Chapter 5 - Origin of Species

the process by which one species splits into two or more species

changes in allele frequency in a population over time

refers to broad patterns of evolutionary change above the species level

• a group of similar living things that ranks below the genus in scientific classification and is made up of individuals able to produce offspring with one another • Used to be based just on physical similarity (morphology). • Now use morphology, physiology, DNA, ecology.

states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with members of other such groups i.e., they are reproductively isolated from other species.

Pre-zygotic Barriers Post-zygotic Barriers

Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic Isolation

• Reduced Hybrid Viability • Reduced Hybrid Fertility • Hybrid breakdown

populations live in different habitats and do not meet

different breeding times, seasons

courtship or mating behaviors

sex organs are mechanically incompatible

gametes (egg and sperm) are incompatible

Genes of the different parent species may interact and impair the hybrid’s development or survival in its environment

a common feature of reproductive barriers separating recently diverged species

Some first-generation hybrids are fertile, but when they mate with each other or with either parent species, offspring of the next generation are feeble or sterile

• Cannot be applied to fossils or asexual organisms • Hybridization sometimes observed in nature

defines a species by structural features - It applies to both sexual and asexual species but relies on subjective criteria

defines a species in terms of its ecological niche

Speciation will occur if environmental conditions are different, and populations are separated for long periods and developed their own unique characteristics.

• Allopatric speciation • Sympatric speciation

geologic barrier divides a population, leaving each group to follow its own evolutionary path.

more common in plants, no geographic barrier. Isolation is due to accidents during cell division resulting in polyploid offspring.

Geographical barrier arises Separated populations may evolve independently through mutation, natural selection, and genetic drift - Causes genetic divergence Reproductive isolation may arise as a by-product of genetic divergence - E.g., geographically isolated populations of mosquitofish have become reproductively isolated as a result of selection under different levels of predation