.10: Male mallard ducks (Anas platyrhynchos) are distinguished from females by
their green heads. The coloration of females is brown with white spots.
Review Questions: questions 1 – 11
pg. 359
Activity 9.1: Identify the Type of Selection
pg. 358
1 – 13
pg. 130 – 131
How Do Populations Evolve? (9.1)
1-5
pg. 132 – 134
Evolution by Natural Selection (9.1)
1–2
pg. 135
Study Guide:
Self Assessment
DATE:
CHAPTER 9
NAME:
A Feathery Tale: Greater Sage
Grouse Silhouette
CLASS:
BLM 9-1