Opener Chapter 22: Origin of Species

Concept of Species Sympatric species: These co-occur and differ phenotypically in their behavior and utilize different parts of habitat. Subspecies: geographically distant populations of same species that are distinct from one another Intermediate populations may be present between geographically separated populations

Fig. 22.1

The Biological Species Concept Biological species are defined as groups of populations that either have the potential to or can interbreed with each other to produce fertile offspring Populations which do not mate with each other or who cannot produce fertile offspring are reproductively isolated (members of different species). Reproductively isolated mechanisms prevent genetic exchange between species Two types – Prezygotic isolating mechanism and Postzygotic isolating mechanism

Prezygotic isolating mechanism Prevents mating and formation of zygotes between two species Ecological isolation-Species occur in same area but utilize different portions of environment. For example: Tigers and Lions in India Geographic isolation- Species occur in different area which are often separated by physical barrier Behavioral isolation-Species differ in mating rituals. Use different signals via sensory modes for communication, sound production, pheromones etc Temporal isolation- Species reproduce in different seasons or different times of the day. Example: Five species of genus Rana Mechanical isolation- Structural differences prevent mating. Example: Structure of male and female copulatory organs may be incompatible Prevention of gamete fusion-Gametes of one species function poorly with gametes of another species or within the reproductive tract of another species.

Fig. 22.3

Postzygotic isolating mechanism Prevent proper functioning of zygotes after they form or prevent development of zygote into reproducing adults Hybrid in viability or infertility – Hybrid adults do not survive in nature or are sterile or have reduced fertility Genetic complements are different that they cannot function in embryonic development Example: Hybridization between sheep and goats produces embryos that die in early developmental stages.

Fig Postzygotic isolation in leopard frogs – mating is rare, fertilized eggs with many problems (in laboratory conditions)

Fig Reinforcement encourages prezygotic isolating mechanisms till they are completely effective

Fig Genetic drift and natural selection can increase reproductive isolation Adaptation to different environments can lead to reproductive isolation through the accumulation of differences Ex: Anolis lizards with their dewlaps How genetic drift and natural selection can influence reproductive isolation?

Fig Allopatric speciation – populations are geographically isolated Colonization of one or few individuals in distant places, barriers to movement can split population, extinction of intermediate populations can leave population isolated. Evolve into separate species because there is no gene flow How geography influences speciation?

Sympatric speciation – Speciation occurs without geographic separation Produced due to polyploidy and disruptive selection Polyploidy – Autopolyploidy and allopolyploidy Autopolyploidy – errors in meiosis creates a new species resulting in tetraploidy Allopolyploidy – result of hybridization of two species Disruptive selection – create population containing individuals exhibiting different phenotypes

Fig

Species Clusters: Evidence of Rapid Evolution Adaptive radiation Species evolving from a common ancestor inhabits a new environment with many resources and few competing species Species evolving from a common ancestor inhabits a new place or old place where abrupt extinction of many species Increased accessibility to new environments (key innovation)

Fig Adaptation occurs as species respond to different environments on different islands Adaptation occurs as sympatric species get driven by need to minimize competition for available resources

Fig Character displacement involves evolution of divergent adaptations that minimizes competition between two species for available resources

Fig

Fig a Gradualism are the slow and steady accumulation of changes over long periods of time The relationship between speciation and evolutionary changes are on a continuum between gradualism and punctuated equilibrium

Fig b Punctuated equilibrium are rapid and discontinuous separated by long periods of stasis

Fig Mass extinctions have happened five times due to asteroids hitting earth and global climate change Human activities are creating a sixth mass extinction Adaptive radiation occurs in remaining species Species diversity rebounds after mass extinctions

The future of evolution Human influences on environment can lead to evolutionary adaptation and formation of new species Decreased population size due to habitat fragmentation and pollution will increase genetic drift and small populations may evolve or become extinct Human gene pool is becoming homogenized as people from different ethnic origins reproduce

Fig

Tab. 22.1

Tab contd.

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