17.3 The Process of Speciation

17.4 Molecular Evolution

MRS. MACWILLIAMSACADEMIC 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 MECHANISMS1. 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 DarwinsA. 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 DuplicationA. 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 ClocksA. 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 PlansA. 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.