Biology 2 Chapter 24

Postzygotic barriers

May contribute to reproductive isolation after the hybrid zygote is formed. For example, developmental errors may reduce survival among hybrid em broys. Or problems may cause infertility.

Prezygote factors

Before the zygoe. Block fertilization from occurring. Three types: by impeding members of different species from attempting to mate, by preventing an attempted mating from being completed successfully (mechanical), and by hindering fertilization if mating is completed successfully (fert).

Reproductive Isolation

Biological speices are defined in terms of reproductive compatibility **Def- The existence of biological factors (bariers) that impede members of two species from producing viable, fertile offspring. Such barreirs block gene flow between the species and limit the formation of hybids (offspring that result from an interspecific mating).

Microevolution Macroevolution

Changes over time in the allele frequencies in a population Broad pattern of evolution over long time spans. Ex. Oriign of new groups of organisms, such as mammals or flowering plants, through a series of speciation events. ****Speciation forms a conceptual bridge between the two

Morphological species concept

Characterizes a species by body shape and other structural features. Pro-applied to asexual and sexual organisms; useful even without information on the extent of gene flow.****In practice this is how most scientists distinguish most species. Con- Relies on subjective criteria.

Phylogenic species concept

Defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life. Biologists trace the phylogenetic history of a species by comparing its characteristics, such as morphology or molecular sequences, with those of other organisms. Con- Difficult determining degree of difference required to indicate separate species.

Biological Species Concept

Described by biologist Ernst Mayr (1942) A species is a group of populations who 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 the members of a biological species are united by being reproductively compatible, at least potentially.

Allopolyploid

Infertile hybrid may be able to propagate itself asexually (as many plants do). In subsequent generation various mechanisms can change a sterile hybrid into a fertile polyploid. **Allopolyploids are fertile when mating with each other but cannot interbreed with either parent species; thus, they represent a new biological species.

Reproductive isolation may then arise by-product of selection or drift having cause the populations to diverge genetically.

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List of reproductive barriers

1. Habitat Isolation (Living in different areas) 2. Temporal Isolation (Breed day or night) 3. Behavioral Isolation (Courtship Rituals) 4. Mechanical isolation (Can't mate correctly) 4. Gametic Isolation (Sperm of one species may not be able to fertilize the eggs of another species). Post-Zygo 5. Reduced Hybrid Viability- Survival or development may be impaired. 6. Reduced Hybrid Fertility-Hybrid may be vigorous but cannot reproduce (sterile) 7. Hybrid Breakdown- First Generation hybrids may be viable and fertile but when they mate with another or with either parent species, offspring of the next generation are feeble or sterile.

Support for allopatric speciation

1. Regions that are highly subdivided by geographic barriers typically have more species than do regions with fewer barriers. 2. Laboratory and field tests show isolation between between two populations generally increases as the distance between them increases.

Possible outcomes for hybrids

1. Reinforcement (stenghening of reproductive barriers-hybrids gradually cease to be formed) 2. Fusion (Weakening of reproductive barriers-the two species fuse). 3. Stability (Continued production of hybrid individuals). E. Appalacian mountain ground crickets. Hybrid zone exists where the fitness of teh hybrid is sometimes better than the parents. Other example. Bombina toads where the hybrid is no good. Has mutations and bad fertility/survival.

Hybrid zone

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

Once the process of speciation begins, it can be completed relatively rapidly- a suggestion confirmed by a growing number of studies.

Range of speciation Average of 6.5 million years and rarely taking less than 500,000 years. Basically on average millions of years may pass before a newly formed species will itself give rise to another new species. Can be as quick as 4000 years as in chichlid example. Can be as long as 40 million years (some bettles). *Implication- Organisms do not have a speciation clock ticking inside them, causing them to produce new species at regular time intervals. Speciation begins only after gene flow between two populations is interrupted, perhabs by an unpredicatable event such as a storm that transports a few individuals to an isolated area. Once gene flow has been interrupted, the populations must diverge genetically to such an extent that they become reproductively isolated-all before another event causes gene flow to resme, reversing the speciation process.

Sympatric speciation

Speciation occurs in populations that live in the same geographic area. *Although sympatric speciation is less common than allopatric speciation, sympatric speciation can occur if gene flow is reduced by such factors as polyploidy, habitat differentiation, and sexual selection (These factors can promote allopatric speciation also).

Fusion

Speciation process reverses, eventually causing the two hybridizing species to fuse into a single species. Ex. Lake Victoria cichlids. Cannot see in lake, mating with many other.

Gene flow has the potential to hold the gene pool of a species together, so long as it is not outweighed by effects of selection or drift, T or F?

T. Widely seperated populations can have low level of gene flow. IF the level is too low you have populations diverge.

Why Punctuated Equilibria

The appearance in fossils may have taken 50,000 years but it is fast in relative terms to the 5 million year existence of the species. **Cannot determine exactly when a new biological species forms when the fossil change is gradual, since information about reproductive isolation does not fossilize. This may have taken millions of years.

Punctuated Equilibria

These periods of apparent stasis punctuated by sudden change. Other species do not show a punctuated pattern; instead, they cange more gradually over long periods of time.

Speciation can occur rapidly or slowly and result from changes in few or many genes, T or F.

True

Even if minimal gene flow occurs, natural selection or genetic drift may outweigh the possible genetic connection, T or F?

True.

Modern testing like DNA sequencing confirmt hat morphologically distinct species are indeed discreet groups, with many differences in addition to morphological ones, T or F.

True.

Separation itself is not a biological barrier to reproduction, T or F

True.

Polyploid speciation occasionally occurs in animals, T or F?

True. Ex. tree fog Hyla versicolor. Far more common in plants thought. More than 80% of plants species alive today are descended from ancestors that formed by polyploid speciation. * Many important agricultural crops- such as oats, cotton, potatoes, tobacco, and wheat- are polyploids.

Sympatric speciation can also occur when genetic factors enable a subpopulation to exploit a habitat or resource not used by the parent population. Such is the case with the NA apple maggot fly, T or F

True. Natural selection operating on these alleles provides a postzygotic barrier to reproduction, further limiting gene flow.

Ecological species concept

Views 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. Pro- Sex and Asex; Emphasizes the role of disruptive natural selection as organisms adapt to different environment conditions.

Autopolyploid

An individual that has more than two sets that are all derived from a single species. Ex. Failure in cell division could double a cell's chromosome number from the diploid number to a tetraploid number. **Just in one generation, autopolyploidy can generate reproductive isolation without any geographic separation.

How does speciation occur

Evolution of reproductive isolation. *This concept cannot apply to fossils bc we cannot see who was isolated to the other. In addition it does not apply to those organisms that reproduce asexually all or most of the time, such as prokaryotes. ***In addition, there are some species that are morphological and ecologically different but have gene flow. **This observation has led some to say that the biological species concept overemphasizes gene flow and downplays the role of natural selection.

Sexual selection driving sympatric speciation

Ex. Cichlids. Differ in coloration of breeding males. Mate choice based on male breeding coloration is the main reproductive barrier that normally keeps these gene pools of these two species separate.

Stability

Ex. of Bombina. Perhaps extensive gene flow from outside the zone leads to the continued production of hybrids and overwhelms selection for increased reproductive isolation inside the hybrid zone. Ex. of the Allonemobius crickets. Event a slight change in the local environment can cause one or the other of the parent species to disappear from that location.

Polyploidy

Extra set of chromosomes that comes from an accident during cell division.

Allopatric speciation

Gene flow is interrupted when a population is divided into geographically isolated subpopulations. *Can occur when river comes and divides two squirrel populations. Can occur without geologic remodeling, such as when individuals colonize remote area and their descendants become geographically isolated from the parent population. Depends on species for what type of geographic barrier will promote allopatric speciation.

Why do members of the same species often resemble each other?

Gene flow. They are connected by gene flow.

Reinforcement

If this is occurring we would predict that barriers to reproduction between species should be stronger for sympatric species than for alloaptric species. Why? Natural selection will strengthen prezygotic barriers to reproduction, thus reducing the formation of unfit hybrids. Ex. Think of flycatcher birds.

How many genes change when a new species forms?

In a few cases, the evolution of reproductive isolation is due to a change in single gene. Ex. Japanese snails. Euhadra. Shells spiraling in different direction. Ex. Monky flower. Both species are isolated both by prez barriers (pollinator choice and partial gametric isolation) and by postzyg (interspecific crosses produce fewer offspring than intraspecific crosses, and F1 hybrids have reduced fertility and survival. **Data suggests that few or many genes can influence the evolution of reproductive isolation and hence the emergence of a new species- a new addition to the great diversity of life.