Macroevolution

Biological Species Concept

• Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.– Breeding behavior can be real or potential, if

two species come together and breed then there was only one species.

– Breeding behavior in nature can be different than in captivity.

How do new species arise?

• Speciation is the development of a new species through evolution.

• Evolution within a species means a change in that population’s allele frequency.

• When two populations are separated their allele frequency changes. Since they no longer have migration between the populations, two separate species develop.

• There are many ways to separate two populations besides geographically.

Allopatric Speciation

• When geographical barriers divide a population, followed by the development of mechanisms in the separated populations that prevent interbreeding.– *Allopatric means ‘of other countries’– Geographical isolation is the most important

factor in starting speciation.– Physical or behavioral changes develop that

will keep the two species isolated from interbreeding.

The isthmus of Panama came into being 3 million years ago. The species of shrimp on either side are now separate species.

• Allopatric Speciation in shrimp

Geographic isolation leads to allopatric speciation.

The Abert squirrel lives on the south rim, the Kaiba squirrel on the north rim

Reproductive Isolation

• Any factor in nature that prevents interbreeding between individuals of the same or closely related species.– Extrinsic isolating mechanism – outside of the

organisms in question• Geographic isolation is extrinsic

– Intrinsic isolating mechanism – internal characteristics that prevent interbreeding

• Differences in anatomy, physiology and behavior

Six Intrinsic Reproductive Isolating Mechanisms

• Ecological • Temporal • Behavioral • Mechanical • Gametic • Hybrid inviability

Ecological Isolation

• When two species have different habitats they will rarely have contact.– Lions prefer open grassland, tigers prefer

forest

Temporal Isolation

• Two species that share the same habitat but do not mate within the same time frame.– Two populations of the same species of plant

release their pollen at different times of the year. If their reproductive periods did not overlap they would have reduced gene flow.

Behavioral Isolation

• Even if populations are in contact and breeding can occur, they must choose to mate.

• Such a choice is based on specific courtship and mating displays.

These albatrosses are behaviorally isolated from other bird species by their elaborate, species-

specific courtship behavior.

Behavioral isolation

Great-tailed Grackle Mating Display

Damsel fly mating behavior

Marine flatworms mating.

Sting rays mating behavior.

Mechanical Isolation

• Reproductive organs differ in size or shape or other feature.– Different species of

Alpine Butterfly look similar but have different reproductive organs.

Gametic Isolation

• Even if mating occurs, offspring may not result if there are incompatibilities between sperm and egg, or between sperm and the female reproductive tract.

Hybrid Inviability or Infertility

• Even if offspring develop, they are most often malformed or sterile

Real Cross-breed Animals

A zebroid is a cross between a horse and a zebra.

Sympatric Speciation

• Intrinsic isolating mechanisms can develop between two populations in the absence of geographical separation.

• Sympatric* speciation - any speciation that does not involve geographic separation.– Sympatric means ‘of the same country’– Two types include polyploidy and speciation

through hybridization

Prior to colonization•Hawthorn trees + Haw fruit flies

Colonization by Europeans•Apple trees + fruit flies (called Apple flies)

Sympatric Speciation in fruit flies

• Only 6% of flies interbreed• Conduct courting, mating, laying eggs in

their specific type of tree• In transition to being two separate

species• Mutation or combination of rare alleles

allowed flies to smell the apple trees

Apples mature earlier than Hawthorne trees.

Fruit flies winter underground as larvae.

The flies that mate on the Apple tree emerge earlier that the Haw flies. They share their variant alleles among themselves.

Temporal Isolation

Speciation through Hybridization

• Where the egg and sperm come together to produce offspring.

• Rarely a healthy hybrid is produced.

Polyploidy

• 100,000 species of plants exist today because of polyploidy

• A multiplication of the normal number of chromosomes in an organism.

• Most often happens in plants that self pollinate

• Rarely happens in animals because they do not self pollinate.

Gametes are formed in separate species, A and B. Each has two chromosomes.

Gametes fuse and form a new, sterile plant.

The zygote doubles its chromosomes for cell division then fails to divide.

Gamete formation takes place

Gametes from the same plant fuse, forming a new plant with 4 chromosomes.

When does speciation occur?

• Consider the horseshoe crab which has not changed much in 300 million years.

• Consider Darwin’s finches, they developed into 13 separate species in 100,000 years.

Generalists vs Specialists

• The horseshoe crabs are generalists – their diet is diverse

• The finches are specialists – their diet is very specific, especially when food is scarce.

• When the food source changes, the finches must adapt or they will not survive. The horseshoe crab just moves on and eats something else.

Adaptive Radiation

• Rapid emergence of species from a single species that has been introduced into a new environment.

• When the finches arrived on the Galapagos there were no other birds of their kind living in the islands.

• It was easy for them to specialize and fill the available niches. This is especially true on the Galapagos because there are 25 separate islands.– Water between the islands is a barrier = allopatric

speciation

Two conditions that contribute to speciation

• Specialization of food source and environment

• Migration to a new environment, especially if there is no competition.

Is speciation smooth or jerky?

• Gradualism model • Punctuated equilibria model