U3L3 Flashcards
Sympatric Speciation
occurs when a new species evolves from a single ancestral species while inhabiting the same geographic area.
Allopatric Speciation
Occurs when a population is geographically divided into two or more isolated groups, typically due to a physical barrier like a mountain, river, or an event that isolates part of the population - leads to separate species
Speciation
origin of new species, is at the focal
point of evolutionary theory
Microevolution
consists of changes in allele
frequency in a population over time
Macroevolution
refers to broad patterns of
evolutionary change above the species level
biological species concept
states a species is a group 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 a species together genetically
Reproductive isolation
existence of biological factors (barriers) that impede two species from producing viable, fertile offspring
Hybrids
offspring of crosses between different species
Prezygotic barriers
block fertilization from occurring by:
– Impeding different species from attempting to mate
– Preventing the successful completion of mating
– Hindering fertilization if mating is successful
Postzygotic barriers
prevent the hybrid zygote from developing into a viable, fertile adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
Limitations of the Biological Species
Concept
cannot be applied to fossils or
asexual organisms (including all prokaryotes). The biological species concept emphasizes absence of gene
flow but gene flow can occur between distinct species (grolar bear)
morphological species concept
defines a species by structural features
– It applies to sexual and asexual species but relies on subjective criteria
Ecological species concept
views a species in terms of its ecological niche (role and position of an organism within its environment)
– It applies to sexual and asexual species and emphasizes the role of disruptive selection
phylogenetic species concept
defines a species as the smallest group of individuals on a phylogenetic tree
– It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species
Speciation can occur in two ways
Allopatric speciation
Sympatric speciation
Regions with more geographical barriers have less or more species than regions with fewer geographical behaviors?
more
Sympatric speciation can occur if gene flow is reduced by factors including:
Polyploidy
Sexual selection
Habitat differentiation
Polyploidy
The presence of extra chromosome sets due to accidents during cell division
autopolyploid
an individual with more than two chromosome sets, derived from a single species
allopolyploid
a species with multiple sets of chromosomes derived from different species
A 4n (68 chromosomes) plant is crossed with a 2n plant (34
chromosomes). What would be the chromosome number in
the resultant offspring?
68/2 = 34 - undergoes meiosis
34/2 = 17 - undergoes meiosis
34+17 = 51
Sexual selection can drive
sympatric speciation
In allopatric speciation, geographic isolation restricts
gene flow between populations
Sympatric speciation can result from
polyploidy, natural selection, or
sexual selection
hybrid zone
region in which members of different species mate and produce hybrids. Hybrids result from mating between species with incomplete reproductive barriers
Hybrids often have….. compared to parent species
reduced fitness
Reinforcement: Strengthening
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
* Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species
Fusion: Weakening Reproductive
Barriers
hybrids are as fit as parents, there can be substantial gene flow between species
* If gene flow is great enough, the parent species can fuse into
a single species
Stability: Continued Formation of
Hybrid Individuals
Extensive gene flow from outside hybrid zone can overwhelm selection for increased reproductive isolation
inside hybrid zone
Hybrid zones can lead to
- Reinforcement
- Fusion
- Stability
punctuated equilibria
periods of apparent stasis punctuated by sudden change
2 models of speciation
Punctual model: Rapid evolution leading to final result
Gradual model: Evolution through small steps
Types of prezygotic barriers
Habitat isolation: 2 species that occupy different habitats within the same area may encounter each other rarely
Temporal isolation: Species that breed during different times, seasons or years
Behavioural isolation: Courtship rituals that attract mates - mate recognition (ex: blue footed bird has a specific script to follow)
Mechanical isolation: Mating is attempted but morphological differences prevent its successful completion - biological differences that does not complete mating
Types of post zygotic barriers
Gametic isolation: Sperm of one species may not be able to fertilize the eggs of another species
Reduced hybrid viability: Genes of different parent species may interact in ways that impair the hybrid’s development or survival
Reduced Hybrid fertility: If the chromosomes of two parent species differ in number or structure, hybrids may fail to produce normal gametes leading to infertile offsprings
Hybrid break down: Some first generations hybrids are viable and fertile but when they mate with another or a parent species, offsprings tend to be sterile.
Possible outcomes for a hybrid zone
Reinforcement, fusion, stability
