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Ernst Mayr

first enunciated a classical species definition known as the biological species concept (1942)

biological species concept

defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with other species

reproductive isolation

separation of species or populations so that they cannot interbreed and produce fertile offspring

prezygotic barriers

impede mating between species or hinder the fertilization of ova if members of different species attempt to mate:
1. Habitat isolation
2. Behavioral isolation
3. temporal isolation
4. mechanical isolation
5. gametic isolation

habitat isolation

prezygotic barrier between species; the species remain isolated because they breed in different habitats.

behavioral isolation

prezygotic barrier in which two populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding

temporal isolation

prezygotic barrier in which two populations reproduce at different times

mechanical isolation

prezygotic barrier in which female and male sex organs are not compatible

gametic isolation

prezyogtic barrier in which sperm of one species may not be able to fertilize the eggs of another species

postzygotic barriers

prevent the hybrid zygot from developing into a viable, fertile, adult:
1. Reduced hybrid viability
2. reduced hybrid fertility
3. hybrid breakdown

reduced hybrid viability

Genes of the different parent species may interact and impair the hybrid's development

reduced hybrid fertility

even if hybrids are vigorous they may be sterile; meiosis in hybrids may fail to produce normal gametes

hybrid breakdown

The first-generation hybrids are viable and fertile, but when they mate the offspring are feeble and sterile

ecological species concept

defines a species in terms of its ecological niche, the set of environmental resources a species uses.

pluralistic species concept

the factors that are most important for the cohesion of individuals as a species vary.

morphological species concept

characterizes each species as a set of organisms with a unique genetic history

allopatric speciation

takes place in populations with geographically separate ranges, likelihood increases when population is small and isolated,

sympatric speciation

takes place in geographically overlapping populations , requires the emergence of some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population

adaptive radiation

evolution of many diversely adapted species from a common ancestor


A chromosomal alteration in which the organism possesses more than two complete chromosome sets.


An individual that has more than two chromosome sets, all derived from a single species.


A fertile individual that has more than two chromosome sets as a result of two different species interbreeding and combining their chromosomes.

punctuated equilibrium

a theory of evolution holding that evolutionary change in the fossil record came in fits and starts rather than in a steady process of slow change


evolution on a large scale extending over geologic era and resulting in the formation of new taxonomic groups

most evolutionary novelties are modified versions of

older structures


A structure that has evolved in one environmental context and later becomes adapted for a different function in a different environmental context.

allometric growth

The variation in the relative rates of growth of various parts of the body, which helps shape the organism.


Evolutionary change in the timing or rate of an organism's development.


The retention in an adult organism of the juvenile features of its evolutionary ancestors.

homeotic genes

Any of the genes that control the overall body plan of animals by controlling the developmental fate of groups of cells

species selection

A theory maintaining that species living the longest and generating the greatest number of species determine the direction of major evolutionary trends.

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