Download Mechanisms of Evolution

Mechanisms of
Evolution
Chapter 17
How Does Evolution Work?
 Individual organisms cannot
evolve. Each individual’s traits
are determined by its genes.
 Natural selection acts on the
range of phenotypes in a
population.
 Evolution occurs as the
frequency of genes in a
population changes.
Evolution-What Happens?
See page 404, Figure 5.
Snapdragon Flowers
Evolution occurs when
there is a change in the
gene pool-specifically a
change in allele frequency.
Definitions
 Gene pool: All alleles of the
population’s genes.
 Allelic frequency: % of a specific
allele in the gene pool.
 Genetic Equilibrium: This exists
when the frequency of alleles remains
the same over generations. The
population is not evolving.
When Evolution Occurs
 Evolution results when there are Forces that
change allelic frequencies and disrupts
genetic equilibrium.
 Forces that cause Evolution:
1. Gene flow: Transport of genes by migrating
individuals.
2. Nonrandom Mating: Mating based on
preferences example: a female may choose
a mate based on male size, color, or ability
to gather food
Forces of Evolution Continued
3. Mutation: Change in DNA
4. Genetic Drift: chance event
changes allelic frequencies –
Greatly affect small populations
such as the animals of the
Galapagos Islands or Amish.
Causes of Genetic Drift

Random mating over a long time
period
 No immigration of males
 No emigration of females
Forces of Evolution Continued
5. The most significant factor causing
changes in the gene pool (evolution) is
natural selection.
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Overproduction of offspring
Some variations increase or decrease
chances of survival. These variations can
be inherited.
So, frequency of alleles changes over
generations due to natural selection.
Adaptation: over time, those traits that
improve survival and reproduction become
more common.
3 Types of Natural Selection
 Stabilizing selection – favors average
individuals
 Directional selection – favors one of
the extreme variations of a trait
 Disruptive selection – favors
individuals with either extreme of a trait
(eliminates intermediate phenotypes)
 Stabilizing selection favors average
individuals
 Directional selection favors one of the
extreme variations
 Disruptive selection or Diversifying
selection favors both extreme variations of a
trait
What is a Species?
 A population or
group of
populations whose
members have the
ability to breed with
one another and
produce fertile
offspring
Evolution of Species
(Speciation)
 Significant changes in the gene
pool can lead to evolution of a new
species over time.
 Speciation occurs when members
of similar populations no longer
interbreed to produce fertile
offspring within their natural
environment.
Why Don’t the Populations
Interbreed?
 Geographic isolation – physical barrier
divides a population.
 Reproductive isolation – formerly
interbreeding organisms can no
longer mate to produce offspring..
 Polyploidy – results in immediate
reproductive isolation. Very common
in plants.
Speciation can occur quickly
or slowly
 Gradualism – idea that species originate
through a gradual accumulation of
adaptations.
 Punctuated equilibrium – hypothesis that
speciation occurs relatively quickly, in
rapid bursts, with long periods of genetic
equilibrium in between.
Patterns of Evolution
 Adaptive Radiation – ancestral species
evolves into an array of species to fit diverse
habitats. This is a type of divergent evolution
where species diverge or become less and less
alike as they adapt to different environments.
 Convergent Evolution – Unrelated species
occupy similar environments in different parts of
the world. Similar pressures of natural selection
lead to similar adaptations.