How do species occur? Concept 24.2: Speciation can take place with or without geographic separation Speciation can occur in two ways: – Allopatric speciation – Sympatric speciation Both work through a block of gene flow between two populations.

Example Pupfish populations in Death Valley. Generally happens when a specie’s range shrinks for some reason.

Fig (a) Allopatric speciation (b) Sympatric speciation

Allopatric Speciation Allopatric = other homeland Ancestral population split by a geographical feature. Comment – the size of the geographical feature may be very large or small.

The Process of Allopatric Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift

Fig A. harrisi A. leucurus

Evidence of Allopatric Speciation Regions with many geographic barriers typically have more species than do regions with fewer barriers

Fig Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs Millions of years ago (mya) mya65 mya 56 mya India Madagascar

Reproductive isolation between populations generally increases as the distance between them increases

Fig Geographic distance (km) Degree of reproductive isolation

Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Fig. 24-9a EXPERIMENT Initial population Some flies raised on starch medium Mating experiments after 40 generations Some flies raised on maltose medium

Fig. 24-9b RESULTS Female Starch Maltose population 1 population 2 Male Starch Maltose Male Starch population 1 population Mating frequencies in experimental group Mating frequencies in control group

Conditions Favoring Allopatric Speciation 1. Founder's Effect - with the peripheral isolate. 2. Genetic Drift – gives the isolate population variation as compared to the original population.

Conditions Favoring Allopatric Speciation 3. Selection pressure on the isolate differs from the parent population. (environment is different on the edges)

Result Gene pool of isolate changes from the parent population and new species can form.

Sympatric Speciation Sympatric = same homeland New species arise within the range of parent populations. Can occur in a single generation. In sympatric speciation, speciation takes place in geographically overlapping populations

Plants Polyploids may cause new species because the change in chromosome number creates postzygotic barriers.

Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division An autopolyploid is an individual with more than two chromosome sets, derived from one species

Polyploid Types 1. Autopolyploid - when a species doubles its chromosome number from 2N to 4N. 2. Allopolyploid - formed as a polyploid hybrid between two species. – Ex: wheat

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Autopolyploid

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n2n Gametes produced are diploid.. Autopolyploid

Fig n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n2n Gametes produced are diploid.. 4n4n Offspring with tetraploid karyotypes may be viable and fertile. Autopolyploid

An allopolyploid is a species with multiple sets of chromosomes derived from different species

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes allopolyploid

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes allopolyploid

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3 allopolyploid

Fig Species A 2n = 6 Normal gamete n = 3 Meiotic error Species B 2n = 4 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10 allopolyploid

Allopolyploid

Polyploidy is much more common in plants than in animals Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

Habitat Differentiation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Fig EXPERIMENT Normal light Monochromatic orange light P. pundamilia P. nyererei

Allopatric and Sympatric Speciation: A Review In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented

In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection

Adaptive Radiation Rapid emergence of several species from a common ancestor ( often Allopatric speciation) Common in island and mountain top populations or other “empty” environments.

Mechanism Resources are temporarily infinite. Most offspring survive. Result - little Natural Selection and the gene pool can become very diverse.

When the Environment Saturates Natural Selection resumes. New species form rapidly if isolation mechanisms work. Examples – Galapagos – Finches – Usambaras Mountains – African violets

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