Morphological novelty (cont.)

Recall the transition of bones forming the ancestral jaw joint into middle ear bones in Mammals.

The transition state between these two different conditions is seen in some Cynodonts, such as Probainognathus, which had a double jaw joint, with one articulation between the Quadrate/Articular, and the other between the Dentary/Squamosal.

When the line leading to mammals went back to a single jaw joint, it kept the Dentary/Squamosal, leaving the Quadrate and Articular bones to move into the middle ear.

This unlikely sounding scenario makes more sense when we realize that:
1) The Quadrate was in contact with the Stapes already and..
2) They were thus already probably functioning to transmit vibrations to the ear, as the lower jaw does in some modern lizards and snakes.

Sexual Selection

Selection can occur at different points in the life cycle.

Our discussion so far has concerned viability selection.
Something else must be going on as well, though, since males and females often look different despite living in the same environment.

Wish to understand:
    Sexual dimorphism
    Extreme, seemingly maladaptive, traits in males

First, some definitions:

Of course, it is possible to have all gametes the same size.
"Isogamy" - All gametes are the same size.
Anisogamy - Some gametes are large and others small.

Many single celled eukaryotes, and some multicellular algae, are isogamous.

By contrast, multicellular animals and plants, as well as most other multicellular eukaryotes, are anisogamous.

Evolution of anisogamy
There is a tradeoff between how big your gametes are and how many of them you can produce (few large vs. many small).

In multicellular organisms, there is generally selection for large zygotes, which favors producing large gametes.

At the same time, there is also selection to produce many gametes, which means making them small.

A consequence of these conflicting selection pressures is that isogamy becomes evolutionarily unstable - meaning that it becomes advantageous for individuals to produce gametes that are either a bit larger or a bit smaller than average.

Say that some individuals are producing larger gametes (which increase offspring survivorship). This creates an opportunity for others to benefit by producing smaller gametes, which reduce offspring survivorship but increase the number of offspring produced.

This process quickly produces a state in which some gametes are very large (eggs or ovules) and others are very small (sperm or pollen).

This difference in gamete sizes creates a situation in which the mating strategies of those producing big gametes are different from the strategies of those producing small gametes. This leads to sexual selection.

General trend: Because they produce a larger number of smaller gametes, males generally show a higher variance in reproductive success than females.
    This is true even without internal fertilization.

Darwin (in "The descent of man and selection in relation to sex") noted that in most cases, one male can (in principle) fertilize many females, who then must devote time and energy to incubating and feeding young.
Concluded that males should compete with one another for access to females, while females should be picky about which males they choose to mate with.

Darwin thus distinguished two modes of sexual selection:
    Male/male competition
    Female choice

We will see that these are general trends that are sometimes reversed.

Breeding systems:
Monogamy = One male with one female
Polygyny = One male with many females
Polyandry = One female with many males

Polygamy = polygyny + polyandry

Among vertebrates, when one parent can raise offspring alone, we most often see polygyny.

Here, a few males father most of the offspring while most males never mate
    This produces high variance in male reproductive success.

Males in such cases are selected to be large, aggressive towards other males, territorial, and often ornamented to attract females.

In such cases, males contribute relatively little to the care of offspring; their contributions to offspring are therefore limited to: 1) their genes, and 2) the territory that they control.

Females in such systems are therefore selected to be picky about whom they mate with and to prefer males with the best genes (more on this later) and the best territory.

These are in fact the traits that we see in polygynous animals:
--Large, aggressive males who try to mate with as many female as possible.
--Smaller, less aggressive females who are picky about which males they mate with.

A test of the theory is to look at cases in which it is the females who have the highest variance in reproductive success.

Example: Spotted Sandpipers.
Here, females mate with many males, who incubate the eggs.

As expected, females are ~20% larger than males and aggressively territorial towards other females.

Here, most females never mate.

This raises a question: How to get to a state of polyandry from the more common case of polygyny?

When conditions are bad, related sandpiper species become monogamous.
When conditions improve, such pairs often produce two clutches of eggs, one incubated by each parent.
This strategy, known as "double clutching" makes it easier for the female to desert and mate with other males, once the first male is sitting on a clutch of eggs.

Reception of sexual selection theory
Sexual selection involving female choice was one of Darwin's least popular ideas. It was rejected by most biologists at the time, including Wallace.

One reason that the idea was unpopular was that it seemed unlikely that females would prefer to mate with males that had traits that would reduce their survivorship.

Extreme dimorphic traits
Almost always in males.
Often maladaptive with respect to survival.

So, why do females like such traits? There are two major hypotheses:

Runaway Sexual Selection
If there is initially some preference among females, then females are selected to mate with males that have the preferred trait just because it is to their advantage to have sons who exhibit that trait (since other females like it.)
If the genes controlling the male trait are genetically correlated with those that control the female preference (i.e. there is gametic disequilibrium between them), then an increase in either the preference or the trait tends to carry the other along with it.
This can lead to extreme traits and preferences, even though the male traits serve no other function and may reduce his viability.

A version of this idea was suggested by Fisher in the early 20'th century, but not widely accepted until the 1980's, when mathematical models showed that it could work.

The necessary condition: Runaway sexual selection can work only if there is some genetic covariance between the trait in one sex and the preference for that trait in the other sex.
The key:If there is any preference to begin with, then this will cause nonrandom mating that creates gametic disequilibrium between the preference and the trait.

Thus, the nonrandom mating inherent in sexual selection creates the condition necessary for runaway sexual selection.

Prior bias in mating preference

An interesting observation is that females sometimes have a prior preference for a stimulus, even if no males posses it.
Example: Female Zebra Finches like males with white crests, even though male Zebra Finches do not have crests.
Females prefer males that have had white crests glued to their heads.
Other colors of attached crests are not preferred.

Among Swordtail fish, males have a long extension on the tailfin, and females prefer males with this extension.
In the most closely related species without a swordtail, females still prefer males to whom a tail augmentation has been attached by researchers.
Only in more distantly related species do females not show this preference.

It thus appears that the female preference for the elongated tail evolved before any males had such a tail.

Note that, in the absence of a swordtail, preference for one is a neutral trait, since it is never expressed.
It thus can evolve by drift.

Indicator Traits
For traits like size and brightness of plumage, or even antler size, the expression of the trait in a male may be a good indicator of other factors that are related to overall high genetic fitness. (Such traits are generally noticeably less pronounced in an unhealthy individual.)

Thus, females may prefer males that show strong expression of such traits because the are correlated with other traits that are desirable, but not directly observable.. Jul 8, 2021