Ch 16: the process of Speciation

• I. A species can be defined as a group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.

– What is the relationship between a species and a population?

• A population is the number of a species in a specific area

– Can an organism belong to the same species but different populations? If so, provide an example in nature.

• YES… deer in Northern Illinois, deer in Southern Illinois

– Describe a scenario in which organisms in a single population could give rise to 2 distinct species. (Think about the definition above and the process of natural selection).

• The Illinois river becomes much deeper and wider and the 2 deer populations have no way of breeding…

• Read the following excerpt from Biology: A Guide to the Natural World by David Krogh.– Q: If the two populations can no longer

interbreed, what has occurred?• speciation

– Q: What was the key event in the scenario described above that allowed this to occur?

• migration

• II. For a new species to evolve, populations of a species must become Reproductively isolated from one another.

• 1.) Once this occurs, the members of the populations can’t interbreed – they have separate gene pools and will respond to natural selection as separate units.

• a.) Over time they will become so different that when they are brought back together, they will no longer be able to mate – they will be different species!

2.) So how do populations/species become (or

remain) reproductively isolated? • 1) Behavioral Isolation: occurs when two

populations are capable of interbreeding but have differences in courtship rituals or other behaviors.

2) Geographic Isolation: occurs when two populations are separated by geographic barriers.Ex: Canyon, mountain, body of water.

3) Temporal Isolation:• occurs when two populations reproduce

at different times.• Ex: orchids

• Mechanical isolation: structure of reproductive organs is different enough that mating cannot occur.

• Gametic isolation: gametes are incompatible – even if act of mating occurs, sperm cannot fertilize egg.

• 3.) So, Darwin’s BIG IDEA was that natural selection takes place in populations that are isolated from one another and over LONG periods of time, small changes in allele frequencies from generation to generation can add up to BIG changes in species. This leads us to the idea of a universal common ancestor.

• We can represent this idea with a diagram such as this. This only depicts MAJOR living categories of organisms and doesn’t even include extinct organisms and their relationships!

• 5.) There is observed and experimental evidence of speciation. One example of experimental evidence follows…

Patterns of Evolution

I. Adaptive Radiation: A single species or a small group of species evolving into several different species that live in different ways.

• Example – Darwin’s Finches on the Galapagos Islands: one species from the mainland evolved into at least 12 different species

II. Convergent Evolution: Results in similarities in body structures among animals that DO NOT share a RECENT common ancestor. This occurs due to similar environmental demands.a. Examples: Shark, whale, seal, penguin.

Have fins, flippers, and paddles & they are all streamlined to reduce drag in water. Look similar due to common environmental history

i. Analogous structures: look similar but have different structure

III. Coevolution:

Process in which two species evolve together in response to change in each other

a. Examples: Orchid and moth, insects and plant poison, hummingbird & flower structure.

IV. How Quickly Does Evolution Happen?

a. Gradualism:

Slow and steady. Small changes in populations over time. This was Darwin’s belief.

b. Punctuated Equilibrium:

long stable periods followed by brief periods of rapid change

• **Note in geologic time “short” and “rapid” can mean hundreds of thousands to millions of years! The fossil record shows both of these theories as possibilities.

V. The Role of Developmental Genes called hox genes:

Small changes in developmental genes during the embryonic stage can have a huge effect on an organism. This adds to the variation involved in natural selection

• http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html

• Hox gene video (~5 min)