Chapter 24
Species – a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
Speciation is a lineage-splitting event that produces two or more separate species
Has produced a vast amount of diversity
Produces new species differ from existing ones
Explains why organisms share features
Ability to Interbreed
Problems:
Some organisms are asexual
Applies to living organisms only
Some species interbreed in captivity but not in nature
Reproductive isolation is not absolute
Reproductive Isolation
The existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring
Block gene flow
Limit formation of hybrids
Prezygotic Barriers
Habitat isolation: Two species occupy different habitats
Example: Aquatic and terrestrial garter snakes
Temporal isolation: Species that breed at different times of the day, different seasons, or different years
Example: Spotted skunk (summer vs. winter breeding cycles)
Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Example: Dance of the Blue-footed boobies
Mechanical isolation: Morphological differences can prevent successful mating
Example: Snail shells spiral in different directions and prevent proper alignment of genetalia
Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species
Example:
Sea urchin release sperm and eggs into the surrounding water
Gametes from red and purple urchin have different surface proteins and cannot fuse
Postzygotic Barriers
Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
Example:
Salamandar species of the genus Ensatina can produce hybrids
Hybrids either do not complete development or are frail adults
Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
Example:
Male Donkey, Female Horse → Mule
Different chromosome number in parents prevents proper meiosis (gamete production) in offspring
Hybrid breakdown: First-generation hybrids may be fertile, but hybrid offspring of the next generation are feeble or sterile
Example:
Cultivated rice strains
Strains carry unique recessive alleles
Hybrid breeding allows recessive alleles to accumulate, leading to small, sterile plants
Allopatric Speciation
Reflects geographic barriers
Depends on the ability of an organism to move about (Bird vs. Squirrel vs. Pollen)
Gene flow between isolated populations stops
Forces of microevolution act independently in each group
Example:
Mosquitofish
Series of ponds that have become isolated (Bahamas)
Environment is similar
Predators vary
High-predation ponds –body shape that enables rapid bursts of speed
Low-predation ponds –body shape that enables fish to swim for long periods of time
Sympatric Speciation
Occurs in a shared habitat
Less common than allopatric speication
Gene flow is reduced by factors such as polyploidy and habitat differentiation
Polyploidy
Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division
Polyploidy is much more common in plants than in animals
An autopolyploid is an individual with more than two chromosome sets, derived from one species
An allopolyploid is a species with multiple sets of chromosomes derived from different species
Habitat Differentiation
Sympatric speciation can also result from the appearance of new ecological niches
For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees
Trees experience different fruit maturation periods
Flies on Hawthorn and Apple trees are becoming genetically diverged
Hybrid Zones
Region in which members of different species mate and produce hybrids
Hybrids are the result of mating between species with incomplete reproductive barriers
Hybrids often have reduced fitness compared with the parents
A hybrid zone can occur in a single band where adjacent species meet
Example: Bombina Toads in Europe
Possible Outcomes of Hybrid Zones
Reinforcement – Strengthening of reproductive barriers; hybrids gradually cease to be formed
Fusion – Weakening of reproductive barriers; the two species fuse
Stability – Continued production of hybrid individuals
Tempo of Speciation
Patterns in speciation can be studied using the fossil record, morphological data, or molecular data
Fossil record supports two patterns of speciation
Punctuated equilibrium
Brief bursts of rapid evolution interrupting long periods of little change
Gradualism
Evolution proceeds in small, incremental changes over many generations
The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years
Important implications for how life recovers from mass extinctions
Indicates organisms do not have a “speciation clock” ticking inside them
Random events drive speciation
Phylogeny is the evolutionary history of a species or group of related species
For example, a phylogeny shows that legless lizards and snakes evolved from different lineages of legged lizards
The discipline of systematics classifies organisms and determines their evolutionary relationships
Phylogenies show Evolutionary Relationships
Taxonomy is the scientific discipline concerned with classifying and naming organisms
Binomial Nomenclature
In the 18th century, Carolus Linnaeus published a system of taxonomy based on resemblances
Two key features of his system remain useful today: two-part names for species and hierarchical classification
The two-part scientific name of a species is called a binomial
The first part of the name is the genus
The second part, called the specific epithet, is unique for each species within the genus
The first letter of the genus is capitalized, and the entire species name is italicized
Both parts together name the species (not the specific epithet alone)