Chapter 24, Species and Speciaton

speciation

the process by which one species splits into two species.

• it has produced the tremendous diversity of life, repeatedly yielding new species that differ from existing ones.

• helps to explain the many features that organisms share (the unity of life): When one species splits into two, the species that result share many characteristics because they are descended from this common ancestor.

• bridginging macro and micro evolution

macroevolution

the broad pattern of evolution above the species level

• An example of macroevo- lutionary change is the origin of new groups of organisms, such as mammals or flowering plants, through a series of speciation events.

the morphology

body form

biological species concept

emphasizes the separateness of different species due to reproductive barriers,

a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring—but do not produce viable, fertile offspring with members of other such groups

thus members in a species are united by being reproductively compatible

• species are designated by the absence of gene flow.

• natural selection can cause such species to remain distinct even though some gene flow occurs between them.

gene flow

the transfer of alleles between populations, gene flow occurs between the different populations of a species.

reproductive isolation

Two species that occupy different habitats within the same area may encounter each other rarely, if at all.

Temporal Isolation, barrier

Species that breed during different times of the day, different seasons, or different years cannot mix their gametes.

prezygotic barrier

Behavioral Isolation, barrier

Courtship rituals that attract mates and other behaviors unique to a species are effective reproductive barriers, even between closely related species. There is little or nos exual attraction between different species

prezygotic barrier

Mechanical Isolation, barrier

Mating is attempted, but morphological differences prevent its successful completion.

prezygotic barrier

hybrids

offspring that result from an interspecific mating.

Gametic Isolation, barrier

male and/or female gametes die beofreuniting or fail to unite

prezygotic barrier

Reduced Hybrid Viability, barrier

The genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment. hybdrids fail to develop or to reach sexual matruity

post zygotic barrier

Reduced Hybrid Fertility, barrier

hybdrids fail to produce functional gametes

post zygotic barrier

Hybrid Breakdown, barrier

Some first-generation hybrids are viable and fertile, but when they mate with one another or with either parent species, offspring of the next generation are feeble or sterile.

post zygotic barrier

Prezygotic barriers

prevent fertilization or mating from occurring done by:

1. by impeding members of different species from attempting to mate,

2. by preventing an attempted mating from being completed successfully,

3. by hin- dering fertilization if mating is completed successfully.

temporal, habitat, behavioral, mechnaical, and gametic

postzygotic barriers

prevent the develoment of fertile adults, reeuced hybrid viability, reduced hybrid fertility, and hydbrid breakdown

may contribute to reproductive isolation after the hybrid zygote is formed. Developmental errors may reduce survival among hybrid embryos. Or problems after birth may cause hybrids to be infertile or decrease their chance of surviving long enough to reproduce.

morphological species concept

distinguishes a species by body shape and other structural features. defines a species in terms of its ecological niche, the sum of how members of the species interact with the nonliving and living parts of their environment

can be applied to asexual and sexual organisms

pros:

can be useful even without information on the extent of gene flow

cons:

it relies on sub- jective criteria; researchers may disagree on which structural features distinguish a species.

allopatric speciation

A population forms a new species while geographically isolated from its parent population.

can also occur without geologic change, such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population.

• Different mutations arise, and natural selection and genetic drift may alter allele frequencies in different ways in the separated populations. Reproductive isolation may then evolve as a by-product of the genetic diver- gence that results from selection or drift.

Sympatric speciation

A subset of a population forms a new species without geographic separation.

requires the emergence of a reproductive barrier that isolates a subset of a population from the remainder of the population in the same area.

polyploidy

a condition where A species may originate from an accident during cell division that results in extra sets of chromosomes

more common in planst and two types of polyploidy is commonly seen:

1. An autopolyploid

2. an allopolyploid

An autopolyploid

an individual that has more than two chromosome sets that are all derived from a single species.

In plants, for example, a failure of cell division could double a cell’s chromosome number from the original number (2n) to a tetraploid number (4n)

• A tetraploid can produce fertile tetraploid offspring by self- pollinating or by mating with other tetraploids. In addition, the tetraploids are reproductively isolated from 2n plants of the original population,

  • autopolyploidy can generate reproductive isolation without any geographic separation.

an allopolyploid

occur when two different species interbreed and produce hybrid offspring.

The allopolyploids are fertile when mating with each other but cannot interbreed with either parent species; thus, they represent a new biological species. The new species has a diploid chromosome number equal to the sum of the diploid chromosome numbers of the two parent species.

• Most hybrids are sterile because their chromosomes are not homologous and cannot pair during meiosis. However, such a hybrid may be able to reproduce asexually.

sexual selection

typically, females select males based on their appearance can act as a reproductive bar- rier that keeps the gene pools of these two species separate.

Habitat Differentiation

Sympatric speciation can also occur when a subpopula-
tion exploits a habitat or resource not used by the parent population.

hybrid zone

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

Hybrid zones typically are located wherever the habitats of the interbreeding species meet.

• Those regions often resemble a group of isolated patches scattered across the landscape

What causes a differing pattern of allele frequencies across a hybrid zone? ( decreease and increase fo population in specific area within the zone)

We can infer that there is an obstacle to gene flow—otherwise, alleles from one parent species would also be common in the gene pool of the other parent species.

Hybrid Zones and Environmental Change

A change in environmental conditions can alter where the habitats of interbreeding species meet. When this happens, an existing hybrid zone can move to a new location, or a novel hybrid zone may form.

a hybrid zone can be a source of novel genetic variation that improves the ability of one or both parent species to cope with changing environmental conditions.

• This can occur when an allele found only in one parent species is transferred first to hybrid individuals and then to the other parent species when hybrids breed with the second parent species.

hybrid zone over time

three common outcomes for a hybrid zone over time: reinforcement of barriers, fusion of species, or stability

Reinforcement: Strengthening Reproductive Barriers, hybrid zones over time

the evolutionary process where natural selection strengthens prezygotic reproductive barriers (like mate choice or fertilization blocks) between two species in a hybrid zone because hybrid offspring have low fitness

barriers to reproduc- tion between species should be stronger for sympatric popula- tions than for allopatric populations.

Fusion: Weakening Reproductive Barriers, hybrid zones over time

Fusion in evolutionary biology occurs when weakening reproductive barriers cause two separate species in a hybrid zone to interbreed so extensively that their gene pools merge back into a single species

Stability: Continued Formation of Hybrid Individuals, hybrid zones over time

two separate species continue to interact and produce hybrid offspring over time without fusing or diverging further

punctuated equilibria

a theory in evolutionary biology stating that species remain stable for long periods, with short bursts of rapid change creating new species

Speciation Rates

the speed at which new species form over time within a lineage, can reveal traits that cause reproductive isolation.

speciation begins only after gene flow between popu- lations is interrupted, perhaps by changing environmental conditions or by unpredictable events

once gene flow is interrupted, the populations must diverge genetically to such an extent that they become reproductively isolated—all before other events cause gene flow to resume, possibly reversing the speciation process

Species

Two groups of organisms that differ from one another in one or more characteristics and do not reproduce extensively or have viable offspring if they occur together in nature are considered to be different 

effect of promiscious for speciation

would increase the rate of speciation, since it provides an opportunity for greater sexual selection which is an enormous driver for diversity

conditions for speciation to occur

• You need genetic variability 

• You need some selective pressures, which favour one or more variants 

• 1st step: you have subspecies formation, still able to interbreed but clearly see they are taking their own way 

• The true step to a new species is separation or isolation of subspecies barriers - migration 

• Then assortative mating: the males and females become picky

• Reproductive isolation: pre- and/ or post-zygotic barriers 

What is the primary difference in origin between autopolyploidy and allopolyploidy?

Autopolyploidy involves chromosome multiplication within a single species. Allopolyploidy involves the combination of chromosome sets from two or more distinct species.

habitat isolation barrier

prezygotic barrier

populations live in different habitats and do not meet