Lesson 6: Speciation

Biological Species Concept

A population whose members have the potential to interbreed in nature and produce viable, fertile offspring

Speciation

process of isolating one group of genes (reproductive isolation) from another, allowing each set of genes to DIVERGE genetically under the pressure of different selective forces in different environments

• Slow process, needs one or both of the populations to diverge

• 2 dif species

2 types of speciation

Allopatric and sympatric

Allopatric

• population becomes divided GEOGRAPHICALLY which then interrupts gene flow

• Geographic barrier’s existence causes gene pools to diverge through mutations, natural selection and genetic drift

• Each subpopulation may be exposed to different selective pressures, leading to divergent evolution and potential formation of new species.

Sympatric

• NO PHYSICAL BARRIER, more difficult to accept

• occurs when populations evolve reproductive isolation while living in the same geographic area, often through mechanisms such as polyploidy, behavioural changes, or habitat differentiation and sexual selection

eg. diurnal vs nocturnal moths

Prezygotic Barrier ( pre - before zygote formation)

Factors that PREVENT mating and fusion of egg and sperm

• Habitat Isolation

-genetic factor causes species within population to exploit a habitat or different resources not used by the parent population (within the same environment) two gene pool do not mix.

eg. one group of snakes live in water, another group lives on the land

• Behavioral Selection (sexual selection)

-species have unique mating behaviors that prevent individuals of DIFFERENT SPECIES from interbreeding

• Temporal Isolation

-species breed at different times of the day, season, or years, preventing mating altogether.

eg. two species of plants flowering at different times.

• Mechanical Isolation

-morphological difference → internal organs prevent mating

• Gametic Isolation

-Sperm of one species cannot fertilize the egg of another species (chemical imbalance) although they make contact

Postzygotic Barriers (after zygote formation → fertilization)

Prevention of a hybrid zygote from developing into a viable, fertile adult

• Hybrid Inviability (3)

-sperm and egg unite but the resulting hybrid fails to develop properly or dies before reaching reproductive maturity, dies in womb (sperm and egg don’t recognize eachother chemically)

• Hybrid Sterility (2)

-Hybrid lives, but organism is unable to produce gametes of its own and is therefore sterile → not able to produce children/offspring (eg. mule)

• Hybrid Breakdown (1)

-some first generation hybrids are viable and fertile, but their offspring are inviable or sterile in subsequent generations. Hence geno 2 breaks down

Patterns of evolution

• divergent evolution

-occur when adaptation to new habitats results in phenotypic diversification

eg. Adaptive radiation → occurs quick - numerous species emerge from a common ancestor, when empty ecological niches need to be filled often after extinction or when colonizing isolated areas

• convergent evolution

-occurs when different species independently evolve similar traits as a result of adapting to similar environments or ecological niches.

-NOT SAME SPECIES

• parallel evolution

-two related species that have independently made similar evolutionary adaptations after their divergence from a common ancestor and driven by similiar environmental selective pressures

Coevolution

-two unrelated species evolve together by developing a reciprocal set of evolutionary traits

eg. pollinators and flowering plants, hosts and pathogens

Rates of Evolution

• Gradualism (accepted)

-species descended from a common ancestor, evolving at a slow and steady pace through gradual changes over long periods of time. Diverge more and more in morphology as they acquire unique adaptations

• Punctuated equilibrium (difficult to accept)

-Periods of stability and stasis (no change), then punctuated by a SUDDEN CHANGE. Changing ecological conditions is the catalyst (factor) for that.