Speciation

Learning objective SWBAT: Describe and identify the various types of reproductive isolation necessary for the formation of new species according to the biological species concept SWBAT: Compare allopatric and sympatric mechanisms of speciation.

Speciation The process by which one species splits into 2 or more species

Different conceptual models to define species Biological species concept  Defines species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring  Limitations to this model:  Does not help to identify fossils as to species  Does not take into account organisms that reproduce asexually

In certain circumstances other models can be used Morphological species concept Ecological species concept Phylogenetic species concept These, however, tend to be much more subjective

Biological Species Concept Reproductive isolation = The existence of biological barriers that impede members of 2 species from interbreeding and producing viable, fertile offspring These barriers do 2 things:  Block gene flow between species  Limit formation of hybrids  Hybrids= offspring that result from mating of 2 different species

2 Types of Barriers Prezygotic= block fertilization from happening in the first place Postzygotic= contributes to reproductive isolation after the zygote is formed Zygote= the fertilized egg

Prezygotic barriers Work in one or more of the following ways: 1. Impede members of different species from attempting to mate. 2. Prevent attempted mating from being successfully completed 3. Hinder fertilization of mating is successful

Figure 22.3 Prezygotic barriersPostzygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown MATING ATTEMPT FERTILI- ZATION VIABLE, FERTILE OFF- SPRING (a)(c)(e)(f)(g)(h)(i)(l) (j) (k) (d) (b)

Postzygotic barriers After fertilization results in : 1. Reduced hybrid viability 2. Infertile hybrids 3. Hybrid breakdown- subsequent generations are weak or sterile

Figure 22.3 Prezygotic barriersPostzygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown MATING ATTEMPT FERTILI- ZATION VIABLE, FERTILE OFF- SPRING (a)(c)(e)(f)(g)(h)(i)(l) (j) (k) (d) (b)

Mechanisms of speciation Speciation can occur with or without geographic separation  Allopatric Speciation  Sympatric Speciation

Allopatric Speciation Allos= other Patra= homeland “different country” = gene flow is interrupted when a population is divided into geographically isolated subpopulations

Once separated, the gene pools may diverge.  Different mutations  Natural selection  Genetic drift Then, over time, reproductive isolation may occur. Important- is is NOT the physical barrier that creates 2 species. It is the biological, reproductive barriers that develop as a result of the changes in allele frequencies.

Sympatric Speciation Syn= together Patra= homeland “same country” Not as frequent as allopatric speciation = gene flow is interrupted NOT by physical barriers, but by factors such as:  Polyploidy  Habitat differentiation  Sexual selection

Polyploidy=an “accident” in cell division creates an organism with extra set(s) of chromosomes.  More common in plants Habitat differentiation= when a subpopulation is better suited to use a habitat or resource not used by the parent population. Sexual selection= though living in the same habitat, mate choice leads to speciation.

Hybrid Zones = a region in which members of different species meet and mate, producing at least some offspring of mixed ancestry

How long? How many? Speciation can occur rapidly (1000’s of years) or slowly (millions of years). Speciation can occur as a result of changes in just a few genes or it may require many gene changes.

Learning objective SWBAT: Describe and identify the various types of reproductive isolation necessary for the formation of new species according to the biological species concept SWBAT: Compare allopatric and sympatric mechanisms of speciation.