Ch.18 Flashcards
Ernst Mayr
In 1927, nearly 100 years after Darwin boarded the Beagle, a young German naturalist named Ernst Mayr embarked on his own journey to the highlands of New Guinea. He was searching for rare “birds of paradise.” On his trek through the remote Arfak Mountains, Mayr identified 137 bird species (including many birds of paradise) based on differences in their size, plumage, colour, and other external characteristics. Mayr found that the native Papuans, who used these birds for food and feathers, had their own names for 136 of the 137 species he had identified. The close match between the two lists supported Mayr’s belief that the species is a fundamental level of organization in nature.In 1942, Mayr published the book Systematics and the Origin of Species, in which he described the role of geography in the evolution of new species; the book quickly became a cornerstone of evolutionary biology.
What mechanisms produce distinct species?
microevolutionary processes alter the pattern and extent of genetic and phenotypic variation within populations. When these processes differ between populations, the populations will diverge genetically, and they may eventually become so different that we recognize them as distinct species. Although Darwin’s famous book was titled On the Origin of Species, he did not dwell on the question of how new species arise, although he clearly saw similar species as having shared inherited characteristics and a common ancestry.
speciation
(the process of species formation) as a series of events that occur through time. However, using a range of approaches, they study the products of speciation, species that are alive today. Because it is difficult to witness the process of speciation from start to finish, scientists make inferences about it by studying organisms in various stages of species formation.
four major topics:
how biologists define and recognize species, how species maintain their genetic identity, how the geographical distributions of organisms influence speciation, and how different genetic mechanisms produce new species.
morphological species concept
•All individuals of a species that share measurable traits (that is qualitative) that distinguish them from individuals of other species with different measurable traits.
•This concept dates to Linnaeus’ classification system based on visible anatomical characteristics (=Morphological characters)
The concept that all individuals of a species share measurable traits that distinguish them from individuals of other species.
Has many practical applications. For example, researchers typically use morphological criteria to identify the species of fossilized organisms. And because we can observe the external traits of organisms in nature, field guides to plants and animals list diagnostic (i.e., distinguishing) physical characters that allow us to recognize them.
Nevertheless, relying exclusively on morphology to identify species can present problems. Some individuals of a single species look very different in size and colouration, for example. Conversely, morphology does not help us distinguish some closely related species that are nearly identical in appearance.
Finally, morphological species definitions tell us little about the evolutionary processes that produce new species.
biological species concept
•Consists of a group of actually or potentially interbreeding individuals that are reproductively isolated from such groups. (= species)
•Reproductive isolation is key.
•A group of actually interbreeding individuals that are reproductively isolated from other such groups = population.
Problems with the biological species concept:
1. Only applies to sexually reproducing species.
2. Suggests the two different species would never make and reproduce offspring.
3. Groups of organisms adapted to a particular set of resources and with the same ecological role ( = niche) in the environment
4. Groups of organisms that share a common ancestry history
The definition of species based on the ability of populations to interbreed and produce fertile offspring.
Ernst Mayr defined biological species as “groups of … interbreeding natural populations that are reproductively isolated from [do not produce fertile offspring with] other such groups.”
The first half of Mayr’s definition notes the genetic cohesiveness of species: populations of the same species experience gene flow, which mixes their genetic material. Thus, we can think of a species as one large gene pool, which may be subdivided into local populations.
The second part of the biological species concept emphasizes the genetic distinctness of each species. Because populations of different species are reproductively isolated, they cannot exchange genetic information. In fact, the process of speciation is frequently defined as the evolution of reproductive isolation between populations.
The biological species concept also explains why individuals of a species generally look alike: members of the same gene pool share genetic traits that determine their appearance.
However, the biological species concept does not apply to the many forms of life that reproduce asexually, including most bacteria and archaeans; some protists, fungi, and plants; and a few animals. In these species, individuals do not breed, so it is pointless to ask whether members of different populations do. Similarly, we cannot use the biological species concept to study extinct organisms because we have little or no data on their specific reproductive habits. These species must all be defined using morphological or biochemical criteria. Yet, despite its limitations, the biological species concept currently provides the best evolutionary definition of a sexually reproducing species.
phylogenetic species concept.
A cluster of populations that emerged from the same short branch on a phylogenetic tree.
•Developed in response to the limitations of the biological species concept.
•Considers populations that share a recent evolutionarily history: based on the reconstruction of the revolutionary relationships among organisms using more logical and molecular data.
One advantage of the phylogenetic species concept is that biologists can apply it to any group of organisms, including species that have long been extinct, as well as living organisms that reproduce asexually.
Proponents of this approach also argue that the morphological and genetic distinctions between organisms on different branches of the Tree of Life reflect the absence of gene flow between them, one of the key requirements of the biological species definition.
Nevertheless, because detailed evolutionary histories have been described for relatively few groups of organisms, biologists are not yet able to apply the phylogenetic species concept to all forms of life.
Subspecies
When geographically separated populations of a species exhibit dramatic, easily recognized phenotypic variation, biologists may identify them as different subspecies, which are local variants of a species. Individuals from different subspecies usually interbreed where their geographical distributions meet, and their offspring often exhibit intermediate phenotypes.
B
Ring Species
Some plant and animal species have a ring-shaped geographical distribution that surrounds uninhabitable terrain. Adjacent populations of these ring species can exchange genetic material directly, but gene flow between distant populations occurs only through the intermediary populations.
Clinal species
When a species is distributed over a large, environmentally diverse area, some traits may exhibit a cline, a smooth pattern of variation across a geographical gradient.Clinal variation usually results from gene flow between adjacent populations that are each adapting to slightly different conditions. However, if populations at opposite ends of a cline are separated by great distances, they may exchange very little genetic material through reproduction. Thus, when a cline extends over a large geographical gradient, distant populations may be genetically and morphologically distinct. Despite the geographical variation that many species exhibit, even closely related species are genetically and morphologically different from each other.
reproductive isolating mechanisms
A biological characteristic that prevents the gene pools of two species from mixing.Thus, by reducing the chance of interspecific (between-species) mating and the production of hybrid offspring (i.e., offspring with parents of different species), these isolating mechanisms prevent the gene pools of distinct species from mixing.
prezygotic isolating mechanisms
A reproductive isolating mechanism that acts prior to the production of a zygote, or fertilized egg
postzygotic isolating mechanisms
A reproductive isolating mechanism that acts after zygote formation.
Table Relative to Fertilization Mechanism
Mode of Action Prezygotic (“premating”) mechanisms
Ecological isolation Species live in different habitats.
Temporal isolation Species breed at different times.
Behavioural isolation Species cannot communicate.
Mechanical isolation Species cannot physically mate.
Gametic isolation Species have nonmatching receptors on gametes.
Postzygotic (“postmating”) mechanisms Hybrid inviability Hybrid offspring do not complete development.
Hybrid sterility Hybrid offspring cannot produce gametes.
Hybrid breakdown Hybrid offspring have reduced survival or fertility.
ecological isolation
A prezygotic reproductive isolating mechanism in which species that live in the same geographic region occupy different habitats.
temporal isolation
A prezygotic reproductive isolating mechanism in which species live in the same habitat but breed at different times of day or different times of year.
Behavioural isolation
A prezygotic reproductive isolating mechanism in which the signals used by one species are not recognized by another.These behaviours (collectively called courtship displays) are often so complicated that signals sent by one species are like a foreign language that another species simply does not understand.
mechanical isolation
A prezygotic reproductive isolating mechanism caused by differences in the structure of reproductive organs or other body parts.
