Download Mechanisms of Evolution

Last week in science:
Mechanisms of Evolution
Populations, not individuals, evolve
• If an organism has a harmful trait, it may be
unlikely to survive and reproduce
▫ The organism is programmed by its genes and can
NOT change the genes nor the trait
• Natural selection acts on a
range of phenotypes in a
population (and not a single
individual)
Gene Pool
• Is the combination of all
the genes in one
population put
together
• The percentage of any
one gene in the gene
pool is called the allelic
frequency
Genetic Equilibrium
• Is when the allelic frequencies are the same
over many generations and are NOT EVOLVING
• Requires the following conditions:
▫
▫
▫
▫
▫
Very large population (No genetic drift)
No immigration or emigration
Random mating
No mutations
No natural selection
Genetic Drift
• RANDOM change in allele frequencies due to
small populations
Video flounder effect & bottlenecking
3 types of Natural Selection
• Stabilizing Selection: Favors average individuals
in the population
• Directional Selection: Occurs when one of the
extreme variations of a trait is favored
• Disruptive Selection: Occurs when either
extreme of a trait is favored
Team Quiz
Determining the mechanism for evolution:
1. Random or nonrandom?
▫
▫
Random = Genetic Drift
Non-Random = Natural Selection
2. If Genetic Drift:
2. If Natural Selection:
•
Founder effect
•
Stabilizing
•
Bottleneck effect
•
Directional
•
Disruptive
What is the mechanism?
• In the 1890s, elephant seals were hunted to a
population fewer than 100. The population has
since rebounded to 127,000, but all are
genetically very similar.
▫ Bottleneck Effect
What is the mechanism for evolution?
• Giraffes with the longest necks have the best
opportunity to reach food, which increases their
survival and reproduction.
▫ Directional Selection
What is the mechanism?
• An island has only very large nuts and very small
seeds. Birds with either small or large beaks are
well adapted, while birds with medium sized
beaks struggle to survive.
▫ Disruptive Selection
What is the mechanism?
•Longer legs allow for faster speeds, so over time, horse
species have evolved to be larger in size
•Directional Selection
What is the mechanism?
• Amish populations in the U.S. began with a very
small number of individuals, and the Amish only
marry within their communities. The Amish
happened to have a higher rate of polydactyly
than the rest of the U.S. population
▫ Founder Effect
What is the mechanism?
• Babies that are too small at birth are unhealthy,
and babies that are too large are at risk during
delivery, so average sized babies are favored
▫ Stabilizing Selection
Fun Science Photo for Feb. 27
Unlike some other insects that
use their hives or nests for
storing food, Honeypot Ants
store their food in their
abdomen. The worker ants
gather food and feed it to the
honeypot ants to store. The
honeypot ants can grow so big
that they can’t move! When there
is a food shortage, the honeypot
ants regurgitate the food to help
feed their colony.
What is the mechanism?
Directional Selection
simulation
What is the mechanism?
Stabilizing Selection
What is the mechanism?
Disruptive Selection
How do we get new species?
• video
Evolution leads to new species
• Species = group of organisms that
can interbreed and reproduce
• Speciation, the evolution of a
new species, occurs when similar
organisms in a population can no
longer interbreed to produce
fertile offspring
Speciation
Speciation
• Is enhanced by geographic isolation
▫ Physical barriers prevent populations from
interbreeding and differences in environment lead
to differences in favored traits for natural
selection
Speciation
• Is enhanced by reproductive isolation
▫ Interbreeding doesn’t occur due to too many
differences to produce fertile offspring
▫ Behavioral isolation leads to different mating
seasons or rituals
Rate of Speciation
• Gradualism: Idea that
evolution takes place in small
gradual steps
• Punctuated Equilibrium:
organisms remain the same
for long periods then undergo
bursts of change in short
periods of time