Transcript Evolution and Speciation

Evolution and Speciation
Classification System
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primate
Family: Hominidae
Genus: Homo
Species: sapiens
Varieties and Breeds
Defining Species
Evolution creates new species, but …
What is a species?
Its not as straightforward a question as most believe.
These are members of different species - eastern (left) and western (right) meadowlark.
What is a Species?
There is only one extant (existing) human species.
What is a Species?
And these are all members of a single species.
Determining What Is and What Isn’t a Distinct Species
Can Be Difficult
Northern spotted owl (left) and barred owl (right).
What is a Species?
Lets start with the most widely accepted definition:
A species is a group of individuals capable of
interbreeding to produce fertile offspring.
This is the
biological
species concept.
Like all attempts
to define a
species, it has a
problem.
Hybrid Infertility
Horse X Donkey =
Mule
Cannot Reproduce
and produce
Fertile offspring.
So a better definition of a species might be:
A group of individuals capable of
interbreeding to produce offspring that are
fertile and these offspring, in turn, can
produce offspring that are fertile.
Another problem with the Biological
Species Concept
For asexually-reproducing organisms, like these bacteria,
what constitutes a species?
How Many Species Are There?
We don’t know.
About 2 million species have been described.
Estimates of existing species number range
from 4 million to 100 million (with 10-15
million being a more commonly considered
upper estimate).
Two Patterns of Speciation
One
species
completely
replacing
another
One
species
evolving
from
another
with both
then
existing at
same time
How Do New Species Arise?
The key to speciation is reproductive
isolation of populations
.
**It is reproduction that makes more
organisms…so mess with
reproductive success and you mess
with population!
What does it mean to have reproductive isolation?
It means that for whatever reason, one or more
organisms are prevented from being able to
reproduce with others in the population of that
species.
1. Organism itself changes because of random mutation(s)
2. Environment changes, causing separation of population into
two or more subgroups of the original population.
Intrinsic Reproductive Isolating Mechanisms
Something changes about the organisms in the
population that interferes with reproductive success
but is not caused by geographic isolation!
Maybe something about mating ritual
Maybe something about sperm or egg
Maybe something about time of year they mate
Etc.
Extrinsic Reproductive Isolating Mechanisms
Something happens to the ENVIRONMENT of
the organism which interferes with reproductive
success.
Reproductive Isolation May Occur With or Without
Geographic Isolation
Allopatric speciation
occurs when geographic
isolation creates a situation
where reproduction can’t
occur (an extrinsic
mechanism).
Sympatric speciation
occurs when reproduction is
not inhibited by geographic
isolation but something
about the individual
organisms of the population
changes and interferes with
reproductive capability. (an
intrinsic mechanism)
Allopatric Speciation: A change in the environment
Harris’ antelope squirrel
White-tailed antelope squirrel
Two species of ground squirrel are believed to have
descended from a common ancestral population that was
separated by formation of the Grand Canyon.
Intrinsic Reproductive Isolating Mechanisms Are Always
Required for Speciation…Even when allopatric speciation
has already occurred.
Harris’ antelope squirrel
White-tailed antelope squirrel
Intrinsic mechanisms involve changes to the organisms that prevent interbreeding.
In allopatric speciation, intrinsic mechanisms come into play once
populations are physically separated.
In sympatric speciation, intrinsic mechanisms are the only ones involved.
Intrinsic Isolating Mechanisms
(different habits within an overlapping range)
Intrinsic Isolating Mechanisms Continued:
Courtship rituals,
are critical for
mating within a
species, but
ineffective for
attracting members
of other species.
Mating Ritual
Blue-footed Booby
Mating Ritual Fiddler Crab Male Waving
Intrinsic Isolating Mechanisms Continued
Salamander Evolution
Subspecies evolving
At different
Locations
in California…
ALL ARE SAME
SPECIES
Speciation Occurs at Widely Differing Rates
A slow rate of speciation is seen with a living horseshoe
crab which has diversified into 13 species within the last
300 million years.
A rapid rate of speciation is seen in Galapagos finches
which have diversified into 13 species within the last
100,000 years.
What else, other than isolating
Mechanisms, can contribute to the
formation of new species?
Sometimes simply CHANCE EVENTS
can contribute to speciation!
Genetic Drift
any change in the allelic frequency in a population that is due
chance events (floods, insects getting sprayed with insecticides,
earthquakes, fires, etc)
Allelic frequency: a measure of how common an allele is in a population
Especially true in small populations because the
chance of losing an allele completely becomes greater!
Type of Genetic Drift
Founder Effect: when a small population
colonizes a new habitat (separate from the rest
of that population) and interbreeds. Alleles that
were once fairly uncommon in a population now
may be more common because this subset of the
population is so small.
Amish population:
Dwarfism, Polydactyly
Migration may contribute to speciation
as well!
Transports genes to different gene pools if
they mate while they are migrating.
b/c their genes are isolated from their
original population and the alleles
enter a different gene pool.
Speciation Rates
Generalists (eat many
different foods, well-adapted
to many different habitats)
like the horseshoe crab, tend
to remain as stable species.
Specialists (eat more specific foods
and only adapted to live in certain
habitat), like the Galapagos finch, tend
to be unstable as a species and
More likely to change!
Speciation also becomes rapid when, new
niches become available to the organisms,
as seen with Galapagos islands.
Formation of New Species- Gradualism or Punctuated Equilibrium?
Gradualism
Punctuated
Equilibrium
Does Evolution Create the Perfect Organism?
No, only organisms which are
better suited to survive and
reproduce in that
environment at that time!
Scenario Groups:
Species Come and Go
Best estimates from the fossil record indicate
that greater than 99% of species that have
existed are now extinct.
A typical “lifetime”
for a species is
about 1 million
years.
Mass Extinctions Are a Fact of Life
The Cretaceous/Tertiary Mass Extinction
65.5 mya
Asteroid
Impact
Theory
85% of all
species died
Gary Larson
Are We Now Causing a Mass Extinction?
Are humans causing the 6th mass extinction?
Current Mass Extinction
Mass Extinction
Humans and Extinction