Macroevolution

Morphological traits: physical characteristics of an organism
Drawbacks for determining species * How many traits to consider * Traits may vary in a continuous way * What degree of dissimilarity to use * Members of the same species can look very different * Members of a different species can look very similar

Biological species concept: 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 other populations
Gene flow between populations holds the phenotype of a population together

Reproductive isolation: prevents one species from successfully interbreeding with other species
Four main problems for determining species * may be difficult to determine in nature * can interbreed and yet do not * does not apply to asexual species * cannot be applied to extinct species

Compare features to identify similarities and differences among different populations * DNA sequences within genes * gene order along chromosomes * chromosomes structure * chromosome number
May be difficult to draw the line when separating groups

Variety of factors related to an organism’s habitat can be used to distinguish one species from another
Many bacterial species have been categorized as distinct species based on ecological factors * Drawback – different groups of bacteria sometimes display very similar growth characteristics, and even the same species may show great variation in the growth conditions it will tolerate

Way to define the concept of a species and/or provide an approach to distinguish one species from another
Biological species concept: species is a group of individuals whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but cannot successfully interbreed with members of other species * Limitations: * The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) * The biological species concept emphasizes absence of gene flow * However, gene flow can occur between distinct species
Evolutionary lineage concept: species should be defined based on the separate evolution of lineages
Ecological species concept: each species occupies an ecological niche– the unique set of habitat resources that a species requires, as well as its influence on the environment and other species

Cladogenetic speciation: occurs when a population is different enough from its ancestral species so that no genetic exchange can occur between them * The appearance of a reproductive isolating mechanism produces 2 or more species where there was one
Two modes * Allopatric speciation * Sympatric speciation

Sympatric speciation: occurs when members of a species that are within the same range diverge into two or more different species even though there are no physical barriers to interbreeding
Mechanisms include * Polyploidy * Adaptation to local environments * Sexual selection
Adaptation to local environments * Geographic area may have variation so that some members of a population may diverge and occupy different local environments that are continuous with each other
Sexual selection * Certain females prefer males with one color pattern, while other females prefer males with a different color pattern
Disruptive selection * Animals diverge mostly due to reproductive isolation. * Reproductive isolation is a result of genetic factors that cause offspring to rely upon resources not used by previous generations.

Hybrid zones reveal factors that cause reproductive isolation
Hybrid zone: a region in which members of different species mate and produce hybrids
Hybrids: the result of mating between species with incomplete reproductive barriers

When hybrids are less fit than parent species, reinforcement of reproductive barriers may occur through strong selection for prezygotic barriers * Over time, the rate of hybridization decreases
Fusion: if hybrids are as fit as parents, there can be substantial gene flow between species * If gene flow is great enough, reproductive barriers weaken and the parent species can fuse into a single species
Stability: extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

Adaptive radiation: produced from a combination of allopatric and sympatric speciation; the rapid evolution of diversely adapted species from a common ancestor * Separation from ancestral species allopatrically * Presence of a variety of open ecological niches provides opportunity for sympatric speciation by disruptive selection
Adaptive radiations may follow * Mass extinction * The evolution of novel characteristics * The colonization of new regions

* Adaptive radiations can occur when organisms colonize new environments with little competition