Biology 5C ~ Lecture 21 ~ Species and Speciation

two main questions:

1. are species entities that actually exist in nature? or are these produced by human perception?

2. if species exist, can we provide a formal definition that allows for identification and study? are they identifiable?

Species: sometimes, it is super easy...

to tell the species apart

-the longer ago two lineages diverged, the easier it can be to distinguish them

*polar bear and desert mouse?
*bacteria and magnolia tree?
*penguin and flamingo?
*slime mold and sea turtle?
*green algae and dragonfly?

species: sometimes it is really hard...

to tell the species apart b/c there are organisms that share a lot of similarities but maybe have some differences (how are we supposed to decide what matters in differentiating the species)

*what if they have only just begun to diverge?

what is so tricky about defining species?

species are discontinuous groups that are produced by a continuous process (and the process can occur in many different ways)

an embarrassing number of species concepts have been proposed

-character-based concepts
-(reproductive) isolation-based concepts
-phylogenetic concepts
-evolutionary concepts

*different concepts yield different answers b/c we want to organize living organisms in a useful way

character-based concepts:

use traits or genes to diagnose similarities between organisms

(reproductive) isolation-based concepts:

uses the potential for gene flow between groups to diagnose species

phylogenetic concepts:

diagnose species as an exclusive group with closely-related members

evolutionary concepts:

species diagnosed through independent evolutionary trajectory

why is the biological species concept so popular?

-provides clear criteria to measure (can identify species)

-reproductive isolation can be measured in natural populations

-works very well for sexually reproducing animals

-works okay for sexually reproducing higher plants

*ultimately, barriers to gene flow is fundamental to the process of speciation

when organisms stop exchanging genes...

genetic incompatibilities build up over time

three types of barriers to gene exchange

-premating
-postmating, prezygotic
-postzygotic

*which one occurs first depends on many factors

premating:

organisms are unable to mate at all

postmating, prezygotic

organisms can mate but no zygote is produced (no fertilization)

postzygotic

a zygote is produced but viability or fertility of hybrid offspring is poor

premating barriers (no mating at all):

-geographic isolation (islands, mountain ranges)

-temporal isolation (differences in mating season, flowers open and different times)

-behavioral isolation (changes in mating calls or behavior, mechanical isolation)

-ecological isolation (pollinator shifts, habitat preferences)

postmating, prezygotic barriers (can mate but no zygote formed):

-sperm and egg incompatibilities

-cryptic female choice (if females are inseminated from multiple individuals but make zygotes only with own 'species')

postzygotic barriers:

-the reduced viability or fertility or hybrid offspring

questions in the study of speciation:

1. what are the barriers that are currently operating between species, and to what extent does each of these barriers currently reduce gene flow?

2. when did each of these barriers arise in the history of the diverging lineages, and therefore which of the barriers was initially responsible for a reduction in gene flow?

3. in what geographic context did the barriers arise?

4. what roles have evolutionary processes (drift, selection) played in driving the divergence?

5. what is the genetic architecture of reproductive isolation?

Californian monkey flowers

-measured barriers to gene flow in two sister species

-mostly ecological distribution

-visited by different pollinators

hawaiian crickets (assortive mating)

-males have mating songs with different pulse rates

-females preference for pulse strongly genetically linked to male pulse

Japanese winter moths (temporal isolation)

-intensity of winter varies by latitude

-northern moths have two temporally isolated populations

-southern sites have no temporal isolation

quick review:

*species are discontinuous groups created through a continuous process

*once the process is complete species are easy to distinguish, but before then it can be challenging

*biologists typically focus on barriers to genetic exchange that can be premating, prezygotic, and postzygotic

*this process has been studied in many organisms and occurs in many ways