Last time, we discussed the mechanisms by which evolutionary change occurs -- these mechanisms included mutation, gene flow, genetic drift and natural selection.
One implicit assumption in the theory of evolution is that if genetic changes in populations are drastic enough, populations may become so different that they become new species.
Figure 17.3 provides a graphical representation of how this might occur.
Let’s now look at how new species are formed.
Species: collections of populations containing individuals that can interbreed.
Each species has a geographic distribution -- locations on the earth where the species can be found -- some species have very large distributions (e.g. humans), while others have very small distributions.
Populations of a species are distributed throughout the species distribution -- populations maybe contiguous (highly connected) or they may be spatially isolated -- spatial isolation means low gene flow between populations.
Genetic variation among populations -- genetic characteristics of populations often vary from population to population -- populations generally display geographic variation through space.
Factors producing genetic differentiation among populations are the forces of evolution -- genetic differentiation (= divergence) among populations is expected because:
1. different mutations occur in different populations producing different phenotypes.
2. genetic drift operating in different populations will cause them to diverge randomly.
3. natural selection -- populations in different parts of the distribution may live in very different environments -- as a result, natural selection will favor different adaptations in different populations and cause differentiation.
Speciation: formation of new species from pre-existing ones -- process that many people incorrectly use as a synonym for evolution.
Before we consider speciation, let’s talk about species.
Biologists are still debating exactly how species are defined -- here are two of the most common species concepts:
Typological species (morphospecies) concept: species designated on basis of visible morphological differences -- each species defined by a "type" specimen.
Problems: 1) variation within populations away from type; 2) at what point are individuals "different" enough to be considered species?
Nonetheless, the typological species concept is still used by biologists today -- more in a moment.
Biological species concept: natural group of actually or potentially interbreeding organisms that are reproductively isolated from other such groups.
Notice: species defined on basis of reproductive isolation -- ability of populations to exchange genes -- not on how different individuals look, behave, use resources, etc.-- these may be suggestive, but reproductive isolation is the ultimate criterion for designating species -- rejection of typological species concept.
"natural groups" -- does not apply to laboratory/zoo/greenhouse hybrids.
"Actually or potentially interbreeding"-- obviously, populations of the house mouse in New York and London are not actually exchanging genes --but they could (potentially) if they weren't seperated by the Atlantic Ocean-- therefore, they are the same species.
Problems: 1) definition ignores time -- difficult to apply to extinct fossil forms -- must resort to typological species; 2) mode of reproduction -- "interbreeding" infers sexual reproduction, yet a large number of organisms reproduce partially or completely by asexual means; 3) don't always know if geographically distinct populations of the same species can actually interbreed.
How does speciation occur? No one has ever recorded the complete process from beginning to end, but there are many examples of groups of organisms at various stages of the process -- probably takes thousands, or hundreds of thousands of years -- most common way we think speciation occurs is via the allopatric model of speciation.
Allopatric Speciation: thought to be the most common way that speciation occurs especially among animals.
1. begin with the geographic distribution of a single species.
2. a barrier arises that fragments the distribution -- a population or populations become geographically isolated so that they can no longer exchange genes (no gene flow) -- river, canyon, mountain range, deserts, may be such barriers.
3. While geographically isolated, each population is undergoing mutation, drift and, most importantly, natural selection which adapts each population to its own environment -- 1) thus, genetic differences begin to accumulate between populations; 2) lack of gene flow (geographic isolation) prevents swamping of differences
4. Genetic differences may become so large that if our populations come back into contact, they may no longer be able to interbreed (reproductive isolation) -- speciation has occurred.
The following diagram illustrates this process:
Summarizing: 1) geographic isolation of populations; 2) genetic divergence while isolated; and 3) reproductive isolation resulting from genetic divergence.
Other models of speciation:
1. Sympatric speciation -- speciation without geographic isolation -- may involve disruptive selection.
2. Speciation by Polyploidy -- common in plants -- a special type of sympatric speciation that occurs when a mutation results in the multiplication of a plants entire set of chromosomes -- e.g. diploid (2N) becomes tetraploid (4N).
Species are reproductively isolated from other species.
Q: What kinds of things keep species from interbreeding?
A: Reproductive Isolating Mechanisms (RIM's) -- result incidentally from genetic divergence of geographically isolated populations.
Hybridization: matings between different species.
I. Pre-Zygotic Mechanism -- prevent hybrid zygotes from being formed -- most efficient because species do not waste gametes breeding with other species.
1. ecological or habitat isolation -- during speciation, species may have evolved different habitats where they live and/or mate -- e.g. mosquitoes have specific aquatic habitats for mating (stagnant, flowing, brackish).
2. seasonal isolation -- species may have evolved differences in the times they reproduce -- e.g. flowering time in plants.
3. behavioral isolation -- in animals, males and females are often attracted to one another by specific sequences of behavior -- differences in these behaviors may evolve between species so that there is no interspecific attraction -- e.g. courtship displays, songs, etc.
4. mechanical isolation -- "lock and key" -- differences in the sizes/shapes of genitals may make copulation impossible -- e.g. insects.
II. Post-Zygotic Isolating Mechanisms -- operate after fertilization occurs and a hybrid zygote is formed -- less efficient than pre-zygotic.
A. hybrid inviability -- hybrid zygote formed, but the hybrid dies some time before it reaches reproductive age due to genetic incompatibility -- e.g. hybrids produced by sheep X goat crosses die as embryos (miscarriages).
B. hybrid sterility -- hybrid offspring survives, but is unable to reproduce either with other hybrids or with parental species -- e.g. horse X ass cross produces a mule -- healthy organism that is sterile -- very wasteful because the sterile hybrid may compete with parents for food, space etc.
Some important general points about speciation:
1. Once two new species have been formed, each is on it’s own evolutionary trajectory -- will evolve independently of each other since they no longer interbreed -- each species may give rise to more new species
2. .As speciation occurs, diversity(= number of species) increases -- assuming speciation has been occurring since the first life evolved on earth 3.5 b.y.a., then the history of life on earth resembles a "bush" and all organisms are related to varying degrees.
Evolutionary relationships of organisms are often presented as evolutionary trees -- these reflect genealogical relationships of species and higher taxa in the same way as "family trees."
Figure 17.11.
Sometimes a species will enter a new adaptive zone -- unoccupied geographic area or way of life not used by other species -- when this happens adaptive radiation may occur.
Adaptive radiation -- single lineage gives rise to many new species as a result of entering a new adaptive zone.
Examples: birds -- once the adaptive zone of flight was entered, the ancestral bird lineage underwent adaptive radiation to produce the various bird species -- diversity of sizes, colors, shapes, etc.
Mammals -- adaptive radiation over the past 65 million years.
Figure 17.12.
Next Time: Evidence for Evolution