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Population Genetics/Natural Selection

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Devin Weiss

on 23 March 2015

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Transcript of Population Genetics/Natural Selection

Environmental factors, such as radiation or chemicals,
Other mutations occur by chance.
Many are lethal
Sometimes a mutation results in a useful variation, and the new gene becomes part of the population’s gene pool by the process of natural selection.
Changes in genetic equilibrium
Some variations increase or decrease an organism’s chance of survival in an environment.
Natural selection acts on variations
There are three different types of natural selection that act on variation:
Any factor that affects the genes in the gene pool can change allelic frequencies, disrupting a population’s genetic equilibrium, which results in the process of evolution.
Evolution occurs as a population’s genes and their frequencies change over time.
Each member has the genes that characterize the traits of the species, and these genes exist as pairs of alleles.
Natural selection acts on the range of phenotypes in a population.
Some evidence from the fossil record supports gradualism.
Gradualism is the idea that species originate through a gradual change of adaptations.
Scientists once argued that evolution occurs at a slow, steady rate, with small, adaptive changes gradually accumulating over time in populations.
Speciation rates
Reproductive isolation occurs when
interbreeding organisms can
no longer mate
produce fertile offspring
As populations become increasingly distinct, reproductive isolation can arise.
Reproductive isolation can result in speciation
A new species can evolve when a population has been geographically isolated. Example: Galapagos Finches
Geographic isolation occurs whenever a physical barrier divides a population.
In nature, physical barriers can break large populations into smaller ones.
Physical barriers can prevent interbreeding
Explain the role of natural selection in convergent and divergent evolution.
Summarize the effects of the different types of natural selections on gene pools.
Relate changes in genetic equilibrium to mechanisms of speciation.
Section Objectives

Convergent evolution
occurs when unrelated species occupy similar environments in different parts of the world.
A pattern of evolution in which distantly related organisms evolve similar traits is called convergent evolution.
Different species can look alike
The Evolution of Species
The evolution of new species, a process called speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.
Recall that a species is defined as
a group of organisms that look alike and can interbreed to produce fertile offspring in nature.
Populations, not individuals, evolve
However, within its lifetime, it cannot evolve a new phenotype by natural selection in response to its environment.
If an organism has a feature—called a phenotype in genetic terms—that is
poorly adapted
to its environment, the organism may be
unable to survive and reproduce
Genes determine most of an individual’s features, such as tooth shape or flower color.
The percentage of any specific allele in the gene pool is called the
allelic frequency.
Picture all of the alleles of the population’s genes as being together in a large pool called a
gene pool.
How can a population’s genes change over time?
Populations, not individuals, evolve
Populations, not individuals, evolve
Populations, not individuals, evolve
Natural Selection:
The selection by nature to allows some things to thrive and other not.
Sexual Selection
: More desirable traits will have more opportunity to mate
: Spontaneous change in a species gene (most are harmful) Evolution depends upon good mutations.
Genetic Drift:
Random chance
first 1:18
Gene Flow:
Changes due to a new set of alleles entering the pool
Start 8:12 to 10:15
Medium is best- not one extreme or the other
Peppered moth from one extreme to another
against those in the middle
Give an example of each from the video
Start 1:30 to 2:50
Lamarck's theory of evolution: Lamarck believed that organisms tried to make themselves better during their lifetimes and that the improvements or changes they made would be passed on to their offspring. This concept is believed to be false, but the study and discovery of Epigenetics is proving Lamarck correct- on a genetic scale.
When an ancestral species evolves into an array of species to fit a number of diverse habitats, the result is called
adaptive radiation
Adaptive radiation is a type of
divergent evolution
, the pattern of evolution in which
species that were once similar to an ancestral species diverge, or become increasingly distinct.
These birds originated from a common ancestor. Explain what this process is called and how it happened.
Speciation through
Geographic barriers
Adaptive radiation
divergent evo.
Population Genetics and Natural Selection
Divergent Evolution
adaptive radiation
Convergent Evolution
Homologous Structures
Recall that there are four parts of natural selection to take place.
A)Above is a series of pictures representing changes in a population of cacti. Pictures 1 and 2 show what happened when a deer came to eat, picture 3 shows the cacti a few weeks later (notice the flowers on the right-hand cactus), and picture 4 shows the situation a few months later.

1) Genetic Variation within the population: In picture 1, what is the main difference between the cactus on the left and the cactus on the right?
2) Struggle to survive: Why would a deer be more likely to eat the cactus on the left than the cactus on the right in picture 1?
3) Differential Successful Reproduction (fitness): What effect does the deer's behavior have on the survival and reproduction of these two types of cactus?
4) Do you think that evolution by natural selection is occurring in this cactus population? (Assume there is actually a large population of cacti, some look like each type of cacti shown above)Explain why or why not.
9.Describe what is happening in figures 1-3 to the right. Is the population of mice different in figure 3 than in figure 1? Explain why.
10.Living things that are well adapted to their environment survive and reproduce. Those that are not well adapted don’t survive and reproduce. An adaptation is any characteristic that increases fitness, which is defined as the ability to survive and reproduce. What characteristic of the mice is an adaptation that increased their fitness in #3?
What factor would most likely devastate a population?
A) The population has more genetic diversity.
B) The population has a few weak individuals, but is a large population.
C) The population is small in size, and only have a few strong individuals.
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