LIFESCI 7B Lecture Notes - Lecture 6: Species Problem, Neanderthal, Ecological Niche
10 Jun 2018
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21.1 The Biological Species Concept
species A group of individuals that can exchange genetic material through interbreeding to produce fertile
offspring.
Species are reproductively isolated from other species.
● On maps comparing genomes/traits, species form non-overlapping clusters
○ Dot distances within clusters is variation between individuals of a species, which are more
similar than individuals of different species
● Species depends on ability to exchange genetic material to produce fertile offspring
○ Represents CLOSED gene pool
○ Mutations can spread within species, not outside of it
● biological species concept (BSC): concept that “species are groups of actually or potentially
interbreeding populations that are reproductively isolated from other such groups.”
○ The BSC is the most widely used and accepted definition of a species, but cannot be applied to
Bacteria or Archaea.
○ Reproductively isolated: describes how individuals from different species cannot reproduce
● Important that can produce viable, FERTILE OFFSPRING
The BSC is more useful in theory than in practice.
● BSC is difficult to actually test!
● morphospecies concept The idea that members of the same species usually look like each other
more than like other species. (based on PHENOTYPE!)
○ Rule of thumb, not always applicable
■ Membes of same species can look different: and members of different species can look
the same
■ cryptic species. Organisms traditionally considered as belonging to one species because
they look similar, but turn out to belong to two species because of a distinction at the
DNA sequence level.
The BSC does not apply to asexual or extinct organisms.
● Only focuses on sexually producing species
● Only depends on reproduction---doesn’t apply to extinct
Ring species and hybridization complicate the BSC.
● ring species Species that contain populations that are reproductively isolated from each other but can
exchange genetic material through other, linking populations.
○ Indirect passing of genetic material of species
■ Ex: populations 1 and 2 do not interbreed (2 different species), but populations 2 and 3
do, and 3 and 4 do, and 4 and 1 do (ring!)
● hybridization Interbreeding between two different (but simiilar) varieties or species.
○ Esp. in plants; but also homo sapiens and homo neanderthalensis at some point
○ Can result in fertile offsrping, but natural selection often acts against the progeny
Ecology and evolution can extend the BSC.
● Species can be characterized by their ecological niche A complete description of the role a species
plays in its environment.
○ habitat requirements, its nutritional and water needs, etc.
○ impossible for two species to coexist in the same location if their niches are too similar →
competition leads to the extinction of one of them
● ecological species concept (ESC) The concept that there is a one-to-one correspondence between a
species and its niche. (set of organisms exploiting a single niche)
○ Key aspects: Resources exploited+habitat occupied
find more resources at oneclass.com
find more resources at oneclass.com
○ Species based on niche; observing nutrition needs of (asexual) bacteria can tel us whether or
not they’re the same species
● phylogenetic species concept (PSC) The idea that members of a species all share a common
ancestry and a common fate.
○ Unique derived characteristics+shared ancestry
○ Useful in observing asexual species but too unspecific about “common ancestor”
■ At one point, all mammals shared common ancestor (all mammals=single species?);
■ family members share a common ancestors (are they their own species?)
● OVERALL, difficult to define “species” w/ single definition, because species change over time and have
to apply to an astonishing variety of living/dead biological forms
Species Concept Video:
● Morphological differences
○ Species can look the same and be very genentically different, or look very different and be
genetiically similar/identical
○ “Subspecies?”--similar appearance but have genetic differences
● Phylogenentic species cocnept: single common ancestor: time scale=problematic
● Biological Species Concept: based on ability to breed
○ Dinorsaurs: too difficult to mate; difficult to apply in practice
○ Reproductive isolation:
■ Pre-zygotic barriers-prevents fertilization from taking place
● Behavioral isolation (e.g. recognition of males within species),
● temporal isolation (some species productive during one time, one another)
● Merchanical isolation (incompatibility--different genitals)
● Gametic islation: gametes only recognize each other within species (proteins on
surface)
● ecologcal isolation
■ Post-zygotic-failure to develop into fertile individual
● Reduced hybrid viability---hybrid dies before reprpoductive age (e.g. during
development)
● Reduced hybrid fertility (e.g. infertile mules)
● Hybrid breakdown--offsrping of hybrids are infertile (infertile F2)
22.2: Reproductive Isolation
● pre-zygotic factors prevent fertilization from taking place, whereas post-zygotic factors result in the
failure of the fertilized egg to develop into a fertile individual.
Pre-zygotic isolating factors occur before egg fertilization.
● behaviorally isolated Describes individuals that only mate with other individuals on the basis of
specific courtship rituals, songs, and other behaviors. (animals)
● gametic isolation Incompatibility between the gametes of two different species. (animals and plants)
● mechanical incompatibility Structural configuration of the genitalia that prevents mating with another
species (animals).
● temporal isolation Pre-zygotic isolation between individuals that are reproductively active at different
times.
● geographic isolation Spatial segregation of individuals.
● ecological isolation Pre-zygotic isolation between individuals that specialize ecologically in different
ways.
Post-zygotic isolating factors occur after egg fertilization.
○ Hybrid inviability (does not survive)
○ Hybrid sterility (cannot reproduce)
○ Hybrid breakdown w/ reduced fitness of F2
find more resources at oneclass.com
find more resources at oneclass.com
● Typically involve some kind of genetic incompatibility (ex: differnet number of chromsomes) of
parents
● Incompatibility in parental genomes can result in failure for zygote to develop after fertilization
○ OR zygote developts into viable but infertile adults
● The more closely related (geneticlaly similar) a pair of species are, the less extreme the genetic
incompatibility of their genomes
22.3: Speciation
THREE OUTCOMES of renewed/continued contact between two populations
● Reinforcement, Fusion, Stability
○ Speciation in progress. Selection for evolution of strong reproductive barriers.
○
● How does speciation occur? How does reproductive isolation arise among populations?
Speciation is a by-product of the genetic divergence of separated populations.
● Genetic divergence: populations split into two that cannot interbreed, random mutations + genetic
drift/natural selection → populations gradually become distinct & eventual reproductive isolation →
SPECIATION! (gradual process)
○ Speciation—the development of reproductive isolation between populations
● Crossing genentically diverged but NOT YET fully reproductively isolated populations
○ partially reproductively isolated Describes populations that have not yet diverged as separate
species but whose genetic differences are extensive enough that the hybrid offspring they
produce have reduced fertility or viability compared with offspring produced by crosses between
individuals within each population.
Allopatric speciation is speciation that results from the geographical separation of populations.
● Geography is the easiest way to separate a population--ensures physical therefore genetic isolation
○ Allopatric populations = geographically separated from each other.
○ Need TIME! Geographically separated populations that come back and are able to interbreed
(fertile offspring) are still same species; time=new mutations → fixed
● subspecies Allopatric populations that have yet to evolve even partial reproductive isolation but which
have acquired population-specific traits.
○ Can potentially interbreed; sitll within same species (speciation hasn’t yet occured)
Dispersal and vicariance can isolate populations from each other.
● Types of geological isolation: outcome=SAME! (genetic divergence/eventual speciation)
○ dispersal The process in which some individuals colonize a distant place far from the main
source population. (could be intentional or accidental)
■ peripatric speciation A specific kind of allopatric speciation in which a few individuals
from a mainland population disperse to a new location remote from the original
population and evolve separately.
■ Island (isolated) population typically small and in slightly different environment from that
of mainland
■ Difficult to date time of separation
○ vicariance The process in which a geographic barrier arises within a single population,
separating it into two or more isolated populations.
■ Easier to study than dispersal; can date time of separation
● adaptive radiation A period of unusually rapid evolutionary diversification in which natural selection
accelerates the rates of both speciation and adaptation in a single lineage.
○ occurs when there are many ecological opportunities (empty niches) available for exploitation
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
21. 1 the biological species concept species a group of individuals that can exchange genetic material through interbreeding to produce fertile offspring. On maps comparing genomes/traits, species form non-overlapping clusters. Dot distances within clusters is variation between individuals of a species, which are more similar than individuals of different species. Species depends on ability to exchange genetic material to produce fertile offspring. Mutations can spread within species, not outside of it. Biological species concept (bsc): concept that species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups. The bsc is the most widely used and accepted definition of a species, but cannot be applied to. Reproductively isolated: describes how individuals from different species cannot reproduce. The bsc is more useful in theory than in practice. Morphospecies concept the idea that members of the same species usually look like each other more than like other species. (based on phenotype!)
