The Origin of Species – Speciation, Concepts, Hybrids, and Natural Selection

Hello everyone! My name is Sai Rachakonda.

Welcome to our continuing AP Biology/ General Biology series here on STEM Talks! Recently our series has been picking up interest so if you want to help write/edit posts, please let me know!

This post will talk about the Origin of Species as it relates to:

  1. Species Concepts
  2. Speciation (reproductive barriers, geographic separation, sympatric )
  3. Hybrids (hybrid zones, reinforcement, fusion, stability)
  4. Speciation speed

The origin of the vast diversity of life on Earth has always befuddled man in our quest to find answers to these complex questions. It has been described as a “mystery of mysteries.”

Darwin formulated his theory of Natural Selection based on the Species and biodiversity of finches he saw on his journeys to the Galapagos islands – a relatively isolated ecosystem.

However,  a good question comes up: how does one define a species and how does a species split to form two new species? Also, what is the nature of the reproductive barriers between species that may allow hybrids in some cases but in others no offspring? We will explore these questions in this post.

First we must go through the various species concepts:

  1. Biological – a group of populations that can make viable (fertile) offspring with each other but not with populations of other groups. The viability of offspring and even the potential to have offspring is covered by reproductive barriers. The downside to this is that it doesn’t work for extinct or nonsexual species as viability cannot be tested.
  2. Morphological – species are distinguished when they have different anatomical shapes or physiological functions. This species concept can be applied to any organism and any reproductive method unlike the Biological concept. The downfall here is that this concept assumes morphological differences are the result of substantial genetic differences.
  3. Ecological – this concept states that each species fulfills a specific niche and species are distinguished from one another if they fulfill a different niche. A con for this is that niches are hard to determine in the field for many species.
  4. Phylogenetic – a species is the smallest group of individual organisms that share a common ancestor. This means that if there is an organism that is slightly different or shares a common ancestor further down, it is not part of the species. However, the problem of how small one should go or how small the group should be arises here.
  5. Recognition – Classification of species based on mate selection. If an individual chooses to mate with another individual, that must mean they are in the same species according to this concept. A shortfall of this is that it cannot apply to asexual organisms.

The Biological Species Concept is the most viable of these 5 and therefore is used most commonly.

In the BSC, speciation (formation of new species) relies on Reproductive Isolation as it decreases the viability between estranged populations due to microevolution of the two populations. Certain barriers exist such that species do not intermingle however some groups who have recently undergone or are undergoing speciation can form hybrids.

We will now explore these mechanisms. They can be assorted into two groups:

  1. Prezygotic Barriers – block fertilization of two different species
  2. Postzygotic Barriers – reduced viability of the offspring of two species such that they may not be able to reproduce but have fertilized

We will now enumerate the types of each:

Prezygotic:

  1. Isolation by Habitat – two species in the same area will not interact, having no gene flow
  2. Behavioral Isolation – mate is species specific (sexual selection) and will not breed with those of other species without the desirable traits
  3. Temporal Isolation – Species breed at different times of day/year so there is limited gene flow
  4. Mechanical Isolation – Occurs when reproductive organs won’t physically function together so fertilization cannot take place
  5. Gametic Isolation – There is no biochemical recognition between sperm and egg so fertilization is impossible

Post Zygotic:

  1. Reduced Hybrid Viability – zygote from fertilization will die or be diseased
  2. Hybrid Infertility – zygote becomes a living organism but it lacks the ability to reproduce
  3. Hybrid Breakdown – Second or subsequent generations have reduced lifespans and may not be able to reproduce.

Note: Hybridization is the process of interbreeding between two populations of organisms that have come together after becoming separated.

Now speciation can occur with or without geographic separation. The following were edits made to the biological species concept over time.

Allopatric speciation is when a population is separated by geology and is isolated. The splinter group will most likely have a different gene frequency. Genetic Drift will then continue until there are significant genotypic and phenotypic differences. Different selection pressures will also be present. This will then cause speciation of the isolated groups.

Sympatric speciation is the formation of a new species from a subpopulation while located amongst a parent population (meaning no geographic isolation). Reproductive barriers will form in sympatric speciation through polyploidy, sexual selection, and habitat differentiation.

Polyploidy is an accident in fertilization of two gametes with different chromosome numbers. For example, a gamete with 3 can fertilize with one of 7 and make an offspring with 10 chromosomes (haploid) or 20 diploid. There are two types of polyploidy – Autopolyploidy and Allopolyploidy.

In Autopolyploidy an individual has more than 2n sets of chromosomes which came from the same species (self fertilization within species).

Allopolyploidy is the formation of a stable hybrid of two different species that underwent fertilization and polyploidy.

Habitat Differentiation is the term given to the situation when a group exploits a resource or fulfills a niche that was not used by the parent group in the same area. Over time, genetic drift here can lead to speciation.

It is important to note that pre and post zygotic barriers are not set in stone. When two species that do not have completely separated barriers come together, they form hybrid zones where offspring of mixed ancestry is produced. Hybrid Zones usually occur along the boundaries of two geographic areas where formerly related species dominate respectively.

Over time, these hybrid zones will change. In Reinforcement, the number of individuals in the hybrid zone will shrink as reproductive barriers strengthen, reducing the viability of hybrids. In Fusion, the already weak reproductive barrier will become weaker allowing the two species to fuse into one again. In Stability, the reproductive barrier strength remains the same and the number of hybrids and the size of the hybrid zone stays constant as well.

I hope you have found this information useful

-Sai Rachakonda

Co-founder STEM Talks NC

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