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Biology 3201 Unit 4
EcologyCh. 21 - Adaptation and Speciation
Speciation
Speciation: the formation of a new species
Biological species: a group of organisms able to
interbreed and produce fertile offspring.
• Ex. Horses and donkeys are separate species.
They are able to interbreed but the offspring
produced are not fertile
How to species form?There are generally two pathways
in which species are formed:
1. Transformation: the formation of
a species because of a series of
accumulated changes over time.
One species changes into
another in this way
2. Divergence: the formation of
species from a parent species
Speciation
Speciation occurs when two groups in a population
become isolated from each other
The separation is caused by one or more
BARRIERS.
Barriers that cause speciation
Geographical barriers: When a population becomes
divided by a geographic boundary such as a canyon,
river, etc. This prevents interbreeding. Over time
natural selection causes genetic differences to
become so large two species form.
• Ex. Giraffes that have become separated by
mountains will eventually develop into separate
species.
Biological barriers: barriers that keep species
reproductively isolated; may be pre-zygotes
barriers or post-zygotic barriers
Pre-zygotic barriersThese are known as pre-fertilization barriers that
impede mating or prevent fertilization
Behavioral isolation: bird songs, courtship rituals,
pheromones, etc. are all species specific and
prevent fertilization
Temporal isolation: these are often timing barriers.
Several species mate at different times during the
year and as such are not able to mate.
Habitat isolation: some species live in the same area
but have different habitats; ex. North American
Garter snakes, one species prefers open areas while
the other prefers water
Pre-zygotic barriersMechanical isolation: some species are anatomically
incompatible thus not allowing them to exchange
sperm and egg
• Ex. The genitals on certain species of insect work
only on a lock-key system. If the lock does not fit
the key, no fertilization can occur.
Gametic isolation: sometimes the gametes from
species do not even meet. This prevents fertilization
• Ex. Sea urchins release their eggs into the water,
but chemicals on the surface of the eggs prevent
sperm from a different species from fertilizing them.
Post-zygotic barriers
These are barriers that prevent a zygote from developing into a
fertile offspring
Hybrid invariability: incompatibility of the two species may cause the
zygote to stop embryonic development
• Ex. Embryos of sheep/goats do not survive
Hybrid sterility: the production of an organism, but it is sterile
• Ex. Horse + donkey = mule (sterile)
Hybrid breakdown: sometimes the first generation of offspring are
viable, but subsequent generations are either sterile or wheat
(cotton plants)
Adaptive Radiation
Adaptive radiation: the diversifying of an ancestral
species into a variety of species.
This usually occurs after a novel characteristic has
evolved or there is a mass extinction
Ex. The finches of the Galapagos islands evolved
from one common ancestor.
Divergent evolution
Divergent evolution:
where species that were
once similar to an
ancestral species diverge
to become different
species
Adaptive radiation is an
example of divergent
evolution.
Convergent evolution
Convergent evolution: evolution where two
completely unrelated species have similar traits
Ex. Birds and bees have wings (similar trait)
Each species develops the same traits because they
adapt to the same type of environmental conditions.
The species DO NOT come from a common
ancestor
Coevolution
This is evolution where two species change together
where each species responds to the changes in the
other
Ex. Milkweed plants and Monarch butterflies. The
milkweed plant has toxins in their leaves. Monarch
butterflies eat the leaves and absorb the toxins
making them toxic. Most birds avoid Monarch
butterflies for this purpose.
The pace of evolution
How fast does evolution occur?
There are two theories/models that explain how fast
evolution occurs. Both examine the fossil record
1. Gradualism
2. Punctuated equilibrium
Gradualism
A model that says change occurs slowly and steadily
before and after a divergence
Proposed by Darwin
Fossils show a slow and repeated change through
the fossil record
Gradualism
Punctuated equilibriumA model proposed by Gould and Eldridge
A model that proposes evolution happens in "spurts"
The model says that species undergo long periods of
"stasis" where they remain unchanged, followed by
short periods of very rapid change (spurts)
The changes are usually brought about by sudden
environmental changes such as volcanoes,
earthquakes, etc.
Species previously disadvantaged could now be
advantaged and new species could develop quickly
Punctuated equilibrium
Gradualism vs
punctuated equilibrium
The origins of lifeMany theories exist that try to explain the origin and development of life on
Earth.
The following theories will be considered:
1. Chemical evolution
2. Panspermia
3. Gaia hypothesis
4. Heterotroph hypothesis
5. Symbiogenesis (Symbiotic theory)
6. Intelligent design
Chemical evolutionA theory of evolution created by Oparin-Haldane
They said that organic molecules (the building blocks of life) could
develop from inorganic compounds present on the surface of the
early earth
• The early earth had an atmosphere that consisted of no oxygen,
but plenty of hydrogen, ammonia, methane, and water vapor
(inorganic molecules)
• These gases condensed and formed a "primordial soup"
• Energy from lightning and UV radiation caused organic molecules
to develop from the inorganic molecules in the soup
• Over time, the organic molecules combined to become an early life
form
Chemical evolution -
Miller-UreyTwo scientists who designed an experiment to prove
Oparin-Haldane's theory:
• They combined methane, ammonia, water vapor,
and hydrogen in a flask and exposed the gases to
an energy source simulating lightning
• The liquid inside the flask changed color and when
examined contained several organic compounds
including amino acids
Panspermia theory
A theory that suggests life began "elsewhere" in the
universe and "migrated" to our planet
For example, it is believed that life originated from
bacterial cells elsewhere and travelled from outer
space to earth on meteorites.
Gaia hypothesisTheory put forth by James Lovelock
Idea that Earth is a "superorganism" called "Gaia"
The Earth has systems that keep a balance between
temperature and atmosphere
After life originated on earth, Gaia came "alive" and
began to regulate earth systems
The systems help provide an environment where life
could exist and survive
Heterotroph hypothesisTheory put forth by Oparin; said first "cells" on earth
had to be heterotrophs that eventually developed
into autotrophs
Primordial soup existed of organic molecules
The environment was oxygen poor
Heterotrophs such as anaerobic bacteria fed on the
organic molecules
The heterotrophs began to release carbon dioxide
into the atmosphere.
Heterotroph hypothesis
The heterotrophs developed into autotrophs and
began using the carbon dioxide
The autotrophs bean to release oxygen into the
atmosphere
This made the atmosphere oxygen rich that could
now support life
Symbiogenesis
Put forth by Lynn Margulis
Theory that attempts to explain the development of
mitochondria and chloroplasts as organelles that
appear in eukaryotic cells
Chloroplasts and mitochondria have their own DNA
and come from the symbiosis (working together) of
prokaryotic cells
Symbiogenesis - how it
happened
1. An anaerobic bacterium ate but did not digest an
aerobic bacterium (called a guest bacterium)
2. The guest provided oxygen to the bacterium. The
guest bacterium eventually became a mitochondrial
3. Other bacteria ate photosynthesizing bacteria. The
photosynthesizing bacteria became chloroplasts
Intelligent design
A theory that suggests life and mechanisms of life
are too complex to have evolved by chance
Believed that the generation and evolution of life
must have been directed by some unidentified
supernatural intelligence
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