Speciation and Reproductive Isolation Flashcards
Biological Species Concept (BSC)
species are groups of interbreeding natural populations that are reproductively isolated from other such groups
- groups of populations that can exchange genes (actually or potentially)
- produce fertile offspring
- reproductively isolated from other such groups
- as long as two forms interbreed, they cannot diverge
- if two forms DO NOT interbreed, they each can develop different adaptations
Pre-Zygotic Isolation
union of egg and sperm never occurs because gametes never encounter one another
Post-Zygotic Isolation
egg and sperm encounter one another but the zygote is either not viable or gives rise to individuals that cannot reproduce or have very low fitness
Zygote
cells formed by the union of male (a sperm) and a female (an ovum) sex cells
Pre-Zygotic Mechanisms for Reproductive Isolation
- ) Geographic Isolation (allopatric speciation)
- ) Gametic Isolation
- ) Mechanical Isolation
- ) Temporal
- ) Behavioral
- ) Ecological Preferences and Mating Isolation
Geographic Isolation (allopatric speciation)
existences of a geographic barrier
- complete geographic isolation
Gametic Isolation
occurs when incompatibility in gametes (egg and sperm) do NOT permit fertilization
- i.e. differences in number of chromosomes or chemical signals
Mechanical Isolation
simply put, male “parts” don’t fit female “parts”
- i.e. dragonflies, beetles, bush babies, flowering plants and specificity in their pollinators
Temporal Isolation
inter-species differences in timing of reproduction
- could be considered sympatriotic because there’s no geographic isolation
- i.e. cicadas
Behavioral Isolation
can occur when mating displays and courtship rituals become intertwined with reproductive act
- mating rituals, displays, songs, etc.
Ecological Preferences and Mating Isolation
occurs when environmental preferences reduces the frequency of interbreeding
- i.e. apple maggots
Post-Zygotic Mechanisms for Reproductive Isolation
- ) Zygote Morality
- ) Hybrid Inviability
- ) Hybrid Sterility
- ) Low Hybrid Fitness (survivorship)
Zygote Morality
occurs when a successfully fertilized egg begins process of development but the zygote is not viable and fails to complete development
Hybrid Inviability
occurs when gametes fuse, development progresses, but offspring cannot survive to adulthood
- i.e. occasional cross between Rana Pipiens and Rana Sylvatica offspring lasts only a few days
Hybrid Sterility
occurs when gametes fuse, development progresses and offspring survive to adulthood but they themselves cannot reproduce
- i.e. the Zorse
Low Hybrid Fitness (survivorship)
occurs when gametes fuse, development progresses and offspring survive to adulthood and can produce offspring but have very low fitness in the wild
- i.e. dog-wolf hybrids
3 Geographic Models of Speciation
- ) Allopatry
- ) Parapatry
- ) Sympatry
Allopatry
occurs when populations become completely, physically separated from one another by geographic barriers
- i.e. isolation on islands (Darwin’s finches)
Parapatry
separated not by geographical areas but an extreme change in habitat
i.e. grass adapting to rivers with heavy metals whereas the grass in clean rivers don’t need to
Sympatry
occurs when there is no spatial separation but reproductive isolation occurs
Ring Species (Parapatry)
two populations that do not interbreed are living in the same region but they are connected by a geographic ring of population that do interbreed
How do geographically isolated populations diverge?
- ) Natural selection adjusts population to local environments
- ) Genetic Drift (mutations)
Allopatric Model for Speciation (how geographically isolated populations diverge)
- ) a single contiguous population in a homogeneous environment
- ) differentiation of environment and migration to a new environment increases phenotypic variation
- ) Founder Effect plus adaptation to local environments results in genetic divergence in geographically isolated peripheral populations
- ) some of these isolated subspecies differentiate with respect to genetic and chromosomal changes that control reproductive isolation mechanisms
- ) changes with the environment permit geographically isolated populations to overlap with parent species
Genetic Drifts (how geographically isolated populations diverge)
all left to chance; unpredictable fluctuations in allele frequencies
- more effective in small populations (sampling errors)
Bottleneck Effect
the large population decreases in size and their survivors lack diversity
- i.e. Florida panthers
Founder Effect
when a few individuals of group separate from a large population and establish a new one
- genetic diversity within the main population is conserved
- i.e. the Amish population
