What is a species and how do they arise? (ch 16)

• a group of organisms that successfully reproduce with one another

• share a common ancestor

• share many alleles

species

divergence of biological lineages and emergence of reproductive isolation between lineages

• how new species form

speciation

classifies species based on physical traits, can be applied to both sexual and asexual organisms

morphological species concept

what are some limitations to morphology

• males and females of a species may be different, and immature individuals may not look like their parents

• cryptic species

are morphologically the same, but do not interbreed

cryptic species

if two organisms can mate and produce fertile offspring, they are considered the same species

biological species concept

defines a species as a single, continuous line of ancestry

• species as branches on the tree of life, includes asexually reproducing species

lineage species concept

What do you think is ultimately the most important factor for

generating distinct species?

Reproductive isolation

Does all evolutionary change result in new species?

No, speciation requires interruption of gene flow

how can one lineage ever split into two reproductively isolated species?

genetic incompatibility must occur

A population is subdivided and then the two groups evolve independently

The Dobzhansky-Muller model of genetic incompatibility

As species diverge genetically, reproductive isolation ______

increases

• genetic incompatibility between the two isolated populations will develop overtime

• chromosomal rearrangements also support _______ _______ model

Dobzhansky-Muller

the more genetic differences the more likely a new ______ will form

species

physical barrier that divides a species range, more common

• typically, a large population (genetic diversity is usually higher and when they get separated, they can evolve in different directions over time)

allopatric speciation

does not require large-scale geographic distance, not as common

• when new species form within the same area, without any physical barrier

• usually happens because different groups within that population start using different resources or develop different behaviors

• overtime these differences can lead to them becoming distinct species

sympatric speciation

exploring new niche, microhabitats, different pollinator glands

disruptive selection

individuals with similar traits tend to mate with each other

• variation in flowering/mating times, sexual selection

assortative mating

two causes for sympatric speciation:

• disruptive selection

• assortative mating

prevents hybridization from happening

• preventing species from being able to mate

prezygotic isolating mechanisms

reduces fitness of hybrid offspring

• offspring will not be able to reproduce/survive

postzygotic isolating mechanisms

• mechanical isolation

• pollen incompatibility

• temporal isolation

• behavioral isolation

types of prezygotic isolation

physical differences (shape, size, etc) in reproductive structures that make it difficult or impossible for individuals from different species to mate successfully

mechanical isolation

pollen can’t successfully germinate and is stopped early on

pollen incompatibility

‘Isolated in time’

• prevents species from mating because they breed at different times

temporal isolation

type of reproductive isolation that happens when two populations don’t interbreed because their behaviors don’t match up

• ex. courting dance vs pointing display

  • female birds will recognize male birds, as a potential mate, if they’re doing the correct dance

behavioral isolation

• low hybrid zygote viability

• low hybrid adult viability

• hybrid infertility

postzygotic isolation

hybrid zygotes may fail to mature normally

low hybrid zygote viability

hybrid offspring may have lower survivorship than non-hybrid offspring

low hybrid adult viability

hybrids may mature into infertile adults

hybrid infertility

ex. tangelos (grapefruit x tangerine)

• flower can produce pollen

• 2 tangelos together won’t flower

• cross pollination is needed for offspring

low hybrid fertility

can form if reproductive isolation isn’t complete

• ex. carrion vs hooded crow

• have substantial overlapped regions

• are they the same species? (if they can produce fertile hybrids most likely, if hybrids are infertile they are most likely different)

hybrid zones