Presentation on theme: "Speciation Diversity. Question It is estimated that of all species that have ever lived on earth are now extinct. A.20 % B.45 % C.80 % D.90 % E.99 %"— Presentation transcript:
Question It is estimated that of all species that have ever lived on earth are now extinct. A.20 % B.45 % C.80 % D.90 % E.99 %
Mass Extinctions Are a Fact of Life It is estimated that 99% of all species that have ever lived on earth are now extinct.
Did the creation end in the Garden? Before Charles Darwin, many naturalist believed the creation of new species ended in the Garden of Eden. By the 18 th century scientist began to understand that extinctions were a common phenomena. Was the Diversity of life winding down?
Charles Darwin There is grandeur in this view of life, with its several powers, having been originally breathed by the creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved. From the Origin of Species by Charles Darwin
Darwin’s Tree of Life The ascent of life through time. Each new branch or species arise from the modification of an ancestral species. The process by which new species arise is called Speciation.
What is a species? Its not as straightforward a question as most believe. Evolution creates (and destroys) new species, but … These are members of different species - eastern (left) and western (right) meadowlark.
What is a Species? There is only one extant (existing) human species.
What is a Species? And these are all members of a single species.
Are these the Same Species?
What is a Species? It is the smallest evolutionarily independent unit –Different species follow different evolutionary trajectories. –The essence of speciation is lack of gene flow It is difficult to determine when populations are actually evolving independently. species form a boundary for the spread of alleles
Proposed species concepts There are three different species concepts currently in use. 1.The Biological species concept 2.The Phylogenetic species concept 3.Morphospecies concept
The Biological Species Concept ( BSC) “ Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” Ernst Mayr 1942 Definition implies: –A population is reproductively isolated if it possesses genetic differences that prevent it from inbreeding with other biological species. –There is no hybridization or hybrids fail to form fertile offspring. –Lack of gene flow ensures independent evolutionary trajectories between populations.
The BSC does not include geographical isolation. Two populations that cannot interbreed because they are separated by some physical barrier such as a river or a mountain range are not defined as separate species. Why? This makes sense, because geographic isolation may be temporary (rivers change their course mountains erode), whereas reproductive isolation caused by genetic differences is usually irreversible.
Problems associated with the BCS plantshybridize freely 1.Many plants hybridize freely fossil forms 2.Cannot test fossil forms asexual populations 3.Irrelevant to asexual populations
The Phylogenetic Species Concept (PSC) Species are identified by estimating the phylogeny of closely related populations and finding the smallest monophyletic groups.
PSC (cont) Taxa labeled A-G represent populations. They re the smallest monophletic groups on the tree and represent distinct species. If populations cannot be clearly distinguished in a phylogeny by unique, derived characters, then they will form clusters like the populations designated B, E, and G. Clusters B, E and G would be considered part of the same species
Explain the Figure below In this example, salamander lineages A and B are separate species. Each has a common ancestor that individuals of other species do not. Even though it has diversified a lot, Lineage C is a single species, according to the phylogenetic species concept. None of the subspecies of Lineage C has a single common ancestor separate from the other subspecies.
PSC Rational Species are formed when a single interbreeding population splits into two lineages that do not exchange genetic material. –Form Sister species (Lineages that diverged from the same ancestral node on a phylogenetic tree) –Identified from diagnostic traits called synapomorphies (shared derived characteristics) Species are identified on the basis of statistically significant differences in the traits used to estimate the phylogeny (ancestry)
PROBLEMS with PSC(cont) Populations must have been independent long enough for diagnostic traits to emerge Phylogenies are only available for a handful of groups Very tiny differences, even a single DNA substitution may be used as a trait that separates groups Could end up doubling the number of species
Morphospecies Define species based on the morphological differences. Commonly used with fossils. Used when we do not have tests for reproductive isolation or well-estimated phylogenies Assignment to species is often arbitrary and cannot distinguish cryptic species cryptic species= ones which are strongly divergent based on non-morphological characters. - Things such as song, temperature or drought tolerance, habitat use, or courtship displays
Application of the 3 species definitions to red wolf Red wolf appeared in 1930 to be a Morphospecies being intermediate in appearance between the gray wolf and the coyote, all 3 appearing to be distinct. Studies have shown that the red wolf is actually a hybrid between gray wolves and coyotes. Therefore its intermediate characteristics are the result of hybridization and not independent evolution. This makes the Red wolf not a distinct species for most biologists because… Neither the BSC or the PSC allow for hybridization However, it is still considered a separate species and the morphospecies is the only one of these 3 definitions that works.
How do you miss a whole Elephant Species? Up until recently, scientists believed there were two species of elephant: the African elephant and the Asian elephant. Geneticists conducting a comprehensive DNA sampling of elephants from across Africa recently found that there are in fact two species of African elephants.
Speciation Classically, speciation has been hypothesized to be a three-stage process –1 Population becomes reproductively isolated. –2 divergence in traits –3 Development of isolating mechanisms (Secondary contact)
Allopatric Speciation Allopatric (Allo= different, Patric=homeland) Population are separated due to geographical barriers
Geographic isolation 1.By dispersal and colonization Dispersal to novel environment such as rafting a portion of a population to an island 2.By Vicariance events Events that physically divide a population into two or more isolated ranges like rising of a mountain range, long term drying trend, or lava flow.
Example of Dispersal and Colonization Question Why do each of the islands of Hawaii have their own drosophilid species? Hypothesis: It is due to the Founder effect. –small group of individuals cut off from the original population colonizes a new habitat –Drift, mutations and selection –Drift, mutations and selection leads to divergence
Predictions Closely related species should be found on adjacent islands The phylogenetic branching sequence should follow island formation Figure 15.7b
The analysis of mitochondrial DNA showed that closely related species were found in the predicted pattern.
Vicariance events Events which split a species into two or more isolated ranges and prevents gene flow between them (or at least greatly reduces it) Can be slow processes like rising of a mountain range, long term drying trend or rapid like a lava flow.
Case study of Mexican Cavefish In Mexico, researchers identify a river running through rocks to a cave system below. In the cave they find fish that are like the population on the outside, however with some important differences
Case study continued The eyes of the cavefish have become vestigial (no longer functional) The fish have also lost their pigmentation. The two populations of fish are no longer able to mate Speciation has occurred.
Example from isthmus of Panama land bridge opened as the isthmus closed about 3 million years ago Found 7 pairs of closely related morphospecies of snapping shrimp. One member of each pair on each side of the land bridge The pairs from either side of the bridge are shown to be sister species (each other’s closest relative) believed to share the same common ancestors which split to form each pair
5 Also, interestingly, shrimp populations would have been isolated in a staggered fashion as the land bridge gradually formed in stages Species 6 and 7 live in the deepest water and were cut off first 1-5 were in shallower water and diverged later
SECOND STAGE OF SPECIATION Genetic Divergence
Mechanisms of Genetic Divergence. If the environments are different between isolated populations then natural selection, mutations, and genetic drift will work independently on each group. This will speed up genetic divergence and speciation.
Genetic Drift This sampling process leads to the random fixation of alleles with the concurrent loss of other alleles Effect most predominant in small populations and founder effect populations. Especially with bottlenecking Because peripheral populations in colonization events are almost always small, genetic drift is hypothesized to be the key to the second stage of speciation
The role of genetic drift is controversial Hundreds of small populations have been introduced to new habitats around the world in the last 150 years, but few dramatic changes in genotypes have occurred. Now a more balanced approach is taken and the role of natural selection is also brought into the equation.
Natural selection novel If one of the split populations occupies a novel environment, Natural selection can quickly lead to divergence
Reinforcement If populations have sufficiently diverged while in allopatry, their hybrid offspring should have markedly reduced fitness when compared to individuals in both parental populations. Parental populations will reduce their fitness if they produce hybrid offspring, therefore this should favor assortative mating Selection that reduces the frequency of hybrids is called reinforcement
Reinforcement finalizes the speciation process by completing reproductive isolation The final stage of speciation, that of establishing reproductive isolation occurs as Isolating mechanisms develop between groups. These are mechanisms that prevent interbreeding between two groups and assure continued genetic divergence.
Isolating mechanisms Mechanisms that lead to, or cause genetic divergence by preventing interbreeding between two groups. –As long as two groups do not interbreed their gene pools will continue to drift further and further apart. –The longer two species are genetically isolated from each other the more different they become from each other.
Isolating mechanisms Isolating mechanisms can be divide into two different types. –Isolating mechanisms that occur before fertilization are called Prezygotic mechanisms –Isolating mechanisms that occur after fertilization are called postzygotic mechanisms
Types of Prezygotic Isolating mechanisms Behavioral isolation: Potential mates meet but cannot figure out what to do about it because patterns of courtship may be altered to the extent that sexual union is not achieved Temporal isolation: (Time) Different groups overlap in range but may not be reproductively mature in the same season. Mechanical isolation: Potential mates attempt engagement but sperm cannot be successfully transferred. This may be due to differences in reproductive organs.
Types of Prezygotic Isolating mechanisms cont. Gametic isolation: Sperm is transferred but sperm and egg are incompatible. Ecological isolation: potential mates never meet because they live in different habitats
Types of Postzygotic isolating mechanisms Zygotic mortality: Egg is fertilized but zygote does not develop properly dies before birth because parents are genetically incompatible. Hybrid inviability: Hybrid very weak and can’t live outside the uterus. Hybrid offspring: Hybrid is sterile.
Sympatric Speciation A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parents.
Sympatric speciation Sympatric (Sym = same, Patric=homeland) speciation occurs in the same geographical region without physical isolation. A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population. Example: Accidents during cell division that result in extra sets of chromosomes (Polyploidy). Self-fertilization can give rise to new individuals that are unable to mate and form fertile off-spring with the parent species.
Sympatric Speciation Accidents during cell division may results in extra sets of chromosomes in the progeny, a condition called polyploidy. The differences in chromosome number act as a reproductive barrier between parent and offspring. This is type of speciation is quite common in plants that are able to self-fertilize.
Sympatric Speciation Botanist estimate that up to 50% of all plant species are polyploids. The evolution of Wheat
Parapatric (Para = near, Patric=homeland) neighboring populations become distinct species while maintaining contact through Hybrid zones
Hybridization Creation of new species through hybridization
There are 3 possible outcomes when hybridization occurs 1.Reinforcement of parental forms as two recently diverged species 2.Formation of a new third species secondary 3.Formation of secondary hybrid zones
New Species through Hybridization Three species of Sunflower –Helianthus annuus, H. petiolaris and H. anomalous. Rieseberg crossed individuals of H. annuus and H. petiolaris to produce three hybrid populations. –The three experimental hybrids were genetically almost identical to the natural H. anomalous. Only certain alleles work in combination and that other hybrids are inviable and have reduced fitness.
Speciation through Hybridization variety of hybrid gene combinationsHybrids can mate with siblings and backcross to their parents the result is a variety of hybrid gene combinations. third species may ariseIf certain of these combinations is best suited to a new habitat, a third species may arise that is genetically somewhere intermediate between the parents. Secondary contact and gene flow between recently diverged species can result in the formation of a new third species.
Size, shape and longevity of hybrid zones are determined by 3 possible outcomes of hybridization 1.When parents and hybrid are equally fit… The zone is wide. Hybrid traits Hybrid traits are found with highest frequency at the center of the zone. Width of the zone is determined by a) distance of dispersal in each generation b) How long zone has existed
Factors (cont) 2.When hybrids are less fit than purebred individuals 2.When hybrids are less fit than purebred individuals (parents) … Fatedepends on the strength of selection Fate of the hybrid zone depends on the strength of selection against them. Strong selectionreinforcement (a) Strong selection leads to reinforcement, with a very narrow and short-lived hybrid zone. selection is weakwider and longer lived (b) If selection is weak, the hybrid zone is wider and longer lived A balance develops between formation of the hybrids and the selection pressure against the hybrids. (similar to the banded and non-banded snakes we used as an example of selection- migration balance)
Factors (cont) 3.When Hybrids are more fit than purebreds Depends on the extent of the environment advantage Depends on the extent of the environment in which the hybrids are at an advantage New speciesif hybrids are more fit outside the range of the parental species New species results if hybrids are more fit in areas outside the range of the parental species advantage is at the boundaries form a stable hybrid zone If the advantage is at the boundaries between the two parental species then will form a stable hybrid zone Often found in areas called ecotones where markedly different plants and animals meet
Example from Big sagebrush in Utah Between the basin and mountain subspecies Hybrids shown to be more fit than parents in transitional zones Showed that the hybrid zone is maintained because the hybrids have superior fitness in the transitional zone
Hybridization Issues Potential practical problems Some crop species are closely related to weed species Herbicide resistance genetically engineered into a crop plant could be transferred to weed plants through hybridization
Genetic differentiation and isolation How much genetic variation is necessary to produce a new species? The BSC requires that no hybridization whatsoever occur, so it predicts that a virtual “ genetic revolution” or radical reorganization of the genome would be required in order for hybrids to survive. Large scale changes in the genome are unlikely They have also been shown to be unnecessary. Fertile hybrid offspring can be found even when the parental populations are markedly different from one another. Current research focuses on the number, location or nature of the genes which distinguish closely related species
Do we see speciation occurring today? Sockeye SalmonSea run vs. lake dwelling Skipper butterfliesDifferent host plant Blueberry and apple maggot flies Different host plant Warbler finchesHabitat selection (high vs low elevation) Heliconius butterfliesDifferent habitats Lake White fishDwarf versus normal sized individuals