CH 13 speciation and extinction

13.1The definition of “species” has evolved over time Macroevolution describes large, complex changes to living things. It takes a long time. Microevolution can be observed in a smaller amount of time. Species are distinct types of organisms. A. Linnaeus devised the binomial naming system- Carolus Linnaeus-1700’s came up with a definition of species, but more importantly came up with a two word naming system (still used today) Each species gets a two word name consisting of the genus and species name. Ex. Homo sapiens He also came up with a hierarchy of classification. It did not take into account evolutionary relationships.

B. Ernst Mayr developed the biological species concept 1940’s- defined a biological species as a population, or group of populations, whose members can interbreed and produce fertile populations. Speciation, the formation of new species. This happens when members of a population can no longer interbreed. Populations can become separated leading to speciation. Once they are so different that they can’t interbreed, then speciation has occurred. 4 problems with biological species concept 1. asexually reproducing species 2. impossible to apply to extinct species 3. some types of species have the potential to reproduce in captivity. 4. For some species, reproductive isolation is not absolute.

Reproductive isolation is the most common criterion used to define a species.

13.2 Reproductive barriers cause species to diverge Biologists divide the many mechanisms of reproductive isolation into two broad groups: prezygotic and postzygotic. A. Prezygotic barriers prevent fertilization Mechanisms of prezygotic reproductive isolation affect the ability of two species to combine gametes and form a zygote. Reproductive barriers include: Ecological or habitat isolation- habitat preference Temporal isolation- active at different times Behavioral isolation- frog calls, mating dances Mechanical isolation- change in shape of gamete delivering/receiving structures Gametic isolation- each species gametes display unique surface molecules that enable an egg to recognize sperm of the correct species.

B. Post zygotic barriers prevent viable or fertile offspring Postzygotic barriers are sometimes called hybrid incompatibility, they include: Hybrid inviability- embryo may die before reaching reproductive maturity(because parents genes are incompatible) Hybrid infertility(sterility)- ex. Mules and ligers. Hybrid breakdown: a hybrid might be fertile, but when hybrids reproduce the offspring may have abnormalities that reduce their fitness. Successful hybridization is rare in animals, but occurs in plants pretty frequently.

13.3 Spatial patterns define three types of speciation A. allopatric speciation reflects a geographic barrier- some geographic formation physically separates a population into two groups that can’t interbreed. The forces of microevolution will act independently on each population. Allopatric speciation has been considered the most common mechanism because the evidence for it is the most obvious and abundant.

B. Parapatric speciation A populations enters a new habitat bordering the range of the parent species. Some gene flow can occur with the original population, but if the two habitats are different enough, disruptive selection can occur. Over time the two populations would become genetically and reproductively isolated.

C. Sympatric speciation occurs in shared habitats In sympatric speciation, populations diverge genetically while living in the same physical area. (can be a controversial idea among evolutionary biologists) Often sympatric speciation reflects the fact that a habitat that appears uniform actually consists of many microenvironments. Ex. Cichlids in African lakes

13.4 Speciation may be gradual or occur in bursts A. Gradualism and punctuated equilibrium are two models of speciation. Darwin envisioned gradualism- evolution proceeds in small, incremental changes over many generations. Punctuated equilibrium coined by Gould and Niles describes relatively brief bursts of rapid evolution interrupting long periods of little change. This fits well with allopatric speciation. The fossil record supports both punctuated equilibrium and gradualism.

B. Bursts of speciation occur during adaptive radiation Adaptive radiation- a population inhabiting a patchy or heterogeneous environment gives rise to multiple specialized forms in a relatively short amount of time. This typically occurs in response to availability of new resources. It is especially common in island groups. Another type of adaptive radiation: some members of a population may inherit a key adaptation that gives them an advantage. A third type of adaptive radiation occurs when some members of a population have a combination of adaptations that enable them to survive a major environmental change.

13.5 Extinction marks the end of the line Extinct- all of the members of the species have died. A. Many factors combine to put a species at risk. Any species will vanish if the gene pool does not have the right alleles for individuals to produce fertile offspring and sustain the population. B. extinction rates have varied over time: Background extinction rate- results from the gradual loss of species as populations shrink in the face of new challenges. Mass extinctions- when a great number of species disappeared over relative short expanses of time.

Two explanations for mass extinctions: 1 Two explanations for mass extinctions: 1. impact theory- meteorites or comets have crashed into the Earth sending dust into the sky to block out sunlight- sets into motion a deadly chain reaction. 2. the movements of the Earth’s crust caused climate changes that were too quick for organisms to adapt to. Today the activities of man have caused many extinctions.

13.6 Biological classification Systems are based on common descent Systematics- study of classification. Taxonomy- science of describing, naming and classifying species Phylogenetics- study of evolutionary relationships among species. A. the taxonomic hierarchy organizes species into groups. Linnaeus’s ideas of classification are still used today. This system organizes living things into nested groups of taxonomic levels, based on similarities. The more features two organisms have in common the more taxonomic levels they share.

B. Cladistics approach is based on shared derived traits Phylogenies depict species relationships based on descent from shared ancestors. Cladistics is a phylogenetic system that defines groups by distinguishing between ancestral and derived characters. Ancestral charatcters are inherited attributes that resemble those of the ancestor of a group Derived characters – an organism has features that are different from those found in the group’s ancestor. Cladistics builds on the idea of homology.

In a cladistics approach, shared derived characteristics are used to define groups. A clade(or monophyletic group) is a group of organisms consisting of a common ancestor and all of its descendants.

C. Cladograms depict hypothesized evolutionary relationships A cladogram is a treelike diagram built using shared derived characteristics. The emphasis is on historical relationships.

D. Many traditional groups are not monophyletic Many traditional groups of species are not monophyletic. The traditional grouping of class Reptilia is paraphyletic. It shows a common ancestor and some but not all of it’s decedents(it leaves out birds). Polyphyletic groups exclude the most recent common ancestor shared by all members of the group.