Download Notes 17_3_4 Speciation_Mol Evolution

17.3 The Process of Speciation
17.4 Molecular Evolution
MRS. MACWILLIAMS
ACADEMIC BIOLOGY
I. Isolating Mechanisms
*Species- a population or group of populations whose
members can interbreed and produce fertile offspring.
*Speciation- formation of a new species
*Reproductive isolation- occurs when a population splits into
two groups and the two populations no longer interbreed.
*When populations become reproductively isolated, they can
evolve into two separate species.
3 TYPES OF ISOLATION MECHANISMS
1. Behavioral isolation- when two populations that are capable
of interbreeding develop differences in courtship rituals or
other behaviors.
Easter and Western meadowlarks are similar birds whose habitats overlap.
The members of the two species will not mate with each other, partly
because they use different songs to attract mates.
II. Speciation in Darwins
A. Current Hypothesis about Galapagos finch speciation
1. FOUNDER AFFECT: A few members of “species M” from
South America arrived on one of the Galápagos islands.
The allele frequencies of this founding finch population
could have differed from those in the South American
population
2. Geographic Isolation: The islands environment was
different from the South American environment. A
combination of founder affect, geographic isolation, and
natural selection enabled “Species M” to evolve into a new
species – “Species A”. Later, a few “Species A” birds crossed
to another island setting up a new population. Birds rarely
cross over water from island to island, thus finch populations
on the two islands were geographically isolated and no
longer share a common gene pool.
2. Geographic isolation- when two populations are separated
by geographic barriers such as rivers, mountains, or bodies of
water.
For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that
formed when a small population became isolated on the north rim of the
Grand Canyon. Separate gene pools formed, and genetic changes in one
group were not passed on to the other.
3. Temporal isolation- when two or more species reproduce at
different times
For example, three species of orchid live in the same rain forest. Each species
has flowers that last only one day and must be pollinated on that day to
produce seeds. Because the species bloom on different days, they cannot
pollinate each other.
3. Changes in Gene Pools- Over time, populations on each
island adapted to local environments.
*Natural selection could have caused two distinct
populations to evolve (A and B), each characterized by a new
phenotype
4. Behavioral Isolation- A few birds from the second island
crass back to the first island. Will Population A birds breed
with Population B birds? Probably not. Finches prefer to
mate with birds that have the same size beak as they do.
Because the population on the two islands have evolved
differently sized beaks, they would probably not mate. This
behavioral isolation then leads to reproductive isolation.
5. Competition and Continued Evolution- Birds that are
most different from each other have the highest fitness.
More specialized birds have less competition for food. Over
time, species evolve in a way that increases the differences
between them, and new species may evolve (C, D, and E).
III. Gene Duplication
A. Copying Genes
1. Homologous chromosomes exchange DNA during
meiosis(sperm/egg cell production) in a process called
crossing-over
*homologous chromosomes- a pair of the same
chromosome, one from mom and one from dad
2. Sometimes crossing-over involves an unequal swapping
of DNA so that one chromosome in the pair gets extra
DNA. That extra DNA can carry part of a gene, a full gene,
or a longer length of chromosome.
B. Duplicate Genes Evolve
1. Sometimes copies of a gene undergo mutations that
change their function. The original gene is still
around, so the new genes can evolve without
affecting the original gene function or product.
2. A gene is first duplicated, and then one of the two
resulting genes undergoes mutation.
C. Gene Families
1. Multiple copies of a duplicated gene can turn into a
group of related genes = gene family
2. produce similar, yet slightly different, proteins
IV. Molecular Clocks
A. Molecular Clock- uses mutation rates in DNA to
estimate the time that the species have been evolving
independently
1. Researchers use a molecular clock to compare
stretches of DNA to mark the passage of
evolutionary time
2. relies on mutations to mark time
3. Neutral mutations tend to accumulate in the DNA of
different species at about the same rate
B. Neutral Mutations as Ticks
1. Comparison of DNA sequences between species can show
how many mutations occurred independently in each
group.
2. The more differences there are between the DNA
sequences of the two species, the more time has elapsed
since the two species shared a common ancestor.
C. Calibrating the Clock
1. Some genes accumulate mutations faster than others, and
there are many different molecular clocks that “tick” at
different rates. These different clocks allow researchers to
time different evolutionary events.
2.Researchers check the accuracy of molecular clocks by
trying to estimate how often mutations occur. They compare
the number of mutations in a particular gene in species
whose age has been determined by other methods.
V. Developmental Genes and Body Plans
A. Hox genes and evolution
1. Hox genes determine which part of an embryo
develops arms, legs, or wings. Groups of Hox genes
also control the size and shape of those structures.
2. Small changes in Hox gene activity during
embryological development can produce large
changes in adult animals.
B. Change in a Hox Gene
1. Insects and crustaceans are
descended from a common
ancestor that had many pairs of
legs.
2. Crustaceans still have lots of
legs. Insects have only three
pairs of legs.
3. A mutation in a single Hox
gene, called Ubx, “turns off”
the growth of some pairs of
legs.
4. Because of mutations in a
single Hox gene millions of
years ago, modern insects have
fewer legs than modern
crustaceans.
5. A variant of the same Hox gene
directs the development of the
legs of both animals.