Macroevolution: How Do Species Evolve?

Chapter 17

Macroevolution

• The origin, multiplication and extinction of species and higher groups of organisms.

• Speciation – the formation of new species.

• The species seen on the Galapagos islands had all descended from mainland species, but had changed over time and become unique to each island.

What is a species?• Over 1.7 million species identified.

• Most of these species were named and grouped according to a few observable (physical) characteristics

• Morphological species

“Think about it, Ed….The class Insecta contains 26 orders, almost 1,000 families, and over 750,000 described species --- but I can’t shake the feeling we’re all just a bunch of bugs.”

Biological Species Concept• Species are groups of actually or

potentially interbreeding populations, which are reproductively isolated from other such groups.

• No matter how extensive the phenotypic variation, individuals will remain members of the same species as long as their form, physiology and behavior permit them to interbreed and produce fertile offspring.

Problems with the biological species concept.

• What about organisms that reproduce asexually (yeast, bacteria, etc.)?

• What about extinct organisms?

• What about organisms that interbreed sometimes but not others?

• Now can use DNA fingerprinting.

Reproductive Isolation• Geographic or physical isolation

• Evolve separate adaptations

• There are two major barriers to gene flow between species:

1. Prezygotic barriers

1. Before or during fertilization

2. Postzygotic barriers

1. Occur after fertilization

Prezygotic barriers• Temporal isolation – separation by time

– Cicada• One species breeds every 13 years• Another species breeds every 17 years• Overlap every 221 years.

Prezygotic barriers• Ecological isolation – adapted to

different microevironments in the same habitat.

• Behavioral isolation – Mating rituals– Vocalizations

• Mechanical isolation - incompatibility of body parts

Prezygotic barriers• Gametic isolation – gametes are

incompatible at a molecular level– Pollen– Enzymes in sperm

Postzygotic barriers• Zygote either dies or fails to reproduce

successfully.

• Hybrid inviability – embryo dies

• Hybrid sterility – offspring infertile

• Hybrid breakdown – offspring of hybrids weak or sterile.

Patterns of descent

• Divergent evolution – one species of organisms changes into two

• Convergent evolution – independent development of similar features in separate groups of organisms– Wings– Blood antifreeze proteins

• Adaptive radiation – ( a form of divergent evolution) a single species gives rise to several, very differently adapted species.– Finches in the Galapagos– Most dramatic cases occur on islands

• Reproductive isolation• Distinctive selection pressures• Small population size• Smaller number of species leaves more ecological

niches open

– Plants also radiate

• Darwin believed evolution was very gradual

• Others believed in jumps or saltations

• “hopeful monsters” – flounders

• Organisms may not change for long periods (stasis) and then evolve rapidly – punctuated equilibrium

Prebiotic Evolution

Chapter 18

Until about 300 years ago people believed in the idea of spontaneous generation –that life comes from non-living material.

1668 Francesco Redi proved that magots did not come from rotting meat.

Could microorganisms arise spontaneously?

1864 Lois Pasteur finally disproved this idea.

How did life arise on Earth?

1. “panspermia” – life arrived here from outer space– Not a testable hypothesis

2. Prebiotic evolution – life arose from non-living matter

For prebiotic evolution to take place, conditions on the early Earth must

have been very different

1. The atmosphere contained virtually no free oxygen

2. There was no life on Earth

3. Life had 300 million years to get its act together

• The early Earth’s atmosphere probably had hydrogen gas, ammonia, methane, and traces of carbon dioxide and hydrogen sulfide.– No rust– No ozone – UV radiation may have been a

driving force– Heat from the earth and lightning also

provided energy to make chemical bonds

• Organic molecules could have come from comets, asteroids or meteors.

• Either way, oceans may have been full of organic molecules.

• More complex molecules needed more help to form, probably on clay and metal ions at the edges of oceans.

• Can make peptides of up to 50 amino acids in length.

First information carrying molecule

• Was probably RNA since it does not need enzymes or a primer– RNA can act as an enzyme

Cyanobacteria