mechanisms and speciation...gene flow is the movement of alleles between populations caused by...
TRANSCRIPT
Mechanisms and
Speciation
Populations evolve, not
individuals.
Natural selection acts on
distributions of traits. – A normal distribution
graphs as a bell-shaped curve.
– highest frequency near mean value
– frequencies decrease toward each extreme value
– Traits not undergoing natural selection have a normal distribution (no selective pressure).
Using your Data
Look at your Lima Bean Lab. Draw on
your graph a “bet fit bell curve.”
What was a limitation of the lab that
maybe decreased the chances of
seeing a bell curve?
Natural selection can change
the distribution of a trait in one
of three ways.
• Microevolution is evolution within a
population of one species.
– observable change in the allele
frequencies
– can result from natural selection
– Only affects one population of species
1-Directional selection favors phenotypes at
one extreme.
Example: Antibiotic drug resistance in
bacteria
Human Example: Lactose Tolerance • Before cattle domestication: tolerance mutation appeared
in only 1/100,000 individuals. Now, lactose tolerance (production of the lactase enzyme in adults) occurs in as much as 96% of the population in intense dairy cultures like Sweden, Denmark, parts of Australia, and Britain. Over 85% of European descent Americans are lactose tolerant too!
Think and Share
What is something in society in which
only one extreme is favorable?
–
2-Stabilizing selection favors the
intermediate phenotype . Example: mid-
size insect galls in plants caused by predator
selective pressure on both ends.
Galls are abnormal growths in plant
resulting from the work of immature
insects. In a way, they are basically "plant
tumors."
• https://braingenie.ck12.org/skills/104145/learn
Think and Share
What is something in society in which
both extremes are not favorable,
but the mean is favorable?
– 3-Disruptive selection favors
both extreme phenotypes.
Example: male bunting bird
plumage/wing coloration. Only
dominant brilliant colored
males and “dull” males are
fittest.
Think/Share
What is something in society in which
both extremes are favorable, but
the mean is not favorable?
Selection type?
Selection type?
http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html
Concept Map
Natural selection is not the
only mechanism through
which populations evolve.
Gene flow is the movement of
alleles between populations caused
by migration of organisms. •Gene flow occurs when individuals join new populations and reproduce.
•Plant seeds/fungi spores are transported to new areas.
•Gene flow keeps neighboring populations similar.
•Low gene flow increases the chance that two populations will evolve into different species.
bald eagle migration
Two types of gene flow
migration: • Immigration (in-coming)– new
individuals come into a population,
with them they bring new genes.
• Emigration (exit) – individuals leave a
population, take genes with them and
can reduce the alleles in the
population.
Example:
Some individuals from a
population of brown
beetles might have joined
a population of green
beetles. That would make
the genes for brown
beetles more frequent in
the green beetle
population.
Genetic drift is a change in allele
frequencies due to chance. • Genetic drift causes a loss of genetic
diversity.
• It is most dramatic in small populations, but happens to all populations over time.
• Due to chance, some alleles may increase or decrease in frequency.
Example:
Imagine that in one generation, two brown beetles
happened to have four offspring survive to
reproduce. Several green beetles were killed
when someone stepped on them and had no
offspring. The next generation would have a few
more brown beetles than the previous generation
— but just by chance. These chance changes from
generation to generation are known as genetic
drift.
Example:
Remember Lactose Intolerance? Sometimes
natural selection and genetic drift combine to
increase the frequency of traits in a population.
Many scientists think that some African
populations became very small at the same time
that the lactose tolerance gene appeared.
• Genetic drift can cause big losses of
genetic variation for small
populations.
• Two types of genetic drift:
– Bottleneck
– Founder Effect
Bottleneck Effect Population bottlenecks occur when a
population’s size is reduced for at
least one generation.
• Reduces genetic variation.
Bottleneck Example • Cheetahs: Cheetahs
are sufficiently closely related to one another that transplanted skin grafts do not provoke immune responses, thus suggesting an extreme population bottleneck in the past.
• The founder effect is genetic drift that
occurs after start of new population. It
occurs when a few individuals start a new
population (like colonists).
• 10%-80% of seeds available at night are taken by bats.
• Bats and other mammals are effective dispersers of
cactus seeds, depositing them under the tree canopies
(Von Rheede Von Oudtshoorn 1999)
Genetic drift has negative
effects on a population.
Why would more
variation be better for a
population?
Genetic drift has negative
effects on a population.
– Less genetic variation for a population
is bad.
– Less likely to have some individuals
that survive so the population can
adapt
– Harmful alleles can become more
common due to chance
New species can arise when
populations are isolated
First, define a species:
• A group of individuals that can
interbreed to produce fertile
offspring.
The isolation of populations
can lead to speciation. • Speciation is the rise of two or more species from
one existing species.
• Populations become isolated when there is no gene
flow.
• However, isolation, by itself doesn’t necessarily
mean that speciation will occur.
There are two main causes of speciation:
A. Geographic Isolation and
B. Reproductive isolation
Geographic isolation
A. Physical barriers divide population
Reproductive isolation
B. Members of different populations cannot
mate successfully. This increases the
chance of speciation-creation of a new
species!
Reproductive Isolation
• Isolated populations adapt to their own
environments in specialized ways (niches).
• Genetic differences can add up over generations.
Reproductive isolation
There are three types of
reproductive isolation:
1-Temporal isolation: Timing of
reproductive periods prevents
mating (examples: nocturnal vs
diurnal or seasonal differences)
Reproductive Isolation
2. Behavioral isolation - includes
differences in courtship or mating
behaviors that prevents reproduction
between the two populations
3. Hybrid infertility Even if fertilization occurs
successfully, the offspring may not
survive, or if it survives may not
reproduce.
https://www.youtube.com/watch?v=JE8Z
5Es-M0E
Species can become extinct.
• Extinction is the elimination of a species from Earth.
• Background extinctions occur continuously at a very low rate.
– occur at roughly the same rate as speciation
– usually affects a few species in a small area
– caused by local changes in environment
Mass extinctions are rare but much more intense.
– destroy many species at global level
– thought to be caused by catastrophic events
– at least five mass extinctions in last 600 million years (1) Ordovician-Silurian mass extinction-all sea life 2) Devonian-sea life extinction 3) Permian extinction 96% species extinct 4) Triassic and 5)Cretacious
dino
Tropical Island
• Climate: This island has a warm average temperature of 80 degrees. Temperature only fluctuates a maximum of 20 degrees +/- on a daily and annual basis. The days are marked with intermittent heavy rain storms, which contribute to this islands lush foliage.
• Food: The most abundant foods are fish and fruits. The fish are all found in the nearby ocean, and the fruit are found at the tops of very tall trees.
• Predators: There are jungle cats in this area. Also if you eat the wrong type of frog or fruit they could be poisonous.
Desert Island • Climate: This island is a dry, dusty island marked with extremely
hot days up to 120 degrees and extremely cold nights to near freezing. There is little rain fall and there are extreme wind storms that create severe dust clouds.
• Food: There are several different species of snakes and rabbits that can be found on the island, however they are difficult to find during periods of extreme heat. Fresh water is found only sporadically, so it must be conserved. The only vegetation available is cactus.
• Predators: There are coyotes in this area as well as cougars.
Arctic Island
• Climate: This island is covered in snow for 10 months out of the year. Winters months are very cold with temperatures rarely going above 10 degrees and only 2-4 hours of daylight. Summer months are marked by almost 20 hours of daylight and temperatures in the 50’s.
• Food: There is a small population of arctic foxes and penguin-like birds. However, it is difficult to hunt them during the winter months. Ice-fishing allows for most of the food supply. Vegetation can only be found during the summer months.Predators: Wolves
Coniferous Island
• Climate: This is a very mountainous terrain, covered in pine trees. The area is covered in snow for 1/3 of the year. There is a rainy season for 3-4 months and a summer season with highs in the 70’s.
• Food: There are many birds that live in the trees, fish in the fresh water river and lakes, and a population of large bear that compete for fish.
• Predators: Large cats, bears and wolves.
Grassland
• Climate: The average temperature is warm (80 degrees), with summer days reaching 110 and winter days reaching 60 degrees. There is moderate rain fall to provide water for many small plants and scattered trees.
• Food: This area has a population of antelope that feed on the grass and a population of mountain lion that feed on the antelope and sometimes people. There are many insects that live in the grasses.
• Predators: Lions, hyenas, and killer bees.
Water World
• Climate: The climate is very stable, with warm comfortable temperatures all the time in the air, but there are no land masses available for habitation. The water a comfortable temperature, but there are occasionally storms in which there are turbulent waves and currents.
• Food: Many varieties of fish if you can swim fast enough to catch them. Crabs and lobster are on the floor of the water, and there is edible seaweed that floats in “forests” throughout the large body of water.
• Predators: Sharks