- Slides: 30
The Origin of Species © 2014 Pearson Education, Inc.
Learning Objectives Describe the conditions under which new species may arise. Explain the biological species concept definition of a species. Describe the various prezygotic and postzygotic mechanisms which maintain reproductive isolation and prevent gene flow between populations. © 2014 Pearson Education, Inc.
The Origin of Species § In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth © 2014 Pearson Education, Inc.
Definition of Species § A species is a group of interbreeding organisms that produce viable, fertile offspring in nature. § Members of a species will interbreed with one another but not other organisms outside of the species. © 2014 Pearson Education, Inc. 4
What do you get when you breed a donkey and a horse? A MULE!!! © 2014 Pearson Education, Inc. Does the mule fit the definition of a species?
Why does a MULE NOT fit the definition of a species? ? ü Can interbreed X Produce fertile offspring ü Do so in nature © 2014 Pearson Education, Inc.
What do get when you breed a tiger and a lion? A LIGER!!! © 2014 Pearson Education, Inc. Does the liger fit the definition of a species?
Why does a LIGER NOT fit the definition of a species? ? ü Can interbreed X Produce fertile offspring X Do so in nature Ligers & mules are hybrids ~ the offspring of crosses between two different species © 2014 Pearson Education, Inc.
Looks Can Be Deceiving! • These meadowlarks look very similar yet they are not the same species. • By contrast, humans look very different from one another, but they are the same species. • Species identification is determined by morphology, physiology, biochemistry, and DNA sequences © 2014 Pearson Education, Inc. 9
The Biological Species Concept § Speciation is the origin of new species, and is the focal point of evolutionary theory § Microevolution consists of changes in allele frequency in a population over time § Macroevolution refers to broad patterns of evolutionary change that explains how a species came into existence © 2014 Pearson Education, Inc.
The Biological Species Concept © 2014 Pearson Education, Inc.
Animation: Macroevolution © 2014 Pearson Education, Inc.
What Causes Speciation? § Speciation occurs when formerly interbreeding organisms are prevented from mating and producing fertile offspring. § These preventions are referred to as reproductive isolation. © 2014 Pearson Education, Inc.
Natural Selection can lead to reproductive isolation § When two populations become reproductively isolated, their gene pools become separated and they become two separate species. • Gene flow between populations prevents speciation © 2014 Pearson Education, Inc.
Reproductive Isolation § Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring § There are two types of reproductive isolations, depending on whether factors act before or after fertilization ØPrezygotic Barriers prevent fertilization ØPostzygotic Barriers prevents viable, fertile offspring © 2014 Pearson Education, Inc.
Prezygotic Barriers § Prezygotic barriers block fertilization from occurring by § Impeding different species from attempting to mate § Preventing the successful completion of mating § Hindering fertilization if mating is successful § There are 5 prezygotic barriers © 2014 Pearson Education, Inc.
Prezygotic Barriers § Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers © 2014 Pearson Education, Inc.
Prezygotic Barriers § Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes © 2014 Pearson Education, Inc.
Prezygotic Barriers § Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers to mating © 2014 Pearson Education, Inc.
Prezygotic Barriers § Mechanical isolation: Mating is attempted, but morphological differences prevents successful completion © 2014 Pearson Education, Inc.
Prezygotic Barriers § Gametic Isolation: Mating is completed, but sperm of one species is not able to fertilize eggs of another species © 2014 Pearson Education, Inc.
Prezygotic Barriers § When the completion of mating is prevented, fertilization cannot occur. © 2014 Pearson Education, Inc.
Postzygotic Barriers § Postzygotic barriers ~ mating and fertilization occurs, but various factors prevent the hybrid zygote from developing into a viable, fertile adult § There are 3 postzygotic barriers § Reduced hybrid viability § Reduced hybrid fertility § Hybrid breakdown © 2014 Pearson Education, Inc.
Postzygotic Barriers § Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development or survival in its environment § Hybrid embryo dies before birth. § Fully developed offspring display mixed traits, forming a frail, often infertile adult. § Hybrid displays reduced fitness, marked by decreased rates of survival and reproduction relative to the parent species. © 2014 Pearson Education, Inc.
Postzygotic Barriers § Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile © 2014 Pearson Education, Inc.
Postzygotic Barriers § Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with each other or with either parent species, offspring of the next generation are feeble or sterile © 2014 Pearson Education, Inc.
Postzygotic Barriers § Mating and fertilization occurs, but does not result in fertile offspring © 2014 Pearson Education, Inc.
Limitations of the Biological Species Concept § The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) § The biological species concept emphasizes absence of gene flow as the main cause of isolation § However, gene flow can occur between distinct species § For example, grizzly bears and polar bears can mate to produce “grolar bears” © 2014 Pearson Education, Inc.
Figure 24. 4 ▶ Grizzly bear (U. arctos) ▶ Polar bear (U. maritimus) ▶ © 2014 Pearson Education, Inc. Hybrid “grolar bear”
Summary of Learning Objectives © 2014 Pearson Education, Inc.