Class Notes (806,820)
Canada (492,456)
Biology (2,220)
BIO120H1 (1,171)
Lecture 10

Chapter 7.doc lecture 10.doc

9 Pages
Unlock Document

University of Toronto St. George
June Larkin

Chapter 7- The Origin of Species 04/06/2012 22:10:00 ← Ernst Mayr set off for the wilds of Dutch New Guinea to collect plants and animals • Mayr brought back many specimens new to science including 26 species of birds and 38 species of orchids • This concordance between two cultural groups with very different backgrounds convinced Mayr, as it should convince us that the discontinuities of nature are not arbitrary but an objective fact • Nature is discontinuous. When you look at animals and plants each individuals almost always falls into one of many discrete groups • Although there is variation among individuals within a cluster, the clusters nevertheless remain discrete in organism space • We see cluster in all organism that reproduce sexually • These discrete clusters are known as species ← Darwin apparently didn’t see the discontinuities of nature as a problem to be solved or thought that these discontinuities would somehow be favored b natural selection. Either way HE FAILED TO EXPLAIN NATURES CLUSTER IN A COHERENT WAY ← A better title for the origin of species then would have been the origin of adaptations: while Darwin did figure out how and why a single species change over time, he NEVER explained how one species splits in two. ← All this diversity comes from a single ancient ancestor ← If speciation didn’t occur there would be no biodiversity at all—only a single, log evolved descendent of that very first species ← Biologist struggled and field to explain how continuous process of evolution produces the discrete groups known as species. The problem of speciation was in fact not seriously addressed until the mid 1930s ← How we recognize species: as a group of individuals that resemble one another more than they resemble members of other grouthis is known as the morphological species concept (tiger as cat) • This definition has problems. In sexually dimorphic species, males and females can look very differefemales of single species as members of two different species • There is also the problem of variation within a interbreeding groups (ex: humans could be classified into few discrete groups based on eye color) ← What point is differences between populations large enough to make us call them different species? • This concept makes designation of species an arbitrary exercise, yet we know that species have an objective reality and are not simply arbitarary human constructs • Some groups that biologist recognize as different species look either exactly alike or nearly alike. These cryptic species are found in most groups of organism including birds, mammals, plants and insects. • Why if these cryptic forms look so similar we think that they’re actually different species. The answer is that they coexist in the same location and yet never exchange genes: the members of one species simply don’t hybridize with member of another • The groups are thus reproductively isolated from one another: they consisted distinct gene pools that don’t intermingle • These cryptic forms are distinct • Brown eye and Blue or Iut and Kung are members of the same species, we realize that its because they can mat with each other and produce offspring that contain combinations of their genes • They belong to the same gene pool. When you ponder cryptic specie and variation within humans you arrive at the notion that species are distinct not merely because they look different but because there are barriers them that prevent interbreeding ← Mayr and Theodious Dobzhansky, proposed a definition of species that has become the gold standard for evoloutonaty biology ← Mayr defined a species as a group of interbreeding natural populations that are reproductively isolated from other such groupthis known as biological species concept BSC ← Reproductively isolated simply means that members of different species have traits—difference in appearance, behavior or physiology that prevent them from successfully interbreeding, while members of the same species can interbreed readily ← What keeps member of two related species from mating with each other? • Species might not interbreed simply because their mating or flowering seasons don’t overlap • Closely related species living in the same are remain distinct because their peak spawning periods are several hours apart, preventing eggs of one species from meeting sperm from another • Animal species often have different mating displays or pheromones and don’t find one another sexually attractive • Species can also be isolated by preferring different habitats so they simply don’t encounter one another, • Many insects can feed and reproduce on only one single species of plant and different species of insects are restricted to different species of plants this keeps them from meeting others at mating time • Isolating barriers can also act after mating pollen from one plant species might fail to germinate on the pistil of another • When they succeed at mating the sperm of one species is poor at fertilizing the eggs of the other so that relatively few offspring’s are produced • The species exchange virtually no genes in nature and we have confirmed this result by sequence their DNA, these can be considered good biological species ← Advantage of BSC • These population might not mate directly with each other but there is potential gene flow from one population to the other through intermediate geographical areas and little doubt that if they did mate they’d produce fertile offspring • Males and females are members of the same species because of their gene unite at reproduction • According to BSC the species is a reproductive community – a gene pool and this means that a species is also an evolutionary community • If a good mutation crops up within a species, say a mutation in tigers that boosts female output of cubs by 10% then the gene containing that mutation will spread throughput the tiger species. But it wont go any further • The biological species the unit of evolution – it is to a large extent the thing that evolves • This why members of all species generally look and behave similar because they all share genes, they respond in the same way to evolutionary forces • It is the lack of interbreeding between species living in the same area that not only maintains species differences in appearance and behavior but also allows them to continue diverging without limits ← Biologist species concept is still the one that evolutionist prefer when studying speciation ← Sister species species that are each others closest relatives were often separated in nature by geographical barriers ← Sister species of sea urchins for example were found on opposite sides of the Isthmus of Panama ← Could this geographic separation have something to do with how these species arose from a common ancestor? Yes • Mayr argued that these barriers were merely the by products of natural or sexual selection that caused geographically isolated populations to evolve in
More Less

Related notes for BIO120H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.