Transcript CHAPTER 24
The Origin Of Species CHAPTER 24 What is Speciation? • Speciation is the origin of new species • Speciation increases our planet’s biological diversity. • Therefore, speciation is an important part of the theory of evolution. Macroevolution • Microevolution is small changes in the gene pool of a species over a smaller period of time. • Macroevolution is cumulative effects of speciation over vast periods of time What is a Species? • A species is a population or group of populations whose members are able to mate and produce fertile offspring. • Cannot produce fertile offspring with other distinct populations or species. Biological Species Concept • What prevents biologically similar species from interbreeding? Answer: Reproductive Barriers. There are 2 kinds of reproductive barriers: 1. Prezygotic Barriers (Before gametes meet) 2. Postzygotic Isolation (After fertilization) Prezygotic Barriers 1. Habitat Isolation Two species live in the same area but in different habitats may encounter each other but will rarely mate. Example: 2 species of garter snakes live in the same geographic area, but one lives in water, the other on land. So they will never mate. Prezygotic Barriers 2. Temporal Isolation Species may mate at different times of the year, day, season, etc. Example: The Eastern and Western Spotted skunks have some overlap of their geographic territories, but one mates in the winter and the other in the summer. Prezygotic Barriers 3. Behavioral Isolation In many animals, courtship displays and rituals are vital to reproduction. Such behaviors can be very specific, varying between closely related species. Male behaviors such as courtship calls, songs, and dances will only be recognized by females of the same species. Prezygotic Barriers 4. Mechanical Isolation- cannot mate because not anatomically compatible. Often this occurs because the genitalia of different species are incompatible. Example: Bush babies, a group of small arboreal primates, are divided into several species based on mechanical isolation. Each species has distinctly shaped genitalia that, like locks and keys, only fit with the genitalia of its own species. Prezygotic Barriers 5. Gametic Isolation The gametes may not be able to fuse and fertilization does not take place. • • • The sperm (or pollen) of one species may not be able to survive in the environment of the female reproductive tract of the other species The female immune system may recognize the sperm (or pollen) as foreign and attack it. The sperm or pollen may not have the correct apparatus/ surface proteins to fertilize the egg or ovule. Postzygotic Barriers Even if the sperm of one species manages to fertilize an ovum of another species, other barriers prevent the development of a fertile adult. Postzygotic Barriers 1. Reduced hybrid viability genetic incompatibility between two species causes abortion of development or the birth of unhealthy, weak individuals Example: 2 particular salamander species mate and produce offspring. The offspring usually die soon after birth or do not survive to sexual maturity. Postzygotic Barriers 2. Reduced hybrid fertility even if hybrid is born, lack of fertility keeps reproductive isolation. Example: Horses mate with donkeys to produce mules. Mules are healthy, but sterile. Postzygotic Barriers 3. Hybrid Breakdown sometimes you will have a fertile hybrid that cannot produce fertile offspring. Example: 2 different species of rice can produce offspring that are fertile. But when those offspring mate, their offspring (F2 generation) are sterile Biological Species Concept doesn’t work??? • Well, it doesn’t work in all situations. • Organisms that reproduce asexually will not be limited by this trend of barriers. • Does not apply to some sexual organisms Other species concepts Table 24.1 Six Concepts of Species Compared Biological species concept Emphasizes reproductive isolation, the potential of members of a species to interbreed with each other but not with members of other species. Morphological species concept Emphasizes measurable anatomical differences between species. Most species recognized by taxonomists have been designated as separate species based on morphological criteria. Recognition species concept Emphasizes mating adaptations that become fixed in a population as individuals "recognize" certain characteristics of suitable mates. Cohesion species concept Emphasizes cohesion of phenotype as the basis of species integrity, with each species defined by its integrated complex of genes and set of adaptations. Ecological species concept Emphasizes species' roles (niches), their positions and functions in the environment. Evolutionary species concept Emphasizes evolutionary lineages and ecological roles. Modes Of Speciation • Allopatric Speciation- a geographical barrier that physically isolates populations initially blocking gene flow. Populations that are segregated by a geographic barrier are known as allopatric populations • Sympatric Speciation- New species arise amidst an existing parent population Allopatric Speciation • If new individuals colonize a new, geographically isolated area, the colonizing population may vary from the parent population • When populations become allopatric, speciation can occur as isolated gene pools accumulate genetic differences by microevolution. Allopatric Speciation Allopatric speciation of squirrels in the Grand Canyon (see Campbell fig. 24.7, p. 451). The canyon is a barrier to dispersal by small mammals, and as a consequence the isolated populations can diverge. First geographic isolation... •Unless populations are geographically isolated they will continue to interbreed ...then genetic isolation •Populations diverge to the point where they no longer interbreed •This may be due to adaptation to different environments, or genetic drift Sympatric Speciation • New species arise within the parent populations • How can reproductive barriers evolve if the species are in contact with each other? How Sympatric Speciation Occurs 1. Polyploidy- accident in cell division resulting in extra sets of chromosomes – Autopolyploid- an individual with more than two – chromosome sets as a result of polyploidy - all chromosomes came from the same species Allopolyploid – results from two different species breeding and producing polyploid hybrids. The chromosomes came from different species – The hybrids may be sterile and reproduce asexually – The hybrids can, in subsequent generation become fertile and reproduce sexually. Polyploidy is more common in plants than in animals Sympatric Speciation, cont’d. 2. Habitat differentiation and Sexual Selection Sometimes, members of a population of the same species select a different niche. This can lead to the formation of certain reproductive barriers such as temporal, behavioral, habitat, etc. Even though they are still part of the same species, this is sympatric speciation in progress Adaptive Radiation • The evolution of many diverse species from one common ancestor • Example: Galapagos finches Adaptive Radiation • Galapagos Island- finches( peripheral isolate formed new species from south America - mainland) • Multiple invasions would eventually lead to coexistence of distinct species • Islands- far enough to evolve in isolation, close enough to occasionally disperse. Adaptive Radiation Gradualism VS. Punctuated Equilibrium Gradualism- species descended from a common ancestor, gradually diverge more and more in morphology as they acquire adaptations Punctuated equilibrium- new species diverge from the parent species in “spurts of rapid change” instead of the gradual change mentioned in gradualism, then there is a long period of equilibrium where no changes occur. In other words – species change the most when they first “bud” from the parent species and then don’t change much for the rest of the existence of that species. Gradualism vs. Punctuated Equilibrium But which theory does the fossil record support? • Paleontologists rarely find gradual transitions in fossil forms. • Instead, when they look in rocks, they find that new species appeared quite suddenly, then stayed for a while and then disappeared rather suddenly • Paleontologists believe (as did Darwin) that the period of time during which a species underwent change were quite short compared to the period when they remained unchanged. – Which theory of speciation does that sound like? Heterochrony • Evolutionary changes in the rate of development of a species Examples of heterochrony: - Allometric growth - Paedomorphosis Heterochrony • Allometric Growth: When different parts of the body grow at different rates in order to give that organism its native and proportionate shape. The head, limbs, and body grow at different rates, resulting in a human adult with proportions completely different from those of the newborn baby: Paedomorphosis • When an organism grows to its full size, reaches sexual maturity and reproduces, while maintaining certain juvenile characteristics. This axolotl salamander is full grown and able to reproduce, but maintains larval characteristics such as its gills Homeosis • The transformation of one body part into another, due to mutations in or misexpression of specific developmentally critical genes. Which set of genes are we talking about? Branched Evolution of Horses • Horses got bigger, developed teeth for grazing and reduced the number of toes THE END